WO1999019751A1 - Method and apparatus for drill stem data transmission - Google Patents

Abstract

A drill stem communication system for transmitting data between downhole and uphole locations in a borehole includes at least one transducer (40, 40') at a first location on a drill stem (18) for modulating motion and/or stress in the drill stem (18), and at least one transducer (42') at a second location on the drill stem (18) for detecting the modulation. Control devices (36, 260, 312, 382) may be provided at both locations for establishing communication therebetween.

Description

METHOD AND APPaSaRATUS FOR DRILL STEM DATA TRANSMISSION

Background of the Invention

This application claims the benefit of U.S. Provisional

Application No. 60/062,214 of Arthur F. Kuckes, filed October 16,

1997, and entitled "Drill Stem Stress Modulation for Data

Transmission" , the disclosure of which is hereby incorporated

herein by reference, and is a continuation-in-part of U.S.

application no. 08/979,991, filed November 26, 1997, the

disclosure of which is hereby incorporated herein by reference.

The present invention relates, in general, to data

transmission in drilling systems of the type used in gas and oil

well drilling and to the generation of electrical power downhole.

More particularly, the invention relates to a method and

apparatus for modulating the motion of a drill stem at one

location on the stem and for detecting the modulation at a second

location for establishing communication between the locations,

and further to a method and apparatus for generating electrical

power downhole and for supplying such power to downhole

electronic equipment.

The need for better telemetry methods and apparatus for

communicating data to downhole drilling equipment or uphole from

such equipment is widely recognized. For example, there is a need to send data representing desired drilling parameters such

as desired borehole inclination, azimuth and tool face from a

surface location to control equipment located downhole at the

drill bit. Similarly, there is a need to send data representing

drilling conditions, such as inclination and azimuth, etc., from

downhole sensing and control equipment to the surface. Such data

needs to be transmitted rapidly and accurately while the drill is

in operation, in order to avoid unnecessary, time-consuming and

expensive down time.

Conventionally, drilling of deep boreholes is carried out

with a pressurized drilling fluid which can be used to drive

drilling equipment at the bottom hole assembly and to carry away

the debris produced by the drilling operation. The dominant

method for borehole communication in use at the present time

involves the generation of pressure pulses in the drilling fluid.

For communication from a downhole location, pressure pulses are

generated downhole in the drilling fluid, as by periodically

interrupting the fluid flow, and the resulting pressure pulses

are carried in the fluid and are detected at the surface. The

most serious deficiency of such pressure pulse systems is their

very slow data rate, which is limited to approximately one bit

per second. .Another deficiency is the inability of such systems

to transmit data when underbalanced drilling fluids containing gas bubbles are employed, for such bubbles absorb the pressure

pulses before they reach the surface.

Electromagnetic systems have been developed in which drill

string currents are generated and modulated, and these have a

higher data rate than is possible with pressure pulse systems;

however, these systems have a very limited range.

.An additional problem encountered in downhole communication

systems is the need for an adequate supply of electrical power

downhole for operating sensing and control circuits and

equipment, not only for operating a communication system, but for

controlling the operation of the drill in response to received

data. Batteries are unsatisfactory for this purpose because of

their limited life, due in part to the adverse conditions at the

bottom of a borehole. Fluid-driven turbines have been used to

generate electrical power, but these require access to the fluid

flowing inside the drill stem.

Thus, there is a need for an alternative source of

electrical power for downhole operations which does not interface

with fluid inside the drill stem. Further, there is a need for

improved communication of sensed data and control information

along a drill string between the surface and a downhole location

in a borehole, and vice versa.

Summary of the Invention The drill stem data communication system of the present

invention has several aspects which may be utilized separately or

in any combination to meet the particular needs of a borehole

drilling situation. In general, the invention relates to a

method and apparatus for transmitting data between locations

along a drill stem. A first aspect of the invention is directed

to the transmission of data from a downhole location along the

drill stem to a surface location. In a second aspect, the

invention is directed to the transmission of data from a surface

location along the drill stem to a location in the borehole. A

third aspect of the invention is directed to the transmission of

data from a downhole location to a surface location, and then

from the surface back to the downhole location to provide a

feedback control of downhole equipment. A fourth aspect includes

a method and apparatus for generating electrical power for the

downhole electronic equipment which is used to receive and

transmit data signals and for the equipment which operates the

electronic controls for directional drilling in the borehole. In

a preferred form of the invention, all these aspects are combined

in a drilling control system.

Briefly, the present invention relates to a method and

apparatus for modulating torsional or axial stress and/or motion

in a drill stem for transmitting data uphole and downhole in a

borehole being drilled. One embodiment the invention relates to a method and apparatus for modulating torsional stress and/or

rotation by varying the rate of rotation of a drill stem. The

invention also relates, in another embodiment, to a method and

apparatus for producing axial stress waves in the form of axial

motion or of variations in tension or compression in a drill stem

to transfer data. In still another embodiment, the invention

relates to a method and apparatus for generating electrical power

downhole and at the same time transmitting data in response to

the rotation of the drill stem. To transmit data from one

location to another along the drill stem, the motion of the drill

stem is modified at one location, such as at a downhole location,

to create motion or stress variations in accordance with the data

that is to be transmitted, and sensors are provided at the other

location, such as at the earth's surface, responsive to such

variations to recreate the transmitted data for monitoring the

downhole operation and for control purposes.

More particularly, a borehole communication system in

accordance with one embodiment of the invention includes surface

apparatus for generating data signals to be transmitted downhole.

These signals are used to impose corresponding stress or motion

variations on the drill stem, and a sensor at a second location

along the drill stem, such as at the bottom hole assembly,

responds to the stress or motion variations to produce

corresponding data signals which are used at the second location for control purposes. In a preferred form of the invention, the

drill stem is rotated by a drive motor at the surface, and data

signals are encoded and used to vary the rotational speed of the

motor to impose torsional stress variations on the drill stem as

its rate of rotation is modulated. The resulting changes in

drill stem rotational speed can be detected at the downhole

location and decoded for regulating the drilling operation.

Alternatively, axially variable stresses can be applied to the

drill string at the surface, as by raising or lowering the drill

string in incremental timed steps to release pressure or tension

in bursts either at the surface or downhole to produce axial

stress pulses, or shock waves, which are detectable as motion or

stress variations at a remote location. The stresses induced in

the drill stem at a first location are detected by a sensor at

the second location, which may be at a bottom hole assembly where

conventional drilling control circuitry is located.

In the case where torsional drill stem stresses are caused

by variations in the rotational speed of the drill stem, the

sensor preferably includes accelerometers or apparent earth's

field sensors. In another embodiment, the sensor may be an

alternator coupled to the drill stem and driven by the stem

rotation to produce an alternating output current having a

frequency corresponding to the rate of rotation. Such an

alternator may be incorporated in a drill stem stabilizer which is fixed to the borehole wall at the downhole location. Such a

stabilizer is illustrated, for example, in U.K. Patent No.

2,177,738B entitled "Control of Drilling Courses in the Drilling

of Boreholes" wherein stabilizers are utilized for dynamic

control of the bend of the drill string to control the direction

of drilling.

When an alternator sensor is used, the alternator rotor, in

the present invention, may consist of plural magnets located on

the drill stem, with stator windings located in the stabilizer,

which surrounds the drill stem. The rotation of the drill stem

produces an AC current output from the stator windings, and this

current is rectified to produce a direct current for powering

downhole circuitry. Changes in the rate of rotation due to

varying torsional stresses produced at the surface, for example

in response to changes in the motor drive current, cause changes

in the frequency of the output current from the downhole

alternator, which changes can be detected and demodulated to

produce data signals corresponding to the data being transmitted

from the surface location. The data signals may then be used to

provide instructions to the downhole equipment.

In another form of the invention, the rotation of a rotary

drill stem can be modulated by turning the drive motor on and off

in a pattern corresponding to the control signals to be transmitted, and the on and off pattern is detected at the bottom

of the drill hole to provide suitable control signals.

