GB2293653A - Method and apparatus for acoustic determination of porosity - Google Patents

Abstract

This invention provides a method and apparatus for non-destructive porosity determination of a reservoir rock sample from contact acoustic pulse travel time (or contact acoustic wave velocity) measurements. The apparatus includes a probe assembly 10 having transmitter 40 and receiver 50 transducers coupled to a computer system 20. The transducers are adapted to be pressed against a core sample 28. The computer system causes the transmitter to induce acoustic waves (shear or compressional) in the core sample, receives the signals from the receiver and determines acoustic transit times, acoustic velocities and porosity according to programmed instructions. The present invention further provides a method for determining the continuous porosity of rock samples to overburden conditions utilizing the measured velocities without the use of analyzing a large number of core plugs. <IMAGE>

Description

CONTACT ACOUSTIC POROSITY SENSOR AND METHOD FOR DETERMINING CONTINUOUS ROCK POROSITY USING SAME The present invention relates to a method and apparatus for use in measuring acoustic velocity. More specifically, the invention pertains to a nondestructive apparatus and technique for determining the porosity and anisotropy of core samples of reservoir rock separately or in conjunction with a permeability measurement.

Porosity and permeability are key descriptors in reservoir characterization.

Often it is desirable to measure both porosity and permeability of the individual sand layers. It is generally unacceptable, however, to cut a sufficient number of core plugs to completely describe such properties of rock formations.

Therefore, simultaneous, non-destructive porosity and permeability measurement techniques are desired.

The concept of using "probe permeameters" for non-destructive measurements of permeability is well known. One such device for unsteadystate measurements is described in U.S. Patent No. 5,237,854 to Jones, which is incorporated herein by reference. Steady-state permeameters can also be used.

The permeameter disclosed in the Jones patent contains a small diameter tube or probe that is pressed against the surface of a core sample removed or cut from the reservoir. The end of the probe is fitted with a rubber gasket or an O-ring, which makes a gas-tight seal between the core sample and the probe. A gas is delivered from the probe through the interior of the seal to the rock sample at a fixed, measured pressure. The gas then diffuses through the rock sample, starting from the spot beneath the seal, diverging in a somewhat hemispherical pattern and ultimately passing from the upper surface of the sample just beyond the outer diameter of the gasket or O-ring and from other surfaces of the sample that are exposed to atmospheric pressure. Pressure measurements taken during the delivery of the gas are used to determine the permeability of the rock sample.

Currently, such probe permeameters are not suitable for making accompanying, non-destructive porosity measurements. Theoretically, it is possible to measure porosity with the same probe if the internal volume of the permeameter probe is made infinitesimally small. This solution has been proposed by Chen, McLemore and Heller (1993, SPE 26508). However, this proposed method of obtaining porosity is technically difficult to accomplish and requires fast-acting, zero-volume valves with rapid-pressure response.

Wireline acoustic transit-time measurements have been used routinely to compute porosities (Wyllie, Gregory and Garner, 1956; Raymer, Hunt and Gardner, 1980) but no suitable apparatus has been available for non-destructive testing in conjunction with permeability measurements. U.S. Patent No.

5,166,910 to M.L. Batzle and B.J. Smith discloses a portable device and method for measuring acoustic velocity which provides parameters to determine porosity. This device and method, however, require dual receiving transducers to determine velocity and use a stand alone portable apparatus that does not provide permeability measurements.

Further, it is often desirable to determine continuous porosity of pay zone (hydrocarbon containing formations) at formation pressures (overburden conditions). Wireline porosity logs are typically used to obtain continuous porosity measurements. In thinly-bedded pay zones, where the vertical resolution of the wireline tool is too large, such tools provide only average porosity. Typically, to obtain accurate porosity measurements at overburden conditions, core samples ranging from a few feet to several hundred feet long are obtained from the formations. Core plugs are drilled from the core sample along its length and tested in laboratories under overburden conditions, typically using helium as a diffusing medium.Such methods are time consuming and expensive as they require drilling and analyzing a large number of core plugs obtained along the core sample length to obtain a relatively accurate measure of continuous porosity for the entire length of the rock sample.

