WO2015112044A1

WO2015112044A1 - Method and apparatus for positioning downhole tool in a cased borehole

Info

Publication number: WO2015112044A1
Application number: PCT/RU2014/000058
Authority: WO
Inventors: Yuliy Aleksandrovich Dashevsky; Gleb Vladimirovich DYATLOV; Elizaveta Vladimirovna ONEGOVA; Alexandr Anatolevich VINOKUROV
Original assignee: Baker Hughes Inc
Current assignee: Baker Hughes Holdings LLC
Priority date: 2014-01-24
Filing date: 2014-01-24
Publication date: 2015-07-30
Anticipated expiration: 2016-07-24

Abstract

An apparatus and method to locate a collar of a downhole casing are described. The apparatus includes a housing (203). The apparatus also includes a first type of sensor (205) disposed in the housing, the first type of sensor operating in the frequency domain, and a second type of sensor (207) disposed in the housing, the second type of sensor operating in the time domain. The apparatus further includes a gamma ray detector (230) disposed in the housing.

Description

METHOD AND APPARATUS FOR POSITIONING DOWNHOLE TOOL IN A

CASED BOREHOLE

BACKGROUND

In downhole exploration and production efforts, in which a borehole is drilled, a casing is typically inserted to maintain the integrity of the borehole. A casing is a pipe assembled in sections. The sections of casing may be joined by a threaded collar. Because the length of the sections is known, the depth of a borehole may be determined or confirmed based on a count of the number of casing collars. A casing collar locator is usually employed for this purpose. A casing collar locator is a magnetic or electromagnetic logging tool that detects the magnetic or electromagnetic anomaly caused by the relatively high mass of the casing collar.

SUMMARY

According to an aspect of the invention, an apparatus to locate a collar of a downhole casing includes a housing; a first type of sensor disposed in the housing, the first type of sensor configured to operate in the frequency domain; a second type of sensor disposed in the housing, the second type of sensor configured to operate in the time domain; and a gamma ray detector disposed in the housing.

According to another aspect of the invention, a method of locating a collar of a downhole casing includes disposing a locator downhole, the locator comprising a housing, a first type of sensor configured to operate in the frequency domain disposed in the housing, a second type of sensor configured to operate in the time domain disposed in the housing, and a gamma ray detector disposed in the housing; and determining, using the locator, a distance from the locator to the collar of the downhole casing.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings wherein like elements are numbered alike in the several Figures:

FIG. 1 is a cross-sectional view of a downhole system according to an

embodiment of the invention;

FIG. 2 is a block diagram of a locator according to an embodiment of the invention; FIG. 3 illustrates sensitivity of transient signals generated using the second type of sensor of a locator to a variation in thickness of the casing;

FIG. 4 is a process flow of an exemplary method of using a locator according to embodiments of the invention;

FIG. 5 illustrates an exemplary locator including a first type of sensor;

FIGs. 6-9 illustrate the dependence of the difference signals on the distance from the locator to the collar for different values of spacing between the receivers and the transmitter;

FIG. 10 illustrates an exemplary locator including a second type of sensor;

FIG. 1 1 illustrates curves showing voltage values obtained with a receiver over a range of times; and

FIG. 12 illustrates curves showing voltage values obtained with a receiver over a range of distances from the collar.

DETAILED DESCRIPTION

As noted above, a casing collar locator may be used to count the number of casing collars as a way of determining or confirming treatment depth. The position of a tool downhole may be determined with the casing collar locator. Some casing collar locators may be speed dependent such that the measured signal is proportion to the speed at which the locator is pulled through the borehole. Some casing collar locators must be decentralized (pressed against the casing wall). In addition, prior locators are not able to look ahead to an approaching collar and evaluate, in real time, the collar- to-locator distance. This collar-to-locator distance can provide the position of a downhole tool when the locator is at the tool position.