In still another embodiment of the invention, the

communication system includes a data source downhole which is

responsive to selected downhole parameters to produce data

signals to be transmitted to a surface location. The data source

may include sensors located at a drill collar, for example, to

measure conditions of operation downhole and at the drill head

and to produce corresponding output signals. These data signals

are used to modulate the rotation of the drill stem, as by

activating and deactivating a mechanical brake for applying a

variable drag force to the drill stem to change its rate of

rotation. This braking sends torsional stress waves along the

drill stem, and such waves are sensed at the surface, as by

strain gauges, accelerometers or the like. The surface sensor

output is then demodulated to provide data signals at the surface

location corresponding to the data signals produced downhole.

The foregoing torsional systems for modulating and launching

stress waves and motion in the drill stem serve to encode digital

information in the steel of the drill string. These waves travel

along the drill string and are detected and processed to recover

the encoded digital data. It is well known that torsional waves

can be transmitted along a drill string, for in the past the

performance of drill bits has been studied by noting at the surface the time variation of torsional noise stresses generated

by the drill bit during operation. Such noise stresses can be

avoided while using the communication system of the present

invention by removing the weight of the string from the drill bit

while transmitting data. Alternatively, the torsional stress

modulation used for the transmission of data can be carried out

in a narrow band frequency channel where there is little noise

from the operation of the drill bit. However, it has been found

that under most conditions there is in fact relatively little

drill bit noise transmitted to the drill stem because of the

weight of the drilling collars, so there is relatively little

interference with the communication system of the present

invention from this source.

.Axial stresses, which may include axial stress waves and

axial motion, may also be used for drill string communication, in

accordance with the present invention. In one form of the

invention such stresses and motion are generated at the surface

by raising and lowering the drill string. The stress waves are

produced by prescribed incremental lifting or lowering steps and

are detected at a bottom hole assembly at the downhole location

by axial accelerometers. The output signals from the

accelerometers are demodulated and twice integrated with respect

to time to recover the encoded data from the surface. Axial stress waves may also be produced downhole for use in

transmitting data to the surface. For example, standard mud

pulsing, which conventionally is detected at the surface by

measuring pressure variations in the drilling fluid, can also be

sensed uphole by detecting associated axial stress changes in the

drill stem. This is effective, since the attenuation of axial

stress in the steel pipe of the drill stem is significantly less

than the attenuation produced by the drilling fluid, particularly

at the higher frequencies desired for good data transmittal .

In another form of the invention, data relating to

conditions and parameters of operation downhole and at the drill

head are converted to electrical signals which are transmitted

along the electrically conductive drill pipe to an axial stress

pulse transducer located, for example, 100 meters above the

sensors at the drill head. The transducer may be an hydraulic

pressure modulator which is controlled by the encoded data

signals to mechanically produce stress pulses in the drill stem.

These pulses can be in the form of axial motion imposed on the

stem or axial stress imposed on the stem, to produce variations

in tension or compression. Such variations travel along the

drill stem and are sensed at the surface. The transducer

preferably includes a pressure chamber and a release chamber,

both containing hydraulic fluid. Downward pressure of the drill

stem during drilling causes fluid to flow into the pressure chamber from the release chamber. To effect communication, the

drill stem is lifted, causing the lower 100 meters of the stem to

be supported by the transducer and thus to exert a force which

pressurizes the hydraulic fluid in the pressure chamber. This

fluid can then be released from the pressure chamber in

incremental bursts by a solenoid valve under the control of

encoded data signals. Each time the valve is opened, the weight

of the 100 meter long drill stem below the transducer causes the

stem to drop a distance corresponding to the amount of fluid

released by the valve. When the valve is closed, the motion of

the stem is halted. The release of the 100 meter section of

drill stem reduces the tension in the portion of the drill stem

above the transducer, and closing the valve restores the tension.

This produces axial shock waves, or stress pulses in the upper

part of the drill stem which travel up the drill stem to the

surface. A suitable sensor such as a strain gauge is located on

the drill stem at the surface to measure received axial shock

waves and the resulting stress signals are then demodulated to

reproduce the downhole data signals at the surface.

In a variation of the foregoing, the hydraulic pressure

chamber in the transducer may be primed by a fluid pump, rather

than by the weight of the drill stem.

In another aspect of the invention, data to be transmitted

downhole is encoded at the surface, and is used to control a surface hydraulic transducer which generates axial stress or

shock waves for transmission downhole. In one form of this

aspect of the invention, a hydraulic pump is operable to pump

hydraulic fluid into a pressure chamber at or near the earth's

surface. The weight of the drill stem and drill head is carried

by the hydraulic fluid in this chamber, thereby placing the fluid

under high pressure. A solenoid valve controlled by the encoded

data is operated to release hydraulic fluid from the pressure

chamber in incremental steps or bursts, each of which serves to

drop the drill stem a short distance to produce corresponding

axial shock waves, or pressure pulses. These shock waves travel

along the length of the drill stem in an axial direction and are

detected by accelerometers or by strain gauges located near the

bottom of the borehole. The variations in stress so detected are

converted to electrical signals which are demodulated to produce

corresponding data control signals at the downhole location.

In accordance with the invention, therefore, data

communication in a borehole is carried out by variations in drill

stem rotation, by axial stress waves generated at the surface, by

axial stress waves generated downhole, or by a combination of

these techniques .

Data transmission between the surface and the downhole

location is preceded by a suitable command sent from the surface

to signal the downhole electronics to start a data transmission, or to prepare to receive a data transmission. Thus, for example,

a specific sequence of starting and stopping of the fluid pumps

and/or drill stem rotation can be used, or a sequence of lifting

or lowering the drill stem can be used. The sequence is specific

to the encoded command, and after the command is given,

transmission of encoded data begins.

The system of the present invention is particularly well

suited to the establishment of communication links along the

drill stem in the controlled rotary drilling of directional

holes, not only because torsional or axial stress can easily be

induced, but because the rotation of the drill stem can be used

as a source of power for the control circuitry downhole.

Detailed Description of Drawings

The foregoing, and additional objects, features and

advantages of the present invention will be more clearly

understood by those of skill in the art from the following

detailed description of preferred embodiments thereof, taken with

the accompanying drawings, in which:

Fig. 1 is a diagrammatic illustration of a rotary drilling

system for oil/gas wells, incorporating a downhole stress

modulator and a surface detector unit for data transmission in

accordance with the present invention; Fig. 2 is a diagrammatic illustration of an hydraulically

controlled drill stem stabilizer brake for producing torsion

modulation for the system of Fig. 1 ;

Fig. 3 is a cross-sectional illustration of an MWD drill

string control unit with an hydraulic controller for the drill

stem torsion modulator of Fig. 2, and further illustrating the

provision of a downhole alternator;

Fig. 4 is a cross-sectional illustration of a second

embodiment of a torsional stress modulator for use in the system

of Fig. 1, utilizing inertial stress modulation;

Fig. 5 is a cross-sectional view taken along line A-A of

Fig. 4, showing a brake for producing stress modulation;

Fig. 6 is a cross-sectional illustration of a surface

detector unit for the system of Fig. 1 ;

Fig. 7 is a diagrammatic illustration of an alternative

surface detector unit;

Fig. 8 is a diagrammatic illustration of another embodiment

of the invention, incorporating a surface axial stress modulator

and a downhole sensor; and

Fig. 9 is a cross-sectional diagrammatic view of downhole

electronics for an axial stress detector;

Fig.10 is a cross-sectional view of a first embodiment of a

downhole stress wave transducer for use in the system of Fig.l; Fig.11 is a cross-sectional view of a second embodiment of a downhole stress wave transducer;

Fig. 12 is a cross-sectional view of a fluid pump for

supplying hydraulic fluid to the stress wave transducer of the

invention; and

Fig. 13 is a cross-sectional view of a surface stress wave transducer.

Detailed Description of Preferred Embodiments:

Turning now to a more detailed description of the present

invention, there is illustrated in Fig. 1 a rotary deep well

directional drilling system 10 for drilling oil or gas wells.

The drilling system includes, for example, a drilling derrick 12

mounted on a drill platform 14 located on the earth's surface 16.

The derrick supports a drill string 18 which extends into a

borehole 20 in the earth, the drill string being supported by a

top drive motor 22 secured to the derrick 12 by a suitable cable

and elevator lifting mechanism generally indicated at 24. The

top drive motor 22 receives power from an alternating current

source 26 through a motor control circuit 28 by way of line 30,

as is conventional in the petroleum industry. The drill string

18 includes, at its distal end, a bottom hole assembly (BHA) 31,

which includes a drill bit 32 driven by rotation of the drill

stem 18 by the motor 22 to produce the borehole 20. Carried by the drill stem near the drill bit 32 is a measurement while

drilling (MWD) control unit 34 for use in providing control

signals for operation of the drill bit, and for producing data

relating to borehole inclination and azimuth, as well as other

parameters, such data being collected for transmission to the

surface for use in monitoring and controlling the operation of

the drill bit.

