Acoustic Logging Research Articles

Secondary porosity classification in heterogeneous carbonate reservoirs based on the sonic log data

Many carbonate oil and gas reservoirs are characterized by the presence of secondary pores: vugs and fractures. To estimate the volume of oil and gas in carbonate rocks with double porosity, the information about secondary pore structure is required. In the present work we propose a simple and easy to code methodology to determine secondary pore types in carbonate sediments using acoustic well log data. The methodology is based on the comparison of the normalized differences between the calculated and measured in a borehole velocity of the compressional and shear waves. The results were obtained for the rocks with polymineral skeleton (limestone and dolomite). The mineral concentration is estimated by the solution of the inverse problem using the data obtained by a neutron, density, and PEF logging. The classification results were adjusted by using the micromechanical methods (the Kuster-Toksöz method and the Effective Medium Approximation (EMA)). Examples of the developed methodology application in the carbonate reservoirs are presented.

Fluid Identification Method of Nuclear Magnetic Resonance and Array Acoustic Logging for Complex Oil and Water Layers in Tight Sandstone Reservoir

In order to improve the logging interpretation accuracy for complex oil and water layers developed in tight sandstone reservoirs, this study takes the Chang 8 member of the Yanchang Formation in the Huanxian area as the research object, and two new fluid identification methods were constructed based on nuclear magnetic resonance (NMR) logging and array acoustic logging. Firstly, the reservoir characteristics of physical properties and conductivity were studied in the research area, and the limitations of conventional logging methods in identifying complex oil and water layers were clarified. Then, the sensitive parameters for identifying different pore fluids were established by analyzing the relationship between NMR logging and array acoustic logging with different pore fluids. On this basis, the fluid identification plate, composed of movable fluid apparent diffusion coefficient and effective porosity difference (Da-Δφe) by NMR logging data of D9TWE3 observation mode, and the other fluid identification plate, composed of apparent bulk modulus of pore fluid and elastic parameter sensitive factor (Kf-Fac), were constructed, respectively. Finally, these two fluid identification methods were used for reservoir interpretation of actual logging data. This study shows that the two new fluid identification methods constructed by NMR logging and array acoustic logging can effectively eliminate the interference of rock skeleton on logging interpretation, which make them more effective in identifying complex oil and water layers than the conventional logging method. Additionally, the two methods have their own advantages and disadvantages when used separately for interpreting complex oil and water layers, and the comprehensive interpretation of the two methods provides a technical development direction for further improving the accuracy of logging the interpretation of complex oil and water layers.

Coal Structure Characteristics of the 2# Coal Seam in the Jiaozuo Mining Area and Its Geological Dependence.

To clarify the coal structure, spatial distribution, and controlling factors of the 2# coal seam in Jiaozuo mining, the drilling coal samples were collected to observe the coal type and coal structure. The coal macerals were identified by a MPVSP microscope photometer, and the spatial characteristics of the coal structure were obtained through interpreting deep lateral resistivity logging, natural gamma ray logging, density logging, and acoustic logging curves. The influence of coal properties, burial depth, geological stress, and faults on the coal structure were discussed correspondingly. The results exhibit that granulitic-mylonite coal was most developed in the 2# coal seam, followed by primary coal and cataclastic coal; the coal type was dominated by semibright coal, followed by clarain and semidull coal. Granulitic-mylonite, cataclastic, and primary coals were the main components of clarain, semibright coal, and semidull coal, respectively. Higher vitrinite and organic matter contents were conducive to the development of granulitic-mylonite. The coal structure combinations were spatially varied, and the granulitic-mylonite combinations were the most common. Granulitic-mylonite coal was developed in the east and south parts of the study area, and the coal structure was fragmented with a greater burial depth and larger thickness. The geological stress is the fundamental cause of coal structure damage as well as the cutting of faults.

