Abstract:
The present invention provides a method that is useful in determining the permeability of rock within a subterranean formation surrounding a borehole. The method involves the establishment of a compressional wave in the borehole rock of interest and the subsequent detection of a slow compressional wave that is directly related to the permeability of the rock. The slow compressional wave typically has a velocity of approximately 900 m/s. In contrast, the compressional waves used to make borehole measurements in the past have, velocities of approximately 2000-3000 m/s.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for use in determining the permeability of rocks in a subterranean formation surrounding a borehole. 
     BACKGROUND OF THE INVENTION 
     Permeability is a measure of the hydraulic conductivity of a rock, i.e., the ease or difficulty with which a fluid (oil, gas or water) flows through porous rock. In oil/gas fields, the permeability of the rock associated with an oil/gas reservoir is an important parameter to measure, because it is directly related to the rate of oil or gas that can be produced. To elaborate, in exploiting an oil/gas reservoir, one or more wells or boreholes are drilled at locations that typically have been determined based upon information obtained from seismic surveys, exploratory wells and the like. Once the wells have been drilled, the permeability of the rock formations encountered in the borehole is an important parameter in managing the well or wells so as to maximize the oil/gas that can be extracted from the reservoir. 
     There are a number of methods presently in use for determining the permeability of the rock in a borehole. One method, known as formation testing, involves using a wireline to lower an instrument to a desired location within the borehole. The instrument is adapted to force a nozzle into contact with the rock. The nozzle is then used to draw fluids from the rock. The ease or difficulty associated with drawing the fluids is indicative of the permeability of the rock. Formation testing, while providing a direct measurement of permeability, is subject to errors attributable to borehole fluid invasion effects and drilling induced formation damage. 
     Production testing is another method of directly measuring permeability. In production testing, a zone of a borehole is hydraulically isolated with packers and the fluid production of the well is guided to the surface through a sting of pipes. The volume of fluids produced and the thickness of the zone are used to derive the permeability of the rock in the isolated zone. Production testing, while typically providing reliable data, is very expensive due to the substantial amount of equipment required and the length of the test period, typically several days. 
     Yet another method involves the use of Stoneley acoustic waves to measure parameters that are related, but not equivalent to permeability. These measurements are then used to infer the permeability or range of permeabilities. Stoneley acoustic waves are waves that travel along the interface between the rock that defines the borehole and the fluid in the borehole. This method also uses a wireline to lower instrumentation into the borehole that causes Stoneley waves to be formed. The same tool detects the Stoneley waves after they have interacted with the rock of interest. Unlike formation testing, the instrumentation is not placed in direct contact with the rock. Consequently, the instrumentation is typically designed so that measurements are taken while the instrumentation is being raised or lowered by the wireline. One of the major shortcomings associated with the Stoneley waves approach is that the relationship of the measurements made with Stoneley waves to permeability can vary substantially from reservoir to reservoir. To address this variability, it is necessary to perform calibration procedures that make use of permeability measurements made on cores taken from the borehole or measurements made during formation testing. 
     Another method that provides an indirect measure of permeability utilizes nuclear magnetic resonance. To elaborate, nuclear magnetic resonance is used to measure the decay times of protons that are indicative of the pore size distribution, which in turn can be used to infer the permeability of the rock of interest. This method of measuring permeability is typically implemented using a moving wireline logging instrument. The limitations associated with nuclear magnetic resonance are substantially the same as those noted with respect to the Stoneley or tube wave approach. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method of measuring the permeability of rock within a borehole that makes use of a slow compressional wave, which propagates through the rock of interest. To elaborate, an acoustic or wave that travels through a porous rock can manifest itself in three modes: (1) a fast compressional wave, which is also known as a p-wave, in which the solid rock material and the fluid in the pores move in phase with one another; (2) a slow compressional wave in which the solid rock material and fluid in the pores move out of phase with one another; and (3) a transverse wave, which is also known as a shear wave or s-wave, in which the particles move perpendicular to the wave propagation direction. The compressional wave presently known to be used in making borehole measurements is the fast compressional wave with a velocity of approximately 2000-3000 m/s. The present invention makes use of the slow compressional wave, which typically has a velocity of approximately 900 m/s. 