To provide communication between the MWD controller 34 and

surface equipment such as a computer 36 or other control

equipment, the present invention provides a drill stem stress

modulator 40 in a downhole location near the control unit 34.

The drill stem stress modulator 40 generates either axial or

torsional stress waves on the drill stem in response to data

signals produced by the control unit. The modulator 40 may

consist of any desired mechanism for generating torsional or

axial stress waves on the drill stem corresponding to the data

which is to be transmitted to an uphole location.

A suitable surface detector 42 may be in the form of a

strain gauge or an accelerometer, for example, which produces

corresponding output signals on line 44 representing the data

signals generated at the modulator 40. These output signals are

supplied to the computer 36 which processes the data in a well-

known manner to provide outputs to a display 46 and/or by way of

line 48 to the motor controller 28 for regulating the operation of the rotating drill string 18. As described above, since the

axial or torsional waves produced the modulator 40 can be

transmitted accurately along the drill stem 18, data signals from

the control unit 34 can be rapidly and accurately communicated to the surface .

Examples of the torsional stress modulator 40 and the drill

string control unit 34 are illustrated in Figs. 2 and 3,

respectively, to which reference is now made. The torsional

stress modulator 40 in one embodiment is a hydraulically-actuated

brake which includes a spiral fin stabilizer body 50 of

conventional design fabricated as a part of a drill stem section

52 connected into the drill stem 18. The stabilizer 40 is shown

above the control unit 34, and is of conventional design,

incorporating, for example, 18 hydraulically-activated pistons 54

located in spiral fins 55. In an exemplary brake modulator, each

spiral fin may carry six hydraulically-actuated piston buttons,

each 1.75 inches in diameter. A stabilizer with such a

configuration is marketed as an adjustable stabilizer for drill

stem stiffness control by Andergauge Drilling Systems. The

pistons, when activated, engage the side wall of borehole 20 and

serve to position the stabilizer body 50 with respect to the

borehole .

In the present invention, an external hydraulic line 56 is

connected to direct pressurized drilling fluid into the cylinders in which the pistons 54 are mounted. The drilling fluid expands

the pistons against the wall of the borehole 20, with the pistons

being dimensioned to produce a significant drag on the borehole

wall when they are activated. By regulating the flow of drilling

fluid in the hydraulic line 56, the pistons are expanded and

contracted to cause the pistons to brake or release the drill

stem and thus cause variable torsional stresses in the drill

string. This braking action produces torsional waves in the

drill stem 18 which can be detected at the surface detector 42.

As illustrated in Fig. 3, the hydraulic line 56 is connected

to an electronically controllable hydraulic valve 60 located in

the drill string control unit 34 which is incorporated in a drill

stem segment 62 connected below the modulator 40 but above the

drill bit 32. High pressure drilling fluid is supplied from the

surface and flows downwardly through the drill stem core 64,

flowing through both the drill string segment 52 illustrated in

Fig. 2 and the segment 62 illustrated in Fig. 3, and thence to

the drill bit 32, in conventional manner. In accordance with the

present invention, however, a small portion of the high pressure

drilling fluid is supplied from the core 64 through an inlet 66

and through the hydraulic valve 60 to the hydraulic line 56, with

excess fluid being bled through outlet 68 to the exterior of the

drill string segment 62 for return to the surface through the

drill string annulus 70. The hydraulic valve 60 is mounted in an MWD electronics

cavity 72 within the segment 62. Also included in the cavity, in

conventional manner, is a battery pack 74 connected to a sensor

module 76 which may include accelerometers, magnetometers, and

the like. The sensor module 76 is connected to a control

electronics module 78 which produces output signals on line 80

for controlling the hydraulic valve 60.

The hydraulic control valve 60 is similar in design to those

found in conventional MWD fluid pulse modulator systems which are

used to generate fluid pressure pulses. However, in the present

invention, the hydraulic valve 60 is used to produce hydraulic

fluid in line 56 under pressure to operate the pistons 54, as

discussed above, to produce a torsional braking effect rather

than to produce pressure pulses in the fluid flowing in core 64

or in the borehole 70.

The control electronics module 78 includes circuitry

responsive to the sensors in module 76 to produce output data

signals corresponding to acceleration values, magnetic field,

gravity measurements, or any other desired parameter, the control

electronics converting the detected parameters into data signals

which are encoded and used to activate the hydraulic valve 60.

The opening and closing of valve 60 under the control of the data

signals produces variations in the hydraulic pressure in line 56

to activate and deactivate the pistons in modulator 40 to thereby produce torsional stress modulation, or stress waves, in the

drill string for detection at the surface. The surface sensor 42

detects the stress waves and produces corresponding signals on

line 44 which are delivered to computer 36, where the data is

utilized in conventional manner to obtain measurement of

conditions in the borehole and to control the operation of the

drill, among other things. Although the control electronics

module 78 is illustrated as controlling only the hydraulic valve

60, it will be understood that the signals may also be used to

provide steering signals to the drill bit 32, and may be used for

various other purposes as is known in the art .

Not only are the stress waves produced in the drill stem by

modulator 40 measurable at the surface, but they are also

measurable by the MWD sensors 76 to provide feedback signals to

the control electronics 78. These feedback signals enable the

control electronics to regulate the hydraulic control valve 60 to

adjust the amplitude of the stresses induced by the modulator 40.

The drill system MWD control unit 34 diagrammatically

illustrated in Fig. 3 is of conventional design, and because it

is adapted to rotary drilling systems it readily receives

communication from the surface. Such communication may be

provided by varying the drill stem rotary speed in a programmed

manner, with the changes in RPM of the stem being detected by the

accelerometers or magnetometers in the downhole sensors 76. The downhole sensors then produce control data signals for the

control electronics 78 to enable the operator of the drill system

at the surface to control the operation of the MWD control unit.

It should also be noted that although the control unit is shown

as being powered by a battery pack, the power for the control

unit can be supplied by other conventional sources, such as by a

fluid-driven turbine alternator in the drilling fluid flow

stream.

The torsional stress modulator 40 described above and

illustrated in Fig. 2 requires contact between the pistons 54 and

the borehole wall to produce the required the stress waves.

However, such contact between the drill stem and the borehole

wall may not always be reliably available, and accordingly an

alternative form of the modulator is illustrated in Fig. 4 by an

inertial modulator 90. This modulator consists of a large

annular mass 92 mounted for rotation about a drill stem segment

94. The mass 92 is mounted on a set of bearings (not shown) to

allow free rotation of the mass about the drill stem segment 94.

As before, high pressure, high velocity drilling fluid, indicated

by arrow 96, flows through a central passageway 98 in the drill

stem segment and, after flowing through and around the drill bit,

flows upwardly around the drill stem in the annular space 70 in

conventional manner. The average motion of the mass 92 is

governed by friction between it and the fluid 96 flowing upwardly in the borehole and by the proximity of the exterior surface of

the mass to the borehole wall. The mass 92, which may have a

weight of about 1000 pounds, will tend to remain stationary as

the drill stem is rotated so that modulation of the torsion in

drill stem 18 can be accomplished by a brake mechanism, indicated

at 100 in Fig. 4, operating between the mass 92 and the drill

stem segment 94, as will be described below.

A preferred alternative to maintaining the mass 92

stationary by friction between it and the fluid within the

borehole is illustrated in Fig. 4, wherein the mass is driven to

rotate at a speed faster than that of the drill stem by means of

turbine blades 102 mounted on the interior surface of the

rotating mass. The drill stem segment 94 in this case will

incorporate a diverter 104 in the path of the high-pressure, high

velocity drilling fluid 96 flowing through passageway 98, this

diverter serving to direct some or all of the downhole drilling

fluid through the turbine blades 102. This causes the mass 92 to

rotate at a substantial speed; for example, 1000 rpm or more, to

provide an inertial mass which can be used to generate torsional

stress waves on the drill stem by means of the brake assembly

100.