Integrated Formation Evaluation for Site-Specific Evaluation, Optimization, and Permitting of Carbon Storage Projects

Participation in over 80 carbon capture and sequestration (CCS) projects spanning 25 years has led to the evolution of a recommended well-based appraisal workflow for CO2 sequestration in saline aquifers. Interpretation methods are expressly adapted for CCS applications to resolve key reservoir parameters, constrain field-scale modeling, provide answers required for the permitting process, and de-risk unique CCS evaluation challenges, such as Storage capacity Injectivity Containment. A challenge complicating all of the above is the eventual impact of three-way interaction among rock matrix, brine, and (impure) CO2 streams. Most logging, sampling, and laboratory techniques are adapted from established domains such as enhanced oil recovery, underground gas storage, and unconventional reservoir evaluation, though some CCS-specific innovation is also needed. Storage evaluation begins with established methods for lithology, porosity, permeability, and pressure, while special core analysis (SCAL) determines CO2 storage efficiency and relative permeability. Containment evaluation spans multiple disciplines and methods: the petrophysicist’s task to quantify seal capacity relies heavily on laboratory analysis, while geologists leverage downhole imaging tools to verify caprock structural/tectonic integrity. Geomechanics engineers define safe injection pressure via mechanical earth models (MEMs) built on advanced acoustic logs calibrated by core geomechanics, wellbore failure observations, and in-situ stress tests. The impact of rock-brine-CO2 interactions is studied via custom SCAL experiments and/or pore-scale digital rock simulations that rigorously represent chemical and thermal processes. Wireline formation tester samples provide representative formation brine as feedstock for SCAL. Water samples also enable operators to prove injection within regulatory limits while establishing baselines for future monitoring programs. Examples applied to recent CCS projects in North America are presented. All of the above data need to be integrated into a CCS model predicting the CO2 plume behavior across the area of interest and within multiple horizons.

Detection of formation and dissociation of CO2 hydrates in fine-sands through acoustic waves

Natural gas hydrates in oceanic and permafrost sediments are considered a future energy resource. Injecting CO2 into these sediments is believed to aid in the safe and efficient extraction of CH4 and the geological sequestration of carbon. The effectiveness of this strategy can be ensured by assessing the formation of CO2 hydrates in the pores of these sediments at in-situ stress conditions by investigating their elastic properties (viz., compressional and shear wave velocities, elastic modulus, and shear modulus) by resorting to a suitable method such as seismic and/or acoustic logging. However, conducting such a study in field is seldom opted due to the requirement of expensive paraphernalia such as seismic equipment, pressure coring tools, logging while drilling tools, modular dynamic tester, etc. With this in view, efforts were made to synthesize CO2 hydrates in partially saturated fine-sands under triaxial conditions by employing the excess gas method. Ultrasonic wave transducers were employed in tandem to measure the compressional (VP) and shear wave (VS) velocities of the sample, both under formation and dissociation processes. These results have been utilized to propose hydrate formation and dissociation models which encompass three phases each. The rapid reduction in VP and VS observed during thermal dissociation indicates loss of cementation between the sand grains and hydrates, and commencement of hydrate dissociation at their interface. It has been demonstrated that the induction time for CO2 hydrate formation increases, and the water-to-hydrate conversion ratio decreases, with an increase in the initial volumetric water content. Further, the results generated in the present study were compared vis-à-vis those available in the literature for CH4 and CO2 hydrate-bearing sediments (HBS) and it has been found that both VP and VS of the CO2- and CH4- HBS are comparable for a given hydrate saturation.

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.

Machine learning-based evaluation of eccentricity and acoustic impedance in oil well using VDL data

Logging procedure and interpretation are complicated mainly because of the large amount of different data inherited. One way to facilitate the problem is to use fewer tools but keep the same amount of information which can be feasible with the assistance of machine learning models. In this paper, Variable Density Logging data was used to evaluate the eccentricity and the acoustic impedance average of the medium behind the casing of oil wells. Different shallow and deep learning architectures were trained, and a Principal Components Analysis feature extraction approach was investigated. Actual data from acoustic logging tools acquired in different wells were adopted. A supervised learning approach was explored, and the K-Nearest Neighbors model obtained the best results when accounting for the cost-benefit and complexity of the models. State-of-the-art hyperparameter optimization techniques were used to tune the non-trainable parameters of the shallow learning algorithms evaluated. The proposed procedure was able to achieve mean balanced adjacency accuracy results greater than 80% for the free pipe pass and repeated pass of a similar well and results greater than 70% for the main pass of a distinguished oil well; which is a more challenging scenario.

