Fluid-filled Borehole Research Articles

Inversion of shear wave slowness and attenuation from dipole acoustic logging data based on an equivalent tool theory

The presence of a sonic logging tool affects the propagation characteristics of dipole-flexural waves in a fluid-filled borehole. As a result, the tool effect should be considered during the processing and inversion of the dipole-flexural waves for the acoustic properties of a shear wave (S wave). We show that the presence of the tool not only changes the slowness dispersion of the flexural wave but also results in a shift of its frequency-dependent attenuation toward lower frequencies. The equivalent tool theory (ETT) can realistically model the effect of an acoustic tool on guided wave propagation. With the ETT framework, we develop a unified workflow for estimating the S-wave slowness and attenuation from dipole array acoustic waveforms. The workflow is an extension of the model-based dispersive processing of dipole logging data. Our workflow involves two inversion procedures. By matching the actual dispersion data with the theoretical dispersion predicted by the ETT, we can invert the S-wave slowness and tool parameters. With the estimated S-wave slowness and other borehole parameters, we further calculate the partition coefficients of flexural waves using the ETT. Based on the concept of energy partitioning, we formulate a linear optimization problem and invert for the S-wave attenuation from the frequency-dependent dipole attenuation. We use synthetic and field data to demonstrate the effectiveness and applicability of the developed method. The results show that it is important to incorporate the tool effect when processing dipole acoustic logging data for S-wave slowness and attenuation, especially in a fast formation.

Inversion of radial formation velocity distribution based on monopole acoustic logging while drilling

Acoustic logging is an effective solution to obtain the radial formation velocity profile near a borehole, and it is critical for borehole stability evaluation and production performance monitoring. Herein, the responses of monopole acoustic logging while drilling (LWD) in a fluid-filled borehole surrounded by homogeneous and heterogeneous formations are numerically simulated using the finite difference algorithm in a cylindrical coordinate system. A stepwise inversion method is proposed to determine radial formation velocity distributions near the borehole based on the acoustic LWD data as follows. The heterogeneous formation around the borehole is approximated as multi-layered media with different elastic parameters, and then the slowness time correlation (STC) algorithm is applied to determine the compressional (P) and shear (S) wave velocities corresponding to different offsets. Finally, the thickness of each layer is determined according to the ray theory. Influences of measurement environment parameters, such as the central frequency of the source, thickness of the formation variation zone and elastic parameter changes of the formation variation zone, on the inversion results are evaluated to confirm the applicability of the proposed stepwise inversion method. The numerical simulation results demonstrate that refracted P-, refracted S- and Stoneley waves exhibit specific responses to the heterogeneity of the near-borehole formation, whereas collar waves are minimally influenced by the formation properties. The formation velocities do not change with the offset in the homogeneous formation models, although stepped changes are observed in the heterogeneous formation models. Short-offset waveforms are affected by the properties of the formation close to the borehole wall, whereas long-offset waveforms are primarily affected by the properties of the original formation. Relationships between the formation velocities and the offset can be converted into the radial formation velocity distributions by calculating the arrival times of waves propagating along different ray paths. Under common survey conditions, the inversion result of each model is similar to its real velocity distribution, which confirms the effectiveness of the proposed stepwise inversion method. This study thus provides a theoretical basis for developing 3D acoustic LWD tools.

Effects of collar eccentricity on azimuthal response characteristics for acoustic reflection measurement while drilling

Acoustic reflection imaging has been successfully applied in postdrilling wireline logging, and the technique is potentially more valuable in acoustic logging while drilling (ALWD), particularly in geosteering while drilling. Because of the common occurrence of tool eccentering in drilling, the impact of drill collar eccentricity on the acoustic wavefield inside and outside the borehole must be considered. Therefore, we analyze the effect of an eccentric drill collar on acoustic reflection imaging while drilling using analytical solutions in a dual-cylindrical coordinate system. The virtual source analogy and the translational addition theorem for cylindrical Bessel functions are used to calculate the response of the fluid-filled borehole with an eccentric collar to the incident wavefield from a formation reflector. The analysis results are validated with those from 3D finite-difference simulations. We also have studied the azimuthal variation of the received wavefield and compared the results with their centralized counterparts, for which the effects of collar eccentricity, frequency of the source, and formation elastic property are analyzed. The results indicate that the asymmetric feature of the wavefield due to the eccentric collar allows for determining the azimuth of the reflector, providing a solution to the azimuth ambiguity problem of borehole acoustic reflection imaging. Our results, besides developing an effective method for eccentric collar ALWD modeling, can be used to provide a theoretical foundation for ALWD tool development and data interpretation.