A suitable brake assembly 100 is illustrated in cross-

sectional view in Fig. 5, the assembly incorporating an

hydraulically actuated brake band 110 secured at a first end 112 to the exterior of the drill stem 94, as by a suitable fastener

114. The brake band extends around the circumference of the

drill stem and is secured at its opposite end 116 to a piston 118

in a hydraulic cylinder 120. The hydraulic cylinder is connected

to the hydraulic line 56, described above with respect to Fig. 3,

which directs fluid into cylinder 120 through fluid inlet 122.

The introduction of fluid under pressure to cylinder 120 causes

piston 118 to expand the brake band 110 against an interior

surface 124 of the rotating mass 92. The brake band, which may

be a relatively thick elastic steel ring with an appropriate

brake lining, when expanded brakes the rotating mass in

accordance with the operation of the electronically controlled

hydraulic valve 60 under the control of the electronic package

78. Variation of the hydraulic pressure in line 56 cause the

brake band to engage the rotating mass to induce corresponding

torsional stress waves into the drill stem segment 94, which is

rotating at a different speed than the inertial mass 92. In the

case where the mass 92 is rotating at a relatively high relative

speed, the brake assembly can produce relatively high frequency

modulation on the drill stem using only simple and efficient

hydraulics. Furthermore, the drill stem itself need not be

rotating for the system to function when the mass 92 is rotating.

In this embodiment, the drill stem does not need to make contact

with the wall of the borehole for the system to operate. In addition, the drill head can be lifted off the bottom and the

drill stem rotational drive turned off to permit transmission of

data by torsional modulation in a virtually noise-free

environment .

The torsional stress modulation waves generated by the

modulators described above may be detected at the surface by the

drill string detection unit 42. Such a detection unit is

illustrated in greater detail in Fig. 6 as including suitable

strain gauge and accelerometer sensors 130 which are oriented on

the drill stem 18 and balanced so as to measure torque. Such

sensors are of conventional design and produce output signals on

line 132 which are delivered to a telemetry unit 134 which

includes conventional strain gauge sensing electronics and, in a

preferred form of the invention, a radio telemetry unit 136 for

transmitting the measured stress changes to the computer 36 by

way of radio receiver 138. The accelerometer and the strain

gauges have a large dynamic operating range so that small stress

changes are readily measured.

By noting the relative phase and amplitude of the strain

gauge and accelerometer signals, noise signals produced by the

drive motor 22 can be suppressed relative to those which are

being propagated from the downhole modulators 40 or 90. Usually,

the sensitivity of measurement uphole will be limited by stresses

induced by sources other than the downhole modulator. The dominant sources of noise are the drill bit and the drill stem

rotation drive. Drill bit noise is usually relatively small, in

most situations, but in those cases where it is not, data can be

transmitted reliably by removing the weight from the drill bit by

slightly lifting the drill string. Since the rotation drive

motor 22 may operate at 1000 horsepower or more, the noise it

induces may be important. However, the noise spectrum of the

drilling motor can be measured and an optimum frequency channel

selected for the transmission of data-carrying torsional stress

waves.

An alternative form of the surface detection unit 42 is

illustrated in Fig. 7, wherein the detection unit utilizes an

induction coil sensor 130 located on the motor control output

line 30 which carries the drive current for drive motor 22. The

induction coil 130 is, sensitive to changes in the current

supplied to the drive motor. Torsional stresses induced downhole

in the drill stem 18 vary the load on the drive motor 22 at the

surface, causing the drive current to vary as the motor attempts

to maintain a constant rotational speed. Such current variations

correspond to the modulation of the drill stem, and accordingly

the output of the sensor coil on line 132 corresponds to the data

input. This output is supplied to computer 36 for use in

providing information to the system operator and for utilizing the transmitted data for controlling the operation of the system,

as previously discussed.

The foregoing description has been directed primarily to

methods and apparatus for transmitting data uphole, as from a

location near the drill bit at the far end of a drill string, and

for sensing that data at the earth's surface. However, equally

important to the reliable operation of a drilling system is

effective and reliable communication in the opposite direction;

that is, the transmission of data from the surface to the

downhole control unit. Such surface data signals may initiate

the transmission of data uphole, but also provide control

commands for drilling direction and dogleg severity; i.e., hole

curvature .

Systems for communicating downhole are^' generally known, and

some of these systems may utilize the rotation or nonrotation of

the drill stem to switch downhole equipment on or off. However,

there is a serious problem with the operation of downhole control

circuitry because of the all-too-common difficulties with battery

power supply units in such circuits. The batteries are often a

weak point because of the adverse conditions that usually exist

in downhole locations. Fig. 3 illustrates a solution to this

problem, while at the same time providing an improved mechanism

for reliably detecting control and data signals transmitted from

the surface by modulation of the rotational speed of the drill stem. Thus, in accordance with the invention, an electrical

alternator is provided downhole and is generally indicated at

140. This alternator is responsive to the rotation of the drill

stem to produce an alternating current electrical output for

providing power to the downhole control unit. Furthermore, the

frequency of the AC output depends on the rate of rotation of the

alternator, so changes in the rotational speed of the drill stem

produce modulations in the output frequency. Such modulations

can be detected and then demodulated to produce transmitted data

encoded in variations in drill stem rotation.

The alternator 140 includes a stator 142 incorporating a

plurality of permanent magnets 143 supported on a collar 144

which is expandable against the inner wall of the borehole 20 by

a plurality of bowed springs 145. These springs hold the collar

stationary with respect to the rotating drill stem 18. A rotor

146 includes conventional motor windings 147 wound on laminations

148 which produce an alternating current output on line 149.

This AC output is connected to a rectifier 150, and the direct

current output of the rectifier is applied by way of line 157 to

the drill string control unit electronics module 78. The DC

current on line 150 acts as a power supply for the electronics

module 78, replacing the battery pack 74 utilized in previously-

described embodiments. The AC output from the windings 142 is also supplied by way

of line 149 directly to a frequency detector in the electronics

module 78 by way of line 152. The frequency of the alternator

output is dependent upon the rate of rotation of the drill stem

18 as noted above, and the speed of rotation of the drill stem is

controllable by the motor control 28 at the surface. Control

instructions and data provided at the surface by computer 36

regulate the speed of rotation of motor 22 by the motor control

28. The resulting changes in rotation of the drill stem 18

produce corresponding changes in the frequency of the output

signal on line 149 which is supplied to the electronics package

78 where the frequency is demodulated to reproduce the control

instructions and data from the surface.

An alternative embodiment to the rotational or torsional

stress embodiments described above is illustrated in Figs. 8 and

9, to which reference is now made. In this embodiment, axial

displacement modulation of the drill stem is utilized in place of

rotational stress through a controllable lift mechanism which

moves the drill stem longitudinally in selected increments .

Thus, the embodiment of Figs. 8 and 9 includes a derrick 12

mounted on a platform 14 at the surface 16 of the earth, with the

drill stem 18 being supported in a borehole 20 and driven by a

top drive motor 22. The motor 22 is secured in the derrick by a

conventional pulley and cable arrangement 24 for raising and lowering the drill string, with the motor 22 being operated under

the control of computer 36 by way of motor control circuit 28.

Additionally, the embodiment of Fig. 8 incorporates a lift

control mechanism 170 which is operated under the control of

computer 36 by way of line 72 for raising or lowering the drill

string 18 in increments of, for example, 3 feet, so that the

drill stem can be moved the length of a conventional drill string

segment in ten steps. Data to be transmitted downhole is

supplied to the computer 36, which provides corresponding

modulation signals on line 172 to cause the drill stem to move

upwardly or downwardly in one or more steps to encode the data in

axial steps of the drill stem. Because the drill stem is

constructed of steel or other materials which are essentially

inelastic in the axial direction, the vertical step motion of the

support mechanism 170 and of the drill string at the surface is

accurately and reliably transferred to corresponding steps at the

drill string control unit 180, where the vertical motion is

sensed and demodulated.

