Acoustic Logging Research Articles

Consideration of various factors influencing acoustic logging equipment production accuracy

To improve quality of technological machines and equipment, quality and reliability of the downhole material obtained using acoustic logging equipment, it is necessary to take into account all factors affecting reliability and accuracy of the tools at all the stages of their life time - development, production, operation, and interpretation of the well material. The analysis results are presented in the diagram generalizing the principles of high-precision acoustic logging equipment production optimization.

Monopole collar wave characteristics for acoustic logging while drilling in fast formations in the frequency and spatial domains

Collar waves generated by the presence of the massive steel collar can interfere with or even cover formation signals, which has been a challenge for designing acoustic logging while drilling (ALWD) tools. In order to seek the means for the suppression of collar waves, it is necessary to study their propagation characteristics. In this study, considering azimuthally symmetric modes only, we compare the radial distribution of the excitation intensity for the first-order collar mode (hereinafter referred to as the collar wave) with varying frequencies obtained by solving elastic wave equations. And the characteristics of the particle vibration trajectories contributed by the collar wave are investigated. It is found that the radial position corresponding to the excitation peak of the collar wave depends on frequencies. As the frequency increases, the peak gradually moves from the inner wall to the outer wall in the collar. According to this feature, it is concluded that interior grooves cut on the drill collar are more suitable for weakening the collar wave than exterior grooves with lower frequencies. On the contrary, in the high-frequency range, exterior grooves attenuate the collar wave more effectively. To confirm these conclusions, we design numerical models and validate the collar wave attenuation by finite difference time-domain simulations. Furthermore, by analyzing vibration properties of particles contributed by the collar wave with varying frequencies, we reveal that the polarization of collar wave motions at the low frequency is similar to that of the longitudinal wave, while at the high frequency it becomes the transverse-wave-like vibration. Inspired by the observation, it is proposed that the collar wave can be weakened by using a steel collar containing an interlayer whose material differ from that of the steel collar. Two layered models are designed and the synthetic waveforms are numerically simulated to examine the proposal. The comparison of wave amplitudes indicates that the collar with an axial interlayer attenuates more collar waves than the collar containing a radial interlayer in the low-frequency range, while in the higher frequency range, the radial layered structures are better for collar wave suppression. Finally, the feasibility analysis of ALWD in the high-frequency range is carried out. The analysis illustrates that when the source frequency is within 20–25 kHz, the excitation amplitude of the second-order collar mode is relatively weak and the first-order collar mode is mainly concentrated near the outer surface of the collar. Appropriate exterior grooves cut on the collar may lead to a clear arrival of formation signals.

Data fusion for resolution improvement by combining seismic data with logging data

Resolution of seismic data is critical to reservoir prediction and the near surface underground structure. Low resolution of seismic data is caused by many factors, such as the surface environment, shielding layer and sand body. To widen the frequency bandwidth and improve the resolution of the post-stack seismic reflection profiles, we analyze the characteristics of amplitude-spectrum of the post-stack profiles and design frequency expanding inverse filter based on the logging data for data fusion, including the loss of longitudinal frequency information and the variation of transverse frequency. This method creates broadband synthetic seismogram by using a wide bandwidth seismic wavelet and a series of reflection coefficients calculated from acoustic logging and density logging data. By analyzing the synthetic seismogram of widening frequency band with borehole seismic traces, the loss regularity of vertical frequency signal generated by low-pass filter of formation is restored. Additionally, to compare borehole seismic traces with arbitrary cross-well trace, the filtering effect about transverse variation of formation is obtained. The successful application of the inverse filter in terms of data fusion to acquire a new profile with better resolution called the seismic-logging profile, demonstrated that the proposed data fusion method has the advantage of being efficient in compensating the high frequency components of seismic section.

