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

The distribution of lower-order faults affects the development of oil and gas and the distribution of remaining oil, which is also the key to the development of fault block reservoirs in eastern China. The lower-order faults are characterized by short extension, small fault displacement, and difficult identification by traditional seismic interpretation methods. Therefore, it is necessary to predict the development law of faults in combination with the paleostress field during the fault formation period to improve the accuracy of fault seismic interpretation. In this paper, on the basis of reservoir structural analysis and rock mechanics experiments, the geological and mechanical model of the Sanduo period is established to predict the simulation of the paleostress field during the development of lower-order faults. The development of lower-order faults in rift basins is mainly controlled by the minimum principal stress and stress difference, and the shear stress in the profile controls the tendency of faults. Controlled by the paleostress field, the occurrence of lower-order faults in the Weizhuang area, Gaoyou Sag, and Subei Basin is very complex, and the strike of the fault is distributed as a tension shear broom on the plane. The prediction results can provide a reference for the excavation of remaining oil and the development of oil and gas. [ABSTRACT FROM AUTHOR]

Understanding existing fractures is critical to achieving high and stable yields in low‐permeability reservoirs, tight sandstones, and shale reservoirs. When multiple fracture sets are present in a reservoir, accurately determining the seepage direction of fluids is critical for the deployment of the well network, fracture reformation, and the design of horizontal wells. In this paper, based on conventional logging data calculations and rock triaxial mechanical experiments, a heterogeneous finite element model was established using the finite element software. The densities and the occurrence of the natural fractures were calculated by numerical simulation of the paleostress field; the fracture apertures were calculated numerically by a current in situ stress simulation. Using a combined static coordinate system and dynamic coordinate system approach, a model suitable for determining the 3D permeability tensor of multiple fractures is established, and the formula of the permeability tensor is given. In addition, by adjusting the rotation angle and flip angle in the dynamic coordinate system, the permeability of elements is predicted for different directions. The results show that fractures mainly trend in the NE (45°) and ESE (120°) directions. Additionally, the results of differential strain, microseismic monitoring, stress relief, and regional borehole collapse observations indicate that the dominant direction of the in situ stress field is ENE (70°). The principal values of maximum permeability in the reservoirs in the Chang 71 layer mainly range from 0.05 to 2 × 10−3 μm2, and in the horizontal plane, the direction of maximum principal permeability ranges from 56 to 124°. [ABSTRACT FROM AUTHOR]

In eastern China, oil and gas reservoirs mainly have fault block traps. Low-order, small-scale faults are well developed, with small fault displacements and short extension lengths, but are difficult to identify by traditional seismic methods. Based on paleotectonic interpretations, rock mechanics experimental results and stratigraphic data, combined with seismic interpretation, structural evolution and fault activity analysis, a geomechanical model of the Majiazui area in the Dongtai Depression in the Subei Basin is established to construct a three-dimensional model of the paleostress field. Finally, the control of the principal stress, shear stress, and strain energy on the occurrence and development of low-order faults is quantitatively analyzed. This research shows that fault activity was weak during the Funing period (65-53 Ma), became strong during the Dainan period (53-46 Ma) and was strongest during the Sanduo period (46-37 Ma). The Sanduo period was the main period during which the low-order faults in the Majiazui area were formed. The minimum principal stress and strain energy controlled the development of low-order faults in the Majiazui area. The low-order faults are most developed in areas that experience high minimum principal stress and strain energy. In the Sanduo period, the minimum principal stress in the Majiazui area was tensile in the north–south direction, and the maximum principal stress was vertical. In an extensional stress environment, lateral shear stress controls the strike of faults, while vertical shear stress controls the inclination of faults. The faults in the northern Majiazui area are inclined mainly to the south, and the faults in the southern Majiazui area are inclined mainly to the north. These results are of practical importance for evaluating the oil–water relationship in oil-bearing fault blocks, solving the conflict between injection and production wells and improving oil recovery. • Quantitative prediction of low-grade faults based on the finite element method. • The regions of low-grade faults development are predicted by the principal stress and the strain energy density. • The shear stresses are used to predict the dip direction of the low-grade faults. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing and horizontal well technology can effectively improve reservoir production but can affect reservoir reconstruction through many factors, including the reservoir lithology, reservoir stress state and natural fractures. Considering tight sandstone as an example, the finite element method (FEM) and the optimal trajectory design scheme for a horizontal well are used in this study to determine the influence of the stress state, mechanical parameters and natural fractures of rock on the effects of hydraulic fracturing. Core reorientation via a paleomagnetic method is used to determine the natural fracture orientations in different well blocks. The in-situ stress magnitudes and directions are determined using microseismic monitoring of hydraulic fracturing, borehole breakout analysis, difference strain analysis and acoustic emission experiments. A heterogeneous geomechanical model is formulated based on log interpretations of the rock mechanical parameters of 111 wells and the results of triaxial mechanical experiments. The FEM is used to determine the in-situ stress field in the Dongrengou block. A predictive model for the hydraulic fracture propagation direction is formulated considering the in-situ stress, geomechanical model and natural fracture orientation. The results show that the mechanical parameters of the Dongrengou block are characterized by planar anisotropy and three-dimensional heterogeneity, where the rock mechanical parameters vary considerably across the plane and are vertically stratified. Using these results in conjunction with the drilling directions and oil production of 14 horizontal wells shows that oil and gas recovery is maximized by orienting horizontal wells perpendicular to the propagation direction of hydraulic fractures. • A method for quantitatively optimizing the drilling azimuth of horizontal wells in tight sandstone reservoirs. • The planar anisotropy of the mechanical parameters leads to the absence of a conjugate fracture set. • The direction of hydraulic fractures is affected by the tensile strength, stress difference and natural fractures. • The in-situ stress and natural fractures are vertically stratified in the western Ordos Basin. [ABSTRACT FROM AUTHOR]