A suitable sensor for the axial position modulation of the

drill stem 18 is illustrated in Fig. 9, wherein the control unit

includes a Z-axis accelerometer 182 which produces an output on

line 182 corresponding to the measured axial acceleration of the

drill stem. This output, on line 184, is supplied through an

integrator circuit 186 which produces output signals on line 188 for the controller module 190 which may be a computer. The

control unit may include additional sensors 192, the outputs of

which are supplied through decoder 194 to the control module 190,

for detecting other parameters such as drill string rotation,

drilling fluid flow and the like. This information may be used

to control the starting or stopping of the controller 190 or for

other purposes, as is known in the art. The axial position

modulation solves the problem of inaccuracies that can occur in

rotational modulation since in the latter case the stem can twist

and thus provide inaccurate rotational data at the bottom of the

drill string.

Axial stress waves can also be used to transmit data uphole

through the provision of a downhole spring-loaded impact

mechanism, somewhat analogous to the "jars", or shock tools,

presently used for loosening stuck drill pipes. Following a

predetermined control sequence of drill stem rotations and

starting and stopping drill fluid flow, a powerful spring is set

in the downhole assembly by applying the weight of the drill stem

to the bit. The bit is then lifted slightly off the bottom to

initiate a timing sequence in the downhole modulator control

circuit. After a time interval which varies in accordance with

the encoded data to be transmitted, the spring is released by a

hydraulic cylinder. This sequence is repeated, to encode and

transmit uphole data words of arbitrary length. The release of the spring causes the drill stem to be impacted by an inertial

weight associated with the spring device, and the resulting axial

stress waves are detected uphole, using strain gauges and accelerometers in the detection unit 42.

The control circuit times the spring release after the drill

stem is raised, and the time delay encodes the data to be

transmitted in the resulting modulations of the axial stress in

the drill stem. Since the timing of the impact after the drill

spring is lifted can be done very precisely, data is transmitted

accurately.

Axial stresses can also be generated downhole, at transducer

40, in a number of ways. Thus, for example, the transducer may

be a modified joint between two adjacent segments of the drill

stem 18, and is illustrated in Fig. 10 as a splined, telescoping

joint 260 between upper and lower drill stem segments 262 and

264. The splined joint includes a plurality of longitudinal,

spaced grooves 266 on the interior surface of the upper segment

262 which receive corresponding spaced, elongated splines 268

formed on the exterior surface of lower segment 264. The splines

268 slidably engage the grooves 266 to allow the drill segment

264 to move axially with respect to the drill segment 262 while

preventing rotational motion therebetween. In this way, the

drill stem can extend and contract at the transducer 40 while

transmitting rotational motion from segment 262 to segment 264. The upper and lower drill stem segments 262 and 264 are

joined longitudinally by a piston and cylinder assembly 270 which

includes a piston 272 movable within a cylinder 274. In the

illustrated embodiment, the cylinder 274 is formed by a housing

276 mounted on or formed as a part of the lower end of drill stem

segment 262, the housing being generally cylindrical and

surrounding the lower end of the drill stem segment. The housing

includes upper and lower radially-extending walls 277 and 277'

which cooperate with the wall of the drill stem segment 262 to

form the annular cylinder 274. The piston 272 is annular and is

slidably mounted in the cylinder 274 to divide it into an upper

chamber 278 and a lower chamber 280, the piston being sealed to

the cylinder wall by 0-ring 282 and to the surface of the drill

stem segment 262 by O-ring 284.

Piston 272 is connected to, or formed as part of, the lower

drill stem segment 264, and thus is secured to segment 264 by a

connecting arm 286 in the form of a cylinder 286 and radial wall

288. The annular piston 272 is positioned within housing 276

with the connecting cylinder 286 extending through the bottom

wall 277' of that housing to connect the segment 264 to upper

segment 262. A pair of O-rings 290 and 292 are fixed in the

lower wall 277' of housing 276 and engage the inner and outer

surfaces of connecting cylinder 286 to provide a seal between the

cavity 280 and the exterior of housing 276. The upper and lower portions 278 and 280 of cylinder 274 are

filled with hydraulic fluid and the two sections are

interconnected through suitable valves to enable the piston 272

to move upwardly or downwardly with the cylinder. Thus, the

upper section 278 is connected by way of hydraulic line 300

through a one-way check valve 302 and through hydraulic line 304

to the lower chamber 280 of the cylinder. A second valve 306,

which is a solenoid-operated spool valve, connects line 304 to

line 300 by way of line 308 which bypasses the check valve 302,

thereby allowing fluid to flow from lower chamber 280 to upper

chamber 278 of the cylinder 274 when the valve 306 is opened.

The check valve 302 allows hydraulic fluid to flow freely

from upper chamber 278 to lower chamber 280 so that when there

is a compressive force on the drill string; that is, when drill

string segment 262 is pressed downwardly onto drill string

segment 264 to compress the transducer joint 260, hydraulic fluid

will flow from chamber 278 through check valve 302 to chamber

280, allowing the piston 272 to move upwardly in cylinder 274.

Such a compressive force would be applied to the joint 260 when

the drill string is used in its normal drilling operation, for

during this time, the lift mechanism 24 on the derrick 12 lowers

the drill string as it is being rotated by drive motor 22. When

the drill 32 engages the bottom of the borehole, the weight of

the drill string applies a compressive force on joint 260 and the piston 272 moves upwardly in cylinder 274. The splined

connection between segments 262 and 264 transmits the rotation of

the drill stem 18 to the drill head 32.

During the drilling operation, or after the drilling has

stopped, sensors 310 in the MWD control unit 34 produce output

signals corresponding to various measured parameters, and these

signals are supplied to a microprocessor 312 in the control unit.

The sensors may include fluxgate magnetometers, inclinometers,

gravity detectors, or like devices for measuring parameters of

interest, and the sensor output signals are converted by the

microprocessor to data signals which are to be transmitted to the

surface. The sensors 310 and the microprocessor (or computer) 312

are conventional and, in accordance with one embodiment of the

present invention, supply output signals by way of line 314 to

the drill stem 18, which is of an electrically conductive

material such as steel. The signals produce a corresponding

electrical current in the drill stem which may be sensed at the

transducer 40 by a torroidal coil 316 surrounding the drill stem

segment 264. These encoded signals are supplied by coil 316

through line 318 to activate the solenoid valve 306 to open and

close this valve in accordance with the encoded signals .

When data is to be transmitted uphole, the drill string 18

is lifted by the hydraulic lifters 24 at the surface so that the

drill bit 32 is moved away from the bottom of the borehole. In the preferred form of the invention, the transducer 40 is spaced

about 100 meters above the drill head and its associated MWD

control unit 34. This causes the weight of the lower end of the

drill stem below the modulator, which may be 5000 lbs., or more,

to tend to extend the joint 260 and thus tend to pull the piston

272 downwardly, applying a high pressure to the hydraulic fluid

in chamber 280. As long as valve 306 remains closed, the

hydraulic fluid will be retained in chamber 280 and the joint

will be held in its collapsed or upward position, with the piston

272 at the top of cylinder 274. The control signals from

microprocessor 312 may then be used to activate and deactivate

the solenoid valve in short, timed bursts, allowing pressurized

hydraulic fluid to flow out of chamber 280, through valve 306 and

line 308 to upper chamber 278. This releases segment 264 in

incremental steps, and the portion of the drill string 18 below

the modulator drops freely until the valve is closed. Each burst

of released fluid thus momentarily releases the tension or axial

stress applied to the upper part 262 of the drill string 18 by

the weight of the drill stem below the transducer, and thus

reduces the apparent drill string weight that is detected by

sensors 42 at the surface. The change in axial stress caused by

opening of valve 306 is transmitted along the length of the drill

string above the transducer for detection at the surface, and

such a release is referred to herein as an axial stress wave or a shock wave. Repeated openings and closings of the valve 306 in

accordance with encoded signals produced by microprocessor 312

produce corresponding stress pulses, or shock waves, which travel

along the length of the drill stem to the surface for detection

at sensor 42 and for decoding in computer 36.

If desired, the drill string can be rotated during the

transmission of shock waves uphole, so that the rotation

eliminates the effects of stick slip friction between the upper

part of the drill string and the borehole wall, thereby

increasing the efficiency of the modulated axial stress

transmission .

Because of changes in ambient pressure as the drill string

is moved downwardly in a borehole, the hydraulic cylinder 274 may

be connected to an accumulator 330 which may be connected to the

upper chamber 278 by way of a small passageway 332. The

accumulator 330 preferably is an elastomer that allows expansion

to accommodate small ambient pressure changes.

