Apparatus for measuring an acoustic propagation property

Abstract: An apparatus is provided for determining the acoustic anisotropy of a sample (14) experiencing simulated in situ pressures and temperatures. A sample of the formation of interest is received by a compliant sleeve (36) disposed in a pressure chamber (22). The compliant sleeve houses a plurality of transducers (18) which are placed in intimate contact with the sample upon pressurization of the chamber. The transducers generate and receive acoustic pulses, the travel times of which through the sample are recorded to determine the anisotropic behaviour of the sample. The velocity variations of the sample can indicate the historical principal stress axes of the formation at depth.

Description: The present invention relates to an apparatus for measuring an acoustic propagation property, e.g. to determine the acoustic anisotropy of materials, and is particularly applicable to an apparatus for determining the acoustic anisotropy of a material while under increased pressure and temperature. In the oil and gas industry, once a well has been drilled and shows the presence of oil or gas, a determination is made on whether it would be economical to produce. A primary factor in determining whether a well may be economical is the flow rate of the product from the formation into the well. For marginally economic wells, the flow rate may be enhanced by fracturing the reservoir formation to expose more surface area of the formation to the well bore. The orientation of induced fractures in a formation may depend upon the amount of overburden, the formation's principal stress axes, bedding planes, formation homogeneity or cementation. In some cases the principal stress axes control the orientation of fractures. Generally, fracture planes induced in the formation occur in the plane of the two greatest principal stress axes. For example, if the greatest principal stress axis of a formation is along the vertical plane, fractures induced in the formation most likely will occur in some vertically oriented plane. Orientation of in-situ principal stress axes can sometimes be determined by measuring the velocity anisotropy of samples taken from the subsurface interval of interest. The technique is based on the theory that an acoustic pulse propagates through a homogenous material of unit dimension at the same velocity along all axes. If the stress is greater along one of the other axes and then relieved, microfractures will sometimes develop in planes substantially perpendicular to that stress axis. The presence of microfractures generally decreases the effective velocity of acoustic waves propagating through the material. Thus it has been found that acoustic velocity can be lower along the axis of greatest stress. Traditionally the engineer would determine the acoustic anistropy of a formation by extracting an oriented core sample from the reservoir formation. At least two additional smaller samples were taken from the larger core along different axis. Each small sample, having the same dimensions, would be placed between two transducers. One transducer would generate an acoustic signal in the sample. This pulse would travel through the core and be detected by the other transducer at the opposite end. The differences in the acoustic wave travel time through the two samples can reveal the orientation of past stress axis. IEEE Transactions on Sonics and Ultrasonics, vol. SU-17, No. 2, April 1970, pp 77-85 discloses subjecting a porous sample to compressional and shear waves to measure bulk velocity whilst applying triaxial stress, fluid saturation and temperature to simulate in situ surroundings. A cylindrical solid sample has transducer assemblies applied to its axial ends by which purely axial waves can be sent out and received. A neoprene sleeve jackets to the cylindrical surface of the sample. Axial stress is applied by a piston to the sample via the transducer assemblies and an external pressure source applies fluid pressure about the neoprene sleeve to apply radial stress. US-A-3537541 discloses a similar axial measurement structure. US-A-4380930 discloses yet another similar structure but one in which the piston is replaced by a spring and possibly by a screw-threaded member. According to the present invention, there is provided an apparatus for measuring a transit time of an acoustic signal in a sample under pressure greater than ambient atmospheric pressure, comprising: a body containing a pressure chamber; a compressive sleeve received within the chamber, an annular volume being defined between the sleeve and the body; means for transmitting pressurized fluid into the annular volume so as to contract the sleeve around a sample when placed in the chamber for applying radial stress thereto; means for axially constraining a sample when placed in the chamber; and means for generating and receiving acoustic signals in the sample; characterised in that the means for generating and receiving acoustic signals in the sample comprises a plurality of transducers, two of which are disposed opposite one another and located in the wall of the compressive sleeve. In one possible embodiment the compressible sleeve contains a plurality of transducers radially disposed at known intervals. This may include transducers which are diametrically opposed. Additionally, a transducer may be located at each end of the sample. Each of the radially disposed transducers is integral with the wall structure of the sleeve and connected by electrical conductors to an external recording and control unit. The sleeve and transducers are in turn surrounded by a fluid tight material to maintain the integrity of the pressure chamber. Pressurization of the chamber forces the compressible sleeve to conform tightly about the sample. At the desired pressure, the transducers may be energized to produce a discreet signal which propagates through the sample and is received by a generally opposing transducer. The travel time of the signal through the sample is recorded. This process is repeated along a desired number of axes passing through the sample. The collected data are analyzed and used to determine the past principle stress axes of the sample. