Borehole Image Research Articles

CALIBRATION OF A ONE-DIMENSIONAL MECHANICAL EARTH MODELS USING GEOMETRIC APPROXIMATION OF BOREHOLE BREAKOUTS

Link for citation: Konoshonkin D.V., Rukavishnikov V.V., Shadrin A.S., Antonov A.E., Kupriyanova K.A. Calibration of a one-dimensional mechanical earth models using geometric approximation of borehole breakouts. Bulletin of the Tomsk Polytechnic University. Geo Аssets Engineering, 2023, vol. 334, no. 7, рр. 102-110. In Rus. The relevance. Mechanical earth modeling is widely used in various fields of science and technology. In the oil and gas industry, one-dimensional mechanical earth models are used to analyze wellbore stability, to design hydraulic fracturing, to assess the probability of sand production with oil and gas flow, and as input data for three-dimensional mechanical earth models. One-dimensional mechanical earth models are very demanding on the volume and quality of the initial data, for example, when calibrating the model to define the uniaxial compression strength and horizontal stresses, borehole imager data are required, which are usually not recorded in all wells and in whole interval. At the same time, almost every well has caliper data. Therefore, solving the problem of calibration of a one-dimensional mechanical earth model using more accessible caliper data is an urgent problem, the solution of which will allow building calibrated geomechanical models for a larger number of wells. The main aim: to develop an approach for calibration of one-dimensional mechanical earth models based on well caliper data. Methods: geometric approximation, analysis of laboratory studies, as well as analytical methods for calculating the stress-strain state near the well wall. Results. The paper introduces the equations and the algorithm to define the stresses and uniaxial compression strength of rocks using geometric approximation of borehole breakouts according to caliper data in wells.

Regional stratigraphically discordant dolomite mapping based on a dolomitization mechanism on the Arabian carbonate shelf

Dolomite mapping, as a first step of carbonate diagenetic modeling, is critical to understanding dolomitization mechanisms and predicting reservoir quality. Stratigraphically discordant dolomite bodies within the Upper Jurassic intervals have long been studied on the Arabian shelf. Our dolomitization mechanisms find that faults/fractures serve as migration conduits and determine dolomite distribution by controlling fluid flow. We establish an innovative approach based on the diagenetic mechanisms and comprehensive characterization of fracture systems at multiple scales to delineate dolomite in the study area. The complex fracture networks in the subsurface are categorized into macroscale, mesoscale, and microscale fractures. Macroscale fractures, i.e., faults, are much greater than the seismic wavelength and are easily observed in seismic sections due to their obvious seismic discontinuities. Mesoscale fractures are slightly greater than or equal to the seismic wavelength and are recognized using seismic attributes. Microscale fractures are significantly smaller than the seismic wavelength and are observed mainly on core samples and thin sections. The dolomite mapping workflow includes three steps: (1) calibrate the seismic response of multiscale fractures with borehole image logs and core interpretations, (2) implement multiscale fracture characterization using multiple seismic attributes, and (3) interpret dolomite bodies based on fracture characterization and log identification. The mapping results show that massive dolomite bodies are heterogeneously developed and distributed mainly in the northeastern part of the study area, with a southward decrease in dolomite content. The application demonstrates that multiscale fracture systems play critical roles in massive dolomitization, and multiscale fracture prediction provides an innovative solution to delineate complex dolomitization systems and the combination of reservoir and seal in diagenetic traps.