Evaluation of the current in situ stress in the middle Permian Maokou Formation in the Longnüsi area of the central Sichuan Basin, China

Abstract The middle Permian Maokou Formation in the Longnüsi area in the central Sichuan Basin is currently a key formation for exploration and development. The evaluation of the current in situ stress in this area is of great significance for fracture prediction, well pattern deployment, drilling and construction, and fracturing stimulation. This study clarifies the current direction and magnitude distributions of the in situ stress by evaluating the Maokou Formation in the Longnüsi area using finite-element numerical simulation, acoustic emission experiments, and logging data (including data from imaging logging, array acoustic logging, conventional logging, and cross-dipole acoustic logging). Specifically, the current maximum horizontal stress of the Maokou Formation in the Longnüsi area is mainly in the NW‒SE direction, and the stress direction is greatly affected by the local fault zone. The current minimum horizontal stress magnitude of the Maokou Formation obtained by acoustic emission experiments is between 96.29 and 114.36 MPa, the current maximum horizontal stress magnitude is between 126.01 and 145.10 MPa, and the current horizontal stress difference is between 25.59 and 32.58 MPa. The current minimum and maximum horizontal stress magnitudes both decrease from north to south. The current horizontal stress parameters calculated by Huang’s model are not significantly different from those experimentally measured: there is a difference of less than 8% in the current minimum horizontal stress magnitude, a difference of less than 9% in the maximum horizontal stress magnitude, and a difference of less than 15% in the current horizontal stress difference. Therefore, Huang’s model has good applicability in terms of calculating the current horizontal stresses in the Longnüsi area. The current horizontal stress parameters, which are numerically simulated with the finite-element method, are also not much different from those experimentally measured: there is a difference of less than 11% in the current minimum horizontal stress magnitude, a difference of less than 10% in the maximum horizontal stress magnitude, and a difference of less than 20% in the current horizontal stress difference. The numerically simulated current horizontal stress also decreases from north to south. Therefore, the simulated results are highly accurate. This study clarifies the directions and magnitudes of the current in situ stress state of the Maokou Formation in the Longnüsi area of the central Sichuan Basin and provides a basis for the formulation of exploration and development plans for the Maokou Formation reservoir in the study area.

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.

Integrating seismic attributes, post-stack seismic inversion, and static modeling for assessing the Pliocene-Pleistocene hydrocarbon potentialities, Offshore North Sinai Field, Egypt

Optimal gas production from marginal fields vulnerable to tidal energetic effects is exceedingly challenging. Conventional seismic analysis is insufficient for a significant understanding of the geomorphological description, volumetric calculations, and microfacies analysis of the anticipated heterogeneous gas-bearing sands. New insights into the offshore North Sinai (ONS) Field based on quantitative and geostatistical interpretations integrated with physical and geometrical attribute analyses are boosted for reconstructing diverse-scale reservoir internal architecture.Deterministic band-limited inversion using acoustic impedance logs contributed to the initial knowledge of facies migration during tidal-dominated delta progradation and revealed four 2nd-order sequence boundaries dependent on P-velocity variations with depth as a function of rock elasticity, low impedance gas content of 2500–3000 kPa s/m and overburden compressional factors. Gradual boundaries confining reservoir bases due to spatiotemporal facies discrepancies triggered the implementation of stochastic algorism as a model norm for tracing thin-bedded and internal heterogeneous reservoirs with no predictable gas-water contacts across subtle dismantled areas. Stochastic inversion with intensive layering below −1 ms along with static property reservoir modeling allowed the evaluation of both fine and coarse-scale reservoirs discovered as extensions of injector-controlled stacked mouth bars, fan-lobes, and distributary channels crosscutting tidal mudflats of delta top facies associations within a system of spatially migrated clinoforms to the north. Stream avulsion-induced and accommodation space inconsistencies across syn-sedimentary faults caused the shifting of the Late Plio-Pleistocene clinoform depocenter northward and deltaic facies variation inside each individual sand system, not eustatic sea level fall.After calculating the biogenic gas reserves occupying each reservoir, the present strategy offers an additional seven broad gas prospects within four dismantled blocks, raising the ONS initial gas in place to 967.95 Bcf. In addition, it depicts five gas shoals emplaced at shallow depths, posing considerable drilling hazards. This paper workflow is the first of its kind in the Neogene section of the OSN Basin.

