US2846019A

US2846019A - Generating seismic waves

Info

Publication number: US2846019A
Application number: US545453A
Authority: US
Inventors: Harold M Lang
Original assignee: Pan American Petroleum Corp
Current assignee: Pan American Petroleum Corp
Priority date: 1955-11-07
Filing date: 1955-11-07
Publication date: 1958-08-05
Anticipated expiration: 1975-08-05

Description

2 Sheets-Sheet 1 Filed Nov. '7, 1955 }TO FIRING CIRCUIT 5O OXYGEN 'SUPPLY TIME BREAK CIRCUIT INVENTOR.

HAROLD M. LANG BY M F FIG. 2

e 6 76 VI mm FIRING CIRCUIT F UEL GAS SUPPLY FLOW METER ATTORNEY FIG.

8 5, 1958 H. M. LANG GENERATING SEISMIC wavas 2 Sheets-Sheet 2 Filed NOV. 7, 1955 n QE ONOOBS 83d 1.33:1 Nl All 3013A NOLLVNOLHO INVENTOR.

HAROLD M. LANG BY W p ATTORNEY U it 'This invention relates to seismic geophysical surveying and is directed particularly s novel methodand fapparatus for'generating seismic waves. More'particularly, it is directed roan improvement in the generation .ofseismic waves by elongated charges of. explosive material having alinear detonation .velocity approximately States Patent ,matehing the seismic-wave-transmission velocity of a.

surrounding earth medium. Ever since the beginningof geophysical surveying by the seismic method, the detonation of explosive charges I in drilled shot holes has been the most commonly used method. of wave generation, -While other methods of;

producing thedesired seismic waves have been proposed and have been used ;on some occasions, tthedetonation of explosive charges-in shot holes continues to lbetthe preferred method, evenzthough it frequently gives rise to undesirable interfering vwaves. v

Some of the latter can be eliminated arrangements of the chargematerial. It has been found,

5 ,for example, that certain types ofinterfering waves can be eliminated by distributing theqexplos'ive charge ma ..terial throughout a considerable length .of theshot 'hole and detonating it from the top downwardly, withthe rvelocity of propagation of the detonation along the length of the charge approximately matching the seismic-wave- ;transmission velocity in the formations surrounding the shot hole.

Such elongated or distributed velocity-matching charges,

however; are not available commercially. and have heretofore been rather expensive andtime-consuming to prepare and use.; One method of preparing such charges is that disclosed in' Silverman Patent 2,609,885 wherein .one or more strands of Primacord detonating fuse are wound in a helix about a supporting means, the helix having a pitch such as to provide alowered effective.

detonation velocity equal to the wave-transmission veflOCllyQOf theformations in the directioncof the helical axis. Only relatively small weights ofexplosive charge ,material can be easily formed intoa helix in this manner, howeven- Furthermore, the danger to personnel is in some degreeflncreased due to the additional handling of explosive materials required-to form them into a helix.

by particular It can be said, inigeneraL that the diificulty of pro- ,viding a 'charge detonation velocity matching seismicwave-transmission velocities arises from the fact that the great majority of explosive compositions that detonate reliably under bore-holeconditions have detonation velocities mnc'hrtoo high tomatch'the formation wavetransmission 'velocity satisfactorily. Thus, where the seismic-wave-transmission velocities below the weathered layer-most frequently 'fallin the range from 5000 to 10,000 feet per second, the detonation velocities of most reliable bore-hole. explosive'materials are considerably '-above 10,000 feet persecond and sometimes are in ex-- "E'pr ovide a novel, method and,,;apparatus for generating.