Imaging and Characterization of Cement Annulus and Bonding Interfaces in Cased-wells with Fully Connected Neural Network

Well integrity evaluation is of great significance to the production life and productivity of oil and gas wells, as well as the protection of reservoir layers. However, traditional sonic logging technology has been unable to meet the current requirement for evaluating unconventional oil and gas resources. For instance, it falls short when it comes to detecting cementing quality between the cement and formation, as well as achieving a quantitative assessment of annulus. In addition, factors such as mud invasion and wellbore damage have the potential to compromise the integrity of the cement sheath and lead to radial alteration in the medium in the wellbore, which can affect cementing quality. To address these issues, a novel approach based on deep learning is proposed to reconstruct slowness models from wellbore data using a fully connected neural network (FCNN) to evaluate the cementing quality. This interpretation workflow requires forward modeling with the three-dimensional dyadic Green’s function to explore borehole acoustic signals and thus generate training and test datasets. Afterward, the spatial feature maps are learned to reconstruct the slowness models by utilizing all the wellbore data. Additionally, the Real-ESRGAN method is applied to borehole imaging to further improve the above inversion. The inversion results regarding the slowness value, wellbore structure, and interface are more consistent with the true model. Finally, the image pixels are optimized using the thresholding method, resulting in an image of superior quality in terms of sharpness and detail compared to previous results. This research reveals that the FCNN-based method can effectively invert the radial profile of slowness around the wellbore from monopole sonic logging data, and the Real-ESRGAN method can further enhance the imaging results to better approximate the true models. This method has also been applied to realistic borehole scenario models, demonstrating its feasibility.

Quantitative characterisation of fracture connectivity from high-resolution borehole image logs

Fractures are fundamental structural elements in brittle rocks. They have a primary role in transporting geofluids, particularly in basement and carbonate rocks, where they have been the focus of reservoir characterisation and modelling for decades. Fracture networks are often described by statistical analysis of their geometrical attributes such as orientation, length and aperture. However, the spatial arrangement (topology) of fractures/fracture sets and its temporal evolution is equally important to better understand physical attributes of rocks. How much is a fracture network connected? Can this be quantified? Does connectivity change in different locations, if yes what is the reason for it? What is the brittle strain distribution in a fracture network? How much are fracture density and connectivity influenced by mechanical stratigraphy?This paper attempts to address these relevant questions and test quantitative fracture connectivity analysis on high resolution electrical and ultra-sonic borehole image logs in an appraisal well (Total depth, TD: 2195.36 mMD), drilled into the Utsira High basement, in the extensional Norwegian North Sea rift basin. Fracture connectivity and fracture size attributes were investigated in Permian Zechstein carbonates and Ordovician-Silurian granitic basement and separately for defined fracture zones at the interval of 2050–2195 mMD. The average degree of fracture/fault connection is good (D = 2.6) for the sediments and excellent (D = 3.5) for the basement. Accordingly, the block intensity (R22) is 0.4 for sediments and 0.2 for basement, which imply a moderately and strongly disintegrated host rock, respectively. There is a large fracture connectivity variation in different fracture zones, i.e. completely isolated fractures (Zone 1) vs highly interconnected and strongly disintegrated basement (Zones 9–10). The connectivity variation is governed by lithology, mechanical properties (stiffness), strain distribution and presence of weakness zones (faults). The study also highlights some possible applications of fracture connectivity results to reservoir modelling and future follow-up trends in quantitative spatial characterisation of fracture networks.

Experimental study of the impact of CO2 injection on the pore structure of coal: A case study from the Bowen Basin, Australia

This study investigates the impact of carbon dioxide (CO2) on the pore structure of coal during CO2 injection to understand the technical challenges associated with CO2 sequestration in depleted coal seam gas reservoirs. In an integrated approach, Micro-Computed Tomography (micro-CT) scanning, helium porosity and air permeability tests are performed on a coal sample prior to and after CO2 flooding experiments to identify both reversible and irreversible changes in cleat and fracture networks. The results indicate that irreversible changes contribute to a 43% reduction in effective porosity, which can be readily observed in the 3D model of the cleat and fracture networks constructed after CO2 flooding. At lower effective stresses, pore compressibility offsets the matrix swelling effect, resulting in improved permeability, which is beneficial for CO2 injection. Additionally, the analysis of borehole image logs of the study well reveals that most fractures and cleats terminate within coal intervals, with very few fractures extending into adjacent strata that are siltstone and fine sandstone with very low permeability.