Variation in the orientation of the maximum horizontal stress in thick channel-fill sandstones with low-permeability: A case of the Bonan Oilfield in the Bohai Bay Basin, eastern China

Measurements of in situ stress in a single well from borehole breakouts, hydraulic fracturing, multipole acoustic logs and differential strain analysis of core indicate that the orientation of the maximum horizontal stress is northwest-southeast in thick channel-fill sandstones in the central-western Bonan Oilfield, while the orientation is northeast-southwest in the thin interbed siltstones and mudstones of the floodplain in the eastern Bonan Oilfield. This result shows that the orientation of the maximum horizontal stress in the thin interbed of siltstones and mudstones is in accordance with the regional stress orientation obtained by the focal mechanism of a natural earthquake, but has a 30°–40° variation with the thick channel-fill sandstones. Finite element simulation suggests that the main reason for the variation in the orientation of the maximum horizontal stress is the diversity of overall mechanical properties, especially modulus of elasticity, of the thick channel-fill sandstones and the interbedded thin layer of siltstones and mudstones. The average Young's modulus over the stratigraphic column governs stress rotations. The transformation of the orientation of the maximum horizontal stress from the central-western to the eastern locations is at the channel boundary. This research provides a case in which the variation in the orientation of the maximum horizontal stress is affected by thick channel-fill sandstones. It shows that lithological changes can cause variations in the direction of in-situ stress.

Stress magnitudes in the Basel enhanced geothermal system

This paper presents the results of an evaluation of stress magnitudes in the granitic EGS reservoir in Basel, Switzerland. The profile of minimum principal horizontal stress, Shmin, is constrained by hydraulic tests, but the magnitude of the maximum horizontal principal stress, SHmax is uncertain. Here we derive estimates for SHmax by analysing breakout width data from an acoustic televiewer logs run in the granitic basement section of the BS-1 borehole. Some 81% of the borehole in the granite is affected by breakouts. The approach employed to derive SHmax magnitude from the estimated breakout widths is taking into account all stress components at the borehole wall including the remnant thermal stress arising from the cooling of the borehole wall by the drilling. In BS-1, breakouts width tends to decrease with depth. Assuming there is no significant systematic change in the strength characteristics of the rock along the length of the hole, for which there is no evidence, the large-scale trend has the consequence of implying a small gradient of the SHmax profile. A low Shmin gradient was also implied by a stress analysis that additionally considered the occasionally coincident presence of drilling induced tension fractures. The absolute values of SHmax depend upon the failure criterion used. Criteria that consider the strengthening effect of the intermediate stress (Mogi-Coulomb and Hoek-Brown 3D) yield profiles that violate frictional limits on the strength of the crust above 4 km, whereas the profiles of the Mohr-Coulomb and Rankine criteria do not. The Mohr-Coulomb criteria profiles indicate a trend in SHmax from favoring strike-slip faulting above 4200 m to strike-slip/normal faulting below. This is consistent with focal mechanisms recorded during the reservoir stimulation which show a mix of strike-slip and normal faulting throughout the depth range considered.

A general experimental system for the development of acoustic logging tools.

Laboratory testing is a pre-requisite for the practical application of new methods and techniques, and it is crucial in the research and development of acoustic well-logging tools. Various tools have been developed based on different acoustic logging theories and methods. Thus, these tools are equipped with different acoustic sonde structures. To meet the test requirements of different tools in a laboratory environment, we designed a general experimental system that includes hardware platform, software platform, and model wells according to the common structure of actual logging tools. Similar to the internal electrical structure of downhole tools, the hardware platform consists of several main parts, such as power supply, control and telemetering, acoustic emission, and data acquisition. The functions of this hardware platform include controlling the working sequence of the experiment, exciting the transmitter sonde, and collecting the acoustic signals received by the receiver sonde. The software platform installed in the host computer provides a human-computer interface for the experimental system to complete the data transmission between the host computer and the hardware platform, store measured data, and process the data in real time. The model wells approximate the actual engineering environment and stratum condition for system testing. A series of practical laboratory experiments is conducted in the model wells by using this experimental system. The process proves that the hardware and software of the experimental system can work in coordination, and the experimental system meets the basic testing requirements of conventional acoustic logging tools.

Simulations of collar waves for acoustic logging while drilling in the frequency and the spatial domains

Tool waves generated by the presence of the drill collar can cover the formation signals we need. The suppression for those collar waves has been a challenge for designing acoustic logging while drilling (ALWD) tools. To study the collar wave characteristics, we evaluate the excitation intensity of the modes with varying frequencies and positions by solving the elastic wave equations. Analysis results show that the collar wave energy concentrates in the cylindrical tool model. And the radial location of its peak gradually moves from the inner surface to the outer wall with the increasing frequency. The particle vibration trajectories contributed by the collar wave are also investigated. It is revealed that the collar mode in the lower frequency range has an approximately longitudinal polarization, while it tends to the transverse-wave-like motion at high frequencies. According to these features, we propose strategies on the tool structure designs to suppress the collar waves in the lower- and the higher- frequency ranges, respectively. Numerical examples further validate that the proposed collar structures with interlayers or grooves can well attenuate the collar wave propagation.

