None.
Disclosed embodiments relate generally to downhole acoustic measurements and more particularly to the determination of subsurface elastic anisotropy using borehole acoustic measurements having wave propagation in both the axial and circumferential directions.
The use of acoustic measurement systems in prior art downhole applications, such as logging while drilling (LWD) and wireline logging applications is well known. In one application, acoustic signals may be generated at one or more transmitters deployed in the borehole. These acoustic signals may then be received at an array of longitudinally spaced receivers deployed in the borehole. Acoustic logging in this manner provides an important set of borehole data and is commonly used in both LWD and wireline applications to determine compressional and shear wave velocities (also referred in the art in terms of their slowness values) of a formation. These velocities may then be used to compute certain elastic constants of the formation.
Many subterranean formations exhibit elastic anisotropy that may be intrinsic, for example, owing to orientation and layering of sediments (e.g., in shales) or may be induced by the presence of fractures or the stress sensitivity of the formation. Such formations are commonly modeled as being transversely isotropic media having five elastic constants. However, conventional axial acoustic measurements only enable three (or occasionally four) of these five elastic constants to be determined. The remaining constant(s) are sometimes inferred from various assumptions about the geophysics of the formation; however, such inferences can be problematic. Therefore, there is a need in the art for a downhole acoustic measurement system and methodology for directly determining the five elastic constants of a transversely isotropic medium.
A downhole acoustic logging tool includes at least one acoustic transmitter and first and second arrays of acoustic receivers deployed on a tool body. The first array of acoustic receivers includes a plurality of axially spaced apart acoustic receivers and the second array of acoustic receivers includes a plurality of circumferentially spaced apart acoustic receivers. The tool may further optionally include a processor configured to process axial and circumferential acoustic logging measurements to compute each of five elastic constants of a transversely isotropic medium.
A method for making downhole acoustic measurements of a subterranean formation includes causing an acoustic logging tool to obtain axial acoustic logging measurements and circumferential acoustic logging measurements. The axial acoustic logging measurements and the circumferential acoustic logging measurements are then processed to compute each of five elastic constants of a transversely isotropic medium.
The disclosed embodiments may provide various technical advantages. For example, the disclosed logging apparatus and method enable the five elastic constants of a transversely isotropic formation to be directly determined without using geophysical inferences or assumptions. Moreover, these constants may be determined and transmitted to the surface in real time while drilling. Furthermore, in certain embodiments, a sufficient number of acoustic measurements may be made so as to over specify the system and improve the reliability and accuracy of the determined elastic constants.
This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.
For a more complete understanding of the disclosed subject matter, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
It will be understood by those of ordinary skill in the art that the deployment illustrated on
With further reference to
It will be understood that transmitters 130, 135, and 170 may include substantially any suitable acoustic transmitter configured for use in downhole logging operations. Moreover, the transmitter(s) may be configured to transmit substantially any suitable acoustic signal having substantially any frequency band. For example, the transmitters 130, 135, and 170 may be configured to transmit ultrasonic acoustic energy (such as having a frequency in a range from about 30 kHz to about 2.0 MHz).
Axial acoustic measurements may be obtained at 320, for example, via firing a transmitter that is axially spaced apart from a receiver array (such as transmitter 130 or 135) to propagate an acoustic signal in the borehole (and subterranean formation). The acoustic signal is then received at individual receivers in the axial receiver array 140. The velocities of various acoustic waves (e.g., the compressional and shear waves) may then be determined using techniques known to those of ordinary skill in the art (e.g., using semblance or phase velocity algorithms).
Circumferential acoustic signals may be obtained at 330, for example, via firing a transmitter that is circumferentially spaced apart from a receiver array (such as transmitter 135 or 170) to propagate an acoustic signal in the borehole (and subterranean formation). The acoustic signal is then received at individual receivers in the circumferential receiver array 180. The velocities of various acoustic waves (e.g., the compressional and shear waves) may be determined at the individual receivers to obtain the velocities as a function of the azimuthal angle (the tool face angle) about the circumference of the tool. These velocities may also be determined, for example, using semblance or phase velocity algorithms. For example, if a velocity at receiver R3 (located at a tool face angle of 90 degrees in
As stated above, a transversely isotropic media may be characterized by five independent elastic constants, for example, as given in Equation 1:
where c(TI) represents the elastic stiffness tensor of the transversely isotropic media and c11, c33, c55, c66, c12, and c13 represent the corresponding elastic constants. Although six constants are listed in the elastic stiffness tensor c(TI) only five of the constants are independent as c12=c11−2c66. Thus, c11, c33, c55, c66, and c13 are generally considered to be the five independent elastic constants of c(TI).