Integrated borehole image and rotary sidewall core data to support infrastructure-led appraisal: Capercaillie Field, Central North Sea

Abstract Quad 29 in the Central North Sea is a focus for bp, with a strategy to identify remaining hydrocarbon accumulations to tieback to existing infrastructure. Capercaillie was appraised in 2017 with well 29/04e-5 and sidetrack 29/04e-5z. A gas cap and thin oil rim were intersected within the siliciclastic Paleocene–Eocene Sele Formation. The reservoir sandstones were deposited in a deep marine setting by sediment-gravity flow processes. Within the context of a relatively detailed knowledge of the surrounding area, the reservoir data-operational risk-cost balance was addressed through acquisition of a full wireline logging suite, pressure data, latest-generation microresistivity images and targeted large-volume rotary sidewall cores (RSWCs), with the latter two favoured over whole core. Mature deepwater descriptive schemes were applied at the sidewall core (lithotype), bed (borehole image facies) and bed-stack (depositional package) scales. This hierarchical approach provided robust sedimentological data to underpin higher-order depositional models, which together were used as a framework to (1) constrain the reservoirs’ mineralogical and textural attributes, (2) establish the main controls on rock quality and (3) explain the resulting variations in porosity and permeability. This study demonstrates that the careful integration of data derived from the latest borehole imaging tools and large-volume RSWCs can be a successful means of characterizing reservoirs from a sedimentological, reservoir quality and reservoir architecture perspective in mature basins. Similar approaches to geological reservoir characterization in such settings are likely to be a common cornerstone of cost-effective development during the energy transition.

Emplacement dynamics of a complex thick mafic intrusion revealed by borehole image log facies analyses: Implications for fluid migration in the Parnaíba Basin petroleum system, Brazil

Sill intrusions may play a significant role in the development of petroleum systems in sedimentary basins by promoting maturation of source rocks, influencing fluid migration and acting as reservoirs, traps, and seals for hydrocarbon accumulations. Within this study, we present a novel approach to interpreting geophysical borehole data for a ca. 174 m thick sill from the Parnaíba Basin revealing new insights into both the emplacement mechanisms and impacts of the sill on hydrocarbon migration and trapping. High-resolution ultrasonic and microresistivity borehole image logs were integrated with conventional log data, petrography of eight side wall cores and total gas data recorded during drilling. The petrographic analysis revealed finer crystalline dolerite at the top and bottom layers of the sill while the upper central portion revealed coarser crystalline gabbroic facies, associated with anomalies in gamma ray, neutron porosity, sonic and geochemical log responses. From borehole image logs we interpreted key facies units including massive, fractured, cooling jointed and layered dolerite along with clear evidence for layers containing diverse inclusions interpreted as enclave structures. The integration and correlation of these data with analogue studies allowed the interpretation of gradational cooling with magmatic differentiation, partially melted host rock linked to sulphide production and organic matter maturation, mafic microgranular enclaves related to mingling processes of magma recharge, banding layers related to cumulate zones, and intense cooling fractures in the central sill body. An interval of ca. 44 m at the top of the sill with notably less jointing appears to be responsible for sealing the gas underneath, whilst the fractured central portion of the sill has enabled gas migration evidenced by increased gas signatures. In addition, this work also reveals for the first-time image log evidence for magma mingling and indications of assimilation and anatexis processes within the Parnaíba Basin. This study has important implications for improving the understanding of emplacement mechanisms of thick parallel layer intrusions and their implications for fluid flow in sedimentary basins worldwide.