Energy of acoustic signals in a borehole

ABSTRACTIn the present article, the dependencies of the acoustic signal total energy and the energies of the wave packets of different types of the waves on the elastic parameters and permeability of rocks have been studied. We have considered traditional logging tools containing acoustical monopole source. Calculations were performed in a frequency range of dozens of kilohertz, typical for acoustic well logging. It was shown that in a typical high‐velocity formation (vs > vf, where vs and vf are the velocities of the shear wave in the rock and of the compressional wave in the borehole fluid, respectively), the pseudo‐Rayleigh waves, whose elastic properties depend mainly on the shear modulus of the rock, contributed significant energy to the total signal energy in the borehole. The energies of different wave packets depend on the permeability in different ways. The greatest sensitivity to permeability change has been shown by the acoustic signal total energy and the energy of the low‐velocity part of the pseudo‐Rayleigh wave packet. The theoretical analysis was illustrated by real sonic log data.

Assessing organic richness of source rocks through integration of acoustic logs and microresistivity images

Assessing organic richness of source rocks through integration of acoustic logs and microresistivity images

Investigation on acoustic reflection logging of LWD using a flexible finite difference technique

ABSTRACTAcoustic reflection logging while drilling (LWD) measurement has overwhelming advantages in detecting reflectors outside the borehole, making it a prominent technology in geosteering during LWD operations. This paper carries out a comprehensive numerical investigation into the acoustic reflection logging responses in LWD environments by combining the analytical real axis integration (RAI) solution and an exquisitely improved finite difference method (FDM). Mutual validation of these two methods using typical and well-known cases ensures the reliability of the modelling methodology. Numerical studies on LWD and wireline acoustic reflection logging reveal that a ring source rather than four azimuthally orthogonal point sources can excite acoustic LWD monopole waves without interference from higher order wave modes. Lower order spatial differential operators are more efficient for FDM simulations of LWD cases when variance in the medium is obvious. Compared with the wireline monopole case, LWD monopole reflection acoustic logging can obtain higher quality reflected waves and has a superior ability to identify the azimuth of a geological reflector away from the borehole.

Assessment of gas hydrate accumulations using velocities derived from vertical seismic profiles and acoustic log data in Krishna-Godavari Basin, India

The second Indian National Gas Hydrate Program Expedition (NGHP-02) was executed in 2015 in four areas termed as Areas A, B, C and E. During this expedition, twenty five research sites were drilled and/or cored in Krishna-Godavari (KG) and Mahanadi Basins, eastern Indian offshore. During NGHP-02, zero offset vertical seismic profile (VSP) data were acquired at three sites: Sites NGHP-02-17, −19 and −22 in Area B of the KG Basin. In this study, we focus on the three sites in Area B of the KG Basin, where, zero-offset VSP and downhole acoustic log data are used to assess and characterize the gas hydrate deposits. Zero-offset VSP data are correlated with wireline log, surface seismic and synthetic seismic data to characterize and delineate gas hydrate accumulations in the KG Basin. Low velocities ranging from 1500 to 1650 m/s are observed in the unconsolidated shallow sedimentary section above gas hydrate-bearing units, whereas, very high velocities are observed in the acquired acoustic log and VSP data. In the gas hydrate-bearing sedimentary sections, the VSP derived interval velocities range from 2000 to 3000 m/s in the depth interval between 267 and 287 m below sea floor (mbsf) at Site NGHP-02-17, between 1650 and 1750 m/s in the depth interval between 306 and 366 mbsf at Site NGHP-02-19 and between 1700 and 1800 m/s in the depth interval between 198 and 290 mbsf at Site NGHP-02-22. Gas hydrates are distributed both as pore-filling and fracture-filling at all three sites, however, high concentrations are only observed as pore-filling morphology. We estimate the amount of gas hydrate considering both isotropic (pore-filling) and anisotropic (fracture-filling) acoustic reservoir models. Our estimations of gas hydrate saturation match well with the available pressure core data and suggest that high gas hydrate concentrations (to nearly 85% of pore space) are distributed in a load bearing morphology.

