Degree Of Fracture Development Research Articles

Tectonic fractures induced by strike-slip faulting in intracratonic ultradeep carbonate rocks: Insights from the finite element method and self-adaptive constraints computational model for boundary conditions

Numerical simulations of the paleostress field during a period of tectonic fracture formation and rock failure criteria are used to quantitatively predict the development and occurrence of tectonic fractures induced by the formation of the SB18 fault zone in the Middle Ordovician Yijianfang Formation of the Shunnan area, Tarim Basin, China. The results of acoustic emission experiments, mechanical property measurements, and tectonic fracture occurrence observations obtained from core descriptions and fullbore formation microimager logs are combined with the Andersonian model of faulting and the finite element method, which is widely used for the numerical simulation of stress fields, to investigate the paleotectonic and in situ stress fields via numerical simulation. The quantitative prediction of the opening pressure and opening sequence of tectonic fractures is based on the occurrence of tectonic fracture, numerical simulation of in situ stresses, and coordinate system conversion. The results show that the width of the fracture zone induced by strike-slip faulting is ∼310 m. The degree of fracture development is significantly increased when the Young’s modulus, paleostress difference, and paleostress difference coefficient of the rock are elevated. The current horizontal principal stress is positively correlated with the distance from the fault, and the elevated areal density of the secondary faults causes a clockwise deflection of the horizontal stress direction. SSE-striking shear fractures with orientations ranging from 140° to 150° and two sets of tensional fractures with orientations ranging from −40° to −35° and 55° to 60° are preferentially opened in the water injection development stage of the reservoir. As the horizontal stress difference, horizontal stress difference coefficient, and angle between the maximum horizontal principal stress and a fracture decrease, the fracture opening pressure decreases. At the structural highs (burial depths <6225 m) and lows (burial depths >6225 m), the fracture burial depth is positively and negatively correlated with the opening pressure, respectively. Quantitative prediction of tectonic fracture developmental characteristics, opening pressure, and the opening sequence and investigation of the main factors that control their development can help to identify and support opportunities for hydrocarbon exploration and development of fractured carbonate reservoirs.

Factors Controlling Productivity – A Case Study on the Qixia Formation of the Shuangyushi Area, Sichuan Basin

The gas reservoir with high temperature and high pressure (HTHP) over 8,000 m is one of the carbonate gas reservoirs discovered in recent years in the Qixia Formation of the Shuangyushi area, NW Sichuan Basin, and the strong heterogeneity makes it difficult to deploy high-yield wells. The relationship between geology and productivity is given by using the research results of reservoir evaluation and statistical analysis of the dynamic analysis of trial production in the last 4 years in this paper. Then, the evaluation index system of high productivity controlling factors in the gas reservoir is given by using the fuzzy clustering analysis method. Finally, the relative importance of each index is quantitatively analyzed and evaluated by using the analytic hierarchy process to determine the key controlling factors affecting productivity. The evaluation results are highly consistent with the current characteristics of high-yield wells. The productivity of the newly completed Well 008 and Well 107 outside the trial mining area exceeded 1 million m3/day. The result shows that the high productivity of ultradeep gas wells in the Qixia Formation is mainly controlled by the degree of dolomitization, the distribution of high-energy shoal-mound complexes, and the degree of fracture development. It can be used to guide the deployment of productive wells in the southern area of the Shuangyushi-producing test area.

Effect of stress on fracture development in the Asmari reservoir in the Zagros Thrust Belt