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example to the accompanying drawings, in which: Figure 1 is a schematic diagram of a device for determining acoustic properties; Figures 2 and 3 are elevational views of the device and means for loading samples into the device; Figure 4 is an enlarged diagram of the pressure chamber of the device of Figure 1; and Figure 5 is a perspective view of two transducers of the device of Figure 1. Figures 1 through 4 generally illustrate a device 10 in which the invention may be employed for testing desired characteristics of a material. Construction and operating details of such a device may be found in greater detail in United States Patent Numbers 4,573,364, 4,627,270 and 4,864,848, all of which are incorporated herein by reference. Although the instant invention has application to a great many materials, for the purposes of this disclosure, the discussion will be limited to the analysis of earthen samples. Figure 1 is a schematic diagram of the instant invention as employed in a device 10 used to measure the permeability and porosity of samples. The device 10 includes a sample holder 12 for receiving the material 14 to be tested. Disposed within the holder 12 is a chamber 22 with means 16 and 17 for applying a radial and axial stress respectively to the material 14. Radial stress may be applied by a cylindrical sleeve 36 discussed in greater detail below and a fluid introduced thereabout through a port or passage 25 coupled to a fluid source. The axial stress on the sample may be applied by a piston 26 which is also discussed below. Enclosed within the holder 12 are means 18, integral with the radial stress applicator 16, for generating and receiving acoustic signals in the material 14. A remote device is operably coupled to the generating and receiving means to gather, store, and process data from the generating and receiving means 18. Figures 2 and 3 are elevational views of the sample holder 12 and a means 26, 27 for loading the samples into the holder. The apparatus may have a carousel 24 capable of carrying a plurality of samples 14 of substantially constant diameter but of varying length. A piston 26 disposed within a cylinder 27 may be located beneath the carousel 24 for selectively raising one of the samples from the carousel 24 into the sample holder 12 as shown in Figure 3. Once the material is loaded in the holder 12, the piston 26 and the sample holder 12 cooperate together to form a test cell or pressure chamber 22 about the held sample. The pressure chamber and piston function simultaneously to apply both axial and radial forces to the sample, simulating the overburden forces experienced in its natural environment. Figure 4 is an elevational view of the sample holder 12 exhibiting the instant invention. The sample holder 12 is comprised of a mass or body 28 having an axial bore 30. The axial bore is closed at one end by an upper plug 32. Fixed to the lower end of the upper plug 32 may be a perforated plate 38. An upper retainer plug 34 anchored to the body 28 by bolts or threads secures the plug 32 within the body. The rubber sleeve 36 mentioned above is coaxially disposed within the bore 30, and has one end sealed about an end of the upper plug. At the bottom of the body 28 and sealing the lower end of the rubber sleeve 36 is a support plate 40. The support plate contains a hole 41 aligned with the axial bore 30 and may be secured to the body by way of bolts 43. The axial bore 30, plug 32, plate 38, and an end 52 of the piston 26 essentially define the pressure chamber 22. The components comprising the sample holder 12 are preferably cast or machined from stainless steel or high-strength aluminum. In one embodiment of the instant invention, a plurality of transducers 18 may be integrally located within the walls of the rubber sleeve 36. Each transducer within the sleeve 36 has a corresponding transducer diametrically located opposite thereto, all of which are designed to emit and to receive acoustic pulses. The emitted pulses are transmitted through the sample surrounded by the sleeve 36. Measurement of the transmission time, together with the known path length, allows calculation of the transmission velocity. Alternatively, at least two transducers may be placed at each desired radial location about the inside of the sleeve 36: one crystal to generate and receive compressional waves and another crystal to generate and receive shear waves. Shear waves and compressional waves propagate through various media at different velocities and aid the scientist in the evaluation of materials. It may be desirable that additional transducers, similar to sensors 18 located in the rubber sleeve 36, be located in the upper plate 38 and in the end 52 of the piston 26. Transducers located about the circumference of the sample and at each end allow velocity calculations for substantially all axes of the sample, thus providing better data sampling and consequently a better estimate of the principle stress axes of the formation nature. Sleeve 36 may be manufactured from rubber or other flexible material adapted to isolate the sample from a pressure medium such as a gas or fluid introduced into the pressure chamber. For high temperature operations (28° C to over 200° C), Viton sleeves may be used. For low temperature operations (temperatures below 65° C), the rubber sleeve may be used. The thickness of the sleeve 36 may vary depending upon the pressure and temperature extremes contemplated to be encountered. A sleeve thickness on the order of 0.471 cm has proven suitable. Other thicknesses and compositions may be found to be suitable and are considered to be within the scope of this invention. Figure 5 is a perspective view of one embodiment of a transducer 18 