Douglas J. Patterson

Single-well S-wave imaging using multicomponent dipole acoustic-log data

Single-well S-wave imaging has several attractive features because of its directional sensitivity and usefulness for fracture characterization. To provide a method for single-well acoustic imaging, we analyzed the effects of wave radiation, reflection, and borehole acoustic response on S-wave reflection measurements from a multicomponent dipole acoustic tool. A study of S-wave radiation from a dipole source and the wave’s reflection from a formation boundary shows that the S-waves generated by a dipole source in a borehole have a wide radiation pattern that allows imaging of reflectors at various dip angles crossing the borehole. More importantly, the azimuthal variation of the S-waves, in connection with the multicomponent nature of a cross-dipole tool, can determine the strike of the reflector. We used our theoretical foundation for borehole S-wave imaging to formulate an inversion procedure for field data processing. Application to field data validates the theoretical results and demonstrates the advan...

Processing array acoustic-logging data to image near-borehole geologic structures

Imaging near-borehole structures using acoustic-logging data depends on the data-processing method used. We demonstrate that extracting the up- and downgoing reflections from the data and using those separately for the imaging can significantly improve the image quality. A parametric wave separation is first applied to array data to separate direct and reflection waves. In particular, we use a transmitter array gathered from successive source positions to extract the upgoing reflection. Reflection data can also be enhanced by stacking data along the reflection moveout in array using approximate structural dip information. Implementation of this method to acoustic-logging data processing improves imaging quality, making it possible to image near-borehole geologic structures using conventional array acoustic-logging. A potentially important application of drilling steering is using logging-while-drilling acoustic measurements.

Multipole Acoustic Logging-While-Drilling

The problem of multipole acoustic Logging-WhileDrilling (LWD) is analyzed. By formulating a multipole acoustic ring source at the rim of the drill collar, we model the characteristics of monopole, dipole, and quadrupole waves in the LWD environment. In particular, we analyze the feasibility of shear-wave velocity measurement using the dipole and quadrupole waves. The results show that the dipole-flexural wave, although it can attain the shear velocity in a fast formation, lags significantly behind the expected shear-wave arrival time in a slow formation. In addition, there is a strong contamination caused by the tool flexural wave traveling along the collar. In comparison, the quadrupole wave is most suited for the shear-velocity measurement. At low frequencies, the quadrupole wave travels at the formation shear velocity and is free of the tool-wave interference.

Mapping formation radial shear-wave velocity variation by a constrained inversion of borehole flexural-wave dispersion data

We have developed a novel constrained inversion method for estimating a radial shear-wave velocity profile away from the wellbore using dipole acoustic logging data and have analyzed the effect of the radial velocity changes on dipole-flexural-wave dispersion characteristics. The inversion of the dispersion data to estimate the radial changes is inherently a nonunique problem because changing the degree of variation or the radial size of the variation zone can produce similar wave-dispersion characteristics. Nonuniqueness can be solved by developing a constrained inversion method. This is done by constraining the high-frequency portion of the model dispersion curve with another curve calculated using the near-borehole velocity. The constraint condition is based on the physical principle that a high-frequency dipole wave has a shallow penetration depth and is therefore sensitive to the near-borehole shear-wave velocity. We have validated the result of the constrained inversion with synthetic data testing. Combining the new inversion method with four-component crossed-dipole anisotropy processing obtains shear radial profiles in fast and slow shear polarization directions. In a sandstone formation, the fast and slow shear-wave profiles show substantial differences caused by the near-borehole stress field, demonstrating the ability of the technique to obtain radial and azimuthal geomechanical property changes near the wellbore.

High-resolution Borehole Acoustic Imaging Through a Salt Dome

Detailed mapping of geologic structures external and internal to salt domes is of great importance for hydrocarbon exploration and storage in the US Gulf Coast and Gulf of Mexico. A single well acoustic reflection survey has recently been conducted to image geologic structures in the vicinity of a borehole drilled through a salt dome. The borehole acoustic wave frequency is in the kilohertz range and acoustic propagation in salt usually has a high velocity and low attenuation. Thus, the borehole survey allows for obtaining high-resolution structural images hundreds of feet across the well bore. The images reveal salt dome structural geology characteristics corresponding to spine boundaries, remnant bedding, and/or gas or brine-filled porosity (e.g., dissolution planes), which are essentially impossible to map with surface or borehole seismic data. This paper describes the data acquisition and processing procedures for the imaging application. An inside dome imaging result is also presented and validated with other measurement results.