The present invention addresses the above-noted deficiencies and provides a non-destructive apparatus and method for rapidly measuring contact acoustic compression and/or shear wave velocities (or the travel times) through core samples and the corresponding porosities. The present invention further provides a method for determining relatively accurately the continuous porosity of such core samples at overburden conditions without utilizing analyzing a large number of core plugs.

The present invention provide a method and an apparatus for measuring acoustic velocity in a rock sample using a transducer assembly with embedded acoustic transducers positioned a fixed distance apart on the rock material to be analyzed. The time-distance measurements of compression and shear waves are used to respectively determine the corresponding compressional and shear wave porosities of the rock sample. One additional angular measurement provides the ability to determine anisotropy characteristics of the core sample.

The transducer assembly can be attached to the probe of a probe permeameter for simultaneous measurements of porosity and permeability.

The present invention further provides a method for determining relatively accurately the continuous porosity of such core samples at overburden conditions. The method contains the steps of: (a) determining contact compressional (Vp) and shear (Vs) wave velocities at a plurality of locations on the rock sample; (b) determining ratio of the compressional and shear wave velocities (Vp/Vs); (c) grouping rock types or facies according to selected values of Vp/V,; (d) obtaining a selected number of core plugs from the rock sample for each rock group; (e) determining the overburden porosity for such core plugs; (f) determining a relationship between the overburden core plug porosities and an acoustic velocity or travel time; and (g) determining core sample porosities from the relationship corresponding to the contact compressional or shear wave velocities.

Examples of the more important features of the invention thus have been summarized rather broadly in order that detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.

For detailed understanding of the present invention, references should be made to the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals and wherein: FIG. 1 shows a cross-sectional side view of an angle-wedge transducer assembly.

FIG. 2 shows a bottom view of a dual pair angle-wedge transducer assembly.

FIG. 3 shows a cross-sectional side view of a point transducer assembly.

FIG. 4 shows an exemplary linear regression plot between measured core plug porosity fraction versus shear wave travel time.

FIG. 5 shows an exemplary linear regression plot between the measured core plug porosity fraction versus contact acoustic porosity fraction determined by utilizing the compression and shear wave travel times obtained by utilizing the apparatus of FIGS. 1 and 2.

FIG. 6 shows an exemplary continuous porosity log (porosity versus depth) of a core sample obtained by the method of the present invention.

Depending on the type of transducers used, shear and/or compressional wave transit-times data can be collected. Shear waves can provide better measurements because shear-wave velocities are less dependent on fluid saturations than compressional-wave velocities and, therefore, can be easier to relate to the porosity.

FIG. 1 illustrates the preferred embodiment of an angle-wedge transducer assembly 10 which is used primarily to measure refracted compressional or mode converted shear wave velocities. The transducer assembly 10 is attached to a probe permeameter 12 by removing a probe tip 14 from the prove permeameter 12, slipping a mounting bracket 16 of the transducer assembly 10 onto the probe 18 and reattaching the probe tip 14.

The transducer assembly 10 is connected to a computer system 20 via communication lines including a transmission line 22 and a receiving line 24.

When the probe 18 is lowered to position the probe tip 14 in sealed communication with a surface 26 of a core sample 28 to make a permeability measurement, a small-scale acoustic transit measurement also is obtained via the transducer assembly 10.

The measurements are related to porosity and anisotropy characteristics of the sample. In addition, the acoustic transit-time measurements can be combined with density data to compute the elastic properties of the sample including Young's modulus, shear modulus, bulk modulus and Poisson's ratio.

The transducer assembly 10 includes a first end 30, a second end 32 and the mounting bracket 16 which has an aperture 34 slightly larger in diameter than the outer diameter of the probe 18 to allow the mounting bracket 16 to slide freely along the length of the probe 18.

A transmitter frame 40, having a transmitter transducer 42, is attached to the first end 30 of the mounting bracket 16 via a transmitter arm 44 having a spring 46 to force a transmitter surface 48 of the transmitter frame 40 into pressure communication with the surface 26 of the core sample 28. The transmission line 22 provides electrical communication between the computer system 20 and the transmitter transducer 40. The computer system 20 activates a pulse generator (not shown), such as one supplied by Matec Instruments of Massachusetts, to drive the transmitter transducer 42 with a voltage spike (approximately greater than 300 volts with extremely fast rise time) for transmission through various core sample types.

