Degree Of Fracture Development Research Articles

Fine Logging Evaluation of Fractures in Continental Tight Oil Sandstone Reservoirs of Yanchang Formation in the Ordos Basin

Fractures are the main seepage channels in tight reservoirs, and they affect the distribution of high-quality reservoirs and the enrichment of hydrocarbons. Fine logging identification of fractures in the strongly heterogeneous continental tight oil reservoirs of the Upper Triassic Yanchang Formation in the Ordos Basin is a hot and difficult point in the field of petroleum geology. In this paper, taking the Yanchang Formation as an example, a fractal model was constructed to identify fractures in tight oil reservoirs using a large number of cores, conventional and micro-resistivity imaging logging data. The results show that high-angle, vertical and bedding fractures are mainly developed in the Yanchang Formation tight sandstones. There is a negative correlation between sand body thickness and fracture development degree. The fracture sensitivity parameters were used to construct a coupled fractal fracture index. The fractal model incorporates logging information from natural gamma, acoustic wave time difference, rock density, and shallow lateral resistivity. In addition, the constructed fracture fractal index realizes the functions of multi-conventional logging information fusion, which can effectively identify fracture development segments in sandstone. According to the statistics, the fracture identification rate is 83.3%. The study also found that with the increase of sandstone brittleness index, the fracture index has a “S” shape increasing trend. Therefore, the content of brittle mineral components in sandstone is an important factor affecting the development degree of natural fractures, and fractures are more likely to occur in high brittle, thin sand bodies. The highly brittle framework minerals have strong stress-supporting capacity, which can keep fractures open by resisting high overlying loads.

True triaxial experimental study on fluid flow in single fracture with different dip angles under three-dimensional stress at different depths

The shape of ore body in deep underground engineering is complex, and its fracture development degree is affected by water abundance, fracture zone, fault and other factors. With the deepening of depth, the vertical and horizontal stress changes significantly, which has a great impact on the fracture opening. Therefore, the fracture dip angle and its three-dimensional stress state are the main factors affecting the fluid flow of fractures in a large area of ore body and its hanging and footwall rock mass. In this study, firstly, the three-dimensional fractal dimension of a single-fracture specimen was obtained by using CT (Computer Tomography) scanning technology and the single-fracture specimen with a similar three-dimensional fractal dimension was selected for testing. Then, the self-developed single fracture stress seepage coupling true triaxial test system is applied to test the seepage of single fracture samples with six different fracture dip angles under different stress states of underground depth. The experimental results show that the fluid flow and strain of fractured rock mass are comprehensively affected by the fracture dip angle and the stress state of the underground depth, and the influence degree of the fracture dip angle changes with the deepening of the underground depth. The above conclusions can be mutually verified in the aspects of stress-strain, water pressure and permeability coefficient. Furthermore, the relationship between the minimum injectable water pressure (MIWP) of fractured rock mass with different dip angles and underground depth is obtained, the expression of seepage coefficient of single fracture specimen with different dip angles and underground depth under three-dimensional stress is summarized, and the relationship between eh and em of single fracture specimen with different fracture dip angles is determined. This study provides a basic reference for the design scheme of hydraulic conductivity of fractured zone or fractured rock mass with different underground depths.

Mechanism of Loading Fracture of Coal Mass and Formation of Oil and Gas Disaster Channel in Coal and Oil Resources Costorage Area

Fracture structure characteristics are crucial for determining the fracture mechanism of coal mass and the migration law of oil and gas disasters. In this study, computed tomography (CT) scanning technology and fractal theory were used to investigate the damage process of coal mass in the common storage area of coal and oil resources under uniaxial compression as well as the degree of damage of high-pressure oil and gas diffusion to the surrounding coal and rock mass. Uniaxial compression and acoustic emission signal acquisition tests of coal mass were conducted. The relationship among the load evolution law of coal samples at different positions around the oil well and the failure mode of key failure positions were further analyzed. Finally, the formation mechanism of coal load fracture and oil and gas disaster channel in coal and oil resource costorage area was investigated. The test results showed the following: (1) High-pressure oil and gas diffusion degrades the mechanical properties of coal mass in varying degrees. The closer the coal sample to the oil well, the greater the fracture development degree, the fracture density, the fracture fractal dimension of coal sample, and the severity of the coal mass damage and the lower the compressive strength, the acoustic emission event number, and the cumulative energy. (2) The lateral diffusion of high-pressure oil and gas changes the failure mode of coal samples in the common storage area of coal and oil resources. The failure modes when the sampling location is away from the oil well are step failure, conjugate shrinkage failure, and high-frequency vibration intermittent microcrack fracture. (3) Coal mass instability and coal mine oil and gas disaster in the common storage area of coal and oil resources can easily be induced by the formation of microfracture expansion, extension, and coalescence caused by coal failure, the formation of network and macrochannels of main fracture, and the instantaneous release of accumulated stress during failure.