     The method involves transmitting a compressional wave into the rock of interest at a fixed location within the borehole. The compressional wave manifests itself both as fast compressional wave and a slow compressional wave. The slow compressional wave is subsequently detected at a second location that is a known distance from the first location. Information associated with the transmitting and detecting steps is subsequently used to determine the velocity of the slow compressional wave. Namely, the elapsed time between the transmission of the compressional wave and detection of the slow compressional wave and the known distance between the first and second locations provides sufficient information to determine the velocity of the slow compressional wave. The velocity of the slow compressional wave is, in turn, combined with other information to calculate the permeability of the rock of interest. 
     Among the other information that, in conjunction with the velocity of the slow compressional wave, is used to determine the rock permeability is the velocity of the fast compressional wave. In some instances, the velocity of the fast compressional wave may be available from prior measurements. However, since the transmission of the compressional wave used to produce the slow compressional wave also produces a fast compressional wave, one embodiment of the invention also detects the fast compressional wave. The velocity of the fast compressional wave is determined from the elapsed time between the transmission of the compressional wave and the detection of the fast compressional wave and the known distance between the location from which the compressional wave was transmitted and the location at which the fast compressional wave is detected. 
     In one embodiment, the locations associated with the transmission of the compressional wave and detection of the slow compressional wave are chosen to be between about 10 cm and 50 cm of one another because of the high degree of attenuation that is likely to be experienced by the slow compressional wave as it propagates through the rock of interest. Further separation of the two locations is feasible. However, more sensitive detectors are required and/or signal processing to separate the slow compressional wave signal from noise. 
     In one embodiment, the location from which the compressional wave is transmitted (first location) and the location at which the slow compressional wave is detected (second location) are chosen so that a permeability measurement is made in a preferred direction or dimension. In one situation, the first and second locations define a line that is substantially parallel to the longitudinal axis of the borehole. In a vertical borehole, this orientation of the first and second locations results in information being obtained that is useful in determining the vertical permeability of the rock of interest. In another embodiment, the first and second locations define a line that is substantially perpendicular to the longitudinal axis of the borehole. This orientation, in a vertical borehole, provides data that is useful in calculating the horizontal permeability of the rock of interest. In yet another embodiment, the slow compressional wave is detected at two locations that, together with the first location, define a right angle. In this instance, the orthogonal permeabilities of the rock of interest are determinable. In the case of a vertical borehole, this means that both vertical and horizontal permeabilities are determinable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic and block diagram of a wireline logging system capable of being used to obtain information useful in determining the permeability of rock within a borehole; 
     FIG. 2 illustrates the tool portion of the wireline tool system shown in FIG. 1 fixed in place in a borehole; 
     FIG. 3 is a block diagram of the components of the tool portion of the wireline tool system shown in FIG. 1 that are used in creating a compressional wave in the rock of interest and detecting the slow compressional wave and if needed, the fast compressional wave; 
     FIG. 4 is a graph of the signal produced by the pressure gauge portion of the tool with the portions of the signal that correspond to the fast and slow compressional waves identified; and 
     FIG. 5 is an alternative embodiment of the tool that includes a second detector that is useful in determining an orthogonal permeability relative to the first detector. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a wireline logging system  10  that is capable of obtaining slow compressional wave related information and, if desired, using this information to determine the permeability of rock located in a borehole  12 . The system  10  includes a tool  14  that embodies the components that are used to produce a compressional wave in the rock of interest and detect the slow compressional wave that has propagated through the rock of interest and if needed, the fast compressional wave. The system  10  further comprises an electric cable  16  that is used to transmit electrical signals between the tool  14  and a controller  18  located on the surface  20 . The electric cable  16  is also used in conjunction with a winch system  22  to raise and lower the tool  14  within the borehole  12  at the direction of the controller  18 . A transducer  24  permits the controller  18  to monitor the position of the tool  14  within the borehole  12 . 
     With reference to FIGS. 2 and 3, the tool  14  includes a tool housing  28  within which most of the other components of the tool  14  are hermetically sealed to protect them from the fluid within the borehole  12 . Located on one side of the tool housing  28  is pad  30  through which a pair of nozzles  32 A,  32 B extend. The pad  30  provides a frictional surface that facilitates fixing the tool  14  at a desired location within the borehole  12 . The first nozzle  32 A is used in to create a pressure step that induces a compressional wave into the rock of interest. The second nozzle  32 B is used in sensing both slow and fast compressional waves that have resulted from the pressure step output by the first nozzle  32 A after these waves have propagated through the rock of interest. The nozzles  32 A,  32 B are oriented such that a line extending between the nozzles is substantially parallel to the longitudinal axis of the borehole when the tool is in use. This orientation of the nozzles  32 A,  32 B, in a vertical borehole, provides information that is used to determine the vertical permeability of the rock of interest. 