• A numerical simulation method based on the finite element method is presented for determining fracture aperture. • The mathematical relationship between multiple factors and fracture aperture is determined. • When the fracture spacing is less than 2 m, the fracture spacing is positively correlated with the fracture aperture. Fracture aperture is the main source of uncertainty in reservoir fracture modeling; this uncertainty is due to the difficulty of accurately quantifying underground fracture aperture and the limited understanding of the main factors controlling fracture aperture and the mechanical deformation process. The theoretical evaluation model of underground fracture aperture is a key aspect of fracture characterization and modeling of tight sandstone reservoirs. Based on the self-similarity theory of rock fractures and considering the 3D morphology of a fracture surface, a finite element geomechanical model of a fracture surface is established. A combination of physical experiments and numerical simulations is used to determine the fracture apertures in underground reservoir conditions. Furthermore, a theoretical model of fracture aperture is proposed under the constraints of the in situ stress, rock mechanics parameters, fracture occurrence, fracture scale, fracture filling characteristics and fracture surface characteristics. According to the vertical distribution model of in situ stress, the theoretical average fracture aperture in rock at different depths is determined. With increasing burial depth, the fracture aperture decreases. When the horizontal stress difference is greater than 10 MPa, the effect of the in situ stress difference on the average fracture aperture decreases. Among the three fracture filling patterns considered, the average effective aperture of fractures with uniform filling is the largest. When the fracture spacing is less than 2 m, the fracture aperture increases with increasing fracture spacing; when the fracture spacing is greater than 2 m, the fracture aperture is basically unaffected by the fracture spacing. [ABSTRACT FROM AUTHOR]

To ensure the effectiveness of fracturing treatment and improve the production reserves of individual wells, it is important to accurately predict the in situ stress distribution to prevent the collapse of horizontal wells. From the perspective of reservoir development, under similar structural environments, microscopic factors, such as reservoir and petrophysical characteristics, are often the key factors that affect reservoir fracturing differences and drilling design in different well areas. Therefore, we need to understand the microscopic factors that affect the distribution of in situ stress. Here, we have determined the three-dimensional distribution of in situ stress in the Ordos Basin of China through the finite element numerical simulation method and revealed the influencing factors. By establishing a reservoir heterogeneity model and a geomechanical heterogeneity model and with the finite element method, the 3D distribution of reservoir physical parameters and in situ stress can be predicted, and the effects of the reservoir physical properties, fluid properties, rock acoustic velocity and mechanical properties on the in situ stress can be determined. The clay content is found to be negatively correlated with the horizontal stress difference (Δ σ) and horizontal principal stress difference coefficient (K h). Rock density, porosity, and permeability have little effect on Δ σ and K h. Fluid properties affect the in situ stress magnitude. If the water saturation is greater than 60%, this parameter has a negative correlation with Δ σ and K h. Poisson's ratio has a weak effect on Δ σ but a significant negative correlation with K h. [ABSTRACT FROM AUTHOR]