To have efficient modulation of axial stress in accordance

with the foregoing embodiment, the spline section of the drill

stem should be a significant distance above the drill bit so that

a large drill string weight will be carried by the hydraulic

cylinder fluid. Although the data transmission from the control

unit 34 is by way of the drill stem in the illustration, the

control unit 34 can be connected to solenoid valve 306 by a direct wire, if desired. Power for the control unit can be by

way of a battery source, or can be supplied by an alternator m

the manner described above .

A modified form of the downhole transducer is illustrated in

Fig. 11 at 40' . In this case, however, the piston and cylinder

arrangement of the embodiment of Fig.10 is reversed so that

hydraulic fluid is transferred between upper and lower chambers

of the cylinder to charge the transducer by lifting the drill

string. Then, when drilling is started the weight of the string

applies high pressure to the fluid in the lower chamber of the

cylinder so that release of that high pressure fluid by a

solenoid-controlled valve produces short bursts of reduced

pressure to incrementally lower the portion of the drill string

and generates axial stress waves which can be detected at the

surface. One advantage of this arrangement is that the

transmission of data can be carried out during drilling.

As was the case with the embodiment of Fig. 10, the modified

transducer 40' incorporates a splined joint 130 between an upper

drill stem segment 332 and a lower segment 334. The upper

segment 332 is connected as a part of the drill stem 18 and leads

to the surface, while segment 334 is connected through the

control unit 34 (Fig.l) to the drill head 32. In the embodiment

of Fig. 10, the transducer 40 is separated from control unit 34

and drill head 32 by a distance of about 100 meters in order to provide sufficient weight to produce axial stress waves upon

operation of the solenoid valve. In the present embodiment,

however, the transducer 40' can be close to the control unit 34

and drill head 32, since it is the weight of the drill string

above the transducer that produces the desired stress signals

upon the release of pressurized fluid by the solenoid valve.

In the structure of Fig. 11, segment 332 carries on its

interior surface 336 a plurality of longitudinally extending

grooves 338. These grooves receive corresponding splines 340

carried on the exterior surface of lower drill stem segment 334,

thereby forming the splined joint 330. This joint permits

relative longitudinal movement between the lower drill stem

segment 334 the upper segment 332.

Surrounding the spline joint 330 is a cylindrical housing

342 which is mounted on, or is integral with, the stem segment

334 at a lower wall portion 344 and extends upwardly to a top

wall portion 346 which engages the outer surface of stem portion

332. The wall 346 carries a suitable seal 348 such as an 0-ring

to provide a fluid tight transducer cylinder 350, defined by

cylindrical housing 342.

Located within cylinder 350 and dividing the cylinder into

upper and lower chambers 352 and 354 is an annular piston 356

which is secured to or is integral with the bottom end of the

drill stem segment 332. As illustrated, the piston 356 incorporates an 0-ring 358 which seals it against the wall of

cylinder 350 to maintain a fluid-tight seal between chambers 352

and 354. The two chambers are filled with hydraulic fluid such

as oil, with the upper chamber being connected to the lower

chamber by way of hydraulic lines 360 and 362, check valve 364,

and hydraulic lines 366, 368 and 370. The check valve 364

permits the fluid from chamber 352 to flow downwardly into

chamber 354, in the direction of arrow 372, so that when the

drill stem 18 is lifted by the lift equipment 24 (Fig. 1) the

piston 356 will tend to move upwardly in cylinder 350 and force

hydraulic fluid from chamber 352 down through check valve 364

into chamber 354. This charges the transducer to make it ready

for the transmission of data signals to the surface.

Bypassing the check valve 364 is a solenoid-operated control

valve 374 which is connected between hydraulic lines 360 and 368

by way of hydraulic lines 376 and 378. The valve 374 is normally

closed to prevent the flow of fluid between lines 376 and 378,

but upon energization of the solenoid, the valve shifts into an

open position to allow fluid flow.

In operation, after the chamber 354 has been charged with

hydraulic fluid by lifting the drill stem 18, the stem is lowered

by lift equipment 24 and, if desired, can be driven by motor 22

(Fig.l) for continued drilling of the borehole. The weight of

the drill stem which is applied to the drill head 32 is applied through the hydraulic fluid in chamber 354 by way of piston 356,

generating as much as 20,000 lbs./sq.in. of fluid pressure in

chamber 354, which then serves as a pressure chamber. The

splined joint 330 transmits the rotation of drill stem 18 through

the transducer and to the drill head 32.

Downhole sensors 310 in the control unit 34 produce output

signals corresponding to sensed parameters, and these signals are

directed to a microprocessor, or other suitable computer 312, as

described above, which produces encoded data signals on line 314.

In the embodiment of Fig.3, line 314 is shown as being directly

connected to solenoid valve 374 so that the output signals from

computer 312 control the operation of the valve. Thus, the valve

is energized by data signals to shift to an open position for a

short period of time and then to shift back to the closed

position, thereby releasing fluid under pressure from pressure

chamber 354 and allowing it to flow through lines 370, 368, 378,

376, and 360 into the upper chamber 352, which serves as a

release chamber. This allows the piston 356 to shift downwardly

a small amount, the distance depending upon the duration of the

data pulse. The shift of the piston produces an axial stress

wave in the drill stem 18 which travels along the length of the

drill stem and can be detected at the surface by sensor 42, in

the manner described above, thereby accurately transmitting the data from computer 312 at the downhole location to computer 36 at

the surface.

As illustrated, hydraulic system of Fig.11 may incorporate

an accumulator 380 connected to hydraulic line 360 to accommodate

variations in the temperature of the fluid, for example, and may

include a pressure sensor 382 connected to hydraulic line 368 for

measuring the pressure in chamber 354. The electrical output

from sensor 382 may be connected by way of line 384 to computer

312, so that the pressure in chamber 354 may be one of the

parameters included with the data signals on line 314.

Although the preferred method for charging the pressurized

chamber 354 is by lifting the drill stem 18, charging of the

chamber can also be accomplished by supplying hydraulic fluid

under pressure to line 370, as from an hydraulic pump located

downhole. A suitable pump for this purpose is illustrated at 400

in Fig. 12, the pump including an hydraulic cylinder 402 mounted

on and surrounding a section of drill pipe such as the drill stem

segment 334. The cylinder 402 receives a free piston 404 which

is annular in shape, which surrounds the drill stem segment 334,

and which is moved along the length of cylinder 402 by varying

the fluid pressure in the cylinder. The piston includes an inner

annular surface 405 carrying an O-ring 406 for sealing against

the exterior of drill stem segment 134. It also has an outer

annular surface 407 incorporating an 0-ring 408 for sealing the piston against the wall of cylinder 402 so the piston divides the

cylinder into an upper recharging chamber 410 and a lower pumping

chamber 412. The upper chamber contains hydraulic fluid to be

supplied through outlet line 414, and through a check valve 416

to a gear-type pressure multiplier which provides a pressure

increase in the hydraulic fluid. This increased pressure fluid

is supplied from the pressure multiplier through line 370 to the

lower pressure chamber 354 in transducer 40' to recharge it.

Pressurized fluid supplied from pump 400 thus may be used to

charge chamber 354 to a desired pressure level. The multiplier

418 is connected by way of a second outlet line 423 and a second

check valve 424 to return excess fluid to chamber 410.

Pressure is generated in recharging chamber 410 by means of

the conventional drilling fluid, or drilling mud, used in

rotating drilling systems. Thus, drilling fluid flows downwardly

into the bottom of the borehole through the center of the drill

stem 18 and is under about 500 psi at the bottom of the borehole.

A small part of that drilling fluid is supplied from the interior

of drill stem segment 334 through a feed line 430, through a

solenoid valve 432, and through a supply line 434 to the pumping

chamber 412 of cylinder 402. When the solenoid 432 is open, as

illustrated, drilling fluid is supplied to chamber 412 to press

piston 404 upwardly to thereby pressurize the hydraulic fluid in

chamber 410. This causes hydraulic fluid to flow to pressure chamber 354 in the transducer of Fig. 11 to charge the

transducer.