In some applications, the position of the downhole tools must be known with a high degree of accuracy to know the exact depth of the tools. One such application is borehole gravity measurements. Borehole gravity measurements are used to monitor deep water fluids in reservoirs based on tracking the location of oil, gas, or water contact through inversion of repeat differential borehole gravity signals that are usually acquired in cased boreholes. Repeatability does not refer to the absolute accuracy of the measurement but does preclude certain sources of variation from being included in the reported precision of the gravity meter. The repeatability is important when repeat borehole gravity surveys are being conducted. For example, for two successive points of time, it is unlikely to locate the instrument at the same station. Thus, the error in borehole gravity tool positioning (approximately ±0.15 meters (m)) is equivalent to an additional error in the measurement of ±15 μΩεΛ (where 1 μϋζΐ is 10 m/(seconds )).

Embodiments of the system and method detailed herein relate to a casing collar locator that looks ahead while being pulled through a cased borehole to see an approaching casing collar and evaluates the collar-to-locator distance through a realtime inversion of electromagnetic signals acquired by multiple sensors in the frequency and time domains.

FIG. 1 is a cross-sectional view of a downhole system according to an embodiment of the invention. While the system may operate in any subsurface environment, FIG. 1 shows downhole tools 10 disposed in a borehole 2 penetrating the earth 3. The borehole 2 includes a casing 1 10, and an exemplary casing collar 120 is shown. The downhole tools 10 are disposed in the borehole 2 at a distal end of a carrier 5. The downhole tools 10 may include measurement tools 1 1 and downhole electronics 9 configured to perform one or more types of measurements in an embodiment known as wireline logging (WL). According to the WL embodiment, the carrier is, for example, an armored wireline. Raw data and/or information processed by the downhole electronics 9 may be telemetered to the surface for additional processing or display by a computing system 12. The downhole electronics 9 and the computing system 12 may each include one or more processors and one or more memory devices. The borehole 2 may be vertical in some portions as shown in FIG. 1 but need not be vertical in all portions. As detailed below, the downhole tools 10 may be positioned based on a locator 200 (FIG. 2) according to embodiments of the invention.

FIG. 2 is a block diagram of a locator 200 according to an embodiment of the invention. As shown in FIG. 2, the locator 200 includes a housing 203 with a first type of sensor 205 and a second type of sensor 207, as well as a gamma ray detector 230. The locator 200 may include one or more processors 250 and one or more memory devices 255 to process information obtained by the locator 200 and control movement of the locator 200. In alternate embodiments, the locator 200 may convey obtained signals to the downhole electronics 9 or surface computing system 12 for processing and control instead. Although one each of the first type of sensor 205 and the second type of sensor 207 are shown, the locator 200 may include multiples of each of the first type of sensor 205 and the second type of sensor 207. The first type of sensor 205 includes a spaced-apart receiver 220 and transmitter 210. The second type of sensor 207 includes a collocated transmitter 215 and receiver 225. The transmitters 210, 215 and receivers 220, 225 may be coils such that current through the transmitter coils 210, 215 generates an electromagnetic field. The vectors describing dipole moments of the transmitter 210, 215 and receiver 220, 225 coils are oriented arbitrarily with reference to the borehole axis (the transmitters 210, 215 and receivers 220, 225 need not be lined up as shown in FIG. 2). As noted, more than one of the first type of sensor 205 or the second type of sensor 207 may be part of the locator 200. In addition, the transmitter 210 of one or more of the first type of sensors 205 may correspond with multiple receivers 220. The first type of sensor 205 operates in the frequency domain to obtain the first type of measurement as discussed below. The second type of sensor 207 operates in the time domain to obtain the second type of measurement, as discussed below, and transient electromagnetic signals are transmitted by the transmitter 210 and received by the receiver 220. The gamma ray detector 230 obtains a gamma ray log. The gamma ray log is correlated with the locator 200 output to ensure that a casing collar 120 has not been missed in the count by the locator 200, for example. The locator 200 may be connected to a tool 240 (among the downhole tools 10) at both ends of the locator 200. The locator 200 may then be used to precisely position the tool 240 within a borehole 2 casing 1 10 as detailed below. The two types of sensors (205, 207) of the locator 200 obtain two types of measurements. The first type of measurement is obtained by the first type of sensor 205 in the frequency domain. In each of the first type of sensors 205, an oscillating current of different frequencies is generated to energize each of the multiple transmitters 210. Different combinations of amplitudes and phase differences are measured between measured signals of the receivers 220 of the first type of sensors 205. Distance to a casing collar 120 is provided by inverting the measured signals. In addition, as discussed in the examples below, phase difference (in particular, maximum phase difference) may additionally be used for determining collar 120 position, because the phase difference is maximum when the receiver 220 is at a center of the collar 120.