rce'ssof'20,000 feet persecond." r, 1

It is: accordingly, 2. primaryobject of my invention, to

5 2,846,019 Patented Aug. 5,

.2 seismic waves while utilizing the principle .of elongated, velocity-matching charges. Another objectis to provide a novel wave-generating method and apparatus by which elongated, velocity-matching explosive charges areforme'd in a bore hole quickly, inexpensively, and in a' comhole by filling a portion of the borehole with any one of a number of combustible gaseous mixturesfandathen igniting the gaseous column in such a way that detonation occurs. Substantially complete safety is assured because the components of the gaseous mixture. are maintained separate until after they have been introduced intothe shot hole. As the composition of the gaseous mixture chiefly determines the velocity with which detonation proceeds along the bore hole, variations of thecomposition can accordingly be made, to adjust the detonation velocity to any desired value within a-- substantialrange of such values. i

This will be better understood from the description to follow, taken with the drawings forming apart of this application, and illustrating typical embodiments of the invention. In these drawings, 1 Figure 1 shows diagrammatically and incross section an apparatus embodyingthe invention inoperating posi- Figure 3 shows graphically the relation of'detonation velocity and composition for various detonatable gaseous 4O -mixturesof typical fuel gases and oxygen.-

ticularly to Figure 1 thereof, a shothole 10 is shown as I extending from the ground surface 11 to a substantial depth-below the. boundary 12 between the Weathered Referring now to these drawings in detail,iandparupper layer of low seismic-wave-transmission velocity and ,the sub-weathering or lower strata of substantially higher wave-transmission velocity. Seated at some depth within the shot hole 10, preferably near the weathering boundary 12, is an expansible or inflatable packer 15,

attached to and surroundinga cylindrical mandrel 16, which -is preferably closed at the ends and is adapted'to be lowered into shot hole 10 to any depth therein bya flexible cable 17 wound on a reel 18 at groundsurface' 11. A tubing 21 opening into the annularspace surrounding mandrel 16, between it and the inside of sleeve 15, extends to the ground surfaceto a control valve 23 and a pump 22, by which means liquid or gas can be forced into sleeve 15 to expand it, or released therefrom to allow it to contract. After expansion, the fluid pressure .is maintained within the sleeve by closingvalve 23. v Extending through the bodyof mandrel 16 is a hose or tubing 26, open at both ends and preferablyflexible. The upper end of hose 26 terminates just above packer 15, while the lower end extends preferably to a depth near the bottom of shot hole 10; Also passing through the mandrel 16 to a point just below it are a pair of

tubes

28 and 30 which extend upwardly to the. groundv surface 11, wherethetube 28 is attached to a manifold'31, from .which a tubing 32 extends through a fiowmeter 33 and a control valve 34 to an oxygen-supply vessel 35, such as a high-pressure cylinder. From :the manifold131' a second branch line 37 extends to a control valve 38 which may be opened to the atmosphere.

The tubing 30 similarly connects at ground surface 11 through a flowrneter 41 and a control valve 42 to a pressure cylinder 43 containing a supply of fuel gas. Through a packing device 45 on manifold 31 passes a wire 46, containing a pair of insulated electrical leads, extending to an igniting device 47, such as an electric blasting cap, which may thus be lowered through the tubing 28 to a point just below its lower end in the gas space below packer 15.

In operation, packer 15 with its associated tubes, is lowered uninflated, into the shot hole by means of the cable 17 to the desired depth. This will normally be close to the interface 12, at or near which will normally be found the level of the liquid 49 standing in well bore 10. With the valve 23 open, pump 22 is then operated to inject gas or liquid through the line 21 into the interior of sleeve to inflate it and thereby form a firm seal against the wall of hole 10. Closing valve 23 maintains this pressure witihn the sleeve of packer 15.

Valves

42 and 34 are then opened, and, by observation of the

flowmeters

41 and 33, these valves are adjusted to introduce below the packer 15 fuel gas from the supply 43 and oxygen from cylinder 35 in the proper ratio to form a mixture of the desired detonation velocity. As the pressure of the gas mixture increases below packer 15, liquid 49 in the bottom of the shot hole 10 is displaced upwardly through the tubing 26 to a point above the packer 15 or outwardly into porous formations of lower hydrostatic pressure. From the standpoint of introducing the desired quantity of gaseous mixture into bore hole 10, it is not important where liquid 49 goes, but for the purpose of maintaining the seal of packer 15 and withstanding the pressure increase upon detonation, it is preferred that it go upwardly through tubing 26. In fact, to prevent loss of liquid 49, and possibly some of the gaseous mixture also, to porous formations, materials well-known in the drilling-fluid art for forming an impervious filter cake or seal over the porous formation face may be incorporated in liquid 49.