A high-order perfectly matched layer scheme for second-order spectral-element time-domain elastic wave modelling

In this study, we develop a new high-order perfectly matched layer (PML) formulation for the 3D spectral-element time-domain (SETD) method to solve the second-order elastic wave equations in terms of displacements. The high-order PML consists of multiple well-known frequency-dependent complex stretching functions, leading to the complicated temporal convolution which requires large computational resources. To avoid the evaluation of convolution, we deal with the high-order stretching functions by introducing the recursive auxiliary first-order differential equations which can be easily solved in time domain. In addition, an equivalent concept of PML is adopted for absorbing waves, preserving the original form of wave equations in physical domain. Finally, we obtain a high-order, non-convolutional and unchanged-form PML formulation, which can be easily incorporated into the SETD codes designed for the physical domain, to make them applicable for the second-order wave equations in the unbounded media. Numerical experiments demonstrate the accuracy and stability of the high-order PML through the comparison with the reference solution. It is found that the high-order PML provides a much better absorption performance than the first-order PML, particularly for the grazing-incidence waves and surface waves in the large-scale domain. We also give a fluid-filled borehole model to illustrate that the high-order PML works well for absorbing the complex guided waves in the presence of fluid-solid interaction.

Evaluation of the downhole hydraulic fracturing using single-well shear wave scattering imaging -modeling and field applications

Acoustic well logging technology is used to identify the formation heterogeneity (e.g. fracturing cracks). In this paper, the hydraulic fracturing numerical model is equivalent to a random medium model based on the shear wave velocity statistical results. These statistical results are numerically simulated and are based on the hydraulic fracturing field data and elastic wave propagation along a fluid-filled borehole with different fracturing volumes. The results show that significant coda waves are generated in dipole waveforms and the dipole coda is dominated by S-to-S scattering, which can be effectively utilized to diagnose the heterogeneity in the rock formation. The fracturing-induced heterogeneity change can be detected by S-S scattering imaging. We used the field data examples to demonstrate the effectiveness and practicality of using the borehole acoustic techniques for hydraulic fracturing evaluation. Because the hydraulic fracturing occurs in a large volume around the borehole, the application of the method and analysis can significantly enhance the measurement volume of the cross-dipole logging technology, extending its effectiveness for evaluating hydraulic fracturing results for unconventional (e.g. shale gas) reservoirs.

Response analyses on the drill-string channel for logging while drilling telemetry

Downhole acoustic telemetry (DAT), using a long drill string with periodical structures as the channel, is a prospective technology for improving the transmission rate of logging while drilling (LWD) data. Previous studies only focused on the acoustic property of a free drill string and neglected the coupling between pipes and fluid-filled boreholes. In addition to the drill-string waves, a series of fluid waves are recorded in the DAT channel, which has not been investigated yet. Unpredictable channel characteristics result in lower transmission rates and stability than expected. Therefore, a more realistic channel model is needed considering the fluid-filled borehole. In this paper, we propose a hybrid modeling method to investigate the response characteristics of the DAT channel. By combining the axial wavenumbers and excitation functions of mode waves in radially layered LWD structures, the channel model is approximated to the 1-D propagation, which considers transmission, reflection, and interconversion of the drill-string and fluid waves. The proposed 1-D approximation has been well validated by comparing the 2-D finite-difference modeling. It is revealed that the transmitted and converted fluid waves interfere with the drill-string wave, which characterizes the DAT channel as a particular coherent multi-path channel. When a fluid-filled borehole surrounds the drill string, the channel responses exhibit considerable delay as well as strong frequency selectivity in amplitude and phase. These new findings suggest that the complexity of the channel response has been underestimated in the past, and therefore channel measurements on the ground are unreliable. To address these channel characteristics, we apply a non-coherent demodulation strategy. The transmission rate for synthetic data reaches 15 bps in a 94.5 m long channel, indicating that the acoustic telemetry is promising to break the low-speed limitation of mud-pulse telemetry.