Development and production from fractured reservoirs require extensive knowledge about the reservoir structures and in situ stress regimes. For this, this paper investigates fractures and the parameters (aperture and density) through a combination of wellbore data and geomechanical laboratory testing in three separate wells in the Asmari reservoir, Zagros Belt, Iran. The Asmari reservoir (Oligo-Miocene) consists mainly of calcitic and dolomitic rocks in depths of 2000-3000 m. Based on the observation of features in several wellbores, the orientation and magnitude of the in situ stresses along with their influence on reservoir-scale geological structures and neotectonics were determined. The study identifies two regional tectonic fracture settings in the reservoir: one set associated with longitudinal and diagonal wrinkling, and the other related to faulting. The former, which is mainly of open fractures with a large aperture, is dominant and generally oriented in the N45°-90°W direction while the latter is obliquely oriented relative to the bedding and characterized by N45°-90°E. The largest aperture is found in open fractures that are longitudinal and developed in the dolomitic zones within a complex stress regime. Moreover, analysis of drilling-induced fractures (DIFs) and borehole breakouts (BBs) from the image logs revealed that the maximum horizontal stress (SHmax) orientation in these three wells is consistent with the NE-SW regional trend of the SHmax (maximum principal horizontal stress) in the Zagros Belt. Likewise, the stress magnitude obtained from geomechanical testing and poroelastic equations confirmed a variation in stress regime from normal to reverse, which changes in regard to active faults in the study area. Finally, a relationship between the development degree of open fractures and in situ stress regime was found. This means that in areas where the stress regime is complex and reverse, fractures would exhibit higher density, dip angle, and larger apertures.

Fractures in faulted sandstone reservoir

The Chang 8 Member of the Yanchang Formation in southwest Ordos Basin is a typical faulted sandstone oil reservoir. The development characteristics of fractures have obvious controlling effect on the formation of sweet spots of tight reservoir. In this paper, taking the Chang 8 Member of the southwestern Ordos Basin as an example, the development characteristics and controlling factors of fractures in faulted sandstone oil reservoir are systematically studied. The results show that the faulted sandstone oil reservoirs in the study area are distributed along the main strike-slip faults. The fracture system includes vertical fractures and horizontal bedding fractures. The fracture surface of vertical fractures generally has good oil display, and mostly presented as oil spot and oil immersion level; however, the oil level of horizontal bedding fractures is usually presented as oil spot level. The development frequency of horizontal bedding fractures is 62.5 %, while that of vertical fractures is 37.5 %. The fractures are mainly developed in fine sandstone and a small amount of medium-grained sandstone and siltstone. The factors that affect the fracture development degree in faulted oil reservoir include the distance from main fault, sand thickness, lithology and structural location. For the Jinghe and Honghe Oilfields, the degree of fracture development decreases sharply when the distance from the main fault is greater than 1.25 km and 1.5 km, respectively. Single sand body thickness also controls the degree of fracture development. Single sand bodies with thickness within 6 m have more developed fractures, and the fracture development decreases sharply when the thickness exceeds 6 m. The sand body in the wing part of river channel is relatively thin with fine grain size and small compacted space, which is easy to break under tectonic activity. The fractures of the Chang 8 Member in the study area are mainly developed near the faults, the top of anticline and its wing part.

Fracturing parameter optimization technology for highly deviated wells in complex lithologic reservoirs

Highly-deviated wells are the key technology to reduce the risk of drilling accidents and improve the utilization of reservoirs. However, for reservoirs with complex lithology, highly-deviated wells are faced with the problems of geomechanical transformation and fracturing parameter optimization. The research on fracturing parameter optimization technology of high-deviated wells in complex lithologic reservoirs is helpful to the research and application of geomechanics in deep unconventional reservoirs. This paper is based on geological mechanics laboratory experiments and logging interpretation, combined with regional geological background, to clarify the geological and mechanical characteristics of the Fengcheng Formation shale oil region in the Mabei Slope. On this basis, based on the current geostress field and natural fracture distribution pattern of the Mabei Slope, an integrated model of shale oil geological engineering in local well areas was established. Based on the finite element method, optimization design was carried out for the cluster spacing, construction fluid volume, displacement, and sand volume of highly deviated well fracturing, and three-dimensional simulation of fracturing fractures was completed. The research results indicate that: (1) The current dominant direction of the maximum principal stress in the Fengcheng Formation on the Mabei Slope is from northeast to southwest, with the maximum horizontal principal stress generally ranging from 90 to 120 MPa and the minimum horizontal principal stress generally ranging from 70 to 110 MPa. (2) The difference in stress between the two horizontal directions is relatively large, generally greater than 8 MPa. Two sets of natural fractures have developed in the research area, one with a northwest southeast trend and the other with a northeast southwest trend. The natural fracture density of the Fengcheng Formation shale reservoir in the Mabei Slope is 0.32–1.12/m, with an average of 0.58/m, indicating a moderate to high degree of fracture development. (3) The geological model and three-dimensional geo-mechanical model are established according to the actual drilling geological data, and different schemes are designed to carry out single parameter optimization. The optimization results show that the optimal cluster spacing of the subdividing cutting volume pressure of the highly deviated wells in the Fengcheng Formation of the Mabei Slope is 12 m, the optimal construction fluid volume is 1400–1600 m3/section, the optimal construction displacement is 8 m3/min, and the optimal sanding strength is 2.5 m3/m. At the same time, by comparing the fracturing implementation effect with the fracturing scheme design, it is proven that the artificial parameter optimization method for highly deviated wells based on the finite element method based on the regional stress background and the natural fracture development law proposed in this paper is feasible and can provide a scientific basis for the fracturing development of highly deviated wells in complex lithologic reservoirs. This research has been well applied in Mahu area of Xinjiang oilfield.