The value of integrated core–CT–BHI data in characterizing aeolian dune geometries and effects of cross-cutting deformation bands: implications for permeability architecture

Abstract Three-dimensional circumferential CT-scans have transformed how core is described and calibrated with borehole image (BHI) datasets to refine reservoir rock typing and facies description. This paper focuses on the value of circumferential CT-scans in the assessment of plug and bed-scale heterogeneities. It shows how careful re-orientation and calibration with borehole images can help unravel sandbody geometries and orientation, and the potential effects of cross-cutting deformation bands on permeability architecture and sweep efficiency. This is demonstrated using aeolian-dominated core examples, supported with circumferential CT-scans, minipermeability data, conventional logs and BHI data, taken from the Jurassic Norphlet Formation from producing fields in the Gulf of Mexico. The formation overlies Early Jurassic Louann Salt, and syn-depositional halokinesis significantly influenced depositional accommodation space, facies distribution, and preservation potential. Furthermore, deposition during active salt tectonics has resulted in complex deformation band networks within these clean sandstones. CT-scan density contrasts highlight stratification types and deformation bands not always visible on slabbed core. Furthermore, BHI re-orientated CT-scans provide high-resolution dip/azimuth data and aid aeolian bedset bounding surface definition, which is important for determining dune geometry and stacking patterns. Hence, an integrated approach using core, circumferential CT-scan and calibrated BHI has been essential for deciphering the complexity of these deposits.

The Albian – Cenomanian boundary on the southern Tethyan margin: Abu Gharadig Basin, Northern Western Desert, Egypt

Lithostratigraphy and biostratigraphy sometimes provide contradictory interpretations, especially when trying to establish a chrono-lithological boundary between two reservoir formations from different paleoenvironments. One of these contradictions is the definition of the boundary between the Kharita and Bahariya formations and the Albian and Cenomanian Stages in the Abu Gharadig Basin of the Western Desert, Egypt. an integrated approach has been applied to this problem using borehole image logs, palynology, and conventional wireline logs. authors also calibrate the stratigraphic sections with global chronostratigraphic charts, using nannoplankton, planktonic foraminifera, and dinoflagellates to construct a better understanding of this basin. The abundances of pollen, spores, and dinoflagellates in the interpreted reservoir intervals suggest that hydrocarbons trapped in the Albian-Cenomanian reservoirs are composed of type II/III kerogen. The interpreted intervals are subdivided into seven depositional cycles: one in the Kharita Formation, five in the Bahariya Formation, and one in the Abu Roash Formation. The interpreted environments are, from bottom to top: I) fluvio-deltaic distributary channels, II) and III) intertidal to subtidal deposits, IV) offshore bar deposits, V) intertidal to subtidal deposits, VI) shoreface deposits, and VII) shallow marine carbonate deposits. Diagnostic palynomorphs reflect climatic change from arid to humid conditions and also enable changes in salinity to be determined.Much of the published work in the southern Tethyan margin, particularly in the Western Desert, is either related to stratigraphy, palynology, or reservoir evaluation with a focus on a narrow field. Tectonostratigraphic studies and analyses of petroleum system elements have often been very regional in nature. The work presented here is of an intermediate scale linking regional frameworks with local details to provide comprehensive information about the Albian-Cenomanian succession in the Northern Western Desert. The focus has been the Abu Gharadig basin but this integrated approach has implications for other basins in North Africa. A specific example is the similarity of Western Desert and Libyan Sirt basin Albian-Cenomanian key palynomorphs which would facilitate a similar integrated chrono-lithologic intepretation.

Estimating two groups of fracture weaknesses using azimuthal differences in partially incidence-angle-stacked seismic amplitudes