The second type of measurement is obtained by the second type of sensor 207 or by a first type of sensor 205 that has a receiver 220 remote from (not collocated with) the transmitter 210. In each of the second type of sensors 207 or first type of sensors 205 used to obtain the second type of measurement, electronic circuits generate steplike pulses of current to energize the transmitters 215. Thus, the dipole moment M changes as a step function:

In alternate embodiments, a function of arbitrary shape may be used for the dipole moment, as well. The transmitter 215 (or 210) may include an antenna coil coupled with a current source and a magnetic core having a residual magnetization.

Accordingly, a dipole moment of approximately 1000 Ampere square-meters (A*m²) may be achieved using only approximately 50 Watts (W) of direct current (DC) power on a magnetic core with a residual magnetization. Different combinations of transient electromagnetic signals generated in multiple receivers 225 of the second type of sensors 207 (or in the receivers 220 of the first type of sensors 205) are measured. Distance to a casing collar 120 is provided by inverting the acquired transient signals.

The first and second types of measurements may be obtained sequentially and independently. For each of the types, once the measured signals are obtained, an inversion is performed to obtain the collar 120 to locator 200 distance. The inversion for both types of measurements may be implemented by the least squares method or another known method. When the least squares method is used, the distance D that provides the bets fit in a mean square sense may be used. The inversion may be accompanied by an estimate of error (positional error). The error and variance in the determination of the distance D may then be determined as detailed below. By way of explanation, the second type of measurements (transient) obtained with the second type of sensors 207 are used. For the case of the first type of measurements using the first type of sensors 205, the times (ti ...t„) should be replaced with coil spacing and voltage should be replaced with phase difference.

With a function / : O H (V(t„D),...,V(t_H,D)) that relates distance D to the measured signal (voltage V) at times t_l,...,t_n , approximate values J of the signals f_j{D) are given by:

[EQ. 2] where ξ, ' are normally distributed random variables with zero mean and variances

' . The approximate value ^ of parameter U is searched by the least squares method:

That is, the sought value ^ satisfies the equation:

An error P ^~ P D is discussed next. Replacing f^D³) - f_t{D) with ^^D^-SD and ^^D ^ with ^^- = a_i in EQ. 4 yields the following approximate dD dD 3D equation for the error SD . f?L_{8D =}f [EQ. 5] from which the error is obtained as:

[EQ. 6]

That is, SD is the sum of independent normally distributed random variables with zero mean and variances given by:

a,

σ,∑(¾ /σ,)

k=\ [EQ. 7]

Therefore, SD is also a normally distributed random variable with a variance given by:

CT.

The error generally depends on the value of D . To determine the variances ' , with ε ^εο

relative error in percent, and absolute error ^υ in Volts, σ, =

100 [EQ. 9]

D - D.