which may be employed in the invention. The transducers 18, located in the wall of the rubber sleeve 36, preferably are piezoelectric-crystals 42 mounted to a high-strength substrate 44. The crystals 42 may be any of a wide variety of piezoelectric materials including the PZT-5A made by Valpey-Fisher Company. Transducers can be chosen to operate at any practical range of frequencies. Typically a frequency on the order of 1 megahertz (MHz) is used. To assure good acoustic coupling of the acoustic signal with the sample, the substrate 44 may have one side contoured so as to conform to the inner wall of the rubber sleeve 36. As suggested earlier the transducers are incorporated in the wall of the rubber sleeve 36. This may be readily accomplished by molding the sleeve 36 around the transducers or embedding the transducer in a cavity formed in the rubber sleeve 36 to accommodate each transducer 18. Electrical conductors 48 from each transducer pass through the sleeve 34 to join other conductors 48 at a desired location before exiting the sample holder 22. In operation, the material 14 may be loaded into the sample holder 22 by the piston 26 so that an upper end of the sample is urged against the upper end plug plate 38. The end 52 of the piston 26 may also be received within by the rubber sleeve 36. Once the sample is loaded, a fluid is introduced through port 25 in the sample holder to the outside of the sleeve causing the sleeve to collapse and conform to the surface of the sample. The pressure also places the transducer substrate 44 in intimate contact with the sample. The application of pressure to the sample simulates the overburden conditions existing in the formation. With the pressure stabilized, each transducer is energized in any desired pattern, transmitting a pulse into the sample. Each acoustic pulse propagates through the sample and is detected, the travel time of which is recorded for later reference. The process is repeated for substantially all of the transducers located in the pressure chamber. If more than one wave type is generated, e.g. compressional wave and shear waves, the travel time differences between the two waves are also recorded for later reference. It should be noted that no particular sequence is required for the generation of the acoustic pulses, only that the respective axes or radial of propagation be known for each signal. Once the acoustic velocity measurements of a sample have been completed, additional measurements may be performed on the sample such as the porosity and permeability. Conversely, such measurements may be performed simultaneously or prior to the velocity calculations.

Claims: An apparatus for measuring a transit time of an acoustic signal in a sample under pressure greater than ambient atmospheric pressure, comprising: a body containing a pressure chamber (22); a compressive sleeve (36) received within the chamber, an annular volume being defined between the sleeve and the body; means (25) for transmitting pressurized fluid into the annular volume so as to contract the sleeve around a sample when placed in the chamber for applying radial stress thereto; means (17) for axially constraining a sample when placed in the chamber; and means (18) for generating and receiving acoustic signals in the sample; characterised in that the means for generating and receiving acoustic signals in the sample comprises a plurality of transducers (18), two of which are disposed opposite one another and located in the wall of the compressive sleeve. An apparatus as defined in claim 1, wherein the means for axially constraining the sample comprises: a piston (26) axially received by the chamber and by the sleeve, the piston being arranged to contact a sample when disposed within the sleeve; and actuator means (27) for urging the piston (26) against a sample so as to apply an axial load thereto. An apparatus as defined in claim 1 or 2, wherein the means for generating and receiving acoustic impulses in the sample comprises pairs of diametrically opposed transducers. An apparatus as defined in any one of the preceding claims and comprising two transducers (18) arranged to be disposed at respective ends of a sample. An apparatus as defined in any one of the preceding claims, wherein the pressure chamber is defined by a cylindrical bore (30) extending through the body, there being a plug (32) disposed at one end region of the bore and closing the end thereof to define a cavity, at least one passage extending through the body and in fluid communication with the cavity, and means for introducing a fluid under pressure into the cavity. An apparatus as defined in any one of the preceding claims, wherein the means for generating and receiving acoustic impulses comprises a plurality of transducers capable of generating and receiving compressive wave (P wave) and shear wave (S wave) acoustic impulses. An apparatus as defined in any one of the preceding claims, wherein the means for generating and receiving acoustic signals in the core sample comprise piezoelectric sensors. An apparatus as defined in any one of the preceding claims wherein the transducers located in the wall of the sleeve have had the sleeve molded around the transducers. An apparatus as defined in any one of claims 1 to 7, wherein the transducers located in the wall of the sleeve have been embedded in cavities formed in the sleeve. An apparatus as defined in any one of the preceding claims in combination with means (20) operably coupled to the generating and receiving means for measuring the transit time of the acoustic impulse in the sample. An apparatus as defined in claim 10, wherein the means operably coupled to the generating and receiving means, for measuring the transit time of the acoustic impulses, comprises a computer (20).

Inventors: PETERSON PAUL ERIC, PETERSON, PAUL ERIC, Peterson, Paul Eric, c/o Proteus
Applicants: CORE HOLDINGS BV, CORE HOLDINGS B.V.
Application year: 1991
Main classification: G01N29
IPC codes: G01N29, G01N33