Study on the influence mechanism of air leakage on gas extraction in extraction boreholes

Borehole leakage is an important cause of gas extraction failure. In order to study the influence mechanism of borehole leakage on gas extraction, the borehole air leakage mode was studied, and a mathematical model was established to describe the borehole air leakage of the gas extraction. In addition, this paper also analyzed the change rules of borehole air leakage and gas extraction effect at different borehole sealing depth, negative extraction pressure, degree of fracture development and borehole sealing quality. Research shows that: (1) The hole sealing depth has a certain influence on the borehole air leakage and the gas extraction effect. As the hole sealing depth increased, the borehole air leakage gradually decreased. Moreover, the net amount of gas extraction gradually decreased, but the gas extraction concentration increased to some extent, and the increasing range gradually decreased. The reasonable sealing depth of this coal seam was 9∼10 m. (2) The negative pressure of borehole extraction has little influence on borehole air leakage and gas extraction. With the increase of the negative extraction pressure, the gas extraction net amount and borehole air leakage both increased, but the gas extraction concentration decreased. In the later stage of the gas extraction, the negative pressure can be properly reduced to improve the gas extraction effect. (3) The borehole sealing quality has a great influence on the borehole air leakage and gas extraction effect. With the improvement of the sealing quality, the coal-rock permeability gradually decreased. Moreover, the borehole air leakage significantly reduced, and the gas extraction concentration increased significantly. Therefore, attention should be paid to the selection and improvement of sealing materials and sealing techniques in the actual production so that the airtightness and sealing quality of the hole sealing section can be improved.

Development Characteristics and Finite Element Simulation of Fractures in Tight Oil Sandstone Reservoirs of Yanchang Formation in Western Ordos Basin

Structural fractures have a significant control effect on the large-scale accumulation of hydrocarbons in the Yanchang Formation. Previous studies have affirmed the important role of fractures in hydrocarbon accumulations in strongly deformed zones. However, for low-amplitude structural areas, the degree of fracture development is relatively low, and their control on sweet spots of hydrocarbons has not yet formed a unified understanding. In this paper, taking the Upper Triassic Yanchang Formation in the western Ordos Basin as an example, the development characteristics, prediction method, and the distribution of fractures in tight sandstone reservoirs in low-amplitude structural areas have been systematically studied using a large number of cores, thin sections, paleomagnetism, FMI logging, acoustic emission, productivity data, and finite element method. The research results showed that the Yanchang Formation in the study area mainly develop high-angle and vertical fractures, which were formed by regional tectonic shearing. Fractures are mainly developed in the fine-grained and ultra-fine-grained sandstones of the distributary channel and estuary bar microfacies, while the fractures in the medium-grained sandstones of the distributary channel and the mudstones of the distributary bay are relatively underdeveloped. The core fractures and micro-fractures of the Yanchang Formation all have the regional distribution characteristics, and the fracture strikes are mainly between NE50° and NE 70°. Moreover, the finite element method was used to predict the fractures in the target layer, and the prediction results are consistent with the actual distribution results of the fractures. The coupling analysis of fractures and tight oil sandstone distribution showed that the existence of fractures provided conditions for the accumulation of hydrocarbons in the Yanchang Formation. The confluence and turning areas of the river channels were repeatedly scoured by river water, and the rocks were brittle and easy to form fractures. The thickness of the fractured sandstone in these areas is usually greater than 0.4 m. Moderately developed fracture zones are prone to form hydrocarbon accumulation “sweet spots,” and the fracture indexes of these areas are usually distributed between 0.8 and 1.2. However, when the fracture index exceeds 1.2, over-developed fractures are unfavorable for the accumulation of hydrocarbons.