Theoretical dispersion curves for borehole real-valued wave modes in vertically transverse isotropic formations

The dispersion curves of real-valued modes in a fluid-filled borehole are widely used in acoustic well logging. The accurate dispersion curves are the precondition of theoretical analysis and inversion process. Generally, these curves can be obtained by solving the conventional dispersion equation for isotropic formations and most vertically transverse isotropy (VTI) formations. However, if the real-valued solutions exist when the radial wavenumbers for the formation quasi-P and quasi-S equals to each other, the existed methods based on the conventional dispersion equation could lead to incorrect results for some VTI formations. Few studies have focused on the influence of these real-valued solutions on dispersion curve extraction. To remove these real-valued solutions, we have proposed a modified dispersion equation and its corresponding solving process. When solving the dispersion equation, the Scholte wave velocity of VTI formation at high frequency is used as the initial guess. The two synthetic examples including fast and slow VTI formations validate that these real-valued solutions do not contribute to the wavefield, and the new dispersion curve extraction method is suitable for all kinds of VTI formations. Consequently, the method can provide reliable dispersion curves for both theoretical analysis and anisotropic parameters inversion in VTI formations.

Nearly perfectly matched layer implementation for time domain spectral element modelling of wave propagation in 3D heterogeneous and anisotropic porous media

Numerical modelling of wave propagation in the heterogeneous and anisotropic porous media is important for seismic exploration. When modelling time-domain poroelastic waves in an unbounded region, the implementation of perfectly matched layer (PML) is a common technique to absorb the outgoing waves. The PML in terms of first-order equations has been well proposed. However, the first-order PML needs fundamental and nontrivial reconstructions when applied to the second-order systems in displacements, such as finite element method (FEM) and spectral element method (SEM). It is well known SEM is an efficient algorithm for simulating seismic waves for its high accuracy and geometrical flexibility. However, at present, few literature studies the implementation of PML into time-domain SEM for simulating waves in the 3D unbounded heterogeneous porous media. In this work, combining with auxiliary ordinary differential equations (ODEs), we first systematically extend nearly PML (NPML) into time-domain SEM in terms of the second-order poroelastic system. The scheme preserves the original form of wave equations, leading to a simple and flexible incorporation into the existing algorithm. It avoids the temporary convolution through solving a set of first-order ODEs in time domain, resulting in high computational efficiency. Furthermore, to solve the coupling poroelastic/acoustic/elastic waves, we employ domain decomposition technique to handle the different regions independently, and apply the corresponding boundary conditions at the region interfaces to guarantee the coupling. Numerical examples demonstrate the accuracy and stability of the proposed method by comparing the numerical results with the analytical solutions and other independent numerical solutions. Finally, we apply the algorithm to solve wave propagation in a large-scale model involved a fluid-filled borehole embedded in heterogeneous porous formations.

Borehole radiation and reception responses for azimuthal shear-wave reflection imaging with an off-centred dipole acoustic tool

Abstract Radiation and reception responses of a dipole acoustic logging tool placed eccentrically in borehole fluid are an interesting and important topic in acoustic reflection imaging. Herein, we present a thorough research study on these responses. We treat the wave incidence from the reflector as the radiation from a virtual source and use the cylindrical-wave expansion method to solve both the wave radiation and reception problems for the off-centred tool, which, by using the steepest-descent method, yields asymptotic solutions for modelling the radiation and reception wavefield characteristics. The modelling results from the analytical solution and the 3D finite-difference method were in good agreement. Specifically, we analysed the radiation directivity of an eccentric dipole source in a fluid-filled borehole. The results revealed that the radiation pattern was asymmetric with respect to the borehole, and the asymmetry was determined by the eccentric distance, source frequency and formation properties. In particular, for the typical 3 kHz dipole logging frequency, the radiation was stronger in the off-centred direction than in the opposite direction. The asymmetry of the eccentric radiation resulted in a significant amplitude difference relative to its centred counterpart, which provided a potential method for addressing the 180° azimuth ambiguity of the dipole source. We used a theoretical waveform modelling example to demonstrate this advantage. Therefore, the results of this study provide a theoretical foundation for the development and application of dipole shear-wave imaging technology.