Borehole Acoustic Dipole Response With Drilling-induced Vertical Fractures

Understanding the effect of drilling-induced fractures on borehole dipole-flexural waves is important for interpreting the result of cross-dipole anisotropy measurement. This effect was analyzed using a discrete-wavenumber finite element method. We studied the dispersion characteristics of the acoustic dipole modes in an isotropic formation with various drilling-induced fractures along the borehole. Within the frequency range of dipole acoustic logging, these fractures induce two major dipole modes with different phase velocities and distinctive polarization directions along and perpendicular to the fracture orientation. The result indicates that shallow and deep fractures influence the dipole modes differently. Shallow fractures split the dipole velocities at high frequencies while deep fractures cause the splitting at low frequencies. As a result, deep fractures can induce significant shearwave azimuthal anisotropy measured by cross-dipole logging. Other aspects of fractures were also analyzed. Wider fractures cause larger velocity differences than narrower ones. Multiple fractures have a similar effect compared with a single fracture. The result of this work provides a theoretical foundation for interpreting the crossdipole shear-wave anisotropy measurement in the presence of drilling-induced fractures.

APPLICATION OF SPECIAL FILTERING TECHNIQUES IN THE ANALYSIS OF EMAT DATA

The applicability of Electromagnetic Acoustic Transducers (EMAT) for downhole applications in the oil and gas industry is being currently investigated. This application, when compared to conventional usage of EMAT for pipeline inspection, imposes significant engineering and data processing challenges due to difficult downhole conditions, wide variability of casing sizes (both in diameter and thickness) and signal to noise ratio (SNR) limitations. In this paper the investigation of different filtering techniques and methods aimed at analyzing EMAT data for various downhole scenarios, separation and detection of different modes and improvement of SNR is detailed. The techniques being investigated are frequency (FIR) filtering, Gaussian wavelet decomposition, synchronous detection and their combination. The methods and techniques proposed are confirmed and validated based on the results obtained from the numerical simulations and experiments with physical models.

Borehole flexural-wave response to vertical solid-filled fractures

Summary The drilling process, hydraulic fracturing and perforation can create fractures along the borehole, which are often filled with yielded rocks. The yielded rocks have much reduced mechanical rigidity than the original undamaged formation. Understanding the effect of this type of fractures on the acoustic dipole responses not only is important for correctly interpreting the acoustic anisotropy measurement but also will help characterizing the fractures using acoustic logging. We use a finite element method with discrete wavenumbers to model the acoustic modes in a borehole with these fractures. Dispersion properties of the dipole waves are calculated with various fracture parameters. The dipole mode polarized perpendicular to the fracture travels at a lower speed than the dipole mode polarized along the fracture at low frequencies. The velocity difference of the two dipole modes increases with the increasing fracture extension into the surrounding formation. The width of the fracture affects the dipole modes mostly at high frequencies. The effects of the fractures can still be felt in a cased borehole. The result of this study can be used to provide a guideline for understanding and interpreting the cross-dipole measurement in fractured formations.

A curve-fitting method for analyzing dispersion characteristics of guided elastic waves

Summary I n an elastic waveguide such as fluid-filled borehole with a logging tool, the frequency dispersion of a guided wave mode is characterized by a monotonically varying dispersion curve. The proposed method simulates the multiparameter dispersion curve using a simple analytical function that has only a few parameters. By adjusting the parameters to fit the actual wave dispersion data with the simple function, the wave’s dispersion characteristics can be satisfactorily determined. The result of this simplistic approach leads to several important applications in acoustic logging: 1) correcting dispersion effect in the shear-wave velocity from wireline dipole logging, 2) P-wave velocity from the dispersive leaky-compressional wave data from either wireline or logging while drilling (LWD) measurement, and 3), determining shear-wave velocity from LWD quadrupole shear-wave logging.

USE OF ELECTROMAGNETIC ACOUSTIC TRANSDUCERS (EMATS) FOR CEMENT BOND LOGGING OF GAS STORAGE WELLS

According to the Department of Energy (DOE), there are approximately 110 operators maintaining more than 17,000 gas storage wells in over 415 underground storage facilities across the USA. In virtually every application, steel casing, cemented into place, serves to isolate the well from the underground formations. The process of cementing wellbore casing provides two major benefits: 1) cement prevents gas migration between the casing and formation; 2) cement transfers stress from the casing to the formation, increasing the effective strength and working pressure of the casing. Current cement evaluation techniques use an acoustic wave generated and received by a logging tool within the wellbore to detect cement placed outside the casing. These techniques rely on fluid in the casing to provide acoustic coupling between the logging tool and the casing and therefore are unable to operate in gas‐filled boreholes. This paper details efforts to confirm the validity and applicability of the use of EMATs for evaluating cement in gas‐filled boreholes. The methods and techniques proposed for the cement bond logging using EMATs are confirmed and validated based on the results obtained from the numerical modeling and experiments with physical cement models. Partial funding for this investigation was provided by the DOE and Gas Storage Technology Consortium.