Overview of seismic stratigraphic referencing methods in conditions of limited or substandard set of initial well data

Background: Well tying is an integral part of seismic interpretation both at the stage of structural and dynamic analysis. Linking seismic and well data is a critical process that allows you to link geological objects identified from seismic data with actual data obtained from wells, because. it is based on the relationship between the lithological composition of the deposits that make up the section under study and the characteristics of the reflections. Seismic data are in most cases presented on a time scale, while well data are on a depth scale, and in order to calibrate them, it is necessary to select a velocity law and establish a time-depth relationship.
 Aim: This article aims to deepen the understanding of the importance of quality control of initial well data to improve the result of seismic stratigraphic tying and increase the efficiency of geological exploration, as well as to propose methods to improve the quality and efficiency of tying..
 Materials and methods: Methods for correcting the readings of acoustic logging are described, various algorithms for tying seismic and well data are given.
 Results: The paper considers the reasons for the distortion of the values of the initial well data, examples of data quality analysis, suggests options for how to conduct a seismic stratigraphic tie in the conditions of a limited or initially substandard set of initial data and obtain reliable depth-velocity dependences for wells.
 Conclusion: Conducting quality control and, if necessary, correcting the original well data is a critical step to improve the accuracy of the final results of seismic interpretation. It is important to note that even with limited and low quality input data, it is possible to obtain depth-time dependencies. However, the results of the work show that editing and amending the original logs significantly improve the quality and detail of the seismic stratigraphic tie.

Estimation of multistage hydraulic fracturing parameters using 4D simulation

At the present stage, most oil and gas condensate fields in the southern part of the East Siberian oil and gas province are characterized by an increasing proportion of difficult oil reserves in tight reservoirs. Multistage hydraulic fracturing (MHF) is proposed for the offshore Challenger Sea field (Southeast Dome). The implementation of this technique at a shelf will be a source of additional risks. For example, the properties of the RR-2 overlying seal have not been unambiguously assessed, and there are a number of geological uncertainties, such as the tectonic regime. However, there are a number of arguments in favor of MHF: heterogeneity of the reservoir; low permeability; low water cut of the field; sufficient thickness of the pay zone; and the overlying seal. One more positive factor is that sand ingress is not observed in the process of oil production. The selection of a principal well completion scheme on the eastern side of the RR-7 formation is aimed at effectively recovering the remaining reserves. The objectives of the study performed are: to create a geological and hydrodynamic model of the Challenger Sea (Southeast Dome); develop 1D and 3D geomechanical models; evaluate oil production forecasts based on fundamentally different well completion schemes; and determine the optimum parameters for multistage hydraulic fracturing. The research methods included: petrophysical methods; logging methods; core studies; drilling reports and formation testing data; and 3D, 4D geomechanical simulation. Other geophysical methods included acoustic logging, density logging, and gamma-ray logging. After building a geomechanical model of the reservoir at the beginning of drilling, a hydrodynamic calculation was performed. This established the reservoir pressures and saturations at certain points in time. The results made it possible for the principal stress directions, the values of effective and principal stresses, and the values of elastic strains to be determined. In order to assess MGF process efficiency, production forecasts were made using a hydrodynamic model for an exploration well with conventional completion (perforated liner) and with five-stage MGF. In the first case, the accumulated production was 144 kt over 15 years, and in the second case, 125 kt over 17 years. The difference in cumulative production is due to different initial well flow rates, as well as the rate of oil withdrawal during the first few years of development. Thereafter, the production and daily flow rate curves showed similar behavior. In order to select the most effective option, an economic analysis of the efficiency was performed.