A dual-branch fracture attribute fusion network based on prior knowledge

The Tarim Basin region harbors abundant carbonate reservoirs in which fractures throughout the structures play a crucial role in facilitating the transportation of oil and gas. The degree and distribution of fracture development play a crucial role in achieving a high yield and stable production. Seismic attribute analysis is a widely used and efficacious approach for examining fractures in reservoirs. However, different seismic attributes characterize fractures at different scales, and any single seismic attribute may be of poor quality, making it difficult to describe reservoir channel media comprehensively. In the present study, the seismic attribute volume was sliced from three perspectives to obtain complete geological spatial structural information of the seismic volume. This was accomplished using a shifted window (Swin) transformer module guided by a t-distribution before extracting and capturing the prior information and global contextual cues of the seismic attributes. Additionally, a dual-branch fusion (DBFusion) network that combined a convolutional neural network and a Swin transformer to extract the spatial geological structural features of seismic attributes was used to facilitate a comprehensive integration of local and global contextual information. Through the DBFusion network modeling, the network comprehensively extracted local information and integrated complementary global information. Taking three-dimensional seismic data from a unit in the Tarim Basin as an example, this study explores the fusion of three attributes: coherence, ant tracking, and curvature. When compared to individual attributes, the fused attributes can comprehensively leverage the advantages of all three seismic attributes in reflecting fractures. This results in a more precise representation of fractures of varying sizes in the fused output. It was discovered through comparative experiments that the DBFusion network proposed in this paper exhibits the best performance compared to other models.

The effects of various factors on spontaneous imbibition in tight oil reservoirs

Slickwater fracturing fluids have gained widespread application in the development of tight oil reservoirs. After the fracturing process, the active components present in slickwater can directly induce spontaneous imbibition within the reservoir. Several variables influence the eventual recovery rate within this procedure, including slickwater composition, formation temperature, degree of reservoir fracture development, and the reservoir characteristics. Nonetheless, the underlying mechanisms governing these influences remain relatively understudied. In this investigation, using the Chang-7 block of the Changqing Oilfield as the study site, we employ EM-30 slickwater fracturing fluid to explore the effects of the drag-reducing agent concentration, imbibition temperature, core permeability, and core fracture development on spontaneous imbibition. An elevated drag-reducing agent concentration is observed to diminish the degree of medium and small pore utilization. Furthermore, higher temperatures and an augmented permeability enhance the fluid flow properties, thereby contributing to an increased utilization rate across all pore sizes. Reduced fracture development results in a lower fluid utilization across diverse pore types. This study deepens our understanding of the pivotal factors affecting spontaneous imbibition in tight reservoirs following fracturing. The findings act as theoretical, technical, and scientific foundations for optimizing fracturing strategies in tight oil reservoir transformations.

Reservoir Space Characterization of Ordovician Wulalike Formation in Northwestern Ordos Basin, China

