"Apparatus and Method"

The present invention relates to apparatus and method for use in seismic surveying. The invention particularly relates to apparatus for generating a seismic signal for use in downhole surveying, and to a method of using the apparatus in downhole survey techniques.

Seismic sources are well known in the art of well surveying, and known devices can comprise a hammer to impact on the wall of the borehole in order to generate a sound wave that can be detected by seismic detectors positioned elsewhere. Many methods of generating the impact on the borehole wall are known, such as mechanical and electrical vibrators, and pneumatic power sources.

According to the present invention there is provided apparatus for generating a seismic signal in a subterranean substrate, the apparatus comprising a hammer for impacting on a surface of the substrate, wherein the hammer is moved by a hydraulic pressure differential.

The invention also provides a method of generating a seismic signal in a substrate, the method comprising providing a seismic device with a hammer, and moving the hammer to impact it against a surface of the substrate in order to generate the seismic signal, wherein the hammer is moved by a hydraulic pressure differential.

In some embodiments of the invention, the apparatus is adapted for use in an oil or gas well, typically a subsea well, and is adapted to be incorporated within a downhole string; in some embodiments, the apparatus is adapted to be deployed on a wireline string, but other types of strings, e.g. of drill pipe and tubing, can also be used.

In some embodiments of the invention, the hammer comprises a piston that is in fluid communication with a source of pressurized fluid. The fluid is typically pressurized from a device within the downhole string. Typically the fluid is pressurized downhole by an energy storage device such as a spring that can be energised in an earlier step by an external force such as a downhole pump or motor assembly, and locked in the energised configuration until the hammer is to be fired. The spring is typically energised from a hydraulic intensifier from some other source of hydraulic pressure, typically within the same string, and typically downhole. When the spring is released from its energised configuration, and pressurizes the fluid, the fluid pressure drives the hammer against the substrate in order to generate the seismic signal.

Typically the piston is arranged to impact on the surface of the substrate substantially perpendicular to the surface, and can be set to operate in a uni-directional or omni-directional manner. In some embodiments the apparatus can be provided with more than one piston oriented to fire in different radial directions. The number of separate pistons is not limited by the scope of the invention. The orientation of the body, and therefore the direction of impact of the hammer, can optionally be adjusted in use by rotating the tool within the pipe, but in more simple embodiments, the radial orientation of the body (and therefore the orientation of the piston and hammer) is determined upon insertion of the body into the bore at surface, and is maintained by a guide means such as a bow spring device that controls the orientation of the body within the bore. In certain embodiments with more than one hammer piston, the orientation of the hammer pistons can be the same, or in some embodiments it can be different. Typically the apparatus has a clamping device such as a motor driven arm or piston, or a hydraulically powered clamping arm or piston,

extending from one side of the body of the device, that stabilizes the body from which the hammer is moved against the borehole wall, so as to ensure good contact between the hammer and wall upon impact.

Typically, the seismic signal generated by the apparatus is received by a seismic receiver, typically deployed in the same well, and advantageously on the same string as the apparatus for generating the signal. In some embodiments, however, the signal can be generated by a device in a different string, or in a different well, particularly a nearby or adjacent well.

An embodiment of the invention will now be described by way of example, and with reference to the accompanying drawings, in which;

Figure 1 is a schematic view of a seismic surveying string incorporating seismic apparatus;

Figure 2 is a side perspective cutaway view of a typical embodiment of a seismic apparatus employed in the string shown in fig 1 ;

Figure 3 is a side view of the piston arrangement in the figure 2 apparatus;

Figure 4 is a schematic of the piston arrangement of Figure 3; Figure 5 is a schematic perspective view similar to figure 2;

Figure 6 is a schematic graph of seismic signal amplitude against time for a hydraulically operated hammer (solid line) and a pneumatically operated hammer (broken line);

Figure 7 is a side sectional view through a second embodiment of seismic apparatus;

Figure 8 is a plan view of the figure 7 apparatus;

Figure 9 is a side view similar to Figure 2 of a third embodiment of a seismic apparatus;

Figure 10 is a schematic of the Figure 9 apparatus;

Figure 11 is a side view similar to Figure 9, of a fourth embodiment of a seismic apparatus; and

Figure 12 is a detailed side view of a hammer portion of the Figure 11 apparatus.

Referring now to the drawings, a seismic source 5 is deployed in a string of tools S suspended within a well W by wireline or by some other suitable means, such as drill pipe etc. The string S includes the seismic source 5 and at least one receiver R. Typically the string contains more than one receiver R, and typically the receivers R are spaced from the seismic source as shown in fig 1. Seismic signals emitted from the seismic source S travel through the formation being surveyed, and the return signals are gathered by the receivers R for processing and analysis.