Shear wave imaging to determine near-borehole faults for ocean drilling exploration

SUMMARY The Deep Sea Drilling Project (DSDP) or Ocean Drilling Program (ODP), and the recent International Discovery Program (IODP) usually acquire borehole geophysical data for planetary exploration. A recently developed shear wave imaging technique was applied to the acoustic waveform log data of the IODP U1473A hole to image a fault system about 40 m away from the borehole. The shear wave imaging not only allowed us to delineate the fault structure in the vicinity of the borehole, but also to determine the orientation of the fault system, providing much needed information for the interpretation of the tectonic situation of the well site. Theory and method for the imaging application are elaborated and data processing results are presented along with the supporting borehole geophysical data. Its successful application to the IODP U1473A hole suggests that the shear wave imaging technique can be applied to the previously recorded and future ODP/IOPD borehole acoustic data, promising the extraction of valuable geophysical and geological information.

The research of LWD acoustic isolator based on SAW spatial separation

Acoustic logging while drilling (LWD) can extract P-wave and S-wave information from the formation. However, the transmission of the collar wave propagated directly from the emitter to the receiver may interfere with the P-wave and S-wave and affect the extraction of formation information. Therefore, it is necessary to design a suitable acoustic isolator between the transmitter and the receiver to attenuate the drill waves. The commonly used acoustic LWD isolator is that the outer surface of the drill collar is evenly grooved to attenuate the collar wave. However, there are still disadvantages such as the lack of mechanical strength of the evenly grooved acoustic insulators and the ability to extract clean longitudinal wave under certain circumstances. Therefore, there is an urgent requirement to design a new type of acoustic LWD isolator with sufficient strength and acoustic insulation requirements. In recent years, spoof surface acoustic waves (SSAWs) generated by periodic corrugated surface rigid plates have attracted the attention of many researchers, who can spatially separate the surface waves to attenuate acoustic waves. In this paper, a new type of acoustic LWD insulator based on SAW space separation structure is proposed. The finite element software ANSYS is used for acoustic analysis.

The Use of Coring-Induced Petal Fractures in Coal to Supplement and Ground Truth the Interpretation of Image Logs

Resistivity and acoustic scanner image logs, in both the CSG and coal-mining industries, are the preferred means of determining azimuths of joints and cleat in coal. This paper indicates the need for care when interpreting cleat azimuths from image logs.Image logs of the bore wall often exhibit large fractures (joints) that intersect the entirety of the bore wall. They are visible as sinusoidal traces. Those fractures that have low height (cleat), and intersect the bore wall in one or two places, are represented by vertical to sub-vertical linear traces on an image log. These lineations can be represented on a stereo- or polar net as a point with a plunge and trend. Both joints and cleat generally have high dips (greater than 75°).Resistivity image logs detect both joints and cleat. Acoustic image logs often only record joints. The value of obtaining cleat, joint and horizontal stress azimuths, is in planning the optimal orientation of deviated in-seam (lateral) production wells.An image log of a cleat lineation records the bore wall intersection azimuth (BIAZ), that is an apparent azimuth, as well as the apparent dip (or plunge) of the lineation. The best way to determine true azimuth of cleat from lineations, on an image log, is from a statistical weighted mean of numerous BIAZ measurements. In some instances, cleat azimuth has been calculated as 90° to BIAZ. Some interpreters view the dispersion of azimuths as a natural feature, when the dispersion is an artefact arising from the intersection of cleat of the same azimuth, at varying distances from the centre of a bore wall.Petal fractures (PF) in coal, when combined with breakout information, can also be used to determine or ground truth joint/cleat azimuths of both large and small scale fractures. Bedding-plane observations of core also provides cleat/fracture information not obtainable from an image log.