The Ordovician Wulalike Formation in the northwestern Ordos Basin is a new prospect for exploring marine shale gas in China, facing prominent problems such as unclear reservoir conditions and the distribution of enrichment areas. The types of reservoir space, fracture development, porosity composition, and physical properties of the lower Wulalike Formation are discussed through the multi-method identification and quantitative evaluation of reservoir space for appraisal wells. The Wulalike Formation in the study area contained fractured shale reservoirs with matrix pores (mainly inorganic pores) and permeable fractures. The fracture system of the lower Wulalike Formation is dominated by open bed-parallel fractures that are intermittent or continuous individually, with a width of 0.1–0.2 mm and spacing of 0.5–14.0 cm. The fracture-developed intervals generally exhibit bimodal or multimodal features on NMR T2 spectra and have a dual-track feature with a positive amplitude difference in deep and shallow resistivity logs. The length and fracture porosity of fracture-developed intervals varied greatly in different parts of the study area. In the Majiatan-Gufengzhuang area in the southern part of the study area, the fracture development degree generally decreased from west to east. In the Shanghaimiao area in the central part of the study area, fractures were extremely developed, the continuous thickness of the fracture-developed interval was generally more than 20 m, and the average fracture porosity was higher than 1.3%. In the Tiekesumiao area in the northern part of the study area, the fracture development degree was generally lower than that in the central and southern parts of the study area and also showed a decreasing trend from west to east. The lower Wulalike Formation had a total porosity of 2.46–7.08% (avg. 4.71%), roughly similar to the Longmaxi Formation in the Sichuan Basin, of which matrix porosity accounts for 34.0–90.0% (avg. 61.1%) and fracture porosity accounts for 10.0–66.0% (avg. 38.9%). From this, it could be inferred that the shale gas accumulation type of the lower Wulalike Formation in the northwest margin of the basin is mainly a fractured shale gas reservoir controlled by structure, and its “sweet spot area” is mainly controlled by tectonic setting and preservation conditions. This indicates that the Wulalike Formation in the northwestern Ordos Basin has good shale gas exploration prospects, and a large number of fault anticlines or fault noses formed by reverse dipping faults have the potential of favorable exploration targets.

Experimental Investigation of Stress Sensitivity of Elastic Wave Velocities for Anisotropic Shale in Wufeng–Longmaxi Formation

The shale of the Wufeng–Longmaxi formation in the Sichuan Basin is the preferred layer for shale gas exploration in China, and its petrophysical characteristics are the key to geological and engineering sweet spot prediction. However, the characteristics and impact mechanisms of its acoustic wave velocity and elastic anisotropy are currently unclear. In this paper, the Wufeng–Longmaxi shale is taken as the research object, and the P-wave and S-wave velocities of the samples are tested under the loading and unloading processes of confining pressure. The stress sensitivity variations in parameters such as wave velocity, wave velocity ratio, and anisotropy are discussed. P-wave and S-wave anisotropy parameters are correlated under different pressure conditions. X-ray diffraction, casting thin sections, scanning electron microscopy, micron CT scanning, and other analytical techniques are used to explore the mechanisms of stress sensitivity of elastic parameters. The research results indicate that: (1) the acoustic velocities of samples from different angles are V90° > V45° > V0°, and there is a positive correlation between the wave velocity and the confining pressure. After unloading the confining pressure, irreversible plastic deformation occurs due to the closure of some microfractures in the rock core, causing the wave velocity to be higher than the initial value. (2) The stress sensitivity coefficient of the P-wave (The mean is 3.00 m·s−1·MPa−1) is higher than that of the S-wave (the mean is 1.23 m·s−1·MPa−1), and the stress sensitivity coefficient of the compacted stage (the mean is 3.02 m·s−1·MPa−1) is higher than that of the elastic stage (the mean is 1.21 m·s−1·MPa−1). (3) The anisotropy of the P-wave and S-wave is negatively correlated with the confining pressure. When the confining pressure is loaded to 65 MPa, the change rate of the P-wave anisotropy coefficient is 23%, and its stress sensitivity is higher than that of S-wave anisotropy coefficient (the change rate is 13.7%). After unloading the confining pressure, the degree of anisotropy is reduced due to the closure of some microfractures. The empirical formula of P-wave and S-wave anisotropy parameters under different pressures is established through linear regression, which can provide a reference for mutual predictions. (4) The variation in wave velocity anisotropy with stress can be divided into stress and material anisotropy, which are related to the directional arrangement of microfractures and clay minerals, respectively. The quantitative characterization of shale anisotropy can be realized by evaluating the development degree of reservoir fractures and mineral components, providing a reference for logging interpretations, sweet spot prediction, and fracturing construction of shale gas reservoirs.