Referring now to figures 2, to 5, a seismic source 5 has a body 6 housing a radially extending clamp arm 7, a firing piston 4 and a hammer piston 10. The hammer piston 10 is recessed laterally within the body, and is radially movable within a bore 15 when fired from body, so as to impact its end face laterally against the side of the pipe P within which the source 5 is deployed. The hammer piston 10 is sealed within the radial bore 15 within the body 6, and the motive force for movement is provided by a hydraulic pressure differential across the seals of the hammer piston 10.

As shown schematically in figure 3, the hammer piston 10 is sealed within a stepped recess 15 within the body 6. The stepped recess 15 has an outer portion 15a, and inner portion 15b formed within the bore of a piston housing 9 that is connected (at screw thread 9t) and sealed (at seal 9s) to the body 6. The outer portion 15a is disposed radially outward from the second portion 15b with respect to the pipe P, is open to the outer surface of the body 6, and has a larger diameter than the inner portion 15b. The

hammer piston 10 has a matching outer portion 10a, and an inner portion 10b. The outer and inner portions of the recess 15 and the hammer piston 10 are sealed at 10as and 10bs respectively. The outer seal 10as has a larger diameter than the inner seal 10bs, so the hammer piston 10 is sealed across different surface areas on its opposing faces. The hammer piston 10 has a protruding portion 10p that has a narrower diameter than the portions 10a and 10b, and is sealed to the bore 15 in the body 6 at 10ps. The protruding portion 10p extends through the wall of the body 6 to impact against the pipe P.

The recess 15 containing the piston 10 is in fluid communication with one end of a hydraulic piston chamber 20 through conduits18a and 18b. Conduit 18a connects the piston chamber 20 to the inner face of the piston 10 between the seals 10bs and 10as, and conduit 18b connects the piston chamber 20 to the outer face of the piston 10 between the seals 10as and 10ps. The two conduits are interconnected through the piston chamber 20, permitting fluid communication between them. The piston chamber 20 houses a piston 22 which is movable axially therein and is sealed against the inner surface of the chamber by seals 22s. The piston 22 is biased towards the said one end of the chamber 20 by a stack 25 of cup springs or Bellville washers etc, so that hydraulic fluid 19 between the piston 22 and the recess 15 is compressed between the seals 22s of the piston 22, and the seals on the hammer piston 10. Thus, force exerted by the stack of springs 25 is transferred via the piston 22 and the hydraulic fluid 19 through each of the conduits to the hammer piston 10. The pressure across the hammer seals is equalised during charging by the interconnection of the conduits through the piston chamber 20.

The hammer piston 10 is prepared for firing by pressurizing both sides of the seal 10as by injecting pressurised fluid through a control line 30 that

communicates with the two conduits 18 adjacent to the lower end of the piston chamber 20. Typically the pressure is applied to the control line 30 from a downhole hydraulic pressure pump and/or intensifier (not shown) of known design, that is typically carried on the same string as the rest of the tools, or even on the body of the same tool. The pressure in the control line 30 is raised gradually until it exceeds the force applied by the spring stack 25. Because the conduits and the piston chamber are all interconnected, the pressure equalises in the system, and the high- pressure forces the compression of the spring stack 25, moving the piston 22 axially upwards in the chamber 20. The piston 10 is subjected to the same pressure, and because the surface areas of the seals 10as and 10bs are different, different radial forces are applied to the piston 10 to move it radially inwards within the recess 15. The piston 10 continues to move radially inwards until the wider portion 10a shoulders out on the housing 9. At this point, the elevated pressure within the system is stored by closing off the control line 30 by means of an elongate plug 29 that extends downwards through the control line 30 and isolates the two conduits 18a and 18b from one another, or by means of an isolation valve 31. The firing piston 4 remains in the closed position as shown in Fig 3 during charging and by means of two o-ring seals 4s on the piston, it seals off the conduit 18b leading to the outer face of the seal 10as, so that the pressure is held within the conduit 18b by means of seals 10as, 10ps, 4s, and the plug 29. At this point, although the conduits 18a and 18b are isolated from one another, the pressure within the system is still equalised, and the piston 10 is still held radially inwards within the bore 15, as shown in Figure 3.

