Dipole Acoustic Logging Research Articles

Research on Iron Gallium Dipole Logging Transducers

Abstract Orthogonal dipole acoustic logging is an indispensable means to study the mechanical properties of formation rocks, estimate formation porosity and permeability, and carry out out-of-hole remote detection, and the broadband high-power dipole acoustic logging transducer is one of its core components, which is an urgent and unsolved international problem in actual logging. Therefore, it is of great application background and important academic significance to study the new dipole acoustic logging transducer and overcome the related scientific problems. On the basis of full investigation and study of the characteristics of iron gallium materials, this project first designed the iron gallium bending beam oscillator to verify the feasibility of iron gallium bending vibration, and then designed the iron gallium dipole acoustic logging transducer, optimized the driving magnetic circuit, simulated the driving performance of the transducer, and preliminarily verified the feasibility of the design.

Shear wave velocity prediction based on 1DCNN-BiLSTM network with attention mechanism

The Shear wave (S-wave) velocity is an essential parameter in reservoir characterization and evaluation, fluid identification, and prestack inversion. However, the cost of obtaining S-wave velocities directly from dipole acoustic logging is relatively high. At the same time, conventional data-driven S-wave velocity prediction methods exhibit several limitations, such as poor accuracy and generalization of empirical formulas, inadequate exploration of logging curve patterns of traditional fully connected neural networks, and gradient explosion and gradient vanishing problems of recurrent neural networks (RNNs). In this study, we present a reliable and low-cost deep learning (DL) approach for S-wave velocity prediction from real logging data to facilitate the solution of these problems. We designed a new network sensitive to depth sequence logging data using conventional neural networks. The new network is composed of one-dimensional (1D) convolutional, bidirectional long short-term memory (BiLSTM), attention, and fully connected layers. First, the network extracts the local features of the logging curves using a 1D convolutional layer, and then extracts the long-term sequence features of the logging curves using the BiLSTM layer, while adding an attention layer behind the BiLSTM network to further highlight the features that are more significant for S-wave velocity prediction and minimize the influence of other features to improve the accuracy of S-wave velocity prediction. Afterward, the nonlinear mapping relationship between logging data and S-wave velocity is established using several fully connected layers. We applied the new network to real field data and compared its performance with three traditional methods, including a long short-term memory (LSTM) network, a back-propagation neural network (BPNN), and an empirical formula. The performance of the four methods was quantified in terms of their coefficient of determination (R2), root mean square error (RMSE), and mean absolute error (MAE). The new network exhibited better performance and generalization ability, with R2 greater than 0.95 (0.9546, 0.9752, and 0.9680, respectively), RMSE less than 57 m/s (56.29, 23.18, and 30.17 m/s, respectively), and MAE less than 35 m/s (34.68, 16.49, and 21.47 m/s, respectively) for the three wells. The test results demonstrate the efficacy of the proposed approach, which has the potential to be widely applied in real areas where S-wave velocity logging data are not available. Furthermore, the findings of this study can help for a better understanding of the superiority of deep learning schemes and attention mechanisms for logging parameter prediction.

Joint inversion of formation radial shear-velocity profiles by dipole acoustic logging while drilling

Formation properties near a borehole are frequently altered by mechanical damage from drilling, leading to variations of formation elastic velocities in the radial direction. Inversions of radial shear profiles are necessary for a real-time evaluation of the drilling conditions. In this study, we investigate acoustic wave propagation in a logging-while-drilling (LWD) model excited by a dipole transducer in cylindrically layered media. It is revealed that the dispersion characteristics of the dipole waves of the LWD are sensitive to the radial formation velocity variation. Accordingly, we develop a joint inversion method that evaluates radial shear-velocity profiles using the dipole modes of the first and second orders. The propagation of these two modes is strongly influenced by formation alterations, but the inversion results obtained using a single dipole mode indicate ambiguity caused by the nonuniqueness problem. Through our joint inversion, the additional second-order dipole wave can effectively help address the nonuniqueness issue in dipole acoustic logging, although its dispersion features differ in fast and slow formations. It is validated that the joint inversion has considerably improved convergence and accuracy in evaluating the thickness and shear velocity of the altered zone, compared with the estimation by a single acoustic mode. Furthermore, we develop numerical results based on synthetic logging data to demonstrate the feasibility of this inversion in a continuously varying profile, regardless of whether the formation surrounding the borehole is fast or slow. Even when strong drilling noises of low frequencies are added to the receiver signal arrays, the means of joint inversion can remain stable, and the shear-velocity profiles can be evaluated with good precision.