The sensor unit 310 may incorporate a sensor for detecting

the location of piston 404 in cylinder 402 to enable the computer

312 to regulate the operation of solenoid control valve 432. If

the piston 404 shifts upwardly too far, valve 432 may be shifted

to the left (as viewed in Fig. 12) to connect the fluid line 434

through the solenoid valve to an outlet line 436. This allows

the drilling fluid in chamber 412 to bleed out of the system and

to return to the borehole annulus surrounding drill stem 18. The

piston 404 then falls to expand chamber 410, with fluid from

accumulator 422 filling the chamber. When sufficient hydraulic

fluid has been supplied to chamber 410, valve 432 is shifted to

the right to open line 434 to inlet line 430 to thereby allow

the drilling fluid to pressurize chamber 412. The pump 400 thus

is operated by means of the drilling fluid circulating into the

borehole through the drill stem to keep the axial stress

transducer 40' charged and activated.

Axial pressure waves generated by released pressure in

downhole transducers 40 or 40' may be detected at the surface by

sensor 42, which may incorporate suitable strain gauges, as

described above. However, axial stresses generated downhole can

also be measured at the surface as pressure variations in a

hydraulic transducer such as that illustrated at 42' in Fig. 13. This transducer is similar to that described above with respect

to Fig.11, but utilizes a pressure sensor coupled to the upper

chamber for detecting changes in hydraulic pressure due to axial

stress waves received along drill string 18. The output of the

pressure sensor may then be transmitted to the uphole computer 36

by way of line 44 or by any other conventional communications

link.

The uphole transducer 42' includes a cylinder 440

surrounding a telescoping splined joint 442 formed between

adjacent upper and lower drill stem segments 444 and 446.

Segment 444 includes a plurality of longitudinal, spaced grooves

448 located at the lower end of its inner surface, while the

upper end of segment 446 includes a corresponding plurality of

longitudinal splines 450. As previously described, the splined

joint allows relative longitudinal motion between segments 444

and 446, while transmitting rotary motion from one to the other.

Cylinder 440 receives an annular piston 452 which may be integral

with the bottom of drill stem segment 444 and which divides the

cylinder 440 into an upper pressure chamber 454 and a lower

chamber release chamber 456. Suitable O-rings 457 and 458 seal

the piston against the wall of cylinder 440 and seal the cylinder

440 against the outer surface of segment 444. The segment 444 iε

supported by the lift mechanism 44 and is connected to the drill

stem 18 extending into the borehole by the transducer 42' . Thus, the hydraulic fluid in chamber 454 between piston 452 and

cylinder 440 supports the weight of the drill and as a result is

under high pressure .

In operation, stress pulses from the transducers 40 or 40'

received at the surface tend to reduce or increase the pressure

of the hydraulic fluid in chambers 454 and 456 as segment 446

tends to shift axially with respect to segment 444. Such changes

in pressure in chamber 454 and 456 are detected by a pressure

sensor 460 connected by way of line 462 to the upper chamber 454,

for example, with the output of sensor 460 being transmitted by

way of a communication link 44 to the computer 36.

The transducer 42 ' can also be used to transmit data

downhole by generating axial stress waves . In this mode of

operation, the hydraulic fluid pressure in cylinder 440 iε

released in short, timed bursts under the control of a solenoid

valve, operated by encoded signals from computer 36 generally in

the manner described hereinabove with respect to Figs. 2 and 3.

The resulting stress signals, or shock waves, are transmittec

through the drill stem 18 to the downhole transducer 40 or 40' .

These stress signals produce pressure changes in the hydraulic

fluid in the downhole transducer, such as the transducer 40'

discussed with respect to Fig. 11, and such pressure changes are

detectable by the pressure sensor 382 (Fig.11). The resulting

output signals from sensor 382 are supplied to computer 312 at the downhole location for demodulation to allow the computer to

exercise control over the downhole operations .

Stress signals are generated in the drill string at the

uphole transducer 41' by releasing the pressure chamber 454 by

way of hydraulic lines 462 and 466, a solenoid-controlled spool

valve 468, and hydraulic lines 470 and 464. Encoded data or

control signals to be transmitted downhole are provided by

computer 36 on line 472, and these signals operate solenoid valve

468. The normally closed valve is activated by the encoded

signals to allow fluid to flow from upper chamber 454 to lower

chamber 456 in timed bursts to produce incremental motion or

variations in longitudinal tension or compression in drill stem

segment 446 to produce axial stress signals.

Chamber 454 can be charged with hydraulic fluid and the

location of piston 452 in the cylinder 440 can be adjusted by-

lowering the drill stem 18 to the bottom of the borehole and then

further lowering the upper drill segment 444 while opening

solenoid valve 468, thereby allowing a reverse flow of fluid

from chamber 456 to chamber 454. Fluid may also be transferred

to chamber 454 from an accumulator 474 connected to line 470.

Alternatively, the upper chamber can be charged by means of an

hydraulic pump 476 connected to the lower chamber 456 through

accumulator 474 and hydraulic line 464, the pump delivering

pressurized fluid through line 478 and a second solenoid- controlled spool valve 480, the output of which is connected by

way of lines 482 and 462 to the upper chamber 454. Upon

operation of pump 476, under the control of computer 36,

hydraulic fluid under pressure may be supplied to the upper

chamber to raise the drill segment 446 with respect to drill

segment 444 and to thereby charge the upper chamber 454.

Sufficient power to operate such a pump is normally available at

the surface .

By providing transducers such as 40 or 40' at the bottom of

the drill string and a transducer such as that shown at 42 ' at

the top of the drill string, two-way communication within the

borehole by means of axial stress waves is accomplished. Such a

communication system provides rapid feedback control of the

weight on the drill bit at the bottom of the drill string by

controlling the position of piston 452 in cylinder 440. This

permits rapid control of stick-slip problems in the drill bit and

permits control and modulation of the torque applied to the drill

by raising and lowering the drill stem.

The rapid communication between the surface and the drill

controls at the bottom of the borehole provided by the present

invention also permits conservation of the energy required for

such communication, for if strong signals are received at the

surface, computer 36 can transmit instructions to the downhole computer to change the frequency and strength of the pulses to reduce the energy required.

The downhole solenoid valves are battery-operated, in the

preferred form of the invention, and as discussed above, the

downhole units send data by releasing small volumes of hydraulic

fluid in timed increments using these solenoid-operated valves to

impulsively lower the drill string and generate axial stress

waves which propagate to the surface. At the surface the stress

waves are demodulated by strain gauges or by an axial stress

transducer located at the top of the drill string. This upper

transducer is of heavier construction than the downhole units

because of the large weights which it must handle. In a

preferred form, it does not require a check valve for recharging

the hydraulic chambers, but instead is kept activated by a

secondary source of pressurized hydraulic fluid under the control

of a second solenoid valve.

Incoming axial stress wave pulses to the uphole transducer

are sensed by pressure changes, with the pressure sensor sending

corresponding signals to the computer for demodulation. By using

two axial stress demodulators, one at the top of the drill and

one at the bottom, a two-way communication system is provided.

The hydraulic energy for the downhole transducer is provided by

the downward pressure of the drill string or, in the alternative,

by a secondary source of pressurized hydraulic fluid such as a pump driven by the drilling fluid which flows to the drill bit

through the center of the drill string. This allows the downhole

axial stress transducer to continually send data pulses to the

surface while the drill is operating, without the need to re-

energize the unit by lifting the drill string at the surface.

An important feature of the invention is that the intrinsic

fast response and huge power gain which can be built into

electrically-controlled hydraulic valves, coupled with the

intrinsic high frequency wave propagation characteristics of the

drill stem, enables drilling parameters to be controlled by a

fast acting communication system.

An important application of this system is the possibility

of significantly reducing the effects of erratic stick-slip

behavior in aggressive PDC drilling bits. Thus, the transmission

of data from the downhole transducer has the effect of modulating

the drilling weight on the bit. As the weight on the bit is

changed, its bite into the formations being drilled also changes,

producing an immediate change in drilling torque which can be

sensed by a strain gauge on the drill stem near the drilling bit .

Such a strain gauge is diagrammatically illustrated at 490 in

Fig. 1, with the output of the strain gauge being connected to

the microprocessor, or computer, 312 in the control unit 34. By

noting the ratio between the applied drilling weight fluctuations

and the resulting torque fluctuations, a continuous measurement of the dynamic aggressiveness of drilling can be measured. This

parameter can be transmitted uphole to the surface transducer,

where it is received within a second, and can be used to change

immediately the weight on the bit by activating the solenoid-

controlled spool valve 480 which controls the position of the

piston 452 in the uphole axial stress transducer 42 ' from which

the drill stem hangs.