Geophysical and geologic data, e.g., results obtained using rock cores, outcrops, and borehole images, reveal the presence of multiple sets of fractures in rock. To analyze how fracture interactions affect seismic anisotropy and dispersion, we consider an effective model that contains two sets of orthogonal fractures. In seismic amplitude analysis, we focus on the case of subsurface target zones containing a fracture network composed of primary and secondary fracture sets. Focusing on gas-bearing rocks with small fracture densities, we formulate simplified stiffness parameters in terms of two groups of fracture weaknesses, which are related to the primary and secondary fractures, respectively. Using the simplified stiffness parameters, we derive the PP-wave reflection coefficient and the azimuthal elastic impedance (AEI) as functions of the two groups of normal and tangential fracture weaknesses. Based on the derived PP-wave reflection coefficient and AEI, we propose a method and workflow in which the azimuthal variations of the partial incidence-angle-stacked seismic data are used to estimate the AEI and differences in AEI (which we refer to as DEI) is input to a nonlinear inversion for two groups of fracture weaknesses. In the nonlinear inversion, the initial values of fracture weaknesses are obtained based on a two-term approximation of the PP-wave reflection coefficient. First- and second-order derivatives of DEI with respect to fracture weakness parameters are calculated to generate the update in the unknown parameter vector. Synthetic seismic gathers with varying signal-to-noise ratios are used to validate the robustness of the estimation. Applying the inversion method to a real data set, we obtain what we interpret to be reliable fracture weakness estimates that produce azimuthal amplitude differences matching those extracted from real seismic data. The results provide a potential tool for determining if two sets of fractures have developed in a reservoir.

Petrophysical properties of igneous rocks in continental rift basins – a case study from the Mesozoic sequence of the Browse Basin, North West Shelf of Australia

Capturing the ubiquitous distribution and properties of igneous rocks is necessary to succeed in hydrocarbon exploration, field development and underground gas storage when dealing with magma-rich rift margins. In recent years, numerous researches have revealed detailed morphologies of intrusive and extrusive rock units embedded within sedimentary basins owing to advances in 3D seismic data. Outcrop studies have also provided deeper understandings of their occurrence and geometries. However, there has been comparatively little focus on the rock physics of igneous bodies and their relationships to seismic expression. To bridge this gap between the outcrop and the seismic information, we undertook a well-log-based petrophysical study of igneous rocks using subsurface dataset from the Browse Basin located in the Australian North West Shelf. In this contribution, we describe a classification of volcanic facies (e.g. lava flows, volcaniclastics, intrusive sills) based on well log motifs and textures apparent in borehole image data. Statistics of the petrophysical properties of each volcanic facies are also analysed in order to examine their correlation to seismic patterns. Our study implies the importance of detailed characterisation of igneous petrophysical properties of igneous rock units where concentrated within a sedimentary basin, which will help reconcile interpretations of seismic data.

Subsurface fracture characterization in a folded ultra-deep tight-gas sandstone reservoir: A case study from the Keshen gas field, Tarim Basin, China

In the Keshen gas field in the northern Tarim Basin, the main production interval is the ultra-deep (>6000 m), ultra-high pressure Lower Cretaceous Bashijiqike Formation. Understanding the characteristics and role of natural fractures in the reservoir is essential for efficient exploration and development. Data from borehole images (42 vertical wells) and cores (12 vertical wells) enabled a comprehensive characterization of fracture attributes, including mode, orientation, fill, density, and clustering as well as variations with respect to depth and structural position. Using image logs from 20 vertical wells and cores from 12 wells, we find that steeply dipping (>60°), E-W and N–S-striking fractures are the most common across the studied anticline. Fracture density is highest in the forelimb and near faults (2.85 fractures/m), compatible with fracture formation during folding and associated faulting. Fracture clusters 10+ m wide are also preferentially located within these structural zones and are spaced around 5–15 m apart. Fracture orientations, fill, and density varies with depth. We identify a depth transition horizon above and below which fracture characteristics, specially strike, dip, and resistivity change dramatically. We infer those changes are due to fold geometry, rheological differences associated with increasing pressure and temperature at extreme depths. We observe anhydrite-filled fractures located near the top of the formation, whose presence is associated with overlying evaporites. Vertical and lateral variations of fracture characteristics and distribution discovered in this reservoir may aid the interpretation and prediction of the fracture network in other ultra-deep sandstone reservoirs with a comparable setting or well performance.