After the desired amount of fuel-gas and oxygen mixture has been thus introduced into the shot hole 10 and the liquid 49 therein displaced, so that the desired length of the shot hole below the packer 15 is filled by the gaseous mixture,

valves

34 and 42 are closed, and the insulated leads of wire 46 are connected to a firing circuit 50 (Figure 2). Then, when generation of seismic waves is desired, circuit 50 is actuated to detonate cap 47 and create a shock wave in the gaseous mixture. This causes substantially immediate detonation of the column of gas below packer 15, with the detonation wave traveling in a downward direction.

The velocity with which this detonation proceeds through the gaseous mixture is predetermined by choice of the ratio of fuel gas to oxygen in the mixture, allowing for the effect of any diluent gas present, so that the final detonation velocity substantially matches the seismic-wave-transmission velocity of the formations penetrated by bore hole 10 below weathering interface 12. Upon the occurrence of detonation, the frictional force between the sleeve of packer 15 and the wall of Well 10, and the weight and inertia of the liquid 49 above the packer combine to hold in the pressure of the exploding gas column.

As compared with a column of solid explosive material, an important factor of improvement in the efficiency of energy transfer from the exploding gas to the surrounding formation is due to the lower peak pressure in the gas detonation-wave front, plus the fact that this gas is in intimate contact with the formation face throughout the gas-filled borehole length. Consequently, a substantially greater percentage of the energy released by the chemical reactions of the detonation is transferred to the 4. formation in the gaseous detonation than is true of the detonation of a solid explosives column.

At the conclusion of the travel of the detonation wave and resulting combustion of the gaseous mixture, rapid cooling and condensation of the reaction-product gases take place to reduce the pressure below packer 15. By opening valve 38 the non-condensable combustion products of the gaseous detonation are vented to the atmosphere, with the aid of the pressure of liquid 49 which then flows downwardly through the tube 26 and refills the well-bore space below the packer 15. As many subsequent charges may be shot as desired simply by repeating the foregoing steps of filling the well bore below packer 15 with a gaseous mixture of the proper composition and lowering a new igniting device 47 through the tubing 28 each time ignition is desired.

In this embodiment the use of the blasting cap 47 is ordinarily sutficient to insure that detonation of the gaseous mixture takes place promptly. Other ways of igniting the gaseous mixture in the well bore 10 may be used, however, such as that shown in Figure 2 where the principle of spark ignition is employed. Thus, there is mounted on the hose 26 a short distance below the lower errds of tubes 28 and 30 a spark gap 61, having one electrode connected by an insulated lead 62 to a high-voltage firing circuit 50 at the ground surface. The other electrode of gap 61 is grounded. At some additional distance below the spark gap 61, also mounted on hose 26, is an ionization gap 64, similar to spark gap 61, having one electrode grounded in the same manner as gap 61 and the other connected by an insulated lead 65 to a timebreak circuit 66 at the ground surface.

With a detonatable gaseous mixture present in bore hole 10 below packer 15, application of a sudden high voltage impulse from circuit 50 over lead 62 to the gap 61 creates a spark through the gaseous mixture and ignites it. Detonation of the mixture, however, does not ordinarily ensue immediately, but there is a brief interval of time, which is frequently somewhat variable, during which propagation of the combustion in the gaseous mixture occurs at flame, rather than detonation, velocity. This velocity of propagation of the combustion initiated at gap 61 increases until a point is reached at which shock-wave generation begins, and thereafter the detonation wave front proceeds with the characteristic detonation-wave velocity.