Fracture Detection With Azimuthal Seismic Amplitude Difference Inversion in Weakly Monoclinic Medium

The rock with a vertical group of aligned fractures in the tilted transverse isotropy (TTI) background medium is regarded as an effective monoclinic medium. Thomsen parameters and fracture weaknesses can reflect the development degree of TTI background anisotropy and vertical fractures, respectively. Thus, this paper first establishes the approximate stiffness matrix of the monocline medium through the Bond transformation from Thomsen weak anisotropy theory and effective medium theory. Moreover, considering the Born scattering theory, a linearized approximate formula of PP wave reflection coefficient is further deduced. This approximate formula contains the parameters of incident angle, azimuth, tilted angle, compressional and shear moduli, density, fracture weaknesses, and Thomsen parameters. Then, the contribution of each parameter on the reflection coefficient by numerical simulation is analyzed respectively. Finally, from the perspective of Bayesian inference, an azimuthal seismic amplitude difference inversion approach incorporating Cauchy prior information and low-frequency regularization of model parameters is proposed to realize the simultaneous estimation of fracture weaknesses and Thomsen parameters. The synthetic test demonstrates that the inversion approach proposed in this article can obtain information on vertical fractures and TTI background medium and can be applied to azimuthal seismic data with moderate noise well. Furthermore, the inversion results of the synthetic test and field data illustrate the stability and feasibility of the inversion approach and also confirm the fracture weaknesses and Thomsen parameters of the monoclinic medium can be reasonably estimated by the inversion equation derived in this paper.

Numerical simulation of geothermal energy productivity considering the evolution of permeability in various fractures

The optimization of geothermal exploitation model and simulation of productivity is a hot spot in the thermal industry recently. Previously, the fracture permeability was considered as a constant during the thermal exploitation. However, the fracture permeability varies constantly during the thermal exploitation, and this causes an error in the numerical simulation of geothermal productivity. Here, the experiment in evolution of the fracture permeability in granite was performed, and the equations for evolution of the permeability in major fractures, branch fractures and natural fractures were established. On this basis, the thermal-hydro-mechanical coupling geothermal model was established to investigate the effect of well profiles, conditions for evolution of fracture permeability and natural fracture development degree on geothermal exploitation capacity. The results show that when considering the evolution of fracture permeability, the vertical well model is affected after 20 years of exploitation, and the produced thermal energy in the vertical well with developed fractures is about 4% (0.4 MW) lower than in the conventional vertical well model. Under the same conditions, the difference between horizontal wells is within 1%, which can be ignored. More heat is outputted in reservoirs with developed fractures than in those without fractures.

Identification of fractures in tight-oil reservoirs: a case study of the Da'anzhai member in the central Sichuan Basin, SW China

The Da'anzhai Member of the Jurassic Ziliujing formation in central Sichuan is a typical tight-oil reservoir with porosity and permeability less than 2% and 0.1 × 10–3 μm2, respectively. Fractures in this formation are well developed in micro- and nano-scale. However, the factors that control the fracture distribution are unclear. Additionally, the uncomprehensive and ineffective identification and evaluation of fractures in the early stage of tight-oil development makes it difficult to meet the requirements of tight-oil development. In our work, we used cores, thin sections, and a scanning electron microscope (SEM) to study the influence of the microscopic rock composition, including the shelly grains, calcite grains, and clastic grains, on the fracture development. We found that the microscopic composition of shelly grains and calcite grains separately control the development of inter-shelly fractures and shelly fractures, and intergranular fractures, and tectonic fractures. Except for a small number of dissolution fractures found in mudstone, the fractures are not well developed in the formations with clastic grains. According to the characteristics of the development degree of fracture and the resolution of the well-logs, the fractures are divided into large scale, small scale, and micro-scale. By a newly established level-by-level constraints method, we systematically identified the scale, occurrence, filling characteristics, and development degree of fractures in the Da'anzhai member by well-logs. Moreover, a quantitative model is also proposed for identifying the angles and development degree of fractures. The results show that the scale of fractures can be effectively identified by the shapes and values of resistivity logs; the occurrence, development, and filling characteristics of fractures can be semi-quantitatively evaluated by the relative amplitude difference between the matrix resistivity (Rb) and formation resistivity (RT). The results are consistent with the interpretation results by formation micro-resistivity imaging (FMI) log, which further demonstrates that the level-by-level constraint method by conventional well-logs can be used to systematically and effectively predict the fracture characteristics in tight-oil reservoirs.