Measurement resolution of acoustic reflection imaging logging with a cylindrical phased array acoustic receiver station

Azimuthal acoustic reflection imaging logging plays an important role in the exploration and development of fracture-cavity reservoirs, because it can image geological structures within tens of meters around the borehole. However, minimal evaluations have been conducted for the detection of small-scale geological anomalies, particularly the simultaneous measurement of multiple small-scale scatterers. In this study, the scanning-reception implementations of phased array acoustic receiver stations in the Borehole Azimuthal Acoustic Reflection Imaging Tool (BAR) were developed based on the beamforming principle. The measurement responses of BAR were simulated and the scanning-reception performances of arcuate phased array (APA) and cylindrical phased array (CPA) acoustic receiver stations were compared using a three-dimensional (3D) finite difference algorithm for one or two small-scale caves in a formation away from a fluid-filled borehole. The azimuth and elevation measurement resolutions of CPA acoustic receiver stations were determined by analyzing the scattered wavefields under the circumstances of two caves on the same horizontal or vertical plane. Numerical simulation results showed that the echo waves scattered from small-scale caves were similar to those reflected by large-scale formation interfaces outside the borehole. The azimuth angle of a single cave could be obtained by calculating the amplitudes of P–P echoes at different azimuths received by the APA acoustic receiver station. Compared with the APA scanning-reception mode, the 3D scanning-reception mode based on the CPA acoustic receiver station was more applicable when two caves were simultaneously located outside the borehole. Under the calculation conditions of this study, the CPA acoustic receiver station had a limit azimuth resolution of ∼60°–90° and a limit elevation resolution of ∼8.8°–17.4° when two scatterers were located on the same horizontal or vertical plane. This work verified the superiority of the CPA scanning-reception mode for the detection of small-scale geological anomalies and provided a theoretical basis for the wider application of BAR.

Influencing Factors of Shear Wave Radiation of a Dipole Source in a Fluid-Filled Borehole

In shear wave far detection logging, dipole-source radiation is the main factor influencing the amplitude of the reflected shear waves. In this paper, a method is derived with the far-field asymptotic solution to calculate the dipole-source radiation of shear waves in a fluid-filled borehole. Then the dipole-source radiation of the shear waves is simulated under both low and high frequencies. In addition, the influences of formation elastic parameters on the dipole-source radiation of the shear waves are analyzed and the variations of the radiation characteristics of the shear wave with source main frequency and borehole radius are compared. Results show that the density and compressional wave velocity of the formation have little effect on the dipole-source radiation of the shear waves. However, the shear wave velocity not only affects the shear wave amplitude radiated to the formation by the dipole source (radiation performance), but also affects the energy distribution of the shear wave at different locations in space (radiation direction). The dipole source has better radiation performance and radiation coverage at low frequency and the optimal excitation frequency in different formations is very close, which is good for the application of this technology under different circumstances. At low frequency, the borehole has little influence on the dipole-source radiation, no matter how large the borehole radius is. However, at high frequency, the borehole modulation of the dipole-source radiation cannot be ignored, especially at large borehole radius.