     An actuator mechanism  34  is used to fix the tool  14  at a desired location in the borehole  12  and place the nozzles  32 A,  32 B into fluid pressure contact with the porous rock of interest. The actuator mechanism  34 , when activated, creates a normal force between the wall of the borehole  12  and the tool  14  that holds the tool  14  in place so that the necessary permeability related information can be obtained. In the illustrated embodiment, the actuator mechanism  34  mechanically brings the pad  30  into contact with the wall of the borehole and thereby nozzles  32 A,  32 B into fluid pressure contact with the porous rock. There are, however, many other kinds of actuator mechanisms that allow a tool to be selectively fixed in place in a borehole. For instance, large springs and bladder mechanisms have also been used. 
     The tool  14  includes a pressure step generating system for inducing a compressional wave in the rock of interest. With reference to FIG. 3, a pressure step generating mechanism  38  is provided that includes a sample bottle  40  which is used in establishing a pressure sink in the rock of interest. Also part of the mechanism  38  is a pump  42  that is used to establish a low pressure in the sample bottle  40 . First and second sample bottle valves  44 A,  44 B are used to: (1) establish a low pressure in the sample bottle  40 ; and (2) create a pressure step that is transmitted by the first nozzle  32 A into the rock of interest. To elaborate, the establishment of a low pressure in the sample bottle  40  is accomplished by: (1) opening the first sample bottle valve  44 A to establish a fluid communication between the sample bottle  40  and the pump  42 ; and (2) closing the second sample bottle valve  44 B to disconnect the sample bottle  40  from the first nozzle  32 A. With the first and second sample bottle valves  44 A,  44 B in these states, the pump  42  is activated to evacuate fluid from the sample bottle  40  and establish a low pressure in the sample bottle  40 . In one embodiment, the low pressure in the sample bottle  40  is in the range of approximately 10-50 bar below the pressure in the borehole  12 . Typically, the pump  42  provides an indication of when the desired low pressure is achieved within the sample bottle  40 . However, it is also feasible to use a pressure gauge that is part of the tool to determine when the desired low pressure is attained. Once the desired low pressure has been established in the sample bottle  40 , the first sample bottle valve  44 A is closed. With the first and bottle valve  44 A now closed, a pressure step is established in the rock of interest by opening the second sample bottle valve  44 B which, in turn, induces a compressional wave in the rock of interest. 
     The tool  14  also includes a slow compressional wave detector. In the illustrated embodiment, a slow compressional wave detector  48  is provided that is also capable of detecting a fast compressional wave. As a consequence, the slow compressional wave detector  48  is hereinafter referred to as the compressional wave detector  48 . Included in the compressional wave detector  48  is a pressure gauge  50 . Suitable pressure gauges are quartz pressure gauges, which are capable of detecting pressure to an accuracy of 0.01 psi. A compressional wave detector valve  52  is used to connect/disconnect the pressure gauge  50  to/from the second nozzle  32 B. To elaborate, when the pressure gauge is not being used, the valve  52  is used to disconnect the pressure gauge  50  from the second nozzle  32 B and thereby protect the gauge from being subjected to pressures within the borehole  12  that could bias or damage the gauge  50 . The valve  52  is opened when the pressure gauge  50  is needed to detect the slow and, if needed, the fast compressional waves that have propagated through the rock of interest. Since the slow compressional wave is highly attenuated by the rock of interest, the second nozzle  32 B is located relatively close to the first nozzle  32 A to insure that a detectable slow compressional wave is received at the second nozzle  32 B. Due to the limitations of presently available instrumentation, the second nozzle  32 A is located no more than about 50 cm away from the first nozzle  32 A and, preferably, in the range of 10-50 cm away from the first nozzle  32 A. 