Typical misinterpretations and scientific concepts in well-logging geologic studies

The method and theoretical system of well logging geology have been widely used in the fields of basic geology, petroleum geology and engineering geology, but the different response sensitivity of different well logging series to geological information and the mismatching between geophysical properties of multiple well logs and geological genesis of rocks frequently result in misunderstandings in the research process of well logging geology. Therefore, it is in an urgent need to analyze the typical misunderstanding cases in the research of well logging geology and explore the corresponding scientific ideas and countermeasures. After analyzing the typical misunderstandings in the research of well logging geology, this paper investigates vertical resolution scale of various logging series and its contradiction with detection depth and illustrates the importance of the integration of different scales of data. In addition, the factor inducing “fake logging data” and its influence on interpretation evaluation are clarified and a set of ideas for well logging evaluation of geological interpretation is put forward. And the following research results are obtained. First, the typical misunderstandings in the research of well logging geology can be classified into two categories, namely geological body interpretation misunderstanding and reservoir property parameter calculation misunderstanding. Second, special geological phenomena, such as high-density and high-resistivity mudstone can lead to logging data ambiguity, so attention shall be paid to petrophysical response mechanisms during geological logging interpretation. Third, to carry out well logging evaluation of unconventional oil and gas, it is necessary to integrate new technologies of electric imaging logging, dipole acoustic logging and nuclear magnetic resonance logging, and the calibration of core data and the integration of geological ideas can improve the interpretation accuracy. Fourth, In the process of borehole structural logging analysis, sedimentary response, geostress evaluation and fracture identification, geological ideas shall be integrated to avoid the logging interpretation misunderstanding caused by the same response of different geological phenomena in well logs. In conclusion, the dialectical and systematic thinking from geology to logging and then to geology, from practice to recognition and then to practice and from “a narrow view” to “a broad view” can provide a scientific ideas for the comprehensive research of well logging geology.

Study on Shear Velocity Profile Inversion Using an Improved High Frequency Constrained Algorithm

The formation shear-wave (S-wave)’s velocity information around a borehole is of great importance in evaluating borehole stability, reflecting fluid invasion, and selecting perforation positions. Dipole acoustic logging is an effective method for determining a formation S-wave’s velocity radial profile around the borehole. Currently, the formation S-wave’s radial-profile inversion methods are mainly based on the impacts of radial velocity changes of formations outside the borehole on the dispersion characteristics of dipole waveforms, without considering the impacts of an acoustic tool on the dispersion curves in the inversion methods. Accordingly, the inversion accuracy is greatly impacted in practical data-processing applications. In this paper, a novel inversion algorithm, which introduces equivalent-tool theory into the shear-velocity radial profile constrained-inversion method, is proposed to obtain the S-wave’s slowness radial profile. Based on the equivalent-tool theory, the acoustic tool can be modeled using two parameters, radius and elastic modulus. The tool’s impact on the dipole waveform’s dispersion is eliminated first by using the equivalent-tool theory. Then, the corrected dispersion curve is used to carry out the constrained inversion processing. The results of this processing on the simulation data and the real logging data show the validity of the proposed algorithm.

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.

Delineating a cased borehole in unconsolidated formations using dipole acoustic data from a nearby well

A potential application for single-well acoustic imaging is the detection of an existing cased borehole in the vicinity of the well being drilled, which is important for drilling toward (when drilling a relief well) or away from (collision prevention) the existing borehole. To fulfill this application in the unconsolidated formation of shallow sediments, we have developed a detection method using low-frequency P-waves from dipole acoustic logging. For this application, we perform theoretical analyses on elastic wave scattering from the cased borehole and derive the analytical expressions for the scattered wavefield for the incidence of P- and S-waves from a borehole dipole source. The analytical solution, in conjunction with the elastic reciprocity theorem, provides a fast algorithm for modeling the whole process of wave radiation, scattering, and reception for the borehole acoustic detection problem. The analytical results agree well with those from 3D finite-difference simulations. The results indicate that P-waves, instead of S-waves as commonly used for dipole acoustic imaging, are particularly advantageous for the borehole detection in the unconsolidated formation. Field data examples are used to demonstrate the application in a shallow marine environment, where dipole-P-wave data in the measurement well successfully delineate a nearby cased borehole, validating our analysis results and application.