If were available for discrete values of ^J (uniformly spaced), (i.e., the values f_t.J = flV>_j)

), then the derivatives are calculated by the approximate formula:

[EQ. 10]

σ D = D.

and the accurac is given by:

FIG. 3 illustrates sensitivity of transient signals generated using the second type of sensor 207 of a locator 200 to a variation in thickness of the casing 1 10. Each signal 310 corresponds with a voltage in micro Volts (μ V) on the axis 313 at a different time in milliseconds (ms) 315 for a given thickness of the casing 1 10 in meters (m) on the axis 317. The sensitivity of the voltage measurements obtained by the locator 200 to thickness of the casing 1 10 facilitates the use of the locator 200 in inspecting the casing 1 10, as well as accurately locating a collar 120 to precisely position a tool 10 (240). The casing 1 10 in most wells is formed of metallic materials and, most commonly, steel. Thus, the casing 1 10 is subjected to the possibility of the passage of electrical currents away from it and, consequently, corrosion. While the locator 200 is pulled through the cased borehole 2, far from a collar 120, the locator 200 may in situ record casing 1 10 thickness in order to determine whether and to what extent the casing 1 10 has undergone corrosion. The exemplary casing 1 10 used in the illustration of FIG. 3 has an inner diameter of 0.101 m and a thickness of 0.0065 m. The illustration of FIG. 3 indicates the change in signal (voltage) based on a variation in thickness of the casing 1 10 at t = 10 ms from 40 μν for a thickness of 0.0055 millimeters (mm) to 180 μν for a thickness of 0.0075 mm. Thus, the sensitivity of the exemplary locator 200 to the variation in thickness of the exemplary casing 1 10 is approximately 70 μν/mm.

FIG. 4 is a process flow of an exemplary method of using a locator 200 according to embodiments of the invention. At block 410, performing raw positioning and inspection of the casing 110 with the locator 200 includes using one or both of the first type of sensor 205 and the second type of sensor 207 in conjunction with the gamma ray detector 230. The data obtained with the locator 200 may be matched with previously obtained data. As discussed with reference to FIG. 3, the data obtained by the locator 200 may be used to inspect the thickness of the casing 110 as the locator 200 moves along the casing 110 based on the sensitivity of the data obtained by the locator 200 to variations in thickness of the casing 1 10. Introducing a tool 240

(among downhole tools 10) at block 420 to be positioned at a particular location downhole includes connecting the tool 240 to the locator 200 as shown in FIG. 2, for example. Performing accurate positioning of the tool 240 at block 430 includes the process discussed with reference to FIG. 2. Examples are also provided below. As further discussed below, precise positioning of the tool 240 may be obtained by stopping the tool 240 when the collocated transmitter 215 and receiver 225 coils (of the second type of sensor 207) pass the collar 120 edge or when one of the receiver 220 coils (of the first type of sensor 205) is at the center of the collar 120. At block 440, registering the tool 240 position includes registering the exact locator 200 to collar 120 distance when the tool 240 is stopped and clamped (or otherwise affixed). The registered data may be used to make corrections to gravity data obtained through gravity measurement interpretation.

Two exemplary locators 200 are further discussed below.

FIG. 5 illustrates an exemplary locator 200 including a first type of sensor 205. The exemplary locator 200 is shown with a transmitter 210 and two receivers 220 of the first type of sensor 205. The two receivers 220 have coils with identical windings and are connected out-of-phase so that the received signals from each of the coils cancel each other out. As a result, the combined signals from the two receivers 220 are zero when the coils are inside the casing 1 10 without anomalies. When the receivers 220 approach a collar 120 of the casing 1 10, the balance between the two receiver 220 coils is disturbed in a way discernable by the measuring circuit. The locator 200 also includes a gamma ray detector 230. Table 1 indicates the parameter values used to model the exemplary locator 200 shown in FIG. 5.

Table 1. Model parameters used to model signals received with locator 200

The models generated with the parameters in Table 1 and the arrangement shown in FIG. 5 facilitate studying resolution of the distance to collar 120 estimate. With

