Acoustic Logging Research Articles

Revealing Hidden Information: High-Resolution Logging-While-Drilling Slowness Measurements and Imaging Using Advanced Dual Ultrasonic Technology

A new logging-while-drilling (LWD) acoustic tool has been developed with novel ultrasonic pitch-catch and pulse-echo technologies. The tool enables both high-resolution slowness and reflectivity images, which cannot be addressed with conventional acoustic logging. Measuring formation elastic-wave properties in complex, finely layered formations is routinely attempted with sonic tools that measure slowness over a receiver array with a length of 2 ft or more depending upon the tool design. These apertures lead to processing results with similar vertical resolutions, obscuring the true slowness of any layering occurring at a finer scale. If any of these layers present significantly different elastic-wave properties than the surrounding rock, then they can play a major role in both wellbore stability and hydraulic fracturing but can be absent from geomechanical models built on routine sonic measurements. Conventional sonic tools operate in the 0.1- to 20-kHz frequency range and can deliver slowness information with approximately 1 ft or more depth of investigation. This is sufficient to investigate the far-field slowness values but makes it very challenging to evaluate the near-wellbore region where tectonic stress redistribution causes pronounced azimuthal slowness variation. This stress-induced slowness variation is important because it is also a key driver of wellbore geomechanics. Moreover, in the presence of highly laminated formations, there can be a significant azimuthal variation of slowness due to layering that is often beyond the resolution of conventional sonic tools due to their operating frequency. Finally, in horizontal wells, multiple layer slownesses are being measured simultaneously because of the depth of investigation of conventional sonic tools. This can cause significant interpretational challenges. To address these challenges, an entirely new design approach was needed. The novel pitch-catch technology operates over a wide frequency range centered at 250 kHz and contains an array of receivers having a 2-in. receiver aperture. The use of dual ultrasonic technology allows the measurement of high-resolution slowness data azimuthally as well as reflectivity and caliper images. The new LWD tool was run in both vertical and horizontal wells and directly compared with both wireline sonic and imaging tools. The inch-scale slownesses obtained show characteristic features that clearly correlate to the formation lithology and structure indicated by the images. These features are completely absent from the conventional sonic data due to its comparatively lower vertical resolution. Slowness images from the tool reflect the formation elastic-wave properties at a fine scale and show dips and lithological variations that are complementary to the data from the pulse-echo images. The physics of the measurement are discussed, along with its ability to measure near-wellbore slowness, elastic-wave properties, and stress variations. Additionally, the effect of the stress-induced, near-wellbore features seen in the slowness images and the pulse-echo images is discussed with the wireline dipole shear anisotropy processing.

The effects of tool eccentricity on individual P and S head waves in monopole acoustic LWD

Abstract Velocities of P and S waves are main goals of downhole acoustic logging. In this work, we study the effects of an off-center acoustic tool on formation P and S head waves in monopole logging while drilling (LWD), which will be helpful for accurate interpretation of recorded logs. We first develop an analytic method to solve the wavefields of this asymmetric LWD model. Then using a branch-cut integration technique, we evaluate the contributions of branch points associated with P and S waves, and further investigate the effects of tool eccentricity on their characteristics of excitation, attenuation and waveforms. The analyses reveal that the variation of the excitation and attenuation of both P and S head waves with eccentricity depends on frequencies and receiver azimuths strongly. Besides, new resonance peaks appear in excitation spectra due to influences of poles of multipole modes near branch points when the monopole tool is off-center. According to semblance results of individual compressional and shear waveforms, extracted velocities are not affected by tool eccentricity in both fast and slow formations. In fast formations, spectra analyses indicate that S-wave excitation is more sensitive to tool eccentricity than P-wave. Moreover, resonance peaks in P-wave excitation spectra increase with the increasing eccentricity in all directions. In slow formations, off-center tools almost have no influence on both P and S waves at low frequencies, which suggests that the effects of tool eccentricity can be reduced by adjusting the source's operating frequency.