Pressure attenuation due to radiative exchange in fluid-filled boreholes

The paper deals with the propagation of low-frequency waves in fluid-filled boreholes surrounded by an infinite solid media. Its purpose is to give a better understanding of the pressure attenuation of the acoustic wave due to radiative exchange with the solid in the low frequency range applied to the study of hydraulic facilities prone to earthquakes and sizing of hydromechanical components. We consider the case of low frequency pressure waves generated in a reservoir lake under seismic loading in the frequency range [0–20 Hz] and propagating through the duct mouth in a borehole. At theses frequencies, the radiative exchanges between soil and fluid contained into the bore are very weak, thus modelling and quantifying them constitute a challenge. A semi-analytical model based on the derivation of the equations of motion in solid and of wave propagation in fluid is developed and its results are compared with a FEM modelling. Both methods are applied to the case of a semi-infinite cylindrical buried bore and lead to close results: the strong relationship between soil properties and pressure attenuation and the influence of the Stoneley boundary wave on the acoustic pressure wave celerity. Finally, a study implementing the FEM model highlights the influence of heterogeneous solid medium and non-axisymmetric boreholes sections on the radiative exchange.

A matched filter for S-wave sonic imaging

A matched filter is developed and applied to sonic waveforms for extracting reflected signals and minimizing borehole mode waves for S-wave sonic imaging using dipole sources. Sonic imaging uses waveforms acquired by a sonic logging tool in a fluid-filled borehole. Signals from a source are reflected from geological interfaces and recorded by arrays of receivers of the same tool. The reflected signals in the recorded waveforms are extracted and migrated to image geological structures. Because the reflected signals are covered by the strong borehole mode waves, they often are difficult to extract. To overcome this difficulty, a new matched filter has been developed. In the new matched filter, the second derivative of the driving waveform of the dipole source is used for the reference signal, and an average of spectral densities of array data is used to approximate those of the borehole mode waves, which are treated as noise. Because the spectral density of sonic waveforms generally varies for different time intervals, the matched filter is applied using a windowed approach. The matched filter effectively suppresses the borehole mode waves, and reflected signals are well conserved. Application of the matched filter is followed by additional filtering to further remove remaining borehole mode waves. The resulting reflected signals indicate high signal-to-noise ratio compared with those by existing filtering procedures without the matched filter. The use of the matched filter significantly increases applicability of sonic imaging by extracting weaker reflected signals that cannot be obtained by the existing filtering procedures.

Characteristics of the interface acoustoelectric conversion waveform in fluid-filled boreholes

When acoustic waves meet the side wall media in a fluid-filled borehole, the electromagnetic field at the borehole interface between the formation and borehole fluid called the interface acoustoelectric conversion wave signal is converted. The propagation of the refracted P wave, refracted S wave, Stoneley wave, and other borehole-mode waves in the fluid-filled borehole generates accompanying electromagnetic wave signals corresponding to various borehole-mode waves, called accompanying acoustoelectric conversion wave signals. In this study, we examined the interface acoustoelectric conversion wave. First, we numerically simulated the acoustoelectric wave fields in fluid-filled boreholes using a method of real axis integration and analyzed the effects of the borehole radius, porosity, and pore fluid salinity on the interface acoustoelectric conversion waves. The arrival time of the interface acoustoelectric conversion wave increased as the borehole radius increased, and the amplitude of the interface acoustoelectric conversion wave increased as the porosity increased and decreased as the pore fluid salinity increased. Using the self-developed acoustoelectric effect logging tool, we conducted an actual downhole test in a sand shale well in northern China and observed the interface and accompanying acoustoelectric conversion waves. The observations followed the acoustoelectric effect logging theory. Further, to characterize the interface acoustoelectric conversion wave, we calculated its arrival time using a method of long-short time window energy ratio and calculated its energy by taking the summation of squares of the waveform amplitude in a window, respectively. The arrival time approximated to the time in which the acoustic wave propagated from the radiating surface of the acoustic radiator to the side wall interface, and the energy was consistent with the energy ratio of the accompanying conversion Stoneley wave to the acoustic Stoneley wave. The energy curve of the interface acoustoelectric conversion wave and conventional logging curves were highly correlated. The interface acoustoelectric conversion wave in acoustoelectric logging signals can provide a new reference for evaluating the formation parameters associated with open-hole wells.