representing the difference between phases of voltage (V) induced

receiver 220 coils at distances / and -/ from the transmitter coil,

wher

FIGs. 6-9 illustrate the dependence of the difference signals (indicating difference between phases of voltage (Δφ) recorded by the receivers 220) on the distance D from the locator 200 to the collar 120 for different values of spacing (/=0.3, 0.4, 0.5, 0.6, 0.7 m) between the receivers 220 and the transmitter 210. Each of the figures shows results for a different one of the frequencies: 8, 32, 64, 128 Hz. FIGs. 6-9 show difference signals 610 obtained with five different embodiments of the locator 200 (five different spacings / 607 between the transmitter 210 to the receivers 220) shown in FIG. 5. The difference between phases of voltage (Δφ) 603 is shown on y-axis and the distance D 605 from the locator 200 to the collar 120 is shown on x-axis. As FIG.s 6-9 indicate, the signals 610 are independent of spacing / 607 between the transmitter 210 and receiver 220. As FIGs. 6-9 also indicate, the maximum of each of the signals 610 is at a distance that corresponds with a spacing / 607 of the receiver 220 from the transmitter 210 (signal 610 is maximum when one of the receivers 220 is at a center of the collar 120). In FIG. 6 (128 Hz), for example, the maximum signal 610 for a spacing / 607 of 0.4 m is at a distance D 605 of 0.4 m, and the maximum signal 610 for a spacing / 607 of 0.6 m is at a distance D 605 of 0.6 m. The reason for this is that the signal 610 measured by a receiver 220 at the center of a collar 120 corresponds with a thick pipe (with an outer diameter (OD) of the collar 120) while the signal obtained by a receiver 220 far from the collar 120 corresponds with a thin pipe (without a collar 120). Thus, as noted above in the discussion of block 430 of FIG. 4, precise positioning of a tool 240 connected to the locator 200 may be obtained by stopping the tool 240 when the receiver 220 is at the center of the collar 120. A comparison among FIGs. 6-9 indicates that the signal 610 increases as frequency increases. In general, the spacing / 607 between a transmitter 210 and receiver 220 in a first type of sensor 205 may be selected in the range of 0.3 to 0.7 m.

FIG. 10 illustrates an exemplary locator 200 including a second type of sensor 207. The exemplary locator 200 is shown with a transmitter 215 and collocated receiver 225. The transmitter 215 is supplied with constant current and then the current is switched off. This causes eddy currents to flow mostly in the collar 120 and the casing 1 10. The eddy currents generate a secondary magnetic field which is proportional to the current. The receiver 225 coil measures the rate of change of this secondary magnetic field in the time range [t_\, t₂]. The acquired receiver 225 voltages are inverted to determine distance of the locator 200 to the collar 120. The parameters indicated in Table 1 above apply to the examiner relating to FIG. 10, as well. The models generated with the parameters in Table 1 and the arrangement shown in FIG. 10 are discussed with reference to FIGs. 1 1 and 12 and illustrate the feasibility of the locator 200 according to embodiments of the invention looking ahead to an

approaching collar 120.

FIG. 1 1 illustrates curves 1 1 10 showing voltage 1 103 values obtained with a receiver 225 over a range of times 1 105. The voltage values 1 103 are shown on the y- axis and the time 1 105 (in ms) is shown on the x-axis for different values of distance D 1 107 from the locator 200 to the collar 120. At an early stage of the transient process (t < 2 ms), the curves 1 1 10 have a common asymptote. The reason for this is that induced currents concentrate near the source and do not penetrate into the metal (depth of penetration is less than the casing 1 10 wall thickness) so that the measured field is practically the same as in a uniform casing 1 10 without collars 120. At later times, the induced currents travel farther and penetrate into the collar 120. Thus, the measured voltage 1 103 (curves 1 1 10 at later times) indicate the presence of the metallic article. At the same time 1 105, the voltage 1 103 is higher for a closer distance D 1 107 to the collar 120. For example, at t = 10 ms, going from the curve 1 1 10 for distance D 1 107 = 0.5 m to the curve 1 1 10 for distance D = 0.09 m leads to an increase in voltage 1 103 from 100 micro Volts (μν) to 500 (μν).

FIG. 12 illustrates curves 11 15 showing voltage 1 103 values obtained with a receiver 220 over a range of distances D 1 107 from the collar 120. The voltage values 1 103 are shown on the y-axis and the distances D 1 107 are shown on the x-axis.