Similarly, a receiver frame 50, having a receiver transducer 52, is attached to the second end 32 of the mounting bracket 16 via a receiver arm 54. A receiver spring 56 around the receiver arm 54 forces a receiver surface 58 into pressure communication with the surface 26 of the core sample 28.

The receiving line 24 provides communication between a receiver (not shown) and data acquisition apparatus (not shown) in the computer system 20 and the receiver transducer 50.

By attaching acoustic velocity sensors to the probe 18, as shown in FIG.

1, it is possible to measure transit-time and the pressure decay simultaneously.

The acoustic velocity sensors may either directly generate and receive the desired wave type within the core sample 28 or may impact and measure the desired wave type in the core sample 28 by such means as a compliant acoustic electrical transducer to the sample as shown by the angle-wedge transducer assembly 10.

In the preferred embodiment 10, 1 -mHz angle-wedge piezoelectric transducers 42 and 52 were used to provide a 45-degree refracted shear wave in sandstone or limestone. Another incident angle of approximately 68 degrees was calculated to provide a refracted wave that transmitted laterally through the core sample 28. The relatively small size of the transducers 42 and 52 allow acoustic measurements to be taken in close proximity to the probe tip 14 of the probe permeameter 12 without interference.

The transmitter and receiver transducers, 42 and 52 respectively, are pressed to the surface 26 of the core sample 28 with the same seating pressure, approximately 20 psig, as the probe tip 14. The rubber probe tip 14 dampens the surface waves (Rayleigh waves), which may aid in the shear wave transit-time measurements. Alternatively, it may be desirable to preferentially generate and record surface wave data and relate the surface wave transit-time data to sample porosity and anisotropy.

In addition to the two-transducerconfiguration shown in FIG. 1, a second pair of transducers mounted at an angle, as shown in FIG. 2, would provide a second transit time and a measure of anisotropy. This anisotropy could be used to infer permeability anisotropy and aid in the interpretation of the pressure decay permeability data. This embodiment includes dual pairs of transmitter and receiver transducers, 60-62 and 64-66 respectively, attached to the probe 18 via a mounting bracket 68.

FIG. 3 illustrates a point transducer assembly 70 having a mounting bracket 72 with a central aperture 74 for mounting on the probe 18, a transmitter aperture 76 and a receiver aperture 78. A transmitter transducer 80 is contained within a transmitter body 82 having an arm 84 fitted against an O-ring 88 in the transmitter aperture 76. An end 86 of the arm 84 is machined to form a pointed cone. The point transmits acoustic waves in the needed radial pattern to measure the compressional and shear wave velocities.

Similarly, a receiver transducer 90 is contained within a receiver body 92 having an arm 94 fitted against the O-ring 88 in the receiver aperture 78 of the mounting bracket 72. An end 96 of the arm 94 has a cone shaped point. The transducer 80 is fired when it is pressed against the core by the transmitter spring 89. The O-ring 88 prevents transmission of acoustic waves from the transmitter transducer 80 through the mounting bracket 72 to the receiver transducer 90.

The pointed-ends 86 and 96 provide a proper seal between the point transducer assembly 70 and the surface 26 of the core sample 28 for high resolution measurements of compressional wave velocities. The computer system 20 activates a voltage spike through the transmission line 22 to the transmitter transducer 80 causing acoustic waves (compressional or shear) to propagate through the core sample 28. The receiver transducer 90 detects the wave and the data is conveyed through the receiving line 24 to data acquisition apparatus (not shown) and the computer system 20.

The computer system 20 is programmed to calculate the transit time and to correlate the data to determine the acoustic velocity, porosity and anisotropy information. By combining the data with density data, the independent elastic properties of the core sample 18 can also be determined. Thus, the abovedescribed apparatus and methods provide non-destructive means for measuring compressional wave and shear wave acoustic velocities in rock samples utilizing contact acoustic transducers and the porosity of such the rock sample from such measured "contact" acoustic velocities.