A NOVEL APPROACH BASED ON FEATURE FUSION FOR FRACTURE IDENTIFICATION USING WELL LOG DATA

Accurate identification of fractures is necessary and complex for carbonate reservoir exploration. Using conventional well logs and geological data, we identify various fracture identification methods based on depth point information and waveform processing. The results show that the method based on equivalent medium theory maintains high stability and accuracy in reflecting the secondary pores in cases of unfavorable borehole environments. Both the acoustic log and dual lateral difference fractal dimensions increase in line with the degree of fracture development. The high-frequency energy information shows significantly high values in the fractured zone on a suitable scale. Finally, the fractures are characterized by a novel approach based on feature fusion. The linear predictive relationship for fracture identification via proposed comprehensive factor scores (CFS) avoids the influence of the deviation of a few variables on the stability of the overall results. Our study offers a new framework for fracture identification in the exploration and evaluation of carbonate reservoirs.

Study on Smart Numerical Simulation Mechanism in Complex Reservoir Using Embedded Discrete Fracture Model

The embedded discrete fracture model (EDFM) method is used to study the mechanism model of the reservoir with a complex fracture system, and the sensitivity analysis of the development effect is carried out. The results show that: 1) the EDFM method can directly characterize the fracture morphology and fracture attributes in the reservoir; 2) Through comparative analysis of reservoir type, capillary force, and fracture development degree, it is found that different reservoir types have great differences in development effect, fracture opening has little influence on the reservoir, capillary force and fracture density have a great influence on the reservoir development effect, and with the increase of fracture density, the reservoir development effect increases significantly.

Study on mining area of roof surrounding rock stability of mining fissure development

The “double end sealing device of drilling drilling water injection leakage” section, to the different depth of sealing hole drilling water injection test, observation in different directions and different drilling water Kong Shen missed development in mining fracture analysis at different depth of stratum loss, combined with different directions borehole sections of fissure, preliminary judgment the test site of mining roof surrounding rock fracture stability region boundary of mining; use of field test data, analysis of Yu Yang coal mine N21110 mining fissure development scope and the degree of fracture development in mining area of roof stability.

Factors controlling tight oil and gas reservoir development in the Jurassic siliciclastic-carbonate rocks in Sichuan Basin, China

The prediction of “sweet spot” of multi-lithology composite tight oil and gas reservoirs within mixed siliciclastic-carbonate sequences is a hot topic in oil and gas exploration. There are mixed lacustrine carbonate-siliciclastic rocks in the Da'anzhai Member of Ziliujing Formation in the Eastern Slope of western Sichuan Depression. The reservoir lithology is complex, and the main factors controlling the development of high-quality reservoirs are yet to be known. Based on a large number of drilling, core and thin section observation, well log and seismic data, this paper systematically studies the characteristics and factors controlling the development of the mixed siliciclastic-carbonate reservoir. The results show that the reservoir is composed primarily of coquina, sandstone, breccia and shale in lithology, and its development is mainly controlled by sedimentary microfacies, diagenesis, tectonism (fracturing) and paleogeomorphology during the sedimentary period. The effective fractures in reservoirs of the low-energy shell shoal facies are well developed with relatively good physical properties. With the change of sedimentary microfacies from low-energy shoal to high-energy shoal to arenaceous shoal, the hydrocarbon production capacity worsens step by step. Diagenesis, such as dissolution and fracturing, has a certain effect on reservoir physical properties. The palaeogeomorphic highs and slopes had well-developed fractures during the sedimentary period which are conducive to constructive dissolution, and thus they are favorable zones for reservoir development. The development of structural fractures further amplifies the influence of dissolution; thus the fracture zones are also favorable for the development of high-quality reservoirs. The favorable sedimentary facies zone and fracture development degree serve as the core factors for the formation of high-quality mixed reservoirs. The muddy limestone with certain dissolved pores and structural fractures deposited under the low-energy shell shoal setting is of relatively high-quality reservoir in the Da'anzhai Member.