The effects of near well heterogeneities on single-well imaging: numerical studies

Single-well imaging plays an increasingly important role in the exploration and production of hydrocarbon in horizontal wells. Depending on the scattering strength, reflections from lateral sections below heterogeneous regions around the borehole suffer signal distortion and amplitude loss, which complicates the imaging of deeper structures. Thus, the understanding of scattering phenomena and their impact on single-well imaging is of significant interest for hydrocarbon exploration. This paper establishes a complex horizontal well numerical model with which the effects of heterogeneous overburden on deep reflection imaging are studied using dipole acoustic logging. Synthetic data were analyzed with respect to the observed reflectivity in the heterogeneous layer and along the deep reflection. The results show that strong velocity fluctuations increase the severity of the scattering effect when correlation distances are in the order of the dominant wavelength. The reflection images migrate and reveal additional structural details of the medium. Low frequency band imaging enables the extraction of undistorted real reflectors by using the Reflection Image Spectroscopy method. The results explain the influence of the heterogeneous overburden on single-well imaging results and provide theoretical guidance for logging interpretation.

Borehole elastic wave anisotropic scattering and application to hydraulic fracturing

Anisotropy is an important property of inhomogeneous underground rocks and a central topic for geophysical exploration. Detecting the inhomogeneous anisotropy of rocks is key for rational oil and gas reservoir utilization and well placement optimization. Conventional cross dipole measurements can only evaluate the anisotropy of formation rocks near the borehole wall, and are therefore unable to evaluate the rock anisotropy at a distance from the borehole. This study investigates the anisotropic distribution of rock heterogeneity and the resulting azimuthal variation of the acoustic scattering intensity. The results show that this azimuthal scattering intensity variation of the dipole shear-wave acoustic logging data can be used to evaluate the anisotropic distribution of the rock heterogeneity. A borehole model with a three-dimensional heterogeneous and anisotropic elastic formation was established first, and the dipole acoustic logging along the borehole model was simulated using a finite difference technique. The wave-scattering effect in the borehole dipole wave data, measured in terms of a coda wave attenuation, was analyzed. The scattering attenuations of coda waves measured in different azimuthal directions exhibit significant variations, which are related to the azimuthal variation of the formation inhomogeneity. Based on the results of theoretical modeling, a method for the evaluation of the anisotropic variation of formation inhomogeneity (via azimuthal pattern of the scattering attenuations) was proposed. The method was applied to dipole acoustic logging data, acquired before and after hydraulically fracturing a tight reservoir. The field data results show that the scattered dipole-shear coda wave attenuation of the after-fracturing data exhibits an elongated, anisotropic variation pattern, where the direction of elongation coincides with the fracture extension direction. This indicates good potential for using the proposed method for hydraulic fracture evaluation. Because wave scattering occurs in a large volume around the borehole, application of this method and analysis can significantly enhance the measurement volume of the cross-dipole logging technology. This extends its effectiveness toward the evaluation of hydraulic fracturing results for unconventional reservoirs (e.g., shale gas).

Determining formation S-wave transverse isotropy from borehole flexural-wave dispersion data

Many earth formations are characterized as transversely isotropic (TI) media. In acoustic logging through a vertical borehole, the S-wave TI property has traditionally been determined from borehole monopole Stoneley-wave measurements, but the feasibility of shear-TI estimation from dipole flexural waves has not been fully investigated. We have developed a methodology to determine the TI parameters from borehole dipole-flexural wave data. Our analysis shows that the Stoneley wave is sensitive to the TI property mainly in an acoustically slow formation, and the sensitivity diminishes when the formation becomes faster. The advantage of the flexural wave over the Stoneley wave is that the former wave is sensitive to the TI property in the slow and fast formations, provided the wave measurement is made in a broad frequency range in which the flexural-wave dispersion characteristics from low to high frequencies can be used. By calculating the theoretical flexural-wave dispersion curve for the TI formation and using it to fit the measured wave dispersion data, we can simultaneously determine the vertical and horizontal S-wave velocities, from which the S-wave TI parameter is obtained. Application of our methodology to field data processing shows that the TI parameter estimated from the flexural wave is almost identical to that from the Stoneley wave for a slow formation. For a fast formation, the flexural-wave result is more accurate and reliable compared with the Stoneley-wave result. Our study, thus, introduces a novel application of dipole acoustic logging.

High‐resolution Radon transforms for improved dipole acoustic imaging

ABSTRACTImaging using dipole acoustic logging reflections has become a research topic of increasing interest in recent years. Extracting reflections from the whole waveform is both important and extremely difficult because the reflections are obscured by large‐amplitude direct waves. A method of wavefield separation based on high‐resolution Radon transforms has been applied to separate the reflected waves. First, an analysis of the common offset gathers shows that the linear Radon transform can be used to separate the direct and reflected wave fields. However, traditional linear Radon transforms cannot focus the wave event using the least squares method. An improved high‐resolution linear Radon transform is achieved using the principles of maximum entropy and Bayesian methods based on previous studies. The separation method is tested using synthetic data for hard and soft formations, a void model, and a fault model. The high‐resolution Radon transform method is used to process a field dataset and exhibits improved results compared with those of the standard method.