Automatic facies classification from acoustic image logs using deep neural networks

Borehole image logs greatly facilitate a detailed characterization of rock formations, especially for the highly heterogeneous and anisotropic carbonate rocks. However, interpreting image logs requires massive time and workforce and lacks consistency and repeatability because it relies heavily on a human interpreter’s expertise, experience, and alertness. Thus, we propose to train an end-to-end deep neural network (DNN) for instant and consistent facies classification of carbonate rocks from acoustic image logs and gamma-ray logs. The DNN is modified from the well-known U-Net for image segmentation. The training data are composed of two data sets: (1) manually labeled field data measured by different imaging tools from the geologically complex Brazilian presalt region and (2) noise-free synthetic data. Some short sections of the field data that are challenging for manual labeling due to entangled features and noise or low resolution are left unlabeled for a blind test after training. All labeled data are divided into a training set, a validation set, and a test set to avoid overfitting. We determine that the trained DNN achieves a 77% classification accuracy for the test set and provides reasonable predictions for the challenging unlabeled sets. It is a great achievement given the complexity and variability of the field data. Compared with manual classification, our DNN provides more consistent and higher-resolution predictions in a highly efficient manner and thus dramatically contributes to an automatic image log interpretation.

Removal of Artifacts in Borehole Images Using Machine Learning

In this paper, a supervised machine-learning (ML) method to remove artifacts and noise from borehole images is described. Borehole images may exhibit a variety of issues and artifacts due to reasons such as environmental and thermal noise, imperfect calibration, and current leakage through the tool body. Methods that are currently employed to improve these images are based on traditional signal-processing techniques. Although these methods are capable of removing the artifacts in images and significantly improving image quality, they have some drawbacks as well. These drawbacks include not being entirely suitable for real-time implementation and issues with reproducibility. The alternative method presented here is based on an ML algorithm that is trained using a data set pairing raw data with data processed using a traditional signal-processing-based approach. The resulting ML model is capable of being implemented in near-real time. Furthermore, the application of the algorithm does not require user supervision, increasing the reproducibility of the results.

Ultra-deep carbonate basement reservoirs formed by polyphase fracture-related karstification in the Offshore Bohai Bay Basin, China

The Palaeozoic carbonate basement of the Offshore Bohai Bay Basin (OBBB) presents considerable potential for hydrocarbon exploration. However, the multistage tectonism and complex superimposed palaeo-karstification in the area are unclear, which leads to a lack of understanding on the formation mechanism and distribution of the deep carbonate basement reservoirs. In this study, the occurrence of a fracture-vug network and its fillings in carbonate reservoirs were investigated based on borehole cores, thin sections, and image logs from the southwestern slope of the OBBB's Bozhong Sag. Then the diagenetic fluid properties of the carbonate matrix and fillings were analysed via the data of carbon, oxygen, and strontium isotopes, and major, rare elements from coring intervals. The results revealed that fracture-related karst reservoirs have lithologic selectivity inclined toward dolomite strata. The intersecting relationships, widths, and strikes of the fractures and the regional tectonic background indicate three structural fracture families: NW-, NNE-, and NNW- trending, related to the Indosinian, middle Yanshanian, and late Yanshanian orogeny, respectively. The Indosinian NW- and end-Mesozoic NNE-trending fractures produced by compressional tectonic stress mainly contributed to the formation of the basement reservoirs. The geochemistry of the calcite veins filling these fractures suggests two main types of diagenetic fluids. The fluid of autogenic recharge related to the earlier fills is karstification diffuse flow dominated by internal runoff from rainfall in the highland setting of the Indosinian thrusting orogenic belt. The other fluid of allogenic recharge related to the later fills is the main lateral freshwater flow dominated by external runoff from the catchment in the setting of the horst-lowland within the rifting basin, induced by the Yanshanian destruction of the North China Craton. Finally, the relationship between the three fracture families and two kinds of related fluids is revealed. This allows us to propose a model to understand the polyphase-superimposed fracture-related karst reservoir complexes within the deep carbonate basement of tilting fault blocks that neighbour the Bozhong hydrocarbon kitchen and predict the formation of potential plays with high accuracy.