Source-reservoir rock assemblages and hydrocarbon accumulation models in the Middle-Lower Jurassic of eastern Sichuan Basin, China

The eastern Sichuan Basin in China holds vast potential for oil and gas exploration in the Lower-Middle Jurassic strata. However, the geological characteristics and hydrocarbon accumulation patterns of this region remain largely unclear. During the deposition period of the Lower-Middle Jurassic strata, the eastern Sichuan is characterized by the formation of multiple sets of source, reservoir, and caprock assemblages through depositing lake-delta-fluvial deposits, which have great exploration potential. The Jurassic source rocks in eastern Sichuan are mainly developed in the Dongyuemiao Member, Da’anzhai Member, and Liangshan Formation. These source rocks have a total organic carbon (TOC) content greater than 1 and a varying range of organic matter maturity, with a Ro value of 0.8–2.0. The kerogen in these source rocks is primarily type II, with a smaller proportion being type III. A range of reservoir rocks can be found in the Jurassic strata of eastern Sichuan, with sandstone reservoirs being predominantly found in the Liangshan Formation, Shaximiao Formation, and Zhenzhuchong Member. Shale reservoirs are mostly present in the Dongyuemiao, Da’anzhai, Liangshan, and Maanshan Members, and there is a limited distribution of limestone reservoirs in the Da’anzhai Member and Dongyuemiao Formation. The arrangement of source rocks and reservoir rocks in eastern Sichuan has led to the formation of three types of reservoir-forming combinations, including lower generation and upper storage, self-generation and self-storage, and composite. Sandstone reservoirs are typically of lower generation and upper storage, shale reservoirs are primarily of self-generation and self-storage, and limestone reservoirs are mostly composite. The exploration of Jurassic oil and gas in eastern Sichuan should prioritize “layer and area selections.” The Da’anzhai, Dongyuemiao, and Liangshan shale reservoirs should be the primary exploration targets, with the semi-deep lake deposits in the syncline area being the most favorable. The degree of fracture development in the exploration area also has a significant impact on the shale oil and gas content. The Liangshan Formation and Shaximiao Formation sandstone reservoirs can serve as secondary exploration targets, with anticline areas that have better sealing conditions being more favorable. Limestone reservoirs have limited distribution, and exploration areas with high and steep fractures are relatively more advantageous.

A new method for evaluating the utilization effect of carbonate gas reservoir reserves

After nearly 60 years of development, the carbonate gas reservoirs in Sichuan Basin have entered the middle and late stages of development. Affected by various geological factors such as complex structure, formation water distribution and water invasion intensity, low permeability, fracture development degree, fracture-cavity combination, etc., and the difference of development technical policies, the utilization effect of reserves varies greatly among different gas reservoirs. Moreover, the current indicators for evaluating the utilization effect of reserves are mainly reserve utilization degree, dynamic-static reserve ratio, recovery degree, etc., yet a unified evaluation method has not been formed. In order to effectively evaluate the utilization effect of reserves and improve the development benefit of gas reservoirs, the evaluation of utilization effect of reserves was carried out by comprehensively considering geological characteristics and development technical policies. On this basis, a new method for evaluating the utilization effect of carbonate gas reservoir reserves was formed and applied in specific gas reservoirs. The research results show that: 1) The quality of reserve utilization can accurately evaluate the utilization effect of gas reservoir reserves; 2) By introducing big data analysis technology, comprehensively using ward clustering analysis method and Pearson coefficient to correlate the main influencing factors of reserve utilization effect, a prediction model of reserve utilization effect was established; 3) The WBT Carboniferous gas reservoir was chosen to verify the aforementioned model, and the result shows that the model has high prediction accuracy and strong adaptability, which can accurately evaluate the utilization effect of developed gas reservoir reserves. The model is also applicable to evaluating the utilization effect of undeveloped gas reservoirs. In conclusion, by adopting big data analysis, the established prediction model of reserve utilization effect is suitable for quantitative evaluation and analysis of reserve utilization effect of carbonate gas reservoirs, which can provide a basis for guiding the formulation of reasonable development technical policies and improving the reserve utilization effect for similar types of gas reservoirs.