Formation, preservation and connectivity control of organic pores in shale

In view of strong heterogeneity and complex formation and evolution of organic pores, field emission scanning electron microscopy (FESEM), Raman spectrum and fluid injection + CT/SEM imaging technology were used to study the macerals, organic pores and connectivity of organic pores in the lower Paleozoic organic-rich shale samples from Southern China. Combined with the mechanism of hydrocarbon generation and expulsion and pore forming mechanism of organic matter-based activated carbon, the relationships between organic pore development and the organic matter type, hydrocarbon generation process, diagenesis and pore pressure were explored to reveal the controlling factors of the formation, preservation and connectivity of organic pores in shale. (1) The generation of organic pores goes on through the whole hydrocarbon generation process, and is controlled by the type, maturity and decomposition of organic matter; the different hydrocarbon generation components and differential hydrocarbon-generation evolution of kerogen and solid asphalt lead to different pore development characteristics; organic pores mainly develop in solid bitumen and hydrogen-rich kerogen. (2) The preservation of organic pores is controlled by maturity and diagenesis, including the steric hindrance effect of in-situ hydrocarbon retention, rigid mineral framework formed by recrystallization, the coupling mechanism of pore-fluid pressure and shale brittleness- ductility transition. (3) The R o of 4.0% is the maturity threshold of organic pore extinction, the shale layers with R o larger than 3.5% have high risk for shale gas exploration, these shale layers have low gas contents, as they were in an open state before uplift, and had high hydrocarbon expulsion efficiency and strong aromatization, thus having the “congenital deficiency” of high maturity and pore densification. (4) The pores in the same organic matter particle have good connectivity; and the effective connectivity between different organic matter pores and inorganic pores and fractures depends on the abundance and distribution of organic matter, and development degree of pores and fractures in the shale; the accumulation, preservation and laminar distribution of different types of organic matter in high abundance is the prerequisite for the development and connection of organic pores, grain margin fractures and bedding fractures in reservoir.

Nuclear magnetic resonance experimental study of CO2 injection to enhance shale oil recovery

Factors affecting CO 2 flooding of shale oil reservoir were studied by nuclear magnetic resonance (NMR) experiments, the effects of time, pressure, temperature on the recovery of CO 2 flooding in shale oil reservoir were analyzed based on nuclear magnetic resonance T 2 spectrum, and the effect of fracture development degree on recovery of CO 2 flooding in shale oil reservoir was analyzed based on NMR images. In the process of CO 2 flooding, the recovery degree of the shale oil reservoir gradually increases with time. With the rise of pressure, the recovery degree of the shale oil reservoir goes up gradually. With the rise of temperature, the recovery degree of shale oil increases first and then decreases gradually. For CO 2 flooding in matrix core, the crude oil around the core surface is produced in the initial stage, with recovery degree going up rapidly; with the ongoing of CO 2 injection, the CO 2 gradually diffuses into the inside of core to produce the oil, and the increase of recovery degree slows down gradually. For CO 2 flooding in matrix core with fractures, in the initial stage, the oil in and around the fractures are produced first, and the recovery degree goes up fast; with the extension of CO 2 injection time, CO 2 diffuses into the inside of the core from the fractures and the core surface to produce the oil inside the core, and the increase of recovery degree gradually slows down. Fractures increase the contact area between injected CO 2 and crude oil, and the more the fractures and the greater the evaluation index of fractures, the greater the recovery degree of shale oil will be.

The Effect of Multi-Scale Faults and Fractures on Oil Enrichment and Production in Tight Sandstone Reservoirs: A Case Study in the Southwestern Ordos Basin, China