This above-described apparatus may be further used to determine continuous porosity of core or rock samples at overburden conditions. The first step in this method is to measure contact acoustic compressional (Vp) and shear wave (V5) wave velocities as described above at a plurality of locations along the horizontal bedding plane of the core sample and to compute the ratio of the contact compressional wave acoustic velocity and the shear wave acoustic velocity (Vp/V5) for each such location. As the core samples are generally circular elongated members that may be a few feet to several hundred feet long, the contact acoustic velocities may be obtained on the whole or slabbed core sample.About ten (10) such measurements per foot length of the core sample have been found to provide sufficient data for determining the continuous porosity according to this method. A different number of measurements, however, may also be sufficient.

It is known in the art that VdVI is constant for each rock type. Since the core sample may have different rock types along its bedding plane, the next step is to group or sort the data by VdV, values. For example, group A may contain all rock locations that correspond to the VdV,values that are between 1.6 and 1.62 and group B between 1.7 to 1.72 and so forth.

The next step in the process is to obtain a few, preferably 2-4, core plugs from each rock group and measure the compressional or shear wave velocity and porosity of such core plugs under desired formation pressures, i.e., under overburden conditions, utilizing conventional methods, such as by diffusing helium into the core plugs. Such velocity and porosity measurements are accepted in the industry as accurate measures of such physical properties of the core plugs. It should be noted that for the purpose of this invention any other method may be utilized for determining the acoustic velocities and porosity of the core plugs.

FIG. 4 shows an exemplary distribution of the measured core plug porosity fraction (percent fraction) and the corresponding reciprocal of the shear wave velocity for core sample plugs obtained by the helium diffusion method. As an example and not as a limitation, FIG. 4 shows such distribution for three types of geological formations A, B and C. The measured core plug porosity is shown along the horizontal axis or the x-axis and the reciprocal of the shear wave velocity along the vertical or y-axis.The next step is to determine a linear relationship between the velocity and the porosity for the core sample, which may conveniently be obtained by performing well known best fit linear regression to yield the expression: 1/V = a + b where V is the velocity (compressional or shear), a is the intercept (reciprocal matrix velocity), b is the slope and 9 is the porosity. The linear regression provides the values of the constants a and b. The best fit for the rock samples A-C are respectively shown by lines 110, 112 and 14 and the corresponding linear equations are shown across from the rock types.

The next step is to determine the porosity corresponding to each of the contact acoustic velocities (shear or compressional as the case may be) for a core sample from the linear relationship derived for such a core sample. The porosity values so derived may be displayed as a continuous log as a function of the depth of the rock sample, which is typically known. FIG. 5 shows an exemplary continuous porosity log for a core sample derived by utilizing the above described method. The borehole depth for the core sample is displayed along the left vertical axis, the porosity as a percent of the rock volume is displayed along the top horizontal axis.The solid curve, generally denoted by numeral 120, is the continuous overburden porosity log obtained by joining the porosity values calculated from the above relationship corresponding to the contact acoustic velocities measured along the rock sample bedding plane.

FIG. 6 shows the correlation between the measured porosity values from the analysis of the core plugs of the three types of geological formations shown in FIG. 4 and the overburden porosity values calculated from the relationship of equation 1 (Acoustic Porosity) corresponding to the contact acoustic velocities measured along the core sample bedding. Lines 132, 134 and 136 represent best linear fit between the overburden porosity values calculated according to the present method and the actual overburden porosity values obtained by analyzing the core plugs. The values of the regression coefficient R2 vary from .97 to .98, indicating that for each of the rock samples A-C there is a close fit between the overburden porosities calculated from the contact acoustic velocities utilizing the method of the present invention and the porosities measured from the core plugs.

Thus, the method of the present invention provides a fast and relatively accurate continuous porosity log of rock samples from contact acoustic transit time or acoustic velocity measurements taken along the bedding plane of the rock sample. This method is relatively easy and inexpensive and can be completed prior to the completion of the boreholes. This method is especially useful for determining the continuous porosity of thinly bedded formations, since the porosity logging tools usually have large vertical resolution and provide only average porosity values. The ability to measure shear and compressional wave transit times will enhance 3-D and 4-D seismic modeling and interpretation, combined with bulk density data ailows defemination of elastic rock properties, such as Poisson's ratio, Bulk, Shear and Young's modului.