Characterizing flow in the first hundred-meter depth of a fractured aquifer using hybrid seismic methods, acoustic logging, and flow-log measurements

Understanding subsurface flow, especially in fractured rocks only housing water through a few preferential pathways, is still challenging. The point is mainly associated with the poor accessibility of the subsurface and the lack of accurate representations for both heterogeneity and spatial distribution of water bearing bodies. This notwithstanding, highly-resolved geophysical investigations bring new images of the subsurface. This is exemplified over a fractured limestone aquifer at the site scale (for example, that of the radius of influence of an extraction well). On an experimental site, situated in the Cher region (France), two boreholes have been drilled for field experiments. Full Waveform Acoustic Logging (FWAL) and seismic experiments were conducted. Hybrid seismic imaging, which consists in combining refraction and reflection seismic results, has been carried out. Based on a four-step procedure, the processing of refracted and reflected waves provided two sections. After assemblage, these sections produced in a first step an extended time reflectivity section starting from the surface and, in a second step, a section over depth after calibration with Vertical Seismic Profile (VSP) and acoustic data. However, even the Very High Resolution (VHR) seismic methods do not have a sufficient vertical resolution to describe accurately the geological formation. The acoustic sections were processed to separate the different wave fields, to extract the criss-cross events and to build a criss-cross index log. A log of fracturation index, based on both criss-cross index and P-wave velocity measurements, was computed to detect the presence of fractures. After calibration, and under the assumption that the slower the P-wave velocity, the higher the permeability – porosity, a 3D seismic block of reflection can inform on preferential areas where flow should occur. At the scale of an open wellbore, acoustic loggings that measure wave velocities over a short distance within the well also inform on open features crosscut by the well. Finally, flow log measurements confirm the occurrence of flowing horizons that were previously marked by both seismic and acoustic data. Seismic and acoustic data are therefore suited to image contrasted hydraulic properties over fractured subsurface systems usually poorly documented.

A Novel Adaptive Filter for Cooperative Localization Under Time-Varying Delay and Non-Gaussian Noise

In this study, to cope with time-varying delay in acoustic communication under non-Gaussian noise caused by outliers, a novel robust delay filtering algorithm is proposed for cooperative localization of autonomous underwater vehicles (AUVs). First, the modified measurement equations of the nonlinear cooperative location system with time-varying delay are derived, and the delay caused by the information processing and propagation of the underwater acoustic modem is converted into measurement bias. Second, to improve the robustness of the system with outliers caused by abnormal measurement values of underwater acoustic communication and Doppler velocity log (DVL), the statistical similarity measure (SSM) is introduced to construct the cost function. This is accomplished by quantifying the similarity between the state vector and the predicted state vector, similarity between the delay measurement, and modified predicted delay measurement. Finally, the modified measurement noise variance is obtained by maximizing the cost function, and two algorithms for the approximate solution of the nonlinear cost function are presented. The proposed robust delayed algorithm alleviates the impacts of delayed measurement with outliers on localization accuracy. The effectiveness and potential of the proposed filter are verified by lake trials.

Novel Geophone Array Measures Exact Location of Fluid Movement Behind Casing

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 197560, “Enhanced Well Remedial Decisions From Exact Location of Fluid Movement Behind Casing Identification,” by Marcel Croon, SPE, Perry Huber, SPE, and Jacob Wright, SPE, Weatherford, SPE, prepared for the 2019 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 11-14 November. The paper has not been peer reviewed. Historically, acoustic wellbore monitoring is among the primary methods of detecting fluid movement behind the casing. However, analysis of the complex acoustic environment in the wellbore can be challenging. A standard hydrophone noise tool is unable to measure flow directions (vertical and horizontal) and cannot detect low flow, low-pressure sporadic events, or multiple sources. The complete paper discusses a geophone array, including four, three-component geophones deployed by wireline, that provides a solution for this problem by creating a 3D map of the acoustic environment. Introduction Well-integrity issues such as a poor cement bond can lead to undesirable fluid movements behind the casing, which could have negative effects with regard to health, safety, and the environment. A detailed understanding of annular fluid movement is critical in identifying potential source locations for wells with surface-casing vent-flow (SCVF) issues. Applicable scenarios include the following: Planned well-intervention activities for wells with multiple-source vent/flow situations Confirming the presence of flow (gas/oil/water) behind the casing Identifying areas with suspected crossflow between zones Integrity confirmation for gas storage caverns or zones Identifying unwanted flow for well-abandonment activities Well abandonment is bound by stringent regulations in some countries where the testing and repair of SCVF and gas-migration (GM) issues before final abandonment are required. An ex-ample is western Canada, where surface casing vents must be tested through a bubble test at minimum. This involves connecting a small container with 2.54 cm of water using fittings to the surface casing vent and monitoring for gas bubbles for 10 minutes. Often the flow rates are very low (less than 35 cu ft/d) with minimal pressure buildup. If bubbles are detected during the monitoring period, the well is deemed to have a positive vent flow. Consequently, the source of the vent flow must be identified by a survey as approved by the energy regulator, which could be conducted by means of acoustic logs, temperature logs, or gas- isotope analysis. Additionally, GM measurements must be conducted before final abandonment in specific areas within western Canada but are recommended for all wells. The complete paper details this process.