Model-based dispersive processing of borehole dipole wave data using an equivalent-tool theory

We have developed a model-based processing technique for borehole dipole S-wave logging data to estimate formation shear slowness from the data. During dipole acoustic logging, the presence of the logging tool can significantly affect the dispersion characteristics of flexural waves. Therefore, modeling the effects of the tool is essential for model-based processing. We have determined that an equivalent-tool theory using only two parameters, tool radius, and modulus, can adequately model the flexural-wave-dispersion characteristics. We used this theory, together with a calibration procedure, to determine the tool parameters to formulate an inversion method for the logging data processing. Our use of the equivalent tool theory played an important role in fitting the theoretical dispersion curve to the actual flexural-wave-dispersion data, enabling fast processing of the field acoustic data. An advantage of this model-based method is its prediction power, which, in the absence of low-frequency dispersion data, allows for predicting formation shear slowness from the low-frequency limit of the model-fitted dispersion curve. We have also developed an application procedure of the method for field-data processing and demonstrated its effectiveness in the dispersion correction using field acoustic data from fast and slow formations.

The research on piezoelectric cylindrical oscillating transducer for dipole acoustic logging

Acoustic transducer, which is a core component of acoustic logging tool, determines the development of acoustic logging technology. In order to realize the goal of dipole acoustic logging transducer with the features of low frequency, broadband and high radiation power, a cylindrical oscillating dipole acoustic transducer using an actuator of a group of different trilaminar piezoelectric bending bars is designed and studied. Modal and harmonic response analyses of the whole transducer are conducted by using the finite element method, including analyses of input conductance of transducer in air and water, transmitting voltage response and directivity patterns. Besides, the proposed transducer is fabricated and tested to compare with numerical simulation results. Moreover, the numerical results of the proposed transducer are compared with that of traditional dipole transducer. The comparison result indicates the proposed dipole transducer is characterized by low frequency, broadband, and high radiation power, which is feasible in the application of dipole acoustic logging.

Development characteristics and hydrocarbon accumulation significance of fractures in Lower Silurian Kalpintag Formation, northern slope of central Tarim Basin

Abstract Based on the data of core, microscope thin section, scanning electron microscope, cathode luminescence, and conventional and dipole acoustic logging, the development characteristics and hydrocarbon accumulation significance of fractures in the Lower Silurian Kalpintag Formation on the northern slope of the central Tarim Basin were studied. A large number of mainly high-angle or vertical fractures developed in the study area. They feature low density, long length, medium width and low filling extent. More fractures were found in the relatively weak bands of rock, such as the margins and interlayers of boulder clays. Conventional logging curves showed two peaks and one order of magnitude difference. The anisotropy of fast shear wave in dipole acoustic logging showed that fracture development was influenced by NE trending strike-slip faults and the NE-SW trending maximum horizontal principal stress. The fractures in the Kalpintag Formation played important roles in the formation of tight sand-stone reservoirs. They improved the porosity and permeability of tight sandstones, and the large-scale, high-angle or vertical fractures served as pathways for vertical migration of hydrocarbons. For the small-scale adjustment of hydrocarbon migration, the small-scale, low-angle or horizontal fractures were important.

Numerical simulations of an electromagnetic actuator in a low-frequency range for dipole acoustic wave logging

In dipole acoustic logging, transducers are required to work in a low frequency range, such as 0.5–5 kHz, to measure shear wave velocities so as to accurately analyze the anisotropy parameters of formations. In this paper, an electromagnetic actuator is designed for more effective low-frequency excitations than conventional piezoelectric bender-bar transducers. A numerical model has been set up to simulate electromagnetic actuators to generate flexural waves. The Finite Element Method (FEM) has been applied to simulating the radiation modes and harmonic responses of the actuator in a fluid, such as air and water. In the frequency range of 0–5 kHz, the first ten vibration modes are simulated and analyzed. The simulation results of 3-D harmonic responses of the sound field, such as the deformation, acoustic sound pressure, and directivity pattern, have been conducted to evaluate the radiation performance. From the simulation results, it is concluded that the second asymmetric mode at 670 Hz could be excited more easily than the others. This oscillated-vibration mode is useful to be applied in a dipole source. The frequency response curve is broad and flat and the electromagnetic actuator is beneficial to generate the wideband signal in a required low frequency range, especially below 1 kHz.