The spring stack 25 is also still compressed, and is pressurizing the fluid 19 within the chamber 20. The hammer piston 10 remains stationary despite the high pressure in the fluid 19, because an equal force is being

applied by the hydraulic pressure on the outside surface of the seals 10as. In certain embodiments, such as the one depicted schematically in the drawings, the surface area on the outside of the hammer piston area can be larger than the surface area on the inside of the hammer piston area, so that when the force across the hammer piston 10 is equalized, the pressure in the hydraulic fluid 19 will be greater than the pressure in the fluid outside. However, in certain other embodiments of the invention, the piston area on opposite sides of the hammer piston 10 can be substantially the same. The firing piston 4 is held in the radially extended position shown in Figure 3.

When the hammer piston 10 is to be fired, the clamp pressure on the firing piston 4 is vented so that it is rapidly withdrawn from the bore 4b and conduits 18. Firing piston 4 is thus retracted (to the left as shown in Fig 3) so that when the seal nearest to the tip 4b passes the conduit 18b, the pressure in the conduit 18b is vented rapidly to the external surface of the device through the bore 4b. Typically, the venting is achieved through large bore conduits so that a pressure differential is set up very rapidly across the hammer piston 10 because the hydraulic pressure in the pressurized fluid 19 now greatly exceeds the hydraulic pressure on the other side of the seals 10as. The large and rapidly generated hydraulic pressure differential across the hammer piston 10 causes its rapid movement radially outwards from the recess 15 (to the right in Fig 3) and it's outer face impacts against the inner wall of the pipe P or against another part of the wall of the borehole, thereby generating the seismic signal that can be picked up by the receivers R. Optionally, a foot comprising a pad or a flat or arcuate plate can be mounted on the end of the hammer. The foot can be shaped or otherwise adapted to engage with the casing or directly with the formation. In some circumstances, the foot can have a larger or smaller end surface area to impact with the

casing or the formation, and in cases where the foot is to engage the formation directly, the pad can have a larger surface area in order to dissipate the radial load on the formation and prevent its disruption and damage by the firing of the hammer piston 10. This also reduces the tendency of the end of the hammer piston 10 to embed itself in a soft formation during the firing process. In some embodiments a striking plate can be provided on the outside of the body that can be oriented to be perpendicular to the axis of the hammer piston 10, and can provide a striking surface for the impact of the hammer piston 10, instead of the hammer piston 10 striking the inner surface of the borehole wall, the casing in which the body is located, or the formation. This feature is particularly useful for open hole applications, where the striking surface on the borehole wall may be irregular.

Figure 6 shows a schematic graph of a test performed using a hydraulically actuated seismic source according to the invention (solid lines), and a similar pneumatically actuated seismic source (broken lines). The graph shows the seismic signal amplitude of the signal generated upon firing the hammer in each case, and from this it can be clearly seen that the hydraulically actuated hammer generates a seismic signal having a single peak of high amplitude with low noise and a relatively short persistence. However, the pneumatically actuated hammer generates a first peak of similar amplitude but also generates an additional artefactual peak that tends to interfere with the true signal.

Figures 7 and 8 show a second embodiment 50 having a body 56, clamp arms 57 at opposite ends of the body 56, and a hammer piston 60. The hammer piston 60 is located within a recess within the body 56 in which it is movably sealed in a similar manner as hammer piston 10 is sealed within the recess 15 in the earlier embodiment, and functions as a piston

as previously described. The inner end of the hammer piston 60 is in fluid communication with hydraulic fluid 69 contained within a piston chamber 70 between the seals on the hammer piston 60, and the seals on a piston 72. There is also communication with the recirculation reservoir 80, which is fed through the hydraulic pump 85 when charging the system. The piston 72 is biased towards the hammer piston 60 by a spring stack 75 thereby compressing the hydraulic fluid 69 as in the first embodiment. In this example, the hydraulic fluid is taken from an internal reservoir 80 within the body 56, so that the tool is self-contained and does not affect the surrounding environment. The hydraulic fluid within the internal reservoir 80 is also used to operate the clamp arms 57 by means of solenoids. The initial compression of the spring stack 75 is optionally achieved in this embodiment by the operation of an onboard motor 86 driving an onboard hydraulic pump in order to pressurize the fluid from the reservoir 80 and to compress the stack 75. Thus the signal for the initial preparation for firing, and the firing sequence itself can be electronic rather than hydraulic or pneumatic or mechanical.

The hammer piston 60 has a central portion of increased diameter (and therefore surface area) and a seal between that portion and the outer aperture through which the hammer is fired, unlike the schematic version shown in Fig 4. Thus the recess in which the hammer piston 60 moves and which is pressurized by the control line is sealed from the outside of the tool, so that the hydraulic pressure applied to the hammer to force it backwards into the recess acts only on the high surface area central portion and is sealed from the borehole ambient pressure by the external seal.