Domain Adaptation in Intelligent Ultrasonic Logging Tool: From Microseismic to Pulse-Echo

With high temporal resolution demand under noise contamination, the travel time estimation for the pulse-echo is noticed in the acoustic logging instrument design. To this end, an intelligent ultrasonic logging system is built to collect borehole information, and a framework with domain adaptation theory is proposed. The modified maximum mean discrepancy minimization combined with spatial pyramid pooling is constructed on different deep neural networks, where the transfer from the microseismic P-wave picking to the estimation of echo travel time is achieved. Versus the conventional travel time extraction algorithms, the proposed scheme improves the picking accuracy to 83.55% of the 10-dB signal-to-noise ratio. Experiments over the ultrasonic logging tool demonstrate the feasibility of the confusion domain, the effectiveness of travel time estimation, and the versatility of algorithm application.

Optimization for Emission Frequency of Oceanic Impulse Sound Source Logging

Gao, X.; Zhu, J.; Fu, H., and Hao, X., 2020. Optimization for emission frequency of oceanic impulse sound source logging. In: Liu, X. and Zhao, L. (eds.), Today's Modern Coastal Society: Technical and Sociological Aspects of Coastal Research. Journal of Coastal Research, Special Issue No. 111, pp. 124–129. Coconut Creek (Florida), ISSN 0749-0208.In the exploration and development of offshore oilfields, the requirements for the detection distance and accuracy of underground geological structures are increasing day by day. When the traditional acoustic logging technology is used in oceanic logging, the formation structure is complex, and the reflected signal is very weak, which makes it difficult to extract the reflected signal and accurately identify the reflector orientation. The plasma impulse sound source is used as the emission source of the technology of ocean exploration while drilling, which has the characteristics of high power, wide frequency band, energy accumulation, and controllable and repeatable excitation, which can not only enhance the detection distance of traditional acoustic logging, but also detect the geological structure, fracture types, and strike around the well. In this article, sound depth detection technology is applied to study the selection of the dominant emission frequency band of the stimulated sound source based on the establishment of the geological interface depth detection model with different borehole diameter parameters. The results show that: when the well diameter is 0.1524, 0.2159, 0.2445, and 0.3112 m, 5, 3.8–4.2, 3.5–4, and 3 kHz are selected as the dominant excitation frequency bands, and the detection distance can reach tens of meters or even hundreds of meters. This research provides a reference for the subsequent application of impulse source in offshore logging.

Experimental studies on the mechanism of seismoelectric logging while drilling with multipole source

SUMMARYWhen a seismic wave propagates in a fluid-saturated porous medium, a relative movement forms between the solid and the fluid and induces an electric current due to the electronic double layer. As a result, two kinds of seismoelectric coupling responses are generated in this procedure: the localized electric/magnetic field and interfacial electromagnetic wavefield. One important potential application of these two seismoelectric conversions is used for measuring formation P and S waves in well logging. Considering that the strong collar wave seriously affects the velocity measurements of formation P and S waves in current acoustic logging while drilling (LWD), the seismoelectric LWD method, which combines seismoelectric conversion and acoustic LWD technique, was suggested to be a novel method in oil and gas exploration. The collar wave cannot induce any seismoelectric signal on the metal collar since there is no double layer formed on a metal surface. In this paper, acoustic and seismoelectric LWD measurements are conducted in the laboratory. We build a scaled multipole acoustic LWD tool to conduct acoustic measurements in a water tank and a sandstone borehole model. We also build a multipole seismoelectric LWD tool and record the seismoelectric signals induced with the same acoustic source. Then, we compare the recorded acoustic and seismoelectric signals by using the experimental data. The result indicates that the apparent velocities of seismoelectric signals are equal to the formation P- and S-wave velocities and the collar waves do not induce any visible electric signal in the full waveforms. We further analyse the mechanism of seismoelectric LWD by a quantitative comparison of the amplitudes between the inner collar wave and outer collar wave. The results show that the amplitude of outer collar wave decreases significantly when it radiates out of the tool, so that the seismoelectric signals induced by collar waves are too weak to be distinguished in the full waveforms of seismoelectric LWD measurements. Thus, the formation P- and S-wave velocities are detected accurately from the recorded seismoelectric LWD data. These results verify the feasibility of the seismoelectric LWD method for measuring acoustic velocities of the borehole formation.