Curves 1 15 for different time 1 105 values are shown. As FIG. 12 illustrates, the voltage 1 103 measurement increases as distance D 1 107 decreases. Thus, for example, the sensitivity of the voltage 1 103 to the distance D 1 107 is approximately 10 μν/cm when the voltage 1 103 is acquired at t = 10 ms and the distance D 1 107 to the collar 120 is 0.25 m. With either the first type of sensor 205 or the second type of sensor 207, error models obtained for a two-dimensional environment and a collar 120 to locator 200 distance less than 0.6 m indicate an error of no more than 1 centimeter (cm).

While one or more embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.

Claims

1. An apparatus to locate a collar of a downhole casing, the apparatus

comprising:

a housing;

a first type of sensor disposed in the housing, the first type of sensor configured to operate in the frequency domain;

a second type of sensor disposed in the housing, the second type of sensor configured to operate in the time domain; and

a gamma ray detector disposed in the housing.

2. The apparatus according to claim 1, wherein the first type of sensor includes a first transmitter coil disposed remotely from a first receiver coil in the housing.

3. The apparatus according to claim 2, wherein the first type of sensor includes at least two of the first receiver coils.

4. The apparatus according to claim 3, wherein oscillating current of different frequencies is input to the first transmitter coil, and different combinations of amplitudes and phase differences between measured signals acquired by the at least two of the first receiver coils are measured.

5. The apparatus according to claim 4, wherein a distance from the apparatus to the collar of the downhole casing is determined based on an inversion of the measured signals.

6. The apparatus according to claim 1, wherein the second type of sensor includes a second transmitter coil is collocated with a second receiver in the housing.

7. The apparatus according to claim 6, wherein current is input to the second transmitter in pulses to generate a dipole moment change according to a step function, and the second receiver measures transient electromagnetic signals.

8. The apparatus according to claim 7, wherein a distance from the apparatus to the collar of the downhole casing is determined based on an inversion of the transient electromagnetic signals.

9. The apparatus according to claim 1, wherein the gamma ray detector is used to correlate a distance from the apparatus to the collar of the downhole casing obtained by the first type of sensor or the second type of sensor, and the apparatus records a thickness of the downhole casing as the apparatus moves along the downhole casing.

10. The apparatus according to claim 1, wherein the apparatus is connected to a tool and positions the tool within the downhole casing based on a location identified according to a distance from the collar of the downhole casing.

1 1. A method of locating a collar of a downhole casing, the method comprising: disposing a locator downhole, the locator comprising a housing, a first type of sensor configured to operate in the frequency domain disposed in the housing, a second type of sensor configured to operate in the time domain disposed in the housing, and a gamma ray detector disposed in the housing; and

determining, using the locator, a distance from the locator to the collar of the downhole casing.

12. The method according to claim 1 1 , wherein the disposing the locator downhole includes disposing a first transmitter coil remotely from a first receiver coil in the housing as the first type of sensor.

13. The method according to claim 1 1 , further comprising disposing at least two of the first receiver coils as the first type of sensor.

14. The method according to claim 13, further comprising inputting an oscillating current of different frequencies to the first transmitter coil and measuring different combinations of amplitudes and phase differences between measured signals acquired by the at least two of the first receiver coils.

15. The method according to claim 14, wherein the determining the distance is based on inverting the measured signals.

16. The method according to claim 1 1, wherein the disposing the locator downhole includes disposing a second transmitter coil collocated with a second receiver coil in the housing as the second type of sensor.

17. The method according to claim 16, further comprising inputting current pulses to the second transmitter to generate a dipole moment change according to a step function and measuring transient electromagnetic signals at the second receiver.

18. The method according to claim 17, wherein the determining the distance is based on inverting the transient electromagnetic signals.

19. The method according to claim 1 1, further comprising inspecting a thickness of the downhole casing based on moving the locator along the downhole casing and using a sensitivity of the locator to the thickness.

20. The method according to claim 1 1, further comprising positioning a tool within the downhole casing using the collar as a reference location based on connecting the tool to the locator.