Wave propagation in a fluid-filled shell excited by a dipole source

Wave propagation in a fluid filled cylindrical shell excited by a dipole source is investigated for the design of a new kind of calibration pit for dipole acoustic logging tools. Based on classical elasticity wave equations, phase and group velocity dispersion curves of each mode, excitation spectra and mode contributions to wave field in the shell are presented. The dispersion curves show that the lowest mode exists in the entire frequency range with the phase velocity smaller than shear wave velocity of the shell, higher-order modes have cutoff frequencies, below the cutoff frequencies these modes are non-propagating. Analysis on mode excitation spectra and their contributions suggest that the lowest mode has potential to be used in calibrating the velocity measurement accuracy of dipole acoustic logging tools quantitatively, provided that the contributions of this mode to the wave field dominating the first arrivals gathered by a specific dipole acoustic logging tool. [Work supported by National Natural Sciences of China, Grant No. 11134011.]

A curve-fitting technique for determining dispersion characteristics of guided elastic waves

We have developed a novel curve-fitting method to estimate dispersion characteristics of guided elastic waves and investigate its application to field wireline and logging while drilling (LWD) acoustic data processing. In an elastic waveguide such as a fluid-filled borehole with a logging tool, the frequency dispersion of a guided-wave mode is characterized by a monotonically varying dispersion curve bounded by its low- and high-frequency limits. The detailed behavior of the curves relates to various elastic/acoustic parameters of the complicated waveguide structure. The novelty of the proposed technique is that it simulates the multiparameter dispersion curve using a simple analytical function that has only four parameters. By adjusting the four parameters to fit the actual wave dispersion data, the wave’s dispersion characteristics can be satisfactorily determined. The result of this simple approach leads to several important applications in acoustic logging. The first is to correct the dispersion effect in the shear-wave velocity from wireline dipole acoustic logging. The second application obtains P-wave velocity from the dispersive leaky compressional-wave data from wireline or LWD measurements. Third, the technique is applied to obtain shear-wave velocity from LWD quadrupole shear-wave logging. Finally, the technique is applicable to layered waveguide structures encountered in surface seismic exploration.

Borehole flexural modes in transversely isotropic formations: Low-frequency asymptotic velocity

Dipole acoustic logging is widely used for in situ measurement of formation shear velocity. This technique is based on the theoretical result in isotropic formations that the velocity of the dispersive flexural mode approaches the shear velocity at the low-frequency limit. Transversely isotropic (TI) formations frequently are encountered in petroleum engineering, so it is necessary to determine if shear velocity along the borehole can be determined from the low-frequency flexural wave in TI cases. The flexural wave in a borehole parallel to the symmetry axis of a TI formation is investigated. The borehole acoustic field is formulated in the frequency-wavenumber domain as the sum of a direct field from the source and a reflected field from the borehole wall. Poles and branch points of the reflection coefficient are analyzed, and component waves contributed by those singularities are calculated. The dispersion feature of the flexural mode in some TI formations differs from isotropic cases. The low-frequency limit of flexural-mode velocity trends to a value lower than the shear velocity when parameters of the formation satisfy [Formula: see text], where [Formula: see text] and [Formula: see text] are Thomsen parameters and [Formula: see text] and [Formula: see text] are the elastic moduli. That asymptotic value corresponds to a newly found branch point of the reflection function. Numerical results show that the low-frequency flexural wave travels slower than the shear wave in the synthetic full waveforms in Mesaverde clayshale 5501. Extracting the shear velocity from the flexural arrival leads to inaccuracy on the order of 10% in this formation.

Simultaneous inversion of formation shear‐wave anisotropy parameters from cross‐dipole acoustic‐array waveform data

This study presents an effective technique for obtaining formation azimuthal shear‐wave anisotropy parameters from four‐component dipole acoustic array waveform data. The proposed technique utilizes the splitting of fast and slow principal flexural waves in an anisotropic formation. First, the principal waves are computed from the four‐component data using the dipole source orientation with respect to the fast shear‐wave polarization azimuth. Then, the fast and slow principal waves are compared for all possible receiver combinations in the receiver array to suppress noise effects. This constructs an objective function to invert the waveform data for anisotropy estimates. Finally, the anisotropy and the fast shear azimuth are simultaneously determined by finding the global minimum of the objective function. The waveform inversion procedure provides a reliable and robust method for obtaining formation anisotropy from four‐component dipole acoustic logging. Field data examples are used to demonstrate the application and features of the proposed technique. A comparison study using the new and conventional techniques shows that the new technique not only reduces the ambiguity in the fast azimuth determination but also improves the accuracy of the anisotropy estimate. Some basic quality indicators of the new technique, along with the anisotropy analysis results, are presented to demonstrate the practical application of the inversion technique.