     The tool  14  also includes a processor for managing communications between the controller  18  and the various elements within the tool  14 . The processor can take any number of forms. At one end of the spectrum, the processor is simply an interface that relays commands and data between the tool  14  and the controller  18 . At the other end of the spectrum, the processor is capable of performing an entire test and recording the data once the controller  18  has informed the processor that the tool  14  is at a desired location within the borehole  14 . In the illustrated embodiment, a processor  56  is provided that is capable of performing an entire test once informed that the tool  14  is at an appropriate location in the borehole  14  by the controller  18 . However, the processor  56  relays the data to the controller  18  for further relay, recordation and/or processing to determine the permeability of the rock of interest. The processor  56  controls: (1) the actuator  34  to fix the tool  14  in a desired location in the borehole  12 ; (2) the pump  42  and the first and second sample bottle valves  44 A,  44 B to establish a low pressure within the sample bottle  40 ; (3) the first and second sample bottle valves  44 A,  44 B to generate a pressure step that is applied, via the first nozzle  32 A, to the rock of interest; (4) the pressure gauge  50  and the detector valve  52  to detect slow and/or fast compressional wave data received via the second nozzle  32 B; and (5) the detector valve  52  to protect the pressure gauge  50  when not in use. The processor  56  also relays pressure data produced by the pressure gauge  50  during a test to the controller  18 . The controller  18  can, in turn, relay the data to another location, record the data for later processing at the site of the controller or a remote site, and/or process the data to determine the permeability of the rock of interest. 
     Having now described a system capable of: (1) applying a pressure step to the rock of interest in a borehole that induces a compressional wave in the rock; and (2) detecting a resulting slow compressional wave that has propagated through the rock of interest, the method of the invention is now described in the context of the wireline tool system  10 . It should, however, be appreciated that the method of the invention is not limited to the system  10 . For instance, there are a number of ways in which a compressional wave can be established in the rock of interest. For instance, a high pressure step, rather than a low pressure step, can be used to create the compressional wave. The low pressure step, however, serves to preserve the seal of the pad  30  against the borehole wall. Another way to establish a compressional wave in the rock of interest is to use of an impact or hammer device. Furthermore, there are potentially several ways to detect the slow compressional wave, including piezo-electric transducers and strain gauges. In summary, the method of the present invention is applicable regardless of the manner in which a compressional wave is established in the rock of interest and the slow compressional wave detected. 
     Initially, the tool  14  is lowered to a desired location in the borehole  12 . To lower the tool  14  to a desired location, the controller  18  monitors the amount of electric cable  16  that is dispensed by the winch system  22  and controls the winch system  22  accordingly. 
     Once the tool  14  is at the desired location in the borehole  12 , the tool  14  is used to cause a compressional wave to propagate through the rock of interest. At the outset, this involves fixing the tool  14  in place within the borehole and establishing fluid pressure contact between the first and second nozzles  32 A,  32 B and the wall of the borehole  12  by activating the actuator mechanism  34 . If the sample bottle  40  is known to have the required low pressure, the second sample bottle valve  44 B is opened to create a pressure step that induces a compressional wave in the rock of interest. However, if it is known that the sample bottle  40  does not have the necessary low pressure or there is uncertainty as to the state of the sample bottle  40 , the processor  56  operates to establish the needed low pressure in the sample bottle  40 . As previously noted, this involves closing the second sample bottle valve  44 B, opening the first sample bottle valve  44 A and activating the pump  42 . The desired low pressure is detected by opening a third sample bottle valve  44 C (with valve  44 B closed) to establish a hydraulic connection between the sample bottle  40  and the pressure gauge  50 . Once the required low pressure is established, the processor  56  causes the first sample bottle valve  44 A and third sample bottle valve  44 C to close. At this point, the pressure step is generated in the rock of interest by opening the second sample bottle valve  44 B. 
     After the tool is at the desired location in the borehole  12 , the tool  14  is used to detect a slow compressional wave that has propagated through the rock of interest in response to a pressure step applied to the rock. To detect the slow compressional wave, the detector  48  is also placed in an active state by opening the compressional wave detector valve  52  to establish a fluid communication path between the pressure gauge  50  and the second nozzle  32 B. Typically, the valve  52  is opened after the actuator  34  has established fluid pressure contact between the second nozzle  32 B and the wall of the borehole  12  to avoid unnecessarily exposing the gauge  50  to the fluid pressure in the borehole  12 . In any event, the detector  48  must be activated in time to receive the slow compressional wave produced after the pressure step is applied to the rock of interest by the compressional wave generating mechanism  38 . If the velocity of the fast compressional wave is not already known, the detector  48  is also used to detect information relating to the fast compressional wave. In this case, the detector  48  must be enabled in time to detect the fast compressional wave. FIG. 4 illustrates a pressure signal  60  produced by the gauge  50  when enabled in time to detect both the fast and slow compressional waves. The pressure signal  60  includes a first steep drop  62  that is representative of the fast compressional wave and a second, less steep drop  64  that is representative of the slow compressional wave. 