Multi-component induction logging detection characteristics of the fracture azimuth and dip angles and the fast extraction methods of fracture distribution parameters

In the field of log interpretation of tight sandstone and shale reservoirs, it is difficult to quantitatively evaluate the fracture distribution parameters (fracture azimuth angle and fracture dip angle) near the borehole. Multi-component induction logging can measure the apparent resistivities of the reservoir in different directions by an orthogonal coil system, which lays a foundation for the quantitative evaluation of the fracture distribution parameters near the borehole. First, we derive the finite element equations for numerical calculation from the Maxwell equations for the time-harmonic field and construct a three-dimensional (3D) vector finite element numerical algorithm for the fractured formation. Second, we quantitatively analyze the multi-component induction logging detection characteristics of the fracture azimuth and dip angles. Third, we establish a fast extraction method of the fracture azimuth angle based on coordinate system rotation and an analytical calculation method of the fracture dip angle using the combination of cross components and coplanar components of multi-component induction logging. Finally, we use field data to test the fracture distribution parameter extraction method, and the fracture dip and azimuth angles calculated by multi-component induction logging are consistent with those calculated by borehole imaging logging, which realizes the fast extraction of the distribution parameters of fractures in tight sandstone and shale reservoirs.

Active Fault Detection by an Automatic Breakout Geometry Characterization Algorithm from Ultrasonic Borehole Imager

Summary An underground stress state might be disturbed in an area, particularly adjusting to an active shear zone. Borehole breakouts (BOs) that appeared in a circular hole excavated in an inhomogeneous stress field might be tracked to identify the active shear zone. The present study aims to develop the breakout morphology analysis (BMA) algorithm to exploit the valuable attributes of borehole BOs including azimuth, width, and intensity (depth of elongation) of failure from wellbore ultrasonic imaging tools. In the current study, the extracted azimuthal information was surveyed to detect the active shear zone along the well. Ultrasonic data from five wells drilled in the doubly plunging Ahvaz Anticline located in Iran were collected for the purpose of algorithm verification. The multiwell correlation of the BOs’ azimuth generated by the algorithm in the Ahvaz Anticline suggests a shear plan dipping southwest-northeast direction is possibly active in the deep vertical wells. Similarly, the seismic reflection profile of the Ahvaz Anticline shows a track of detachment faulting system in the mid-Cretaceous sediments. The finding confirms that the raw ultrasonic traveling time is more applicable than other borehole image data, such as static and dynamic images of ultrasonic amplitude, in BO characterization.

Consistency Discrimination and Experimental Study of Rock Discontinuities Based on Borehole Optical Images

The structural plane is a geological interface of a particular scale, and how to determine the extension of the structural plane in space is an essential issue in the study of rock structure. This paper analyses the structural plane information of multiple boreholes collected by the digital panoramic borehole camera system to determine the structural plane consistency. The discriminative conditions of structural plane consistency in multiple boreholes are deduced through mathematical analysis methods such as spatial geometry and vector. The structural planes that may be connected are quickly screened out. The consistency is further judged by combining the relevant characteristics of the structural planes, thus forming a complete method based on digital borehole images. The method has been validated by a series of indoor tests. Structural planes with specific parameters were presented in the test blocks in these tests, and several boreholes were drilled. The experimental results show that (1) the structural plane consistency discrimination method can be applied to both flat structural planes and structural planes with certain undulation angles and roughness; (2) the tilt of the borehole does not affect the structural plane consistency using the borehole camera, and the structural plane consistency can be judged by this method in any two boreholes in space; and (3) the analysis method proposed in this paper can quickly complete the structural plane consistency discrimination, and is also suitable for the consistency discrimination and screening of a large number of structural planes in deep boreholes. Finally, the method based on the optical borehole images is successfully applied in the slope survey of the Xichang iron ore mine.