Research on the Estimate of Gas Hydrate Saturation Based on LSTM Recurrent Neural Network

Gas hydrate saturation is an important index for evaluating gas hydrate reservoirs, and well logs are an effective method for estimating gas hydrate saturation. To use well logs better to estimate gas hydrate saturation, and to establish the deep internal connections and laws of the data, we propose a method of using deep learning technology to estimate gas hydrate saturation from well logs. Considering that well logs have sequential characteristics, we used the long short-term memory (LSTM) recurrent neural network to predict the gas hydrate saturation from the well logs of two sites in the Shenhu area, South China Sea. By constructing an LSTM recurrent layer and two fully connected layers at one site, we used resistivity and acoustic velocity logs that were sensitive to gas hydrate as input. We used the gas hydrate saturation calculated by the chloride concentration of the pore water as output to train the LSTM network. We achieved a good training result. Applying the trained LSTM recurrent neural network to another site in the same area achieved good prediction of gas hydrate saturation, showing the unique advantages of deep learning technology in gas hydrate saturation estimation.

Deep learning assisted well log inversion for fracture identification

ABSTRACTManual fracture identification methods based on cores and image logging pseudo‐pictures are limited by the expense and the amount of data. In this paper, we propose an integrated workflow, which takes the fracture identification as an end‐to‐end project, to combine the boundary detection and the deep learning classification to recognize fractured zones with accurate locations and reasonable thickness. We first apply the discrete wavelet transform algorithm and a boundary detection method named changing point detection to enhance the fracture sensibility of acoustic logs and segment the whole logging interval into non‐overlapping subsections by estimating boundaries. The deep neural network based auto‐encoders and the convolutional neural network classifier are then implemented to extract the hidden information from logs and categorize the subsections as the fractured or non‐fractured zones. To validate the feasibility of this workflow, we apply it to the logging data from a real well. Compare with the benchmarks provided by the support vector machine , random forest and Adaboost model, the one‐dimensional well profile predicted by the proposed changing point detection‐deep learning classifier is more consistent with the manual identification result.

ROCK QUALITY GROUPING IN SANDSTONE FORMATION USING A CRITICAL POROSITY APPROACH AT FORMATION PRESSURE CONDITIONS

The methods of grouping reservoir rock types based on the physical properties of rocks that have been studied by previous researchers still have a relatively large value of uncertainty. This uncertainty arises in conditions where rock type grouping is carried out in wells that do not have core sample data. Where we know that in the oil and gas field, not all wells in the field are subjected to rock sampling, either routine core analysis or special core analysis. From these problems, the authors feel the need to carry out this study to create a method for classifying reservoir rock types based on the physical properties of rocks that can be used in wells that do not have core sample data. The rock types in the wells that do not have core samples will be grouped based on the critical porosity value of the rock obtained from the vp value in the acoustic log data owned by these wells. By making an approach model through the critical porosity of rocks from wells that have core sample data, wells that only have acoustic log data can be grouped by using the critical porosity approach which is generated from the vp value of the acoustic log.

