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The evaluation of gas content plays a key role in determining the efficient development of shale reservoirs. To better understand the gas–water miscible competitive adsorption mechanism in the pore-fracture system of water-bearing shale, the water vapor adsorption experiment of dry shale and the methane gas adsorption experiment of water-bearing shale were carried out to compare and analyze the competitive adsorption laws of formation water and methane gas on the shale surface under different temperatures, pressures, and water saturation conditions. Moreover, based on the molecular layer adsorption theory, the gas–liquid–solid interface competitive adsorption model considering the coupling of temperature, pressure, and water saturation was established to predict the actual gas content of water-bearing shale reservoirs. The results show that the main controlling factors affecting the adsorbed gas content of water-bearing shale are specific surface area, temperature, and water content in turn according to weight coefficients; Compared with the gas–solid adsorption model, the gas–liquid–solid competitive adsorption model has higher accuracy in calculation results, which are much closer to the desorption data of shale cores in Jiaoshiba area, Fuling, China. Hence, the competitive adsorption model provides a theoretical foundation for reasonably evaluating the exploitation potential of shale gas reservoirs. [ABSTRACT FROM AUTHOR]

There are still some major issues in the study of fracture network evolution during multi-cluster fracturing in shale reservoirs, such as the simplistic assumption that reservoir rocks are homogeneous and completely brittle, without considering that real rocks are often aggregates of different minerals and may present ductility and heterogeneity. It is important to study the evolution of the multi-cluster fracture network in quasi-brittle shale reservoirs to improve the stimulated reservoir volume (SRV). For this purpose, a 2D, coupled stress-seepage-damage field multi-cluster fracturing numerical model with global cohesive zone was developed in this paper. We conducted triaxial compression acoustic emission experiments using real shale samples and developed a generic trapezoidal TSL softening model that includes triaxial effects. The globally embedded 0-thickness cohesive elements ensures that hydraulic fractures can propagate randomly and thus reflect stress interference among multiple clusters. At the same time, the X-ray diffraction (XRD) experiment was used to determine the mineral content of rock, and the finite element mesh was then processed using the Weibull distribution probability density function to simulate the mineral heterogeneity of rock. In addition, the dynamic distribution of injection fluid flow during multi-cluster fracturing is implemented based on the Bernoulli equation. The cohesive parameters were validated by Brazilian splitting test, and the model was then used to parametrically study the evolution law of the fracture network and the variation characteristics of the flow rate into each cluster. The results show that using conventional bilinear TSL will result in a larger SRV than trapezoidal TSL, and reservoir heterogeneity may further exaggerate the drainage area when using bilinear TSL model. In addition, compared with a homogeneous isotropic reservoir, a highly heterogeneous reservoir has a more balanced flow rate into each cluster during multi-cluster fracturing, and this can significantly increase the complexity of fracture network. By increasing the number of clusters, it is possible to alleviate the stress interference on some internal clusters, which may also promote frequent fracture merging around the wellbore. Compared with increasing the number of clusters, reducing the perforation diameter can better compensate for the stress interference suffered by internal clusters, and make the flow rate into each cluster more balanced, thus improving the SRV. Highlights: The trapezoidal traction separation law can describe the damage softening behavior of quasi-brittle reservoirs. The fracture network formed using the bilinear traction separation law is more complex than the trapezoidal traction separation law. Heterogeneous reservoirs may form a more complex fracture network than homogeneous reservoirs during multi-cluster hydraulic fracturing. Uniformly increasing the number of clusters with small spacing has a limited effect on the uniformity of the flow rate into each cluster. Reducing the perforation diameter is an effective method to balance the flow rate into each cluster by weakening the effect of stress interference. [ABSTRACT FROM AUTHOR]

The injection of high‐pressure fluid and the propagation of hydraulic fractures (HF) can impact the stress state of natural fractures (NF) in shale reservoirs, which will lead to shear slip in NF that are in a critical stress state. However, the NF mechanics response mechanism during hydraulic fracture (HF) approximation has not been well investigated, especially under triaxial stress and injection conditions. This work focuses on the NF shear‐slip characteristics during HF through experimental investigation. Each shale sample contains a single rough NF in the tests, and high‐pressure fluid is injected through a pre‐drilled hole at the top of the sample. Meanwhile, the deformation (axial deformation and radial deformation), axial stress, and injection pressure of the sample during the experiment were recorded in real‐time. The experimental results show that the NF will initiate in tension or shear mode during the HF approaching process, and the fracture will produce shear slip in both initiation modes. In addition, the deviator stress, confining pressure, hydraulic fracture approach angle, and fracture surface roughness all affect the degree of shear slip in natural fractures (NF). The results further reveal the mechanical response mechanism of NF during hydraulic fracturing and provide a reference for optimizing hydraulic fracturing process measures. [ABSTRACT FROM AUTHOR]

• Topology structure and occurrence strike of natural fractures are determined based on geo-statistics. • Fracture propagation model in transversely isotropic shale reservoir is established under seepage-stress coupling. • Effect of natural fractures with different topology structures on hydraulic fracture propagation is studied quantitatively. Natural fractures tend to further promote fracture network which is initiated by hydraulic fracturing. However, how natural fractures with various topology structures impact hydraulic fracture propagation in continental shale reservoir is still unclear. With Fullbore Microscan Imager (FMI) and geo-statistics of Chang 7 reservoir in Yanchang Formation, typical natural fractures are classified into three topology structures, which are natural fractures with type I, type II and type III nodes respectively, and the average occurrence strikes of natural fractures with type I, type II and type III nodes are SW260°, SW255° and SW255°. Coupling with seepage field and stress field, a model simulating fracture initiation and propagation is established considering the transversely isotropic property of continental shale reservoir. On this basis, the role of typical natural fractures with topology structure for hydraulic fracture propagation is studied qualitatively and quantitatively by Finite Element Method (FEM) with globally embedded cohesive element. Natural fractures with type II nodes have largest effect on inducing hydraulic fracture propagation, hydraulic fractures can communicate most easily with them, and also the length and velocity of hydraulic fracture propagation are the longest and fastest, so fracture network is the most complex after fracturing in naturally fractured continental shale reservoir with type II nodes, which follows by the effect of hydraulic fracture propagation in naturally fractured continental shale reservoir with type III nodes. In comparison, natural fractures with type I nodes have insignificant impacts on hydraulic fracture propagation. The research results are conductive to fracturing design in continental shale reservoir. [ABSTRACT FROM AUTHOR]

In different geological and engineering conditions, it is important to study the stress interference of multiple fractures and interaction of natural fractures in "well factory" fracturing to improve the complexity of fracture networks. For this purpose, a 2D, coupled stress-seepage-damage field cohesive model with random propagation of hydraulic fracture was developed, and a random function was used to simulate the natural fractures distribution in a real reservoir. Orthogonal test was designed to investigate the geological and engineering parameters impacting the evolution of zipper fracturing and modified zipper fracture networks. The results show that in naturally fractured reservoirs, hydraulic fracture can open natural fractures and promote the formation of complex fracture networks by combined, induced manner. Higher stress differences lead to simpler fracture networks and straighter hydraulic fracture propagation paths; When pre-existing natural fracture is longer, the propagation direction of hydraulic fracture is more likely to be paralleled with natural fracture; However, the stress shadow effect of multiple fractures may potentially lead to failure of some hydraulic fracture initiation. In addition, the fracture initiation sequence of multiple fractures has a significant effect on the evolution of the fracture networks. The results of the study have important significance for parameter optimization of well factory fracturing in naturally fractured reservoirs. [ABSTRACT FROM AUTHOR]