This wave front is detected as it passes the ionization gap 64, to which a potential is applied over the lead 65. As the ionized gases in the detonation-wave front sharply reduce the resistance between the electrodes of gap 64, there is created a surge of electric current in the timebreak circuit 66 when the detonation wave passes the gap. This impulse, together with the known depth of the ionization gap 64, is used as the time reference or time break on the resulting seismic record, rather than the time of energizing the spark gap 61. Instead of being located close to gap 61, the ionization gap 64 may of course be located near the bottom of the gaseous column in well bore 10, where it will serve to provide not only time-break information but further to assure, by the time interval between the energizing of gap 61 and the arrival of the detonation-wave front at the gap 64, that detonation of substantially the entire length of gas column has taken place satisfactorily.

Figure 3 shows graphically the relationship between gas-mixture composition and the resulting detonationwave velocity for a few typical mixtures of different fuel gases and oxygen. One convenient gaseous mixture which has been found to detonate reliably under borehole conditions with a good release of energy comprises chiefly acetylene and oxygen. The solid-line curve 70 is plotted from data obtained in bore-hole detonationvelocity measurements. This shows that the detonation velocity of a mixture of acetylene and oxygen varies almost linearly from about 6500 to about 9000 feet per "Q. second, as the concentration of acetylene in the mixture varies between about percent; and about 40 percent, the

as occurs due to the bore-hole walls, being water-wet.

This range of detonation velocities is clearly adequate to agree reasonably well, the velocity in the bore hole being generally slightly less than that under laboratory conditions', the difierence being possibly due to experimental error, but more probably to the effect of the water vapor known to be present. The major difierence to be noted appears to be that, in the bore hole, the peak velocity obtainable is lower and occurs at a lower relative concentration of acetylene, being 40 percent as compared to about 50 percent. This is not serious, however, as velocities of detonation in excess of about 9000 feet per second are rarely needed to match formation wave velocities, and as long as the mis-match between detonation and wavetransmission velocities does not exceedabout 10 percent, the major eifects of velocity matching are preserved.

Of a considerable number of other fuel gases which will detonate with oxygen in the proper concentrations, two are shown in'Figure 3. Thus, propane in amounts varying from about 5 percent to about 25 percent or 30 percent will detonate with oxygen at velocities ranging between about 6000 and 8500 feet per second. Hydrogen is another possible choice of fuel gas, as, over the range balance of the mixture being oxygen and such water vapor of hydrogen concentrations from about 30 percent to about 80 percent, the detonation velocity of the mixture will vary from about 6000 to nearly 11,000 feet per second.

Over most of the velocity range of interest the curves of Figure 3, which are representative of a great many fuel gases, may be considered to be straight lines of the 40 general formula F the volume percent of fuel gas in admixture with oxygen, 1

V is the sub-weathering seismic-Wave velocity to be matched by the detonation velocity, and

K and K are constants for any particular fuel gas.

2 'For the three fuel gases of Figure 3, K and K are shown in the following table:

V and m are also given in the table. In is the average slope of the detonation velocity-percent fuel gas curve in the region of interest where it is approximately linear, and V is the zero-percentage intercept of the assumed straight-line curve. The last column of the table shows the range of seismic-wave velocities for which the above general formula holds. It will be apparent that the great majority of sub-weathering seismic-wave velocities are included.

In general, the effects of varying the temperature or. pressure of the gaseous mixture over a substantial range are small'compared to the' eflect of varying the relative concentration of fuel gas to oxygen in the mixture. The

effect of diluent gases, however, is' more'prominent, the t most marked effect being to lower the detonation-velocity values somewhat below those shown by the dashed-line curves, which thus represent probable maximum values of this velocity for the composition obtainable under borehole conditions. i That is, the velocities obtainable Where water vapor is normally present are somewhat lower than those shown by the dash-line curves, but within the plus or minus 10 percent allowable mis-match of detonation and formation velocities it is considered satisfactory to use the compositions shown by the dash-line curves.