Referring now to Figure 9, a modified version of the Fig 7 embodiment has some of the internal components rearranged in order to allow hydraulic

conduits within the device to be straighter, shorter and wider, in order to improve on hydraulic efficiency, and to bring the spring stack closer to the hammer. The modified version 150 has a body 156, clamp arms 157 at opposite ends of the body 156, and a hammer piston 160. The hammer piston 160 moves as a piston sealed within a recess in the body 156. The inner end of the hammer piston 160 is in fluid communication with hydraulic fluid 169 contained within a piston chamber 170 between the seals on the hammer 160, and the seals on a piston 172. The recessed end of the hammer piston 160 is also in fluid communication with the recirculation reservoir 180, which is fed through the hydraulic pump 185 when charging the system. The piston 172 is biased towards the hammer piston 160 by a spring stack 175 thereby compressing the hydraulic fluid 169. In this example, the hydraulic fluid is again from an internal reservoir 180 within the body 156. The hydraulic fluid within the internal reservoir 180 is also used to operate the clamp arms 157 by means of solenoids. The initial compression of the spring stack 175 is optionally achieved in this embodiment by the operation of an onboard motor 186 driving the onboard hydraulic pump 185 in order to pressurize the fluid from the reservoir 180 and to compress the stack 175. Thus the signal for the initial preparation for firing, and the firing sequence itself can be electronic rather than hydraulic or pneumatic or mechanical.

The hammer piston 160 has a central portion of increased diameter (and therefore surface area) and a seal between that portion and the outer aperture through which the hammer piston 160 is fired. Thus the recess in which the hammer piston 160 moves and which is pressurized by the control line is sealed from the outside of the tool, so that the hydraulic pressure applied to the hammer piston 160 to force it backwards into the recess acts only on the high surface area central portion and is sealed from the borehole ambient pressure by the external seal.

As shown in Figure 10, the pump 185 draws fluid from the reservoir 180 through a filter 181. The output of the pump 185 is connected through a two-way line 162 to the outer face of the hammer piston 160, and to a firing valve 188, but there is a one-way valve 187 in the line 163 that prevents fluid returning to the pump, but allows communication between the firing valve 188 and the hammer piston 160. The firing valve controls fluid communication between the outer face of the piston 160 and the reservoir, and is normally closed (as shown in Figure 10) when charging the hammer piston 160.

The hammer piston 160 has a line 161 that permits communication from the outer to the inner side of the hammer piston 160 through a one-way valve 162. A second line allows very restricted two-way flow to prevent hydraulic lockup of the hammer piston 160. Thus the same pressure applied to the outer face of the hammer piston 160 is also applied to the inner face, but since the surface area of the outer face is larger than the surface area of the inner face, this results in the inward movement of the piston 160 when the pressure is applied from the pump 185. The inner face of the hammer piston 160 is in two-way fluid communication with the fluid 169 in the firing reservoir and the inward movement of the piston 169 during charging thereby charges the firing reservoir.

The pump 185 also charges the clamp arms 157 through clamp pistons 156 which are controlled by a clamping valve 155 shown in Figure 10 in the open position to extend the clamp pistons 156 and raise the clamp arms 157. The pump 185 feeds pressure to the clamping valve 155 through a one-way valve 154. The valve 154 dumps fluid back into the reservoir 180 when the arms close.

When the system is to be charged the firing valve 188 is closed as shown in Figure 10 and the pump pressure is wholly routed to the impact piston 160. By means of the non-return valve 162, the same pressure is applied to each face of the piston 160, resulting in the inward movement of the hammer piston 160 due to the surface area differential, charging of the firing reservoir 169, and equalising of the pressure across the hammer piston 160. The equalised pressure in the fluid adjacent to the hammer piston 160 is reported by a pressure transducer monitored from the surface. When the hammer piston 160 is to be fired to create a seismic signal, the firing valve 188 is opened to vent the pressure on the outer face of the piston and discharge the fluid in the line 163 into the reservoir 180. The pressure differential that then exists across the hammer piston 160 cannot pass the one-way valve 162, and cannot equalise quickly enough across the flow restriction, and so the hammer piston 160 fires as a result of the force applied by the high-pressure differential across it. The non-return valve 154 isolates this hydraulic activity from the clamps 157.

Referring now to Figures 11 and 12 a modified version 250 of the Fig 9 embodiment shows a further rearrangement of the internal components. The modified embodiment 250 has substantially the same internal components as the previous embodiment 150, and similar reference numbers have been used with 100 added to the reference numbers for the previous embodiment 150. In the modified embodiment, the piston has been moved between the spring stack and the reservoir, so that the hydraulic conduit connecting the spring stack and the hammer piston can be straight, in order to reduce flow resistance and increase hydraulic efficiency.