     After the slow compressional wave and, possibly, the fast compressional wave have been detected, the data is processed to determine the permeability of the rock of interest. As previously noted, the data can be processed either on site by the controller  18  or at a remote location. In any event, the velocity of the slow compressional wave is determined by dividing the known distance between the first and second nozzles  32 A,  32 B by the elapsed time between the application of the pressure step to the rock that induces the compressional wave and the detection of the onset of the slow compressional wave. If the velocity of the fast compressional wave if not already known, it is determined in the same manner. Once the velocities of the slow and fast compressional waves are known, the following equation is solved to determine the steady state permeability k 0 :                   p2   2     =       -       i                 η                 φ         f        (   ω   )            K   o                       p1   2          ρ   B           K   f          (       K   b     +     4        G   /   3         )                   (   1   )                                
     where 
     ζ p1  is the velocity of the fast compressional wave; 
     ζ p2  is the velocity of the slow compressional wave; 
     η is fluid viscosity; 
     φ is porosity; 
     ρ B  is bulk density; 
     K f  is fluid bulk modulus; 
     K b  is the bulk modulus of the porous rock; 
     G is the shear modulus; and 
     f(ω) is a complex function that is dependent on the frequency content of the pressure step. 
     The values for the parameters η and K f  can be estimated from fluid samples; the value of the parameter ρ B  can be estimated from a density log; and the values of the parameters K b  and G can be estimated from a dipole sonic log and density log combination. If the rock formation of interest is unconsolidated and the fluid in the reservoir is highly compressible (e.g., gas), equation 1 can be simplified to:                   p2   2     =     -       i                 η                 φ         f        (   ω   )            k   o          K   f                   (   2   )                                
     In many instances, it is desirable to determine the permeability of the rock of interest in more than one direction. For instance, it may be desirable to determine the vertical and horizontal permeability of the rock of interest in a borehole. The detection of information relating to the slow compressional wave and, if needed, the fast compressional wave in these other directions is achieved by the addition to the tool of appropriately positioned nozzles and compressional wave detectors that perform comparably to the previously discussed second nozzle  32 A, pressure gauge  50  and valve  52 . With reference to FIG. 5, a tool  68  is illustrated that includes the first and second nozzles  32 A,  32 B and related internal componentry noted with respect to tool  14 . The tool  68  further includes a pad  70  and a third nozzle  72 . Associated with the third nozzle  72  is a pressure gauge and valve assembly that function in substantially the same fashion as the pressure gauge  50  and valve  52 . The third nozzle  72  is positioned such that a first line extending between the third nozzle  72  and the first nozzle  32 A is substantially perpendicular to a second line extending between the second nozzle  32 B and the first nozzle  32 A. With this arrangement of nozzles, information relating to the permeability in orthogonal directions is obtainable. In the case of a vertical borehole, the information obtained would relate to both the vertical the horizontal permeability. The resulting vertical and horizontal permeability measurements are combinable into a vertical to horizontal permeability ratio that is also of considerable importance in the management of an oil/gas reservoir. 
     A number of variations in the instrumentation used to obtain information relating to the slow compressional wave are feasible. As previously noted there are a number of alternative ways to generate a compressional wave, detect the slow compressional wave and fix a tool at a desired location in a borehole. The invention also contemplates that other ways to perform these functions will be developed in the future. The method of is not, however, constrained to any particular apparatus for accomplishing these functions. It should be noted, however, that it is feasible to integrate the componentry for generating a compressional wave and detecting a slow compressional wave into a drilling string so that permeability related information could be obtained while the well is being drilled rather than with a separate wireline tool system after the well has been drilled. 
     The foregoing description of the invention has been presented for purposes of illustration and description. Further, the description is not intended to limit the invention to the form disclosed herein. Consequently variations and modification commensurate with the above teachings, and the skill or knowledge in the relevant art are within the scope of the present invention. The preferred embodiment described hereinabove is further intended to explain the best mode known of practicing the invention and to enable others skilled in the art to utilize the invention in various embodiments and with the various modifications required by their particular applications or uses of the invention. It is intended that the appended claims be construed to include alternate embodiments to the extent permitted by the prior art.