In general, the total energy released is directly proportional to the amount of reacting gases present. vThus,

for any given length of bore hole, increasing or decreasing the pressure of the gas mixture correspondingly increases or decreases the seismic-wave-creating energy with negligible change of detonation velocity. Conversely, at constant pressure, the energy will vary in proportion to the length of bore hole filled.

While I have thus described my invention in terms of the foregoing specific embodiments thereof, it is to be understood that other and further modifications occur to those skilled in the art. In particular, many other gaseous compositions than those disclosed specifically herein can be made to detonate under bore-hole conditions and with the desired velocities for the generation of seismic waves. The invention therefore should not be considered as limited to the details and compositions specifically set forth, but its scope is properly to be ascertained by reference to the appended claims.

I claim:

l. The method of generating seismic waves in a bore hole in the earth which comprises the steps of filling a sub stantial length of said borehole below the weathering velocity interface with a mixture of fuel gas and oxygen capable of combustion by detonation, during said filling step adjusting the relative flow ratesof said fuel gas and said oxygen so that the volume percent F of fuel gas in the mixture with oxygen is equal to X V-K where K and K are constants for any given fuel gas, and V is the seismic-wave-transmission velocity of the subweathing formations penetrated by said borehole, and initiating detonation of the column of gases at the upper end thereof.

2. The method of generating seismic waves in a bore hole in the earth extending to a substantial depth below the weathering interface therein, which method comprises the steps of establishing a partition in said bore hole at approximately the depth of said weathering interface, separately introducing through said partition to a point therebelow a fuel gas and oxygen capable of forming a detonatable mixture, adjusting the relative flow rates of i said fuel gas and said oxygen to produce a mixture in which the percentage F of fuel gas is equal to X V-K where K an K are constants for any given fuelgas and V is the seismic-wave-transmission velocity of the subweathering formations penetrated by said bore hole, filling a substantial length of said hole below said interface with said mixture, and initating a detonation wavefront in the gaseous column at a point immediately below said partition.

3. The method of claim 2 wherein said fuel gas is chiefly acetylene, K and K have values approximately .0100 and 49.0, respectively and V is between about 6500 and'9000 feet per second.

4. The method of claim 2 wherein said fuel gas is chiefly propane, K and K have values approximately .0088 and 48.4, respectively, and V is between about 6500 to 8500 feet per second.

5. The method of claim 2 wherein said fuel gas is chiefly hydrogen, K; and K; have values approximately 7 .0121 and 42.3, respectively, and V is between about 6500 and 9000 feet per second.

6. The method of claim 2 including also the steps of producing an electrical impulse coincident with the passage of said detonation wave front past a point at a known substantial depth below said partition, and recording the time of occurrence of said impulsc'as a reference point.

References Cited in the file of this patent UNITED STATES PATENTS Kow'astch Jan. 7, 1913 Owen May 10, 1927 Scott June 12, 1951 Piety May 25, 1954

Claims

1. THE METHOD OF GENERATING SEISMIC WAVES IN A BORE HOLE IN THE EARTH WHICH COMPRISES THE STEPS OF FILLING A SUBSTANTIAL LENGTH OF SAID BOREHOLE BELOW THE WEATHERINGVELOCITY INTERFACE WITH A MIXTURE OF FUEL GAS AND OXYGEN CAPABLE OF COMBUSTION BY DETONATION, DURING SAID FILLING STEP ADJUSTING THE RELATIVE FLOW RATES OF SAID FUEL GAS AND SAID OXYGEN SO THAT THE VOLUME PERCENT F OF FUEL GAS IN THE MIXTURE WITH OXYGEN IS EQUAL TO K1V-K2, WHERE K1 AND K2 ARE CONSTANTS FOR ANY GIVEN FUEL GAS, AND V IS THE SEISMIC-WAVE-TRANSMISSION VELOCITY OF THE SUBWEATHING FORMATIONS PENETRATED BY SAID BOREHOLE, AND INITIATING DETONATION OF THE COLUMN OF GASES AT THE UPPER END THEREOF.