Modeling Seismic-Acoustic Fields in Axisymmetric Absorbing Media: the Finite Difference Scheme

The finite difference scheme for modeling seismo-acoustic field propagation in axisymmetric absorbing media excited by an emitter in the borehole fluid (a monopole, a dipole, or a quadrupole) during the acoustic logging, or by an emitter in an elastic medium (a concentrated force, a dipole, or a center of expansion) during the seismic prospecting is presented. The explicit finite difference scheme approximating the equations of the modified Biot’s model, which describes the acoustic wave propagation in an isotropic porous viscoelastic medium saturated with viscous fluid, is proposed.

Interpretation of borehole flexural measurements in high-angle wells using 3D spatial sensitivity functions

Undulating wells are routinely drilled to improve reservoir exposure across hydrocarbon-bearing zones. Although conventional acoustic-log interpretation methods are reliable in vertical wells, they often yield inaccurate results when applied to high-angle or horizontal wells due to azimuthal asymmetry, spatial averaging effects, and wave-mode interference. Three-dimensional finite-difference and finite-element algorithms are typically used to quantify the aforementioned effects on acoustic logs, but they are extremely demanding on the central processing unit’s (CPU) time and memory. We develop a fast algorithm to simulate flexural slownesses acquired in high-angle wells using 3D linear spatial sensitivity functions. Spatial sensitivity functions quantify the variation of phase slowness measured by the sonic tool due to spatial perturbations of elastic properties. First, we construct 3D frequency-dependent sensitivity functions of flexural modes using the product of 1D axial, radial, and azimuthal sensitivity functions obtained from first-order approximations. Then, we simulate frequency-domain flexural logs acquired with wireline tools and dipole sources in isotropic and vertical transversely isotropic (VTI) formations penetrated by high-angle wells. For the examined examples, simulated flexural logs exhibit root-mean-square errors less than [Formula: see text] when compared with those calculated with a 3D time-domain finite-difference (3D-TDFD) algorithm. We found that in isotropic formations with layers thinner than the length of the receiver array, flexural slownesses acquired separately with cross dipoles are different because of geometric asymmetry, whereas in VTI formations we observed differences between cross-dipole slownesses because of effective anisotropy. Furthermore, the flexural logs are affected by spatial averaging introduced by the acoustic wireline tool, especially in the vicinity of layer boundaries. Three-dimensional sensitivity functions reduce the computation time of the flexural logs from an average of 15 h of CPU time per depth (using 3D-TDFD methods) to less than 3 min. The fast simulation algorithm disregards wave reflections, wave-mode conversion, and wave-mode interference occurring at layer boundaries.

A new fracture permeability model of CBM reservoir with high-dip angle in the southern Junggar Basin, NW China

Predicting the permeability of coalbed methane (CBM) reservoirs is significant for coalbed methane exploration and coalbed methane development. In this work, a new fracture permeability model of coalbed methane reservoir with high-dip angle in the southern Junggar Basin, NW China is established based on the Poiseuille and Darcy laws. The fracture porosity in coalbed methane reservoir is calculated by applying 3D finite element method. The formation cementing index m was calculated by combining fractal theory and the data of acoustic logging, compensated neutron logging, and density logging with the space method. Based on Poiseuille and Darcy laws, the curvature τ is introduced to derive this new method for obtaining the permeability of the original fractures in coalbed methane reservoirs. Moreover, this newly established permeability model is compared with the permeability from the well testing, which shows a very good correlation between them. This model comprehensively includes the effects of fracture porosity, reservoir pore structure, and development conditions on fracture permeability. Finally, the permeability prediction of coalbed methane reservoir with high-dip angle in the southern Junggar Basin, NW China is conducted, which correlates very well with the well test permeability ( R2 = 0.83). Therefore, this model can be used to accurately predict the coalbed methane reservoir permeability of low rank coals in the southern Junggar Basin. The permeability of the No.43 coalbed methane reservoir for the coalbed methane wells without well testing data is evaluated, which ranges from 0.000251 to 0.379632 mD. This significant change in permeability may indicate a complex coalbed methane reservoir structure in the southern Junggar Basin, NW China.