The increasing complexity of reservoirs, particularly shale reservoirs, demands accurate information on the formation composition, mineralogy, and mechanical parameters for effective exploitation. The development of geochemical and geophysical logging technology allows the investigation of the correlations between the elemental, mineral contents, and the mechanical parameters of shale. Taking the Lower Cambrian Niutitang Formation shale in the Fenggang block in northeastern Guizhou as an example, 889 data sets of the main elemental (Si, Ca, Fe, S, Ti, Gd, K, Mg, and S) concentrations and minerals (carbonate, pyrite, clay, quartz, feldspar, and mica) from Elemental Capture Spectroscopy logging and rock mechanics parameters (dynamic Young's modulus, dynamic Poisson's ratio, shear modulus, bulk modulus, and brittleness index) determined via Sonic Scanner logging data in the target zone of the study area are collected. We investigated the relationships between the mechanical parameters and the elemental, and mineral concentrations by Spearman rank correlation analysis. The influence of different Si concentrations on the mechanical parameters of the rock (positive/negative, and significance) is discussed based on the mineral contents determined via X-ray diffraction of 24 experimental samples. When 22.1% < DWSi (dry weight fraction of the silicon content as a percent) < 28.40% and 28.40% < DWSi <34.20%, the static Young's modulus, shear modulus, bulk modulus, and brittleness index exhibit a highly positive/negative correlation that is significant with the increase/decrease in DWSi, WCLA (dry weight fraction of clay), and WQFM (dry weight fraction of quartz, feldspar, and mica) contents, respectively. DWSi = 28.4% is suggested as the turning point for the correlation change (positive/negative, significance). Thus, there exists uncertainty in the determination of the mechanical parameters of rock within certain elemental content ranges. More mathematical models and factors need to be considered to adequately characterize the mechanical parameters of rock. • Rock's elemental and mineral and mechanical parameters characterization in the Lower Cambrian Niutitang Formation shale. • Spearman rank analysis on elements, minerals and mechanical parameters of a rock. • Discuss the degree of Spearman rank correlations regarding elements and minerals on mechanical parameters. • Discuss the possible mineral sources of silicon in different ranges. [ABSTRACT FROM AUTHOR]

The Lower Paleozoic Silurian Longmaxi Formation shale in the Sichuan Basin of southern China has achieved commercial exploitation in recent years, and has become a rapid growth point in China's oil and gas reserves. However, the Longmaxi Formation on the peripheral margin of the Sichuan Basin has not made substantial progress. Fractures, that as fluid reservoirs and migration channels, play an important role in shale gas exploration and development. Based on data from field outcrop observations, drilling cores, well logging, geochemical tests, microscopic thin section observations, and focused-ion-beam scanning electron microscope (FIB-SEM) combined with energy dispersive spectrometer (EDS) analysis and cathodoluminescence microscope (CLM) technology, the development characteristics of fractures in the Longmaxi Formation SY-5 well in the Sangzhi block of Sangzhi-Shimen synclinorium, NW Hunan Province, are described systematically, and their important roles, primary dominant factors and effectiveness in the shale gas reservoir are discussed. The results showed that tectonic factors are the dominant factors of shale fracture development, followed by nonstructural factors. The fractures were mainly ductile shear fractures, with short extension distances and small apertures, and they had at least two openings in the burial history. The statistical fracture density of drilling cores was no obvious correlated with the contents of total organic carbon (TOC) (0.18%–2.49%) and clay minerals (13.0%–34.0%), and the same with brittle quartz (32%–72%), which indicates that they have controlled the development of fractures together. These results could be used for estimating effective permeability, fracture distribution, numerical simulation, well deployment, hydraulic fracturing and reserves calculation in the processes of exploration and development. • Shale reservoir and fracture characterization in the Lower Silurian Longmaxi Formation. • The fracture characterization method employs from field outcrop and laboratory observations, and geochemical tests, using CLM and FIB-SEM technology combined with EDS analysis are employed to study the relationship between fractures. • Discuss the main controlling factors of fractures, relationships and effectiveness in shale. [ABSTRACT FROM AUTHOR]