Stimulation of unconventional tight oil formations via horizontal wells has seen increasing cases of fracturing infill wells in recent years. The effectiveness of such a strategy is mainly dependent on the proper characterization of the stress evolution and an accurate forecast of the subsequent fracture propagation in the region neighboring the infill wells after considering the production performance and the injection schemes of the parent wells. In this respect, a comprehensive approach was proposed to simulate the stress evolution caused by the production and injection of the parent wells. The approach can also predict the upcoming fracture propagation behavior of the infill well. It was found that depletion in the parent wells can result in dramatic changes of the stress field, highlighted by apparent decreases in the magnitude of the minimum horizontal stress and changes in its orientation. Controlled injection in the parent wells can reproduce the original stress field, which favors the transverse extension of the fracture network in the infill well. In contrast, soaking well alone has little effect on improving the stress field. Therefore, this study suggests optimal injection schemes for parent wells and provides insights for fracture cluster designs in an infill well, eventually leading to maximized productivity of the infill well.

Large-scale hydraulic fracturing is a prevalent technique for exploiting low-porosity and low-permeability shale reservoirs. The propagation and morphology of the hydraulic fracture in the laminated shale formations are significantly influenced by densely developed bedding planes, which can be classified into three categories: continuous, transitional, and discontinuous, with each characterized by distinct properties. This categorization complicates the prediction of the fracture propagation and the optimization of fracturing plans. In this research, a comparative study was proposed to describe fracture propagation and morphology in laminated shale with different types of bedding planes, employing the hydromechanically coupled discrete element method (DEM). The simulation results revealed that bedding planes of different types produce distinct impacts on the fracture propagation, leading to diverse fracture morphologies. In particular, it was found that the plane thickness affected the fracture propagation under low permeability, but the impact was insignificant under high permeability. Under different orientation angles, the continuous bedding planes showed distinct impacts on fracture propagation, while the transitional and discontinuous bedding planes consistently captured the hydraulic fracture. Moreover, the fluid viscosity and injection rate significantly influenced continuous and transitional bedding planes while having a minor effect on the discontinuous bedding planes. The optimal injection schemes incorporating varying injection rates or fluid viscosities were investigated. In addition, the impacts of small-scale bedding planes on fracture propagation were revealed. Furthermore, the bottom hole pressure variation and seismic event distribution were presented to provide complementary evidence of the fracture propagation. The simulation results can promote a comprehensive understanding of the fracture development in shale reservoirs.

The sweet spot and fracability evaluation of shale reservoirs are crucial benchmarks for shale reservoir exploration. Currently, the the similarity in the parameter selection and mathematical approache for sweet spot and fracability evaluation gives rise to several issues, including conceptual confusion, ambiguous interrelationships, and redundant evaluations. There is an urgent need to clarify the connection between sweet spots and fracability while establishing a standardized evaluation method. This study systematically investigates the commonly-used sweet spots and fracability evaluation methods and clarifies the relationship among geological sweetness index (GSI), engineering sweetness index (ESI), and fracability based on conceptual understanding and evaluation methods. Building upon this groundwork, a comprehensive sweet spot-based fracability evaluation method is proposed. This method leverages on-site geology and engineering parameters to calculate GSI and ESI, and then constructs the reservoir fracability index (FI) and regional compressibility evaluation plate, which facilitates a rapid and quantitative assessment of the fracability of single wells. By selecting the geological and engineering parameters of 10 shale gas wells in the Weiyuan block of the Sichuan Basin, GSI, ESI, and FI are calculated using the established method. It is observed that FI has a positive correlation with the actual testing well gas production with an equal proportion for different wells. This method can provide effective scientific guidance for optimizing the well trajectory, increasing the drilling encounter rate with high-quality reservoirs, and optimizing the construction plan of reservoir fracturing.

Understanding the changes in the pore structure of reservoirs in the presence of CO₂ is critical for carbon neutrality, especially for shale oil reservoirs with ultra-low permeability and porosity. However, studies examining the alteration in the pore structure of shale oil reservoirs that have been treated with CO₂ remain limited. Thus, in this paper, nuclear magnetic resonance (NMR) and low-temperature nitrogen adsorption (LNA) technologies were employed to address this issue. The results show that the permeability and porosity of shale oil reservoirs increase after exposure to CO₂. The permeability improves by 49.03%, and the porosity increases by 29.54%. The NMR results reveal that the pore structure of shale oil reservoirs is altered. Specifically, increases of 11.14%, 74.54%, and 990.02% in the presence of CO₂ are observed for micropores, mesopores, and macropores, respectively. CO₂ is more sensitive to macropores, followed by mesopores and micropores. Furthermore, the LNA results indicate that some small pores expand into larger pores, leading to a decrease in the number of small pores and an increase in the number of larger pores. Combining the results of NMR and LNA, it is found that the increase in big pores is the reason behind the enhancement in permeability and porosity. This paper sheds light on the change in the pore structure of shale oil reservoirs after exposure to CO₂, further guiding the evaluation of CO₂ storage capacity.

The increasing application of pad drilling technology in unconventional reservoirs has significantly enhanced development efficiency and reduced operational costs. However, as well spacing decreases, hydraulic fracturing interference between wells has emerged as a critical factor affecting fracture geometry, propagation, and production performance. Despite the adoption of advanced multiwell fracturing techniques, such as zipper fracturing (ZF) and modified ZF (MZF), the mechanisms of stress interference and their influence on fracture propagation remain poorly understood. This study employs the Extended Finite Element Method coupled with the Cohesive Zone Model (XFEM‐CZM) to analyze the effects of stress interference and fracture geometry variations under different fracturing scenarios. The results reveal that when the fracture spacing is large, the stress interference induced by two fractures conforms to the principle of superposition. However, at smaller fracture spacings, stress interference significantly impacts the propagation process and geometry of subsequent fractures, resulting in nonlinear stress interference characteristics. Using the M block in Xinjiang as the study area, this research further analyzes the effects of ZF, MZF, and different fracturing sequences on fracture geometry in multiwell systems. The findings demonstrate that MZF generates longer fractures while ZF exhibits more symmetric propagation characteristics. Moreover, combining MZF with an optimized fracturing sequence maximizes the stimulated reservoir volume (SRV), minimizes casing deformation, and avoids frac‐hit. These results provide theoretical guidance and practical insights for optimizing hydraulic fracturing designs in multiwell pads within tight reservoirs.