Characterization of subaerial volcanic facies using acoustic image logs: Lithofacies and log-facies of a lava-flow deposit in the Brazilian pre-salt, deepwater of Santos Basin

Abstract Volcanic rock facies characterization in subsurface log data have always being challenging. Even though considerable types of well logs are acquired, the results achieved on facies characterization with the conventional log suits are very limited. Conversely, high-resolution borehole image logs calibrated with side wall core samples can provide the necessary structural and textural information for facies definitions. In this case study, an integration of good quality acoustic image log data, side wall core petrography and geochemical analyses provided a good understanding of volcanic facies and stratigraphic relationships. Additionally, outcrop data from the analogous Serra Geral Formation and other Large Igneous Provinces were used for comparison. In the studied well, from Santos basin, Brazil, it was possible to identify several kinds of subaerial basaltic lava flow units, such as compound pahoehoe, sheet pahoehoe and rubbly pahoehoe lava flows. Vesicles, amygdales, vesicle cylinders, sub-horizontal vesicle sheets, autobreccias and entablature are some of the structures described in this study. As a result, 2 image catalogues of subaerial volcanic rocks were produced characterizing facies and flow units along with a stratigraphic model of the history of this volcanism. This is the first time that pahoehoe lava flow units could be characterized at an offshore Brazilian basin. The results achieved are important for the understanding of the Cretaceous volcanism events in the pre-salt layer and also provide support for the evaluation and geological modelling of the volcanic rocks in Santos Basin oil fields.

Acquisition of Logging-While-Drilling (LWD) Multipole Acoustic log Data during the India National Gas Hydrate Program (NGHP) Expedition 02

Abstract Gas hydrates are ice-like crystalline minerals with high energy concentrations, which generally occur in deepwater environments, at low temperature and high pressure, usually close to the seafloor. Gas hydrate research wells are normally drilled riser-less, with multiple wells being drilled in rapid succession. Logging-while-drilling (LWD) is the preferred logging method as the borehole condition in such shallow formations deteriorates very rapidly after drilling. LWD helps minimize the time between drilling and logging. The LWD sonic measurement is key in such projects and is used typically for geomechanics and formation evaluation purposes. The water-bearing sedimentary section within the shallow gas hydrate stability zone is often shale-rich, unconsolidated and the measured acoustic wave slowness is “extremely slow”, with shear slowness above 1400 μs/ft and bulk density of around 1.7 gm/cc. Quadrupole measurements are commonly used in the industry to log shear slowness with LWD tools in slow formations, where the shear slowness is slower than the borehole fluid slowness. However, it is very challenging to log shear slownesses with LWD sonic tool in these extremely slow formations, even with a quadrupole measurement. This paper details the challenges related to borehole acoustics logging in gas hydrate research wells, outlines the key technological enablers and gives an overview of the performance of LWD sonic tools during the National Gas Hydrate Program Expedition 02 (NGHP-02) in the offshore of India. It will spotlight the innovative approach followed to modify the LWD sonic acquisition and processing to adapt to the extremely slow acoustic wave speeds, which facilitated recording LWD shear slowness up to approximately 1400 μs/ft.

Mechanisms for overpressure generated by the undercompaction of paleogene strata in the Baxian Depression of Bohai Bay Basin, China

Abstract Undercompaction is an important mechanism for developing fluid overpressure in the mudstone strata. The main factors that affect undercompaction are the deposition rate (DR) and mudstone content (MC); however, the manner in which the DR and the MC influence the development of undercompaction is still unclear. To clarify the pressurization mechanism of undercompaction and the influence of DR and MC, the acoustic and density logging as well as the sonic-velocity–effective-stress schematic are used to identify the undercompaction overpressure. Furthermore, the PetroMod software is used to simulate the development of overpressure and to analyze the influence of the DR and MC on the undercompaction of Paleogene strata in the Baxian Depression of Jizhong Subbasin. The Baxian Depression causes the development of overpressure in the Dongying Formation, the first member of the Shahejie Formation, the third member of the Shahejie Formation, the fourth member of the Shahejie Formation, and the Kongdian Formation, whereas the undercompaction overpressure is observed to mainly develop in the Dongying Formation and in the first member of the Shahejie Formation. A prerequisite for performing the simulation is that the total thickness and deposition time of the formation should remain constant. Further, the effects of the varying DR and MC on the development of undercompaction are simulated. The results exhibit that the DR and MC are prerequisites for developing undercompaction overpressure. However, the DR only affects the undercompaction development period and exhibits little effect on the final overpressure amplitude. Meanwhile, the amount of MC not only increases the amplitude but also results in an early development of the undercompaction overpressure.