Analytical and numerical modeling of through-tubing acoustic logging

Analytical and numerical modeling of through-tubing acoustic logging

Characteristics of sustained annular pressure and fluid distribution in high pressure and high temperature gas wells considering multiple leakage of tubing string

Tubing leakage is one of the main reasons to cause annular pressure in HPHT gas wells. This kind of annular pressure is called “sustained” because it is long-lasting and can not be eliminated by releasing. Sustained annular pressure is adverse to well barrier reliability, gas well management and repair. Even fire hazard and tubing fracture may happen. Therefore, it is necessary to study related characteristics of sustained annular pressure and fluid distribution, thus reducing potential risk. However, nearly all available research concentrates on cement integrity failure. So this paper develops a model of sustained annular pressure caused by multiple tubing leakage points. The wellbore temperature and pressure are calculated first as the gas leakage power, which considers the impact of pressure and temperature on gas properties. And then, tubing leakage point is classified as gas-leak and gas-liquid-leak. Fluid Leakage rate is calculated. Finally an equation is proposed to calculate the annular fluid distribution and annular pressure. To solve this model, the coupling relationship between leakage rate, sustained annular pressure and fluid distribution is analyzed. And then the well space and leakage time are divided into short segment. Two typical situations are selected as example. One is multiple gas leakage points. The other is combination of gas-leak and gas-liquid-leak points. By the proposed model, characteristics are analyzed, including annular pressure, pressure balance position, annular gas volume and liquid level. Also, sensitivity of factors is evaluated, including leakage point position, size, production rate and bottom pressure. The results indicate that rising process of sustained annular pressure can be divided into three zones (a, b, c). Annular pressure should be released to zone c when acoustic log is applied to locate leakage points. Method based on U-principle is not suitable to locate multiple tubing leakage points. Both pressure and liquid level should be monitored to evaluate tubing integrity. For multiple gas-leak points, position and size of leakage point have impact on maximum annular pressure and final liquid level. When gas-liquid-leak point exists, sustained annular pressure and liquid level continue changing, so annular fluid distribution and its change law can help to judge the existence of gas-liquid-leak points. Increase of production rate and decrease of bottom pressure can be considered as potential measures to reduce the rising rate and maximum sustained annular pressure under both two situations. This paper can help to diagnose tubing integrity by the characteristics of annular pressure and fluid distribution. Acoustic log is recommended to locate tubing leakage point. Potential measures are discussed to reduce maximum annular pressure. The proposed model can also be applied to calculate maximum annular gas volume and gas leakage rate when parameters of tubing leakage points are known, thus helping to assess potential risk.

Study on identification method of gas-bearing carbonate reservoirs based on joint acoustic-resistivity experiments – an example from the Sichuan Basin of China

Cracks and pores coexist in carbonate reservoirs, and this complex pore structure has a significant impact on acoustic and resistivity logging. Making full use of acoustic and electrical well logging data can help to identify carbonate gas reservoirs. First, this paper studies the controlling effects of different lithologies on the acoustic velocity according to the elastic theory of porous and fractured media based on joint acoustic-resistivity rock physics measurement experiments. At the same time, a theoretical gas-bearing reservoir identification template is established based on acoustic experimental data, and the gas-bearing reservoir identification template is used to qualitatively identify a gas reservoir. Then, the empirical relationship between the acoustic velocity and resistivity is derived based on the Archie equation. It is found that the formation P-wave velocity and resistivity have a power function relationship in the research area. Based on the above research, a quantitative identification template for gas-bearing reservoirs is established. The case study has verified the reliability of the gas-bearing carbonate reservoir identification method based on the above two methods.

Investigation the relationship between nuclear magnetic resonance and acoustic velocity for improving the evaluation of tight gas reservoirs

ABSTRACT Nuclear magnetic resonance (NMR) and acoustic logging play different roles in formation evaluation. Establishing a relationship model between the two is helpful for joint inversion or evaluation of the physical properties of the formation content. The rock pore structure is used as a link to analyze the theoretical relationship between the transverse relaxation time (T2) spectrum of NMR and acoustic velocity. In this paper, the relationship between NMR T2 and shear-wave velocity is determined by using NMR-acoustic velocity joint experiment. Considering that both NMR T2 spectrum and Shear-wave velocity (Vs) are highly related to porosity and pore structure, a formula based on the relationship between Vs and T2gm (NMR T2 geometric mean) was established. It is found that the T2gm is a power function of the shear-wave velocity. The respective coefficients in this formula for different lithologies were derived through petrophysical measurements of core samples (both NMR and ultrasonic experiments). The relationship model displays the same power-law relations as fitting expression from the experimental data.After that, there is a relation-model application for field data further validated effectiveness and reliability by contrasting the Shear-wave velocity determined by NMR logging with the direct measurements. This model establishment also helps to predict the mutual prediction of NMR and acoustic information of rocks. Based on the sensitivity of the vertical and transverse wave ratios of the formation to the gas-bearing properties of the reservoir, a NMR-acoustic velocity joint gas-bearing identification map was established. An extended case study demonstrated that the cross-plot between NMR logging and acoustic logging is also applicable to natural gas-bearing reservoir identification. As an important supplement and perfection of the existing methods, the relation model proposed in this paper offers a new thought for the petrophysical and in-situ field studies.