The Chang 8 Member of the Upper Triassic Yanchang Formation in the southwestern Ordos Basin is a typical tight sandstone reservoir and has an average porosity of 8.60% and air permeability 0.20 mD. Multi-scale faults and fractures are widely developed in these reservoirs. In this study, three-dimensional seismic data, outcrops, cores, imaging logs, and thin sections were used to classify faults and fractures at multiple scales. Combined with the oil production data, the influence of multi-scale faults and fractures on the oil enrichment and production was analyzed. The results show multi-scale faults and fractures can be divided into six levels: type-I faults, type-II faults, large-scale fractures, mesoscale fractures, small-scale fractures, and micro-scale fractures. As the scale decreases, the number of fractures increases in a power function. Type-I faults cut the caprocks and are not conducive to the preservation of oil. Type-II faults connect the source rocks and reservoirs and are migration channels of the oil source. Large-scale fractures cut the mudstone interlayer and are the seepage channel inside the reservoir. Mesoscale fractures are controlled by thick interlayers, and small-scale fractures are restricted by thin interlayers or layer interfaces. These fractures are the main seepage channels and effective storage spaces. Micro-scale fractures serve as important storage spaces for these reservoirs. The case study of oil reservoir development proves that type-I faults have the greatest impact on fluid flow, while wells drilled into the type-II faults zone have a higher oil production capacity. The oil production changes with the development degree of fractures in different scales, strikes, and positions of faults. Meso- and small-scale fractures are the key to influencing the early single-well production, and micro-scale fractures are conducive to the stable production of single wells. Consequently, multi-scale faults and fractures have significantly different effects on the oil enrichment and production of tight sandstone reservoirs, and the research conclusions can guide to the exploration and development of such similar reservoirs.

Study on the Influences of Empty Hole and In Situ Stress on Blasting-Induced Rock Damage

Blasting has been used extensively in deep underground excavation. In this paper, a numerical simulation LS-DYNA was carried out to investigate the influence of in situ stress and empty hole on rock fracture development. Based on the simulation process and existing laboratory data, the micromechanical properties of the Riedel-Hiermaier-Thoma (RHT) model were calibrated. By then, this model was used to study the effects of in situ stress and empty hole on the degree of damage and fracture development in the rock. Simulation results revealed that the difference between horizontal and vertical principal stresses can influence the degree of damage as well as the fracture initiation and development, which in turn leads to the fracture development towards a high-stress concentration area. Numerical simulation also showed that the total area of fracture development gradually decreases with the increasing in situ stress magnitudes. Under the same stress conditions, empty hole can facilitate the fracture development at 90°, while preventing the subsequent fracture development at 60° and 120°. This would result in further fracture propagation at 90°, such that the control of fracture direction can be achievable.

Quantitative Characterization and Determination of the Main Factors That Control Fracture Development in the Lower Paleozoic Shale in Southeastern Chongqing, China

The Longmaxi and Niutitang Formations are typical shale reservoirs in southeastern Chongqing. Since the Paleozoic, southeastern Chongqing has experienced multistage tectonic movement and diagenesis, resulting in the formation of a large number of natural fractures. In shale reservoirs, fractures not only provide seepage channels for oil and gas migration but also play an important role in the oil and gas reservoir space. Natural fractures provide a flow path from source rock to reservoir during oil filling and connect hydraulic fractures, matrix pores, and boreholes during production. Therefore, identifying the main factors that control the development of natural fractures in the Longmaxi and Niutitang Formations in southeastern Chongqing has a guiding significance for the efficient development of shale reservoirs in this area. By considering the Longmaxi and Niutitang Formation shales as case studies, using field outcrops, drilling cores, and other data, and conducting X-ray diffraction (XRD) analysis as well as total organic carbon (TOC) measurements, the quantitative parameter characteristics and main factors that control the development of natural fractures in reservoir shales were examined in detail. The results obtained showed that there are three types of fractures in the lower Paleozoic shale in southeastern Chongqing, which are structural, diagenetic, and abnormal high-pressure fractures. Among them, the fractures in the Longmaxi Formation shale, which are relatively abundant, predominantly consist of low-angle and high-angle inclined fractures, while the Niutitang Formation predominantly consists of high-angle fractures. Additionally, the investigation of fracture size and fracture density, as well as correlation analyses, showed that the fractures of the Lower Paleozoic shale are predominantly micro fractures that play a key role in improving reservoir seepage. It was also noted that the development of fractures is affected by several factors, including tectonic stress, mineral composition, organic carbon content, and rock thickness. The degree of fracture development was found to be positively correlated with brittle mineral and quartz contents, and high organic matter contents also significantly favored the formation micro fractures.