Simulation of borehole acoustic wavefields in fractured media by combining the spectral-element method and linear-slip model

We have used the spectral-element method (SEM) to simulate 3D acoustic wavefields in the fluid-filled borehole embedded in fractured media. The fractures are characterized by the linear-slip model (LSM), which is incorporated into the surface integral of the SEM weak form, avoiding meshing individual fractures, thus reducing the degrees of freedom of the fractures compared with meshing each fracture directly. For the fracture-free case, we validate SEM through the comparison with the real-axis integration method for monopole and dipole sources. For the case with a fracture, we compare the SEM-LSM solutions with the reference numerical solutions of a thin-layer model using the finite-difference method. Good agreement is achieved between the results from our method and the reference finite-difference solutions. We find that the acoustic wavefields excited by a dipole source are more sensitive to the fractures than those by a monopole source. To show the ability of the approach to handle complex problems, we simulate the cases with a tilted fracture and multiple fractures. Based on the simulated results, we investigate the influence of the fracture parameters (e.g., stiffness, tilt angle, azimuth, thickness, number, and spatial intervals of fractures) on the scattered wavefields. We find that the tilt angle has an obvious influence on the scattered waveforms and amplitudes. The results also demonstrate that the wavefields are quite sensitive to the number of fractures. The magnitudes of the horizontal components’ transmitted wavefields decrease linearly with the number of fractures. Through analyzing the synthetic data in the time and frequency domains, we discuss how to evaluate the properties of fractures intersected by a borehole.

Ice drilling on Skytrain Ice Rise and Sherman Island, Antarctica

AbstractTo understand the long-term climate and glaciological evolution of the ice sheet in the region bordering the Weddell Sea, the British Antarctic Survey has undertaken a series of successful ice core projects drilling to bedrock on Berkner Island, James Ross Island and the Fletcher Promontory. A new project, WACSWAIN, seeks to increase this knowledge by further drilling to bedrock on two further ice rises in this region. In a single-season project, an ice core was recovered to bedrock at 651 m on Skytrain Ice Rise using an ice core drill in a fluid-filled borehole. In a second season, a rapid access drill was used to recover ice chips to 323 m on Sherman Island in a dry borehole, though failing to reach the bedrock which was at an estimated depth of 428 m.

First order born scattering away from a fluid-filled borehole

In this paper we solve the forward scattering problem in the first order Born approximation as it applies to the multipole borehole acoustic measurement configuration. This configuration consists of a multipole source and an array of multipole receivers, both deployed on a mandrel (The tool), and where the latter is axially offset from the former by a certain distance. The tool is deployed in a fluid-filled borehole, penetrating the earth formations. Assuming an arbitrary elastodynamic scatterer away from the fluid-filled borehole, we derive explicit expressions for the far-field scattered wavefield in the borehole fluid. In principle, these expressions could be used as a fast forward operator in the development of a full waveform inversion technique. Subsequently, to further our understanding, we derive analytical expressions for the borehole fluid scattered pressure at ‘small’ source-receiver offsets, and where the scatterer is represented by a Heaviside step change in elastodynamic formation properties, having an arbitrary orientation relative to the borehole axis. Our expressions apply to tools having a multipole source and receiver of order n. Such a source/receiver consists of 2n sources/receivers, located on the circumference of the tool and azimuthally separated from one another by π/n radians. All sources may be fired with equal polarity or adjacent sources may be fired with alternate polarity. In the latter case we compare our analytical solution with 3-D finite difference simulations. We do this for the practically relevant dipole (n = 1) and quadrupole (n = 2) excitation, for the (specular) PP, SVSV and SHSH borehole acoustic scattered pressure contributions. We obtain excellent results as long as the elastodynamic contrast between scatterer and embedding is small (<30%).