An integrated study of core and borehole image data: palaeokarst development in late Permian siliceous dolomites and the structural evolution of the Loppa High from the Gohta discovery well, SW Barents Shelf

Abstract The Gohta discovery well 7120/1-3, drilled on the Loppa High in the SW Barents Shelf in northern Norway encountered a meteoric karst system hosted by the late Permian lower Røye Formation that formed during a period of sub-Triassic exposure. The karst system is preserved as a series of interbedded collapse breccias representing formerly open palaeocaves, matrix-rich breccias that represent palaeocave fills with in situ and reworked stalactites and stalagmites. Layers of ‘Swiss cheese’ texture are interpreted as zones of dissolution in marine-meteoric mixing zones that may have controlled the levels of palaeocave development within the Gohta structure. These other cavities and the matrix of karst breccias were infilled by layered fine doloarenite matrix interpreted as an internal fill generated by collapse. This was followed by a later matrix of dark grey argillaceous mudstone that is interpreted as an external fill that pre-dates the overlying Klappmyss Formation. The karst system has undergone complex early diagenesis that includes extensive dissolution, early leaching of silica, the precipitation of pyrite on internal surfaces and sediment and the precipitation of carbonate and silica as speleothems and vadose cement. A borehole imaging log (BIL) was run across the lower Røye and Triassic Klappmyss formations, including the cored interval that provides a record of inclination and azimuth of the karst fills and in situ Røye Formation. The core provides information about the origin and timing of the different karst fills, and the BIL provides information about their azimuth and inclination. This integrated use of core and BIL data allows the incremental tilting history of the Loppa High structure during exposure to be reconstructed. At present, the Gohta structure has a finite tilt of 5–10° to the SE. The plunge of in situ stalagmites and stalactites suggests that the karst system was initiated while the Røye Formation was slightly tilted to SE. The dip of karst components and the host stratigraphy indicate several phases of incremental tilting during deposition of the doloarenite and argillaceous mudstone karst matrix. There appears to have been a reversal of tilt of the Gohta structure prior to its onlap by the Klappmyss Formation.

Present-Day In Situ Stress Analysis in Shale Reservoir of Haiba Block, Southern Sichuan Basin, South China

Natural gas from shale gas reservoirs has been an important contributor for reserve growth, deliverability construction, and profits growth in natural gas industry in the world. Hydraulic fracturing is commonly required in the shale gas commercial development, and thus understanding the present-day in situ stress field is greatly significant for the hydraulic fracturing and efficient development in shale gas reservoirs. However, there are no systematic investigations on the present-day in situ stress field in the Haiba Block from the Sichuan Basin, South China. In this study, the present-day in situ stress orientations and magnitudes in shale reservoir of Haiba Block are investigated based on the well interpretations from borehole image log and geomechanical modeling. Then, the effects of stresses on hydraulic fracturing, horizontal wells, and natural fracture reactivation were discussed. The results indicate that the horizontal maximum principal stress (SHmax) orientation is mainly in the NE-SW-trending, NW-SE-trending, and WNW-ESE-trending in the Haiba Block. The magnitudes of horizontal maximum and minimum principal stresses are 13.5 MPa~85.5 MPa and 2.8 MPa~31.6 MPa, respectively. In the Haiba Block, the differential stress is generally low in the northern part, which indicates that complex hydraulic fracture networks may be produced. While the natural fractures are generally stable under the present-day in situ stress field. When the increase of pore pressure gradient is about 30 KPa/m, nearly all natural fractures in the Longmaxi Formation may be reactivated. The results can provide the insights into a better understanding of the present-day in situ stress distribution so as to optimize perforation orientation, hydraulic fracturing design, and enhance gas production in shale gas reservoirs.