Distribution pattern of natural fractures in lacustrine shales: a case study of the Fengcheng formation in the Mahu Sag of the Junggar Basin, China

The Lower Permian Fengcheng Formation in the Mahu Sag develops a set of organic-rich alkaline lacustrine shale strata, which is a key area for shale oil exploration and development. As an important storage space and seepage channel for shale reservoirs, natural fractures have an impact on shale oil enrichment, production and development effect. In this study, the types and characteristics of natural fractures were first analyzed using core, thin section and imaging logging data. On this basis, combined with the distribution of fractures in single wells, the vertical distribution law of fractures is discussed. Finally, the planar distribution of fractures is evaluated using different seismic attributes such as coherence, curvature, likelihood, and AVAz. The results showed that three types of fractures are existed, including transformational shear fractures, intraformational open fractures and bed-parallel shear fractures, with intraformational open fractures being the most developed. The development degree of fractures in different layers has obvious differences, mainly controlled by lithology and brittle mineral content. The basalt and tuff are developed in the Feng 1 Member, with low carbonate mineral content, resulting in a relatively low degree of fracture development. The dolomite and argillaceous dolomite are developed in the Feng 2 Member and the Feng 3 Member, with high carbonate mineral content and brittleness, resulting in a high degree of fracture development. Additionally, the closer to the fault, the higher the degree of fracture development. On the plane, the fracture zone develops near the main and secondary faults, with the trend mainly oriented in the E-W direction and approximately parallel to the direction of the faults. The width of the fracture zone is largest in the central and southern part of the study area. These fractures are fault-related and are caused by regional stress fields resulting from the activity of the main-secondary faults.

Measurement of fracture development evolution of coal samples under acid-alkaline by three-dimensional reconstruction and AE time-frequency characteristic analysis

One of the key factors affecting the stability of coal pillar dam at the boundary of underground reservoir is pH value of mining water resources. So, the comprehensive characterization of micro-pore structure and fracture evolution characteristics of coal pillar dam samples with different pH values were studied and measued in the thesis. Combined with XRD results, the clay minerals are mainly kaolinite. Based on three-dimensional CT reconstruction, the development degree of coal pore fractures were: strong acid > strong alkali > neutral mine water > natural drying, the longer the roar tends to develop in lower section with high H+ concentration. Based on AE Fourier transform, the higher the H+ concentration, the higher the development degree of macro-large fractures; the higher the OH– concentration, the higher the development degree of small-sized fractures. However, the higher the H+ concentration, the higher the macro-orderliness of the crack system.

Numerical simulation of the palaeotectonic stress field and prediction of the natural fracture distribution in shale gas reservoirs: A case study in the Longmaxi Formation of the Luzhou area, southern Sichuan Basin, China

Natural fractures have an important relationship with shale gas production. This study selected the reservoir of the Wufeng‐Longmaxi Formation in Luzhou in southern Sichuan as an example. Based on the seismic reflection map of the formation, using the mechanical parameters obtained from the core sample experiment and using the finite element numerical simulation method, a three‐dimensional geological model is established, the tectonic stress field is inverted and the fracture distribution is predicted according to the Mohr‐Coulomb fracture criterion of the rock fracture, which provides theoretical support for the subsequent well pattern layout and is conducive to improving the efficiency of shale gas exploration and development. The analysis shows that the study area is dominated by NE reverse faults, with an inclination of more than 40°. The core observation results show that the filling degree of fractures is relatively high, and the width is mostly less than 1 mm, mainly caused by tectonic shear fractures. The tectonic stress in the sedimentary belt where the syncline structure is deeply buried is higher, the stress in the northwest and southeast tectonic ridges is relatively low, and the differential stress in the northeast tectonic ridge and near the fault is high. The NE fault and the surrounding rock mass have the highest fracture degree; the fracture proximity coefficient is >1.5 and the fractures are the most developed. The degree of fracture development in the area with a gentle structure is relatively low, but it is far from large faults, which is conducive to preserving shale gas.