Unreliable determination of in situ stress orientation by cross multipole array acoustic logging in fractured shale reservoirs

AbstractWith the development of shale gas exploration in China, the use of conventional logging tools has been introduced, and cross multipole array acoustic logging tools have gradually been used to determine the stress orientation in shale. The direction of fast shear waves (FSWs) is generally parallel to the horizontal maximum principal compression stress (SHmax). However, the azimuth of FSWs is found to be parallel to the main strike (but not to the SHmax) direction of structural fractures in shale reservoirs. Outcrop and image logging data indicate that the natural fractures in this area strike NE‒SW. If the shear wave anisotropy is caused by only the stress around the borehole and the FSWs are known to be NE‒SW, SHmaxshould be parallel to NE‒SW; however, according to statistics of land movement in adjacent areas, anelastic strain recovery, earthquake focal mechanism, borehole breakouts, hydraulic fracturing data, deviated well data, and drilling‐induced fracture data in local regions, SHmaxis oriented in the NW‒SE direction, and the directions of FSWs are generally parallel to the structural fracture direction. This contradiction indicates that the development of structural fractures may affect the orientation of FSWs. Therefore, it is not reliable to use XMAC (Cross‐Multipole Array Acoustilog) logging only to determine the direction of in situ stress in fractured shale reservoirs. In addition, the direction of the FSWs in the middle of thick mudstones is NW‒SE, which may represent accurate information about the in situ stress direction.

Eocene Statistics of the Semi-Deep-Deep Lake-Level Evolution of the Jijuntun Formation in the Fushun Basin, Northeast China.

The fluctuation of lake levels in semi-deep and deep lake environments has long been a central topic in the study of ancient lake evolution. This phenomenon has a significant impact on the enrichment of organic matter and the overall ecosystem. The study of lake-level changes in deep lake environments is hindered by the scarcity of records in continental strata. To address this issue, we conducted a study on the Eocene Jijuntun Formation in Fushun Basin, specifically focusing on the LFD-1 well. Our study involved finely sampling the extremely thick oil shale (about 80 m), which was deposited in the semi-deep to deep lake environment of the Jijuntun Formation. The TOC was predicted by multiple methods, and the lake level study was restored by combining logging INPEFA and Dynamic noise after orbital tuning (DYNOT) techniques. The oil shale of the target layer is type I kerogen, and the source of organic matter is basically the same. The γ ray (GR), resistivity (RT), acoustic (AC), and density (DEN) logging curves are in the normal distribution, indicating that the logging data are better. The accuracy of TOC simulated by improved Δlog R, SVR, and XGBoost models is affected by the number of sample sets. The improved Δlog R model is most affected by the change of sample size, followed by the SVR model, and the XGBoost model is the most stable. In addition, compared with the prediction accuracy of TOC by improved Δlog R, SVR, and XGBoost models, it is shown that the improved Δlog R method has limitations in the prediction of TOC in oil shale. The SVR model is more suitable for the prediction of oil shale resources with small sample size, and the XGBoost model is applicable when the sample size is relatively large. According to the DYNOT analysis of logging INPEFA and TOC, the lake level changes frequently during the deposition of ultra-thick oil shale, and the lake level has experienced five stages of rising-stabilizing-frequent fluctuation-stabilizing- decreasing. The research results provide a theoretical basis for revealing the plane change of stable deep lake lakes and provide a basis for the study of lake levels in faulted lake basins in Paleogene Northeast Asia.

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.