Numerical simulation of shear wave attenuation in borehole inserted by a horizontal fracture

The amplitude of shear waves is attenuated when passing through horizontal fractures crossing a borehole. In this study, we investigate the amplitude attenuation of shear waves throughout simulation of full-wave acoustic logging with the finite-difference method. As the fracture aperture is very small, it needs to be represented in a very fine gird when carrying out finite-difference simulation. Therefore, the variable-grids finite-difference method is adopted to avoid over-sampling in the non-fracture regions, yielding substantial savings in computational cost. We demonstrate the accuracy of waveform modeling with the variable-grid finite-difference by benchmarking against that obtained with the real-axis integrating method. We investigated the effects of several important parameters including fracture aperture, distance from receiver to fracture, borehole radius and extended distance utilizing that benchmarked variable-grid finite difference code. We determined a good linear relationship between the attenuation coefficient of shear wave amplitude and the fracture aperture. Then, the effects of distance from receiver to fracture, the borehole radius and the extended distance of fracture on shear wave attenuation are also studied. The attenuation coefficient of shear wave becomes smaller with the increasing borehole radius. While, it increases as the distance from receiver to fracture and the extended distance of fracture increase. These effect characteristics are conducive to the use of shear wave to evaluate fractures.

An innovative application of borehole acoustic image and amplitude logs for geotechnical site investigation

Down-hole acoustic televiewer (ATV logging) is a geophysical well-logging device known to be able to reveal underground in-depth image. However, reviewing the current developments in the area of geotechnical engineering, ATV logging data were predominantly used for the purpose of visualizing and calculating the specific structural features in an open borehole, and nothing more. Noticeably, to the majority of engineering applications, it appears that ATV logging played a minor role compared to other geophysical borehole logging tools. Our study does not take it for granted. A systematic study is presented of the significance and consequent applications of ATV logging. The significance of correlation between acoustic amplitude and different geotechnical parameters is discussed and found to be consistent with previous findings. The result tells that ATV logging is actually able to provide timely and ground-truth information during site characterization, if properly used. Additionally, in our study, a simple rock mass evaluation index, Aco-RQD, is proposed, which is applicable to all aspects of geotechnical engineering as long as the continuous ATV logging data are available.

Prediction of gas hydrate saturation using machine learning and optimal set of well-logs

Resistivity and acoustic logs are widely used to estimate gas hydrate saturation in various sedimentary systems using one of the two popular methods ((1) acoustic velocity and (2) electrical resistivity), but the limitations of these two methods are often overlooked, which include (i) well-specific calibration of empirical exponents in the electrical resistivity method, (ii) assumption of known pore morphology for gas hydrates in the acoustic velocity method, and (iii) presence of unknown mineralogy and bulk modulus terms in the acoustic velocity method. NMR-density porosity-derived gas hydrate saturation based on the analysis of the transverse magnetization relaxation time (T2) is considered the most precise method, but acquisition of NMR-based logs is limited at relatively recent drilled sites; additionally, its use in conventional oil and gas reservoirs is not that common due to higher cost and operational deployment limitations associated with acquiring NMR well-logs. This study proposes a new method that predicts gas hydrate saturation (Sh) for any well using porosity, bulk density, and compressional wave (P wave) velocity well-logs with neural network (or stochastic gradient descent regression) without any well-specific calibration and/or other aforementioned shortcomings of the existing methods. The method is developed by examining the underlying dependency between Sh and different combinations of well-logs, chosen from 6 routine logs, with 12 different machine learning (ML) algorithms. The accuracy of the proposed method in predicting Sh is ~ 84%, which is better than the accuracy of seismic and electrical resistivity methods (≤ 75%) per the results reported by three different studies. The robustness of the method in the specific case of permafrost-associated gas hydrates is demonstrated with well-log data from two wells drilled on the Alaska North Slope.