Dynamic evaluation method of water-sealed gas for ultra-deep buried fractured tight gas reservoir in Kuqa Depression, Tarim Basin, China

The ultra-deep-buried fractured tight gas reservoir in the Kuqa Depression of the Tarim Basin has developed edge and bottom water. Faults and fractures have become “highways” for water invasion, resulting in the “water sealed gas” effect and reducing gas reservoir recovery. At present, there is a lack of effective evaluation methods. Therefore, based on an analysis of water invasion characteristics of the gas reservoir, a dynamic evaluation method for water-sealed gas in a fractured gas reservoir is established. This method considers two factors: fracture development scale and peripheral water body strength. It is then applied to three developed blocks in the Kuqa ultra-deep layer. The effectiveness of the evaluation results is verified by static and dynamic combination, and countermeasures to improve gas reservoir recovery are proposed. The results indicate that: (1) The non-uniform water invasion of fractures is jointly controlled by structural position, fracture development degree, and fracture network combination, which can be divided into three modes: edge water channeling along the large fracture in the core, edge and bottom water invading along the fracture in the wing, and rapid, violent water flooding of the bottom water along the fracture/small fault in the low part. (2) The replacement coefficient of water invasion in the three typical blocks is 0.2–0.3, indicating that they are sub active water-gas reservoirs. However, the severity of water-sealed gas varies greatly. The more severe the water-sealed gas is, the lower the recovery factor of the gas reservoir. (3) For directionally penetrating large fracture gas reservoirs, water shutoff should be carried out. For fracture network gas reservoirs with high fracture density, mild exploitation can control water, and early drainage can reduce the impact of water invasion, improving gas reservoir recovery. It is concluded that the new method of water-sealed gas dynamic evaluation can provide a reliable basis for evaluating fracture non-uniform water invasion dynamics of the ultra-deep gas reservoir and enhancing oil recovery of the gas reservoir in the Kuqa Depression. This method also supports the formulation of water control policies and the economic and efficient development of ultra-deep gas reservoirs in the Kuqa Depression.

Characterization of fractures in low‐permeability thin tight sandstones: A case study of Chang 6 Member, Yanchang Formation, western Ordos Basin

Structural fractures are formed when the underground rock mass deforms and exceeds its own strength limit, so they represent the complex geomechanical properties of rock. Small or core‐scale fractures usually extend less than 10 m in length, which affects the productivity of tight sandstone. Therefore, core‐scale fracture is the core factor for the prediction of reservoir sweet spots. In this paper, the research on the evolution and prediction of natural fractures in tight sandstone is investigated using the core, thin section, logging materials and numerical simulation methods. The results show that a large number of vertical and horizontal sliding fractures with shearing properties are developed in the tight sandstones of the Yanchang Formation. The particle size affects the compactible space inside the reservoir, which then affects the fracture development degree. Since the superimposed thin sand bodies are formed by the frequent lateral migration of the underwater distributary channels in the study area, therefore, the natural fractures in the thin sand bodies at the edge of river channels are relatively developed. According to statistics, the vertical extension distance of fractures is usually less than 2 m. Meanwhile, the fractured zones are mainly located in the cumulative sand body thickness range of 3 ~ 10 m. Moreover, the fractured zones are generally located in the high part of the structure and its wings. Regional structure and thin sand body distribution jointly affect the evolution of fractures in tight sandstones. Through this study, we found that fractures are more developed in thin sand bodies, and the controlling factors include lithology, sedimentary microfacies, sand thickness and local structure. The comprehensive evaluation of fractures in tight sandstones with high accuracy from 1D to 3D was achieved using core observation, logging interpretation, and three‐dimensional (3D) fracture modelling.

Evaluating the Degree of Tectonic Fracture Development in the Fourth Member of the Leikoupo Formation in Pengzhou, Western Sichuan, China

The extent of fracture development is associated with the degree of enrichment of a natural gas reservoir and its productivity. Based on numerical simulation results of the paleotectonic stress field, a set of evaluation methods for determining the degree of development of reservoir tectonic fractures were established using rock rupture criteria. Taking the fourth member of the Leikoupo Formation in the Pengzhou area of western Sichuan as an example, a finite element (FE) method was employed to simulate the paleo-tectonic stress field during the period of fracture development, and the degree of tectonic fracture development was further evaluated using the above methods. The results indicated that effective fractures were created in the Himalayan period. In this time, mainly NE–NEE and nearly E–W strike tectonic fractures were developed in the target layer. The fractures were mainly low-angle and oblique fractures, while the high-angle fractures were less developed. According to the integrative fracture index (F), five typical fracture development areas were determined: the fault zone, and the northern, eastern, southeastern, and central regions of the study area. The reliability of the fracture prediction results was verified using fracture distribution statistics and gas production test results.