The foregoing description is directed to preferred embodiments and methods of the present invention for the purpose of illustration and explanation.

It will be apparent to one skilled in the art that many modifications and changes to the embodiments and methods set forth above are possible without departing from the scope of the invention. It is intended that the claims be interpreted to embrace all such modification and changes.

Claims (13)

ClAIMS \

1. A method of determining porosity of a core sample, comprising the steps of: a) positioning a transducer assembly in pressure communication with the core sample, wherein the transducer assembly comprises a transmitter transducer and a receiver transducer; b) transmitting an acoustic pulse into the core sample through the transmitter transducer; c) receiving the transmitted acoustic pulses form the core sample in the receiver transducer; d) determining the transit time of the acoustic pulse through the core sample from the transmitted and received acoustic pulses; and e) determining the acoustic porosity of the core sample from the determined transit time of the acoustic pulse through the core sample.

2. The method of Claim 1 wherein the transmitter and receiver transducers are anglewedge transducers and the acoustic pulses produce compressional or shear waves.

3. The method of Claim 1 wherein the transmitter and receiver transducers are point transducers and the acoustic pulses produce compressional or shear waves.

4. The method of Claim 2 or 3 wherein the transit time is the transit time of the shear waves or the compressional waves produced by the transmitter transducer in the core sample.

5. The method of Claim 1 wherein the transducer assembly has an aperture for mounting the transducer assembly on a probe of a probe permeameter and wherein the positioning step further comprises positioning the probe in sealed communication with the core sample for simultaneous permeability and acoustic wave transit-time measurements.

6. The method of Claim 5 wherein the probe utilizes a sealing element for providing sealed communication and which also dampens acoustic waves induced by the transmitter receiver that travel along the surface of the core sample.

7. A porosity measuring apparatus for use with a probe permeameter having a prove that may be sealingly placed against a rock sample surface at a predetermined pressure for injecting a fluid therethrough for determining the permeability of the rock sample, a data acquisition and processing equipment including a computer system that has been configured for determining porosity of a core sample from measurements of acoustic wave transit-time through the core sample, said apparatus comprising:: (a) a transducer assembly having a transmitter transducer spaced a known distance from a receiver transducer, said transducer assembly mounted on the probe in a manner that when the probe is placed on the rock sample surface, the transmitter and receiver transducers are in pressure communication with the rock sample; (b) an acoustic pulse generator and receiver for causing the transmitting transducer to generate an acoustic pulse through the rock sample and for receiving signals from the receiver transducer;; (c) a computer system coupled to the acoustic pulse generator and receiver, said computer system being programmed for causing the pulse generator to generate pulses and for determining the porosity of the rock sample from the transit time between the acoustic pulse generated by the transmitter transducer and the corresponding acoustic signals received by the receiver transducer.

8. The apparatus of Claim 7, wherein the transmitter transducer and the receiver transducer are angle-wedge transducers.

9. The apparatus of Claim 7, wherein the transmitter transducer and the receiver transducer are point transducers.

10. A method for determining the porosity along the length of a rock sample, comprising the steps of: (a) determining acoustic compressional (Vp) and shear (V,) wave velocities of an acoustic pulse passed through the core sample at a plurality of locations on the rock sample; (b) determining ratio of the compressional and shear wave velocities (ViV5) for each said location; (c) grouping rock types or facies according to selected values of VdV,; (d) obtaining a selected number of core plugs from the rock sample for each rock group; (e) determining overburdened porosity for such core plugs; (f) determining a relationship between the overburdened porosities and acoustic velocity; and (g) determining the overburdened porosity at each location of the rock sample, such overburdened porosity being the porosity in the relationship that corresponds to acoustic velocity obtained in step (a) for such location.

11. The method as specified in claim 10 further containing the step of plotting the porosities determined for the various locations to obtain a continuous porosity log for the core sample.

12. A method of determining porosity of a core sample substantially as herein described with reference to the accompanying drawings.

13. A porosity measuring apparatus substantially as herein described with reference to Fig. 1, 2 or 3 of the accompanying drawings.