Hydraulic characterization of fault F31 at the Xinchang underground research laboratory site for geological disposal of high-level radioactive waste

As part of the suitability evaluation of the underground research laboratory (URL) candidate sites, the hydrogeological investigation activity was conducted at the Xinchang site in Beishan area from 2016 to 2019. A previous study discovered that the rock masses at the Xinchang URL site, which is surrounded by north–east trend and east–west faults, have low permeability and shortage of water-bearing structures. This article aimed to describe the hydraulic properties of fault F31, which is located on the west boundary of the Xinchang URL site, and evaluate the fault hydraulic characterization methodology for site characterization of the URL and repository sites.The surface hydrology monitoring activity at the Xinchang site showed that the water flow occurring through the gully where fault F31 is located after precipitation is the main recharge resource of groundwater. Accompanied with hydraulic monitoring during the drilling period of inclined boreholes (BSQ04–BSQ06, BS35, and BS36) crossing fault F31, the survey revealed that groundwater was stored in the weathered zones of the middle and north parts along fault 31. Single-hole and interference tests were performed later with the Heavy-Duty Double Packer system. The hydraulic conductivity (K) and storativity (S) were interpreted from the test data using analytical solutions. Diagnostic plots were also used in determining the reliability of results and identifying the flow regime. By considering the interpretation of borehole acoustic television logging as reference, the spatial morphological statistics of the structures are mainly parallel with the fault plane, wherein K is lower than 1 × 10−9 m/s depth to −250 m below the surface. Thus, the strike-dip parameter of the water resistance structures can be roughly confirmed. Six sections were selected for the evaluation of the water-conductive structures; these sections exhibited depths of more than −250 m below the surface and K values of equal to or more than 1 × 10−9 m/s. After the structures with a strike-dip similar to that in the water-resistant sections were removed, the remaining structural planes were used to build a flow model path during the hydraulic test. Compared with flow regime derived from the diagnostic plots, the structural planes fitting the real flow regime were identified as water-conductive structures.The results showed the following: (1) K and S increased from the south to the north in the shallow zone along fault F31, where water-conductive structures were surrounded by water-resistant structures; (2) the section located −250 m below the surface of F31 was water resistant, and no-flow channel existed in the deformation zone along fault F31; and (3) the fault hydraulic characterization methodology combining hydraulic test, hydrological survey, and geophysical logging is suitable for the site characterization of URL and repository.

Section types and oil prospects of the Bazhenov Formation in the Nadym-Ob interfluve

The article presents the results of studying the geological structure of the Bazhenov Formation in the Nadym-Ob interfluve of Western Siberia with the aim of predicting the oil content of this black shale stratum. As a result of interpretation of a wide range of well logging represented by electric, radioactive and acoustic logging, with subsequent matching of these results with paleontological definitions of micro- and macrofauna, the distribution of the Salym, Nizhnevartovsky and Tarkosalinsky types of sections of the Bazhenov Formation was clarified, transitional areas between them were identified. It has been established that the Tarkosalinsky type is more widespread in the western direction than was shown earlier and is also distinguished in the Vengayakhinskaya, Yaraynerskaya and other areas. The Nizhnevartovsky type, on the contrary, has a narrower distribution and stands out directly within the same name arc and to the south by the Variegasky-Tagrinsky megahigh. On the basis of geological, geochemical, geophysical criteria and the results of an inflow test in deep wells, a map of the oil potential prospects of the “classical” sections of the Bazhenov Formation has been compiled. Regional prerequisites (high catagenesis of organic matter, significant modern concentrations of organic carbon, etc.) for the discovery of industrial accumulations of oil in the Bazhenov Formation in the southern regions of the Yamalo-Nenets Autonomous Area are identified. The results of the test for the inflow of the Bazhenov Formation in this area in the 70–90s XX century were analyzed; repeated, interval testing of these deposits using modern methods of stimulation of the inflow is recommended. The necessity of laboratory lithological, petrophysical, geochemical study of the core of the Bazhenov Formation in the southern part of the Yamalo-Nenets Autonomous Area is substantiated with the aim of determining its lithological composition, identifying oil source and oil productive intervals, studying the reservoir structure and the nature of saturation of its void space, developing recommendations for calculating oil reserves and creating technology for its cost-effective production.