Borehole acoustic full-waveform inversion

Full-waveform inversion (FWI) is a technique that has the potential for building high-resolution elastic velocity models. We apply this technique to wireline monopole acoustic logging data to image the near-wellbore formation velocity structures, which can be used in a fluid intrusion evaluation. Our FWI workflow is established in cylindrical coordinates instead of Cartesian coordinates to adapt to the borehole geometry. Assuming that the acoustic logging tool is centralized and formation properties are azimuthally invariant, we can simplify the problem to two dimensions in the inversion. Synthetic tests find that the high-resolution P- and S-wave velocity model around the borehole could be successfully inverted using FWI once a reasonable starting velocity model is given. However, borehole FWI differs from seismic FWI in that strong borehole-guided waves exist near the borehole wall due to the elastic effects. The borehole-guided waves have little sensitivity to velocity away from the borehole wall. In addition, it is difficult to simultaneously match the waveform of the body waves and guided waves in synthetic data and field data. Therefore, for field data applications, we propose to obtain the formation velocity structures around the borehole by FWI using only the first arrived P waves. The field data tests find that the proposed method is applicable for inverting the P-wave velocity structure around the borehole with a similar resolution compared with ray-tracing tomography. FWI may achieve higher resolution in field applications in the future with better assumptions made in the forward modeling and using more information in the field data.

The petrophysical characteristics of Macabu formation (Aptian, Campos Basin) continental carbonates: Core and special well logs (UBI and NMR) analysis integration

To interpret carbonate heterogeneities and understand the correlation between these rock features and their petrophysical parameters, an integrated dataset formed by thin section, core sample description, image log, and nuclear magnetic resonance analysis were used. This interactive approach enables the comparison of depositional and acoustic image log facies and their permo-porous behaviors. The advantage of the proposed integration lies in the ability to expand the interpretation of depositional facies to the entire imaged well interval. The results highlight the occurrence of six depositional facies, four ultrasonic image log facies, and two intervals with different permo-porous behavior present at Macabu Formation (Aptian, Campos Basin). Based on core sample and thin section analysis, three in situ carbonates were described: F1 – shrubstones, F2 - spherulitic grainstone-wackstone and hybrid forms, F3 - stevensitic claystone with spherulites, and others like transported facies: F4 - intraclastic grainstone-rudstone, F5 - laminated Detrital packstone-grainstone, and F6 - laminated dolomudstone. According to ultrasonic signal amplitude and structural patterns, such as descriptive criteria, the ultrasonic image log facies characterization resulted in: UIF1 - bright-heterogeneous, UIF2 - dark laminated, UIF3 - bright massive, and UIF4 - dark dendritic. The NMR well log analysis helped us divide the formation into two main zones, the first with good reservoir behavior and the other with poor behavior. This approach resulted in the correlation between facies and reservoir quality, indicating those with better fluid flow potential, in which facies F1, F2, F4, and UIF4 showed the best results. Finally, the integration between the different facies analysis demonstrated that several lithofacies may have the same appearance in the acoustic image, leading them to integrate the same ultrasonic image facie, this occurs due to their similarity in roughness.

Wavefields in a cased hole surrounded by a formation with transverse isotropy caused by axial stresses

Understanding acoustic field characteristics in the cased hole accompanied by stress is important for acoustical logging. This study investigates the behavior of multipole wavefields of a cased hole with a perfect combination of casing, cement, and formation under the formation borehole-axial stress. The formation can be equivalent to a transversely isotropic medium when uniaxial stress acts on isotropic formation based on nonlinear elasticity theory. The effect of axial stress in the formation on multipole wavefields in cased hole is investigated. Numerical results show that the phase slowness of Stoneley, flexural, and screw waves decreases with an increase in axial compressive stress. With the increase of uniaxial stresses, the arrival times of the full waveforms decrease, and the amplitudes of the full waveforms increase. These characteristics are similar to that of the open hole. For cased holes in soft formations, the excitation curves of flexural and screw waves shift to low frequencies as the stress increases; and the dispersion curves corresponding to excitation curves also exhibit a similar shift, while open holes do not have this pattern. The wavefields excited by a dipole source and quadrupole source in the cased hole in soft formation have more prominent changes in the frequency spectra, and the changes in the integral amplitudes of the full waveforms are larger than those of the open hole. We found that responses of wavefields excited by dipole and quadrupole sources in cased holes in soft formation to the axial stress are more sensitive than those in the open hole.