Referring to Figure 11 , the modified version 250 has a body 256, clamp arms 257 at opposite ends of the body 256, and a hammer piston 260

located between the clamp arms. The hammer piston 260 is movably sealed within radial recess in the body 256. The inner end of the hammer piston 260 is in fluid communication with hydraulic fluid 269 contained within a piston chamber 270 between the seals on the hammer 260, and the seals on a piston 272. The recessed end of the hammer piston 260 is also in fluid communication (on its opposite side) with the recirculation reservoir 180, which is fed through the hydraulic pump 285 when charging the system. The piston 272 is biased towards the hammer piston 160 by a spring stack 275 thereby compressing the hydraulic fluid 269. The hydraulic fluid is fed from an internal reservoir 280 within the body 256.

The hydraulic fluid within the internal reservoir 280 is also used to operate the clamp arms 257 by means of solenoids. The initial compression of the spring stack 275 is optionally achieved in this embodiment by the operation of an onboard motor 286 driving the onboard hydraulic pump 285 in order to pressurize the fluid from the reservoir 280 and to compress the stack 275. Thus the signal for the initial preparation for firing, and the firing sequence itself can be electronic rather than hydraulic or pneumatic or mechanical.

The hammer piston 260 has a central portion of increased diameter (and therefore surface area) and a seal between that portion and the outer aperture through which the hammer piston 260 is fired. Thus the recess in which the hammer piston 260 moves and which is pressurized by the control line is sealed from the outside of the tool, so that the hydraulic pressure applied to the hammer piston 260 to force it backwards into the recess acts only on the high surface area central portion and is sealed from the borehole ambient pressure by the external seal. The figure 11 embodiment has an on board electronics module 273 that contains an on board bus to process signals from the surface and from the tool. The

provision of an on board bus cuts down on the requirement for signal conduits back to surface.

The modified embodiment of Figure 11 is generally operated in the same way as the embodiment of Figure 9. In the Fig 11 variant, the spring stack 275 is compressed by the motor 286 and pump drawing fluid from the reservoir 288, which is in the form of a compressible bladder, which requires fewer moving parts than a piston and chamber as in the Fig 9 variant. When the required pressure is reached in the fluid 269 in the piston chamber 270 and the spring stack is compressed to the correct amount, the spring stack is locked in position for firing. The piston chamber 270 terminates in a conduit block 271 (see Fig 12) that routes fluid through valve-occluded hydraulic conduits to each surface of the hammer piston 260. The conduit and valve connecting the piston chamber 270 to the inner surface of the hammer piston 260 has a large bore allowing rapid dumping of the pressure. The conduit connecting the piston chamber 270 to the outer surface is closed by a pilot valve that is triggered by an electronic signal from the electronics module 273 at the other end of the device, and dumps the pressure applied to the outer surface of the hammer piston 260. Thus a high pressure differential is rapidly set up across the hammer piston by the rapid dumping through the pilot valve, and the fluid 269 in the wide bore conduit can then surge through the valve and drive the hammer piston radially out of the housing 256, to strike against the borehole wall.

Modifications and improvements can be incorporated without departing from the scope of the invention. For example, as an independent aspect of the invention, whether or not the hammer is hydraulically moved, the string S can optionally be provided with a tube wave isolator I between the receivers R and the source 5 as shown in Fig 1 , to reflect tube waves that

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propagate along the pipe; these tend to interfere with the true reflected signal picked up from the formation by the receivers R, and increase the noise to signal ratio, so that the data are obscured. The isolators can comprise double sealed assemblies that define an annular area that can be evacuated or pressurized, as required, independently of the ambient pressure in the wellbore W, in order to resist transmission of the tube wave through the section of wellbore W between the seals of the isolator I. This reduces the noise generated by the source 5 and improves the signal to noise ratio of the system. Typically the isolator I is dimensioned to be slightly longer than the wavelength of the tube wave, so with a wavelength of 50m for an average tube wave, the isolator might have a length of 60 or 70 metres. The isolator can comprise a single component with a double seal, or can comprise two separate seal devices located at the appropriate axial spacing along the wellbore W.

Also, in some embodiments (with or without the hydraulically moved hammer) more than one source can be deployed on a single string. For example, on one string, five or more sources 5 can be located in series on the same string, spaced axially in the well W from one another, and fired independently of one another. This modification has the benefit that multiple readings can be taken from the string at different signal initiation positions along the well, without having to move the string and reposition it in the next location, which saves on deployment time and therefore costs. The sources can be oriented in the same radial direction or in different radial directions.