The velocity of propagation of compressive waves in steel is

about 500 m./sec, the acoustic impedance of steel (density x

velocity of propagation) is 4 x 10⁷, and the cross sectional area

of a drill string is about 0.006 m². The wave power associated

with 1000 lbs. /in² (7xl0⁶ newtons/m²) stress waves on such a drill

string is approximately 10,000 watts; that is, 13 horsepower.

The energy available from lowering a drill stem 1 meter with

20,000 lbs. of force (which is a characteristic weight on a

drilling bit) is 88,000 joules. Measurement of drill bit

vibrations at the earth's surface show that compressional waves

with frequencies as high as 100 Hz are readily transmitted on the

drill stem. The simplest hydraulic spool valve energized by 20

watts of electric power can control a flow of 30 gallons per

minute at 3000 psi; that is, 44,000 watts of hydraulic power.

The response time of such valves is 0.003 seconds, so that 10 Hz

waves are readily generated. The peak drill string movement associated with a 10 Hz, 1000 psi wave is about 3 mm, thus

indicating the practicality of the present communication system.

Thus, there has been disclosed a unique system for data

communication between the surface and downhole locations in a

drill string with a high degree of accuracy and reliability. In

addition, in one embodiment of the invention, the sensor

mechanism also serves as a power source for supplying operating

power to downhole equipment, thus avoiding the need for batteries

and extending the life of the equipment. Although the invention

has been described in terms of preferred embodiments, it will be

apparent that numerous modifications and variations can be made

without departing from the true spirit and scope of the invention

as set forth in the accompanying claims.

Claims

WHAT IS CLAIMED IS:

1. Apparatus for providing a data link borehole, comprising:

a rotatable drill stem;

a data source ;

a modulator connected to said data source and coupled

to said drill stem at a first location for varying stress in said

drill stem in accordance with data to be transmitted;

a sensor coupled to said drill stem at a second

location for detecting variations in stress applied to said drill

stem and for producing an output signal corresponding to said

variations; and

a demodulator connected to said sensor to produce an

output corresponding to said data.

2. The apparatus of Claim 1, wherein said modulator is

coupled to said drill stem through a rotary drive motor to vary

said stress by varying the speed of rotation of said drill stem.

3. The apparatus of Claim 2, wherein said sensor is an

electrical alternator coupled to said drive stem, said alternator

producing an output signal corresponding to the rotation of said

drill stem.

4. The apparatus of Claim 3, wherein said alternator

output signal has an amplitude corresponding to the rate of

rotation of said drill stem and modulated in accordance with said

variation in said speed of rotation.

5. The apparatus of Claim 4, further including:

drill control circuitry at said second location;

a demodulator in said control circuit for receiving and

demodulating said alternator output to provide said data to said

control circuit; and

a power supply circuit for receiving said alternator

output and for supplying operating power to said control

circuitry.

6. The apparatus of Claim 5, further including.*

a second data source in said control circuitry at said

second location;

a second modulator at said second location responsive

to said second data source and coupled to said drill stem to vary

the stress in said drill stem in accordance with said second

data; a second sensor coupled to said drill stem at said

first location to detect variations in the stress in said drill

stem; and a second demodulator coupled to said second sensor to

reproduce said second data at said first location.

7. The apparatus of Claim 6, wherein said first location

is at the earth's surface and said second location is downhole in

a borehole being drilled.

8. Apparatus for providing a communication link in a

borehole, comprising:

a drill stem extending from the earth' s surface into a

borehole;

a source of encoded signals corresponding to data to be

transmitted between first and second locations along said drill

stem;

an hydraulic transducer connected to said data source

and

coupled to said drill stem at one of said first and second

locations for varying axial stress in said drill stem in

accordance with data to be transmitted; and

a sensor coupled to said drill stem at the other of

said first and second locations for detecting variations in axial

stress applied to said drill stem and for producing a

corresponding output signal .

9. The apparatus of claim 8, wherein said transducer

hydraulically couples first and second segments of said drill

stem to provide controlled relative axial motion between said

segments .

10. The apparatus of claim 9, wherein said sensor is a

second

hydraulic transducer producing an output signal corresponding to

axial stress applied to said drill stem.

11. The apparatus of claim 9, wherein said transducer is

coupled to said first and second segments of said drill stem so

as to transmit rotational motion of one segment to the other

segment .

12. The apparatus of claim 11, wherein said first and

second

segments are adjacent to each other, and wherein said adjacent

segments are coupled by a splined joint which permits relative

axial motion.

13. The apparatus of claim 9, wherein said transducer

includes an hydraulic cylinder containing hydraulic fluid and

incorporating a movable piston which divides the cylinder into a pressure chamber and a release chamber, said piston being

connected to one of said drill stem segments and said cylinder

being connected to the other to one of said drill stem segments

and said cylinder being connected to the other of said drill stem

segments.

14. The apparatus of claim 13, further including means

interconnecting said pressure and release chambers and

controllable to regulate the position of said piston in said

cylinder.

15. The apparatus of claim 14, wherein said interconnecting

means includes an hydraulic line including a controllable valve.

16. The apparatus of claim 14, wherein said interconnecting

means includes means responsive to said encoded signals to

produce corresponding movement of said piston.

17. The apparatus of claim 9, wherein said hydraulic

transducer includes means responsive to said encoded signals to

produce corresponding variations in said axial stress.

18. The apparatus of claim 9, wherein said hydraulic transducer includes an hydraulic pressure chamber containing

hydraulic fluid under pressure, and means responsive to said

encoded signals to release said fluid incrementally to produce

corresponding axial stress signals in said drill stem.

19. The apparatus of claim 18, further including means for

charging said hydraulic pressure chamber.

20. The apparatus of claim 9, wherein said transducer is

located in said borehole at said first location and is responsive

to said encoded signals to produce said variations in axial

stress in said drill stem.

_#21. The apparatus of claim 20, wherein said sensor is a

strain gauge mounted on said drill stem at said second location.

22. The apparatus of claim 20, wherein said sensor is a

second hydraulic transducer coupled to third and fourth segments

of said drill stem at said second location.

23. The apparatus of claim 22, wherein said first-named

transducer is located downhole is located at the earth's surface,

each of said first-named and second transducer being operable to generate and to sense axial stress waves in said drill string for

two-way communication.

24. The apparatus of claim 22, wherein said transducer

includes an hydraulic fluid pressure chamber and a controllable

valve for incrementally releasing hydraulic fluid from said

chamber to varying said axial stress in said drill stem.

25. The apparatus of claim 24, further including a

recharging source for recharging said fluid pressure chamber.

26. The apparatus of claim 25, wherein said recharging

source

includes a check valve connected between a fluid supply and said

pressure chamber.

27. The apparatus of claim 25, wherein said recharging

source

includes an hydraulic pump connected between a fluid supply and

said pressure chamber.

28. The apparatus of claim 27, wherein said hydraulic pump

includes a cylinder and a floating piston dividing the cylinder

into a pumping chamber and a recharging chamber, the pumping chamber being connected to a source of drilling fluid in said

borehole and said recharging chamber being connected to said

pressure chamber in said transducer.

29. The apparatus of claim 25, wherein said sensor

comprises

a second transducer having an hydraulic fluid pressure chamber.

30. The apparatus of claim 29, wherein said transducer

further includes a second controllable valve for incrementally

releasing hydraulic fluid from said chamber for varying said

axial stress in said drill stem to provide two-way communication

between said first and second locations.

31. A method for transmitting data along a drill string,

comprising :

applying a stress to said-drill string at a first

location along the string;

sensing, at a second location along the drill string,

said stress waves.

32. The method of claim 31, wherein applying a stress includes applying a rotational force to said drill string to

produce a torsional stress.

33. The method of claim 31, wherein applying a stress

includes applying an axial force to said drill string to produce

an axial force.

34. The method of claim 31, wherein modulating said stress

includes varying the applied stress in accordance with selected

signals.

35. The method of claim 34, wherein sensing said stress

waves

includes measuring variations in said stress to permit

reproduction of said selected signals.

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