Structure- and lithofacies-controlled natural fracture developments in shale: Implications for shale gas accumulation in the Wufeng-Longmaxi Formations, Fuling Field, Sichuan Basin, China

Descriptions, field emission-scanning electron microscopy and geochemical analyses on shale core samples taken from the fractured intervals of the Wufeng-Longmaxi Formations in three wells from the Fuling Field were used to understand the developmental properties of natural fractures and elucidate their controlling factors. Furthermore, the effects of geological fractures on shale gas accumulation were systematically analyzed. The macro-fractures observed in the shale cores are mainly non-tectonic bedding fractures; to a lesser extent, tectonic fractures, including oblique shear, vertical tension and tension-shear fractures, as well as bed-parallel slip fractures are visible. In contrast, micro-fractures are mainly interparticulate, intraparticulate, and grain marginal in nature. The extent of natural fracture development is controlled by external structural and internal lithofacies factors. For shales with similar lithofacies, the development degree of tectonic fractures increases with decreasing distance from the tectonically deformed and faulted zones. Within the same structural zone, the fractures are well developed in shales of organic matter-rich S-3 (i.e., argillaceous-rich siliceous) lithofacies. The study of fracture properties and their controlling factors has resulted in a model for the development of natural fractures in Wufeng-Longmaxi Shales generated by both structural and lithofacies effects. Overall, shale reservoirs in a “fractured but not broken” state are the most favorable ones for gas accumulation.

Fracture Extraction From Logging Image Using a Dual Encoder-Decoder Architecture With Swin Transformer

Imaging logging is a method of imaging the physical parameters of the borehole wall or the objects around the borehole according to the observation of the geophysical field in the borehole. Imaging logging data can determine the dip angle and structural characteristics of the formation and observe the geometry and development degree of fractures. The performance of existing target segmentation networks relies on large volumes of data. However, logging images are expensive to acquire, so how to effectively extract fractures from small samples of logging images is an urgent problem to be solved. Therefore, we developed a dual encoder-decoder structure using the Swin Transformer, which uses the self-attention mechanism of a hierarchical Vision Transformer with shifted window to model the remote context information. It can overcome the limitations of most convolutional neural network-based methods that cannot establish long-term dependencies and global contextual connections in convolutional operations. In addition, the shifted window mechanism substantially improves the computational efficiency of the model, and the hierarchical structure allows flexibility in modeling at different scales. At the same time, skip connections are established between adjacent layers of the structure, and the higher-level feature maps are stitched with the lower-level feature maps in channel dimensions, which can obtain more high-resolution detail information of fractures, and thus improve the segmentation accuracy. The experimental results show that the performance is better than the mainstream segmentation networks under small training sets of logging images. The effectiveness of our method reveals that it is practical in fracture extraction of logging images.

Development characteristics of multi-scale fracture network systems in metamorphic buried hills

Natural fractures are regarded as important reservoir spaces and effective seepage channels at metamorphic buried hills. Continuous networks associated with multi-scale fractures with good connectivity is critical for high-quality reservoirs as well as high and stable production in the tight metamorphic rocks. The multi-scale fractures in Bozhong 19–6 metamorphic buried hills were well characterized through integrating image logs, cores, thin-sections, and scanning electron microscope, etc. After that, power-law distribution of multi-scale fractures was established to understand contribution of fractures to reservoir quality and figure out structure models of fracture networks as well as their impact on production. Results show that parameters of fracture systems vary regularly with fracture scales. Fracture development degree, e.g., cumulative areal density, increases as a power law function with decreasing fracture size from macro to micro (e.g., aperture and/or length), where storage space associated with micro fractures is also increased. Reversely, fracture connectivity and permeability follow a significant decreasing trend. Five structure models of fracture network were established based on combination pattern of multi-scale fractures: multi-scale fracture network with high-density and multi-sets, large-scale fracture network with medium-density and multi-sets, small-scale fracture network with high-density and multi-sets, large-scale fracture network with low-density and multi-sets, and small-scale fracture network with low-density and single-set. The former two fracture networks can be widely developed into high-quality reservoirs, contributing greatly to high and stable yields. Fracturing is required for the third and the fourth fracture networks to obtain stable production, while it is difficult for the fifth fracture network to obtain industrial oil and gas flow.