Physics-Driven Machine-Learning-Based Borehole Sonic Interpretation in the Presence of Casing and Drillpipe

SummaryInterpretation of sonic data acquired by a logging-while-drilling (LWD) tool or wireline tool in cased holes is complicated by the presence of drillpipe or casing because those steel pipes can act as a strong waveguide. Traditional solutions, which rely on using a frequency bandpass filter or waveform arrival-time separation to filter out the unwanted pipe mode, often fail when formation and pipe signals coexist in the same frequency band or arrival-time range. We hence developed a physics-driven machine-learning-based method to overcome the challenge. In this method, two synthetic databases are generated from a general root-finding mode-search routine on the basis of two assumed models: One is defined as a cemented cased hole for a wireline scenario, and the other is defined as a steel pipe immersed in a fluid-filled borehole for the logging-while-drilling scenario. The synthetic databases are used to train neural network models, which are first used to perform global sensitivity analysis on all relevant model parameters so that the influence of each parameter on the dipole dispersion data can be well understood. A least-squares inversion scheme using the trained model was developed and tested on synthetic cases. The scheme showed good results, and a reasonable uncertainty estimate was made for each parameter. We then extended the application of the trained model to develop a method for automated labeling and extraction of the dipole flexural dispersion mode from other disturbances. The method combines the clustering technique with the neural-network-model-based inversion and an adaptive filter. Testing on field data demonstrates that the new method is superior to traditional methods because it introduces a mechanism from which unwanted pipe mode can be physically filtered out.This novel physics-driven machine-learning-based method improved the interpretation of sonic dipole dispersion data to cope with the challenge brought by the existence of steel pipes. Unlike data-driven machine learning methods, it can provide global service with just one-time offline training. Compared with traditional methods, the new method is more accurate and reliable because the processing is confined by physical laws. This method is less dependent on input parameters; hence, a fully automated solution could be achieved.

Acoustic fields for monopole logging while drilling with an eccentric collar

The acoustic problem of an eccentric drill collar in a fluid-filled borehole has been of interest in the field of acoustic logging while drilling (ALWD) in recent years. To reduce the effects of tool eccentricity on ALWD measurements, studies on acoustic responses under such conditions are essential. This study therefore has developed an analytical method to investigate borehole wavefields with an off-center monopole ALWD tool in fast and slow formations. By evaluating the contributions of compressional and shear branch points, the effects of the tool eccentricity on individual formation primary and shear head waves are investigated. Results illustrate that tool eccentricity only affects the excitation properties and has almost no effect on the extracted velocities. The joint analysis of synthetic full waveforms and dispersion diagrams with varying eccentricity degrees indicates that multipole modes are excited when the tool is off center, and their excitation amplitudes gradually increase with the increasing eccentricity, especially in the direction of tool movement. Moreover, the dispersion analysis reveals that the two modes with intersections in the centered case are coupled when the tool becomes eccentric. In particular, the coupling performance between the Stoneley and flexural modes is the most prominent. Furthermore, the effects of tool eccentricity on the monopole acquisition method, that is, the sum of waveforms received at four orthogonal azimuths, are evaluated. Results show that the above summation method can effectively reduce the effects of slight or moderate eccentricity. However, for large or extreme eccentricity, reducing or eliminating the effects of eccentricity on the Stoneley wave is a challenge for this method. Based on the above analyses, measurements may not be reliable for formation evaluation in the case of extreme eccentricity, especially for Stoneley wave applications.

Beamform processing for sonic imaging using monopole and dipole sources

New processing techniques are presented that enhance event signals for sonic imaging using monopole and dipole sources. The techniques use the azimuthally spaced receivers of a sonic logging tool. Sonic imaging, which is also known as borehole acoustic reflection surveys, uses a sonic logging tool in a fluid-filled borehole to image geologic structures. Signals from monopole and dipole sources are reflected from geologic interfaces and recorded by arrays of receivers of the same tool. Because the amplitudes of the event signals are very weak compared with the direct waves, borehole modes, and noise, the event signals are often difficult to extract. To enhance the weak event signals, beamforming techniques were developed to stack the waveforms from azimuthally spaced receivers of the tool for given azimuthal directions. For the incident P-waves from the monopole source, phase arrival times for the azimuthal receivers are time shifted for stacking using properties of wave propagation in the borehole. For the incident SH-waves from the dipole source, the signs of waveforms for the receivers are changed for specified azimuths. When the waveforms are stacked for the back azimuth of the event signals, the signal-to-noise ratio of the event signals is significantly improved because the event signals are enhanced whereas the direct waves are relatively smeared, and random noise is canceled. Therefore, the stacked waveforms also provide accurate back azimuths of the incident waves.