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The temperature of a deep shale reservoir may reach more than 100°C, and the effect of thermal shock on shale hydraulic fracturing has rarely not been considered in previous studies. Based on mesoscopic damage mechanics and the finite element method, a thermo-hydro-mechanical-damage (THMD) coupling model considering temperature, seepage, stress, and damage fields was constructed to investigate the effects of reservoir temperature, convective heat transfer coefficient (h), in-situ stress difference and bedding plane angle (α θ) on shale hydraulic fracturing. The results show that multiple hydraulic fractures (HFs) can occur under thermal shock and that HFs control the distribution of seepage, temperature, and stress fields. Reservoir temperature, in-situ stress difference and α θ are primary factors affecting hydraulic fracturing, whereas h is a secondary factor. When the reservoir temperature rises from 50°C to 150°C, the initiation and breakdown pressures decrease by 65.5% and 16.7%, respectively. HFs cross the bedding plane more easily, and fracture complexity is obviously enhanced. A higher h is favourable for slightly reducing the initiation and breakdown pressures, but it has little influence on the fracture complexity. Once the in-situ stress difference is low, there is a high fracture complexity, but HFs are more easily captured by bedding planes to limit the propagation of fracture height. When the in-situ stress difference is high, HFs are more likely to form bi-wing fractures. Whether α θ is too large or small, it is not conducive to improving the fracture complexity. In this study, when α θ is 30°, HFs and bedding planes intersect to form a fracture network. Essentially, thermal shock plays a key role in reducing the initiation pressure and forming multiple HFs during the fracturing process, and fracture propagation mainly depends on the injection pressure. The results can serve as reasonable suggestions for the optimization of shale hydraulic fracturing. [ABSTRACT FROM AUTHOR]

Shale reservoirs are usually buried deep, and contain lots of complex natural fractures (NFs) under the influence of tectonic stress and faults. Previous studies usually focus on small-scale reservoir models and there is also a lack of research that completely focuses on NF characteristics. Using the continuous-discontinuous element method (CDEM), a Hydro-Mechanical (HM) coupling model was constructed to explore the propagation mechanism of hydraulic fracture (HF) and the effects of NF characteristics on HF propagation. The results show that as the HF approaches the NFs, the NFs preferentially tend to undergo shear failure, resulting in a decelerated HF propagation rate and an increased injection pressure. After water enters the NFs, the HF rapidly propagates, so the fracture area swiftly increases and the HF has a more tortuous fracture morphology. The NF orientation (α) and the internal friction angle (φ) exhibit a pronounced and distinct effect on HF propagation. Specifically, when α is 30°∼60°, the HF propagation is more tortuous and complex, so it is easier to form a larger transverse reconstruction range. When the NF density (η) is no less than 0.12, the HF tends to open the NFs and form a more tortuous morphology, but a higher η does not necessarily imply a higher fracture complexity. A longer NF (l > 12 m) benefits to improve the longitudinal reservoir reconstruction scope, but has little impact on the overall reconstruction effect. In the presence of stress shadow effect, it is improbable for the HF to completely open the intersecting NFs; instead, it typically opens only a segment or one of them. Consequently, it may be challenging to establish a fracture network with the simultaneous development of primary and branching fractures. Additionally, some water leakage into the inactive NFs gives rise to a decline in both the fracture length and fracture area. The findings can offer theoretical insights for refining the fracturing design in shale reservoirs with NFs. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing, a production enhancement technique, is widely used in the development of unconventional oil and gas reservoirs. The formation of a complex network of hydraulic fractures that connect with natural fractures is crucial for hydraulic fracturing in unconventional reservoirs. However, the current understanding of vertical fracture propagation behavior under the influence of variations in the mechanical properties of interbedded rock is insufficient to meet the requirements for simulations of unconventional reservoirs under complex geological conditions. In this study, a three-dimensional discrete grid method was employed to establish a model of three-dimensional fracture propagation. Numerical simulations were conducted to investigate the vertical growth of fractures while considering the influence of bedding planes. The effects of formation factors (stress, Young's modulus) and bedding planes (cohesion, density) on the height growth of hydraulic fractures were explored. The results indicated that the interbedded stress contrast, Young's modulus contrast, and bedding planes collectively controlled the height growth of hydraulic fractures. The height of hydraulic fractures decreased with increasing minimum horizontal principal stress of adjacent layers. When the minimum horizontal principal stress of adjacent layers exceeded the vertical stress, hydraulic fractures gradually deflected into the horizontal plane. Adjacent layers with large values of Young's modulus promoted the height growth of hydraulic fractures, while adjacent layers with small values of Young's modulus inhibited the height growth of hydraulic fractures. The presence of bedding planes further suppressed height growth, and the degree of suppression was related to the cohesion and density of the bedding planes. Weaker cohesion and higher density resulted in greater suppression. The results of this study provide a reference for the design and optimization of hydraulic fracturing treatments in unconventional oil and gas reservoirs. [ABSTRACT FROM AUTHOR]

Methane deflagration fracturing is a new reservoir stimulation method that serves the efficient development of shale gas reservoirs. However, the propagation law of deflagration fractures is still unclear. In this paper, a numerical model considering the effect of stress wave impact and gas drive of deflagration fracturing was established based on the continuum–discontinuum element method (CDEM). The correctness of the numerical model was verified by comparing it with a laboratory experiment, the steady and unsteady analytical solutions of gas flow, and the approximate solution of fracture propagation. Then, numerical simulations of methane deflagration fracturing in vertical wells and horizontal wells under different factors were carried out to analyze the fracture mechanism. The results indicate that deflagration fracturing in vertical wells can break through the stress concentration around the borehole; the initial radial fractures are formed under the action of stress wave impact and then propagate substantially under the driving action of high-pressure gas. The in-situ stress difference affects the deflagration fracture propagation and makes the half-fracture length in the direction of maximum principal stress larger than that in the direction of minimum principal stress. The more significant the stress difference is, the more noticeable this deviation will be. When the deflagration peak pressure is high, the reservoir burst degree is large, which is conducive to enlarging the stimulation range of deflagration fracturing. Staged deflagration fracturing in horizontal wells can form 5–8 obvious fractures perpendicular to the horizontal borehole in each explosion section. A large cluster spacing and explosion section length are conducive to expanding the stimulation scope. Moreover, the propagation of deflagration fractures will be induced by the natural fractures, and the natural fracture with a considerable length or a slight angle between the dip angle and the propagation direction of deflagration fractures is more likely to be activated. • The deflagration fracturing for shale gas reservoir stimulation is proposed. • The numerical model considers the effects of stress wave and gas drive. • The propagation law of deflagration fracture is revealed by numerical simulation. • Deflagration fracturing can break the stress concentration to form complex fracture. [ABSTRACT FROM AUTHOR]