Lamellation Fractures in the Paleogene Continental Shale Oil Reservoirs in the Qianjiang Depression, Jianghan Basin, China

Based on the data of cores, thin sections, well logs, and test experiments, the characteristics and main controlling factors of lamellation fractures in continental shales of the third and fourth members of the Paleogene Qianjiang Formation in the Qianjiang Depression, Jianghan Basin, are studied. Lamellation fractures mainly develop along laminas in shales. They have various morphological characteristics such as straightness, bending, discontinuity, bifurcation, pinching out, and merging. Lamellation fractures with high density show poor horizontal continuity and connectivity characteristics. The average linear density of the lamellation fractures is mainly between 20 m-1 and 110 m-1, and the aperture is usually less than 160 μm. The density of lamellation fractures is related to their apertures. The smaller the apertures of lamellation fractures are, the higher the density is. The development degree of lamellation fractures is mainly controlled by mineral composition, type, thickness, density of lamination, contents of organic matter and pyrite, lithofacies, structural position, etc. Lamellation fractures develop well, especially under the conditions of medium dolomite content, large lamination density, small lamination thickness, and high total organic carbon (TOC) and pyrite contents. The influences of lithofacies on the lamellation fractures are complex. The lamellation fractures are most developed in carbonaceous layered limestone dolomite and carbonaceous layered dolomite mudstone, followed by stromatolite dolomite filled with carbonaceous pyroxene. The fractures in the massive argillaceous dolomites and carbonaceous massive mudstones are poorly developed. No fractures can be found in the carbonaceous dolomitic, argillaceous glauberites or salt rocks with high glauberite content. Structure is also an important factor controlling lamination fractures. Tectonic uplifts are beneficial to the expansion and extension of lamellation fractures, which increases fracture density. Therefore, when other influence factors are similar, lamellation fractures develop better in the high part of the structure than in the low part.

Water abundance prediction of sandstone aquifers based on the distance function

To establish a more accurate roof water abundance evaluation model and mitigate the difficulties in roof water abundance prediction and evaluation in the coal mine production process, the 2101 working face of the Yingpanhao coal mine was studied. Combined with the hydrogeological characteristics and drilling data of the study area and based on the lithology and structure, four controlling factors, including the equivalent thickness of sandstone, the lithological coefficient of sandstone, the degree of fracture development, and the sand-mud interlayer coefficient, were selected as evaluation indexes for the prediction of roof water abundance zones of sandstone aquifers. In the case of limited borehole data, the distance function was used to couple the improved analytic hierarchy process with the entropy weight method. The errors caused by subjective and objective factors were effectively reduced, and on this basis, a water abundance prediction model of roof sandstone was built. With the use of geophysical exploration results, the model predictions were combined with geophysical survey data to generate a comprehensive water abundance zone map, which divided the study area into three zones according to the water abundance. The results show that the high water abundance areas were mainly concentrated in the west of the working face, and the water abundance throughout the study area was generally low. The prediction results were verified by the data from drainage holes exposed by drilling, which provided a scientific and practical method for the prediction of the water abundance of coal seam roofs.

Brittleness evaluation of the Lower Silurian marine shale reservoirs: A case study of Longmaxi shale in Fenggang block, southern China

The brittleness of shale determines the fracturability of the shale reservoir and has a great influence on the exploration and development of shale gas. Therefore, prediction of brittleness and evaluation of fracturability of shale are very important in finding favorable areas for shale gas. We have used the mineral composition, rock-mechanics experiment, logging evaluation, 2D seismic data inversion, and fracture development degree to evaluate and analyze the vertical and plane brittleness characteristics and the main controlling factors of Longmaxi shale. In addition, we established the fracturability index of shale based on the brittleness index, critical strain energy release rate, and fracture toughness. The results indicate that the brittle mineral content of Longmaxi shale in the Fenggang block is between 69% and 90%, and shale samples are prone to brittle deformation by microscopic observation. Brittle deformation has a positive effect on the porosity and percolation ability of shale; the fractures in the upper and lower parts of the Longmaxi shale are relatively developed, and the degree of core fracture development is consistent with rock brittleness. The brittleness distribution of 2D seismic data inversion is in good agreement with the brittleness predicted by well logging; shale reservoirs with low fracture toughness, low critical strain energy release rate, and high brittleness index have high fracturability. Therefore, the research on shale brittleness and fracturability of Lower Silurian Longmaxi shale is expected to have important guiding significance for shale-gas exploration and development in southern China.