Experimental study on the receiving piezoelectric vibrator of azimuthal acoustic logging

Abstract The performance of the azimuthal receiving piezoelectric vibrator has a major impact on the azimuthal acoustic logging tool. The evaluation of the performance and selective preference of piezoelectric vibrators through experimental methods will help improve the measurement accuracy of the tool. A heating tester was developed to test the static capacitance, resonant frequency, admittance and receiving amplitude of the vibrator at different temperatures. Far-field underwater acoustic tests were used to analyse the peak-to-peak amplitude, sensitivity and −3 dB angle of the vibrator and to determine the amplitude correction coefficients of each vibrator. The horizontal directivity of the azimuthally receiving phased subarray was also tested. Compared with the values at room temperature, the resonant frequency of the piezoelectric vibrator decreases by 9.20%, the static capacitance increases by 21.33% and the amplitude increases by 5.29% at a high temperature of 155°C. The underwater acoustic test showed that the main lobe of the receiving directional characteristic of the vibrator is symmetrical along the 0° main maximum direction, the −3 dB angle of the main lobe is 115°–142° and the average sensitivity level is −209.38 dB. The −3 dB angle of the receiving subarray is 66° and 60° without and with phase delay, respectively, and the energy of the received waveform is significantly increased. The piezoelectric vibrator can function stably at high temperatures, and its performance can be recovered after heating. It has good azimuthal resolution and high sensitivity, but the amplitude response has some discretion which needs to be corrected.

In Situ Stress Determination Based on Acoustic Image Logs and Borehole Measurements in the In-Adaoui and Bourarhat Hydrocarbon Fields, Eastern Algeria

The study of in situ stress from image logs is a key factor for understanding regional stresses and the exploitation of hydrocarbon resources. This work presents a comprehensive geomechanical analysis of two eastern Algerian hydrocarbon fields to infer the magnitudes of principal stress components and stress field orientation. Acoustic image logs and borehole measurements were used in this research to aid our understanding of regional stress and field development. The studied In-Adaoui and Bourarhat fields encompass a combined thickness of 3050 m of Paleozoic and Mesozoic stratigraphy, with the primary reservoir facies in the Ordovician interval. The Ordovician sandstone reservoir interval indicates an average Poisson’s ratio (v) of 0.3, 100–150 MPa UCS, and 27–52 GPa Young’s modulus (E). Direct formation pressure measurements indicate that the sandstone reservoir is in a hydrostatic pore pressure regime. Density-derived vertical stress had a 1.1 PSI/feet gradient. Minimum horizontal stress modeled from both Poisson’s ratio and an effective stress ratio-based approach yielded an average 0.82 PSI/feet gradient, as validated with the leak-off test data. Drilling-induced tensile fractures (DITF) and compressive failures, i.e., breakouts (BO), were identified from acoustic image logs. On the basis of the DITF criterion, the maximum horizontal stress gradient was found to be 1.57–1.71 PSI/feet, while the BO width-derived gradient was 1.27–1.37 PSI/feet. Relative stress magnitudes indicate a strike-slip stress regime. A mean SHMax orientation of N130°E (NW-SE) was interpreted from the wellbore failures, classified as B-quality stress indicators following the World Stress Map (WSM) ranking scheme. The inferred stress magnitude and orientation were in agreement with the regional trend of the western Mediterranean region and provide a basis for field development and hydraulic fracturing in the low-permeable reservoir. On the basis of the geomechanical assessments, drilling and reservoir development strategies are discussed, and optimization opportunities are identified.