Four of these five elastic constants (also commonly referred to as moduli) are directly related to the squared speeds for wave propagation in the directions along the TI symmetry axis or orthogonal to it (which may be the vertical and horizontal directions, respectively when the TI axis is vertical in so-called VTI media). The velocity of various acoustic waves may be given, for example in a VTI medium, as follows:
V11=√{square root over (c11/ρ)} Equation 2
V12=√{square root over (c66/ρ)} Equation 3
V13=V31=√{square root over (c55/ρ)} Equation 4
V33=√{square root over (c33/ρ)} Equation 5
where V11 represents the velocity of a horizontally propagating compressional wave, V12 represents the velocity of a horizontally propagating shear wave having a horizontal polarization, V13 represents the velocity of a vertically propagating shear wave having a horizontal polarization (or the symmetrical horizontally propagating shear wave having a vertical polarization), V33 represents the velocity of a vertically propagating compressional wave, and ρ represents the density of the formation. The fifth constant c13 cannot generally be estimated directly from a single measured wave velocity.
For completeness, the inverse of the elastic stiffness tensor is referred to as the elastic compliance tensor and may be expressed for a transversely isotropic medium, for example, as follows:
where s(TI) represents the elastic compliance tensor and s11, s33, s55, s66, s12, and s13 represent the corresponding compliance constants. The elastic compliance tensor may also be expressed as a function of the classical Young's moduli (EH and EV), shear moduli (GH and GV) and Poisson's ratios (vH and vV) for a transversely isotropic medium, for example, as follows:
where VPa represents the velocity of the compressional wave propagating in the axial direction and c11, c33, c55, c13, ρ, and θ are as defined above. The measured shear wave velocities may also be related to certain of the elastic constants, for example, as follows:
where VSVa represents the velocity of a shear wave propagating in the axial direction with polarization in the vertical direction and VSHa represents the velocity of a shear wave propagating in the axial direction with polarization in the horizontal direction.
For circumferential acoustic measurements (e.g., made at 330), the measured compressional wave and shear wave velocities depend on the azimuthal position (i.e., the tool face angle) of the individual receivers. The compressional wave velocity may be related to elastic constants, for example, as follows:
where VPc represents the velocity of the compressional wave propagating around the circumference of the borehole (the direction of propagation may be thought of as being cross-axial) and Θ is the angle between the propagation vector [xR,yR,zR] at the receiver location R (e.g., receiver R3 in
The measured shear wave velocities may also be related to the elastic constants, for example, as follows:
where VSVc represents the velocity of a shear wave propagating in the circumferential (cross-axial) direction with polarization in the vertical direction and VSHc represents the velocity of a shear wave propagating in the circumferential (cross-axial) direction with polarization in the horizontal direction. For example, for receivers that are at the top or bottom of the tool in a horizontal well (i.e. aligned with the vertical axis V that goes through the center of the tool, where Θ=90 or 270 degrees), Equation 12 reduce to c55=ρVSVc2 (i.e. for the SV mode, the SH mode not being excited in this configuration).
It will be understood that at least five independent acoustic measurements are required to provide sufficient information to determine the five elastic constants of the transversely isotropic medium. These five independent measurements may be obtained, for example, by making axial and circumferential acoustic logging measurements in a deviated borehole (e.g., as depicted on
c11=ρVPa2 Equation 14
The measured shear wave velocities may also be related to corresponding ones of the elastic constants, for example, as the tool is rotated around its axis both c55 and c66 may be determined as follows:
c55=ρVSVa2 Equation 15
c66=ρVSHa2 Equation 16
For circumferential acoustic measurements (e.g., made at 330), the measured compressional wave and shear wave velocities also depend on the azimuthal position of the individual receivers. The compressional wave velocity may be related to certain of the elastic constants, for example, as follows:
where VPc and Θ are as defined above. The measured shear wave velocities may also be related to the elastic constants, for example, as follows:
where VSVc is as defined above.
As stated above, at least five independent acoustic measurements are required to provide sufficient information to determine the five elastic constants of the transversely isotropic medium. These five independent measurements may also be obtained for the special case depicted on
c33=ρVPa2 Equation 19
The measured shear wave velocities may also be related to corresponding ones of the elastic constants, for example, as follows:
c55=ρVSVa2=ρVSHa2 Equation 20
Using monopole Stoneley mode excitation and the processing of Stoneley dispersion curves known in the art, one may obtain from the low-frequency tube-wave a velocity that is related to c66
For circumferential acoustic measurements (e.g., made at 330), the measured compressional wave and shear wave velocities are independent of the azimuthal position (i.e., the tool face angle) of the individual receivers when the borehole is aligned with the vertical axis V of the transversely isotropic formation. The circumferential compressional wave velocity may be related to a single elastic constant, for example, as follows:
c11=ρVPc2 Equation 21
The measured shear wave velocity may also be related to a corresponding elastic constant, for example, as follows:
c66=ρVSHc2 Equation 22
As such this specific configuration does not provide five independent velocity measurements and therefore does not provide for a direct measurement of c13 (although c13 may be estimated using a physical or heuristic model).
While not depicted on
Although downhole elastic anisotropy measurements and certain advantages thereof have been described in detail, it should be understood that various changes, substitutions and alternations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims.
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Number | Date | Country | |
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20150168580 A1 | Jun 2015 | US |