Hydraulic fracturing-induced casing deformation and fault activation have greatly hindered the safe and efficient development of shale oil and gas resources. In this study, statistical analysis, physical tests, and numerical simulation methods are used to comprehensively analyze hydraulic fracturing-induced fault activation and casing deformation processes. This study is based on the Longmaxi Formation of LZ block, a deep shale gas reservoir in the southwest Sichuan Basin (China), as a geological background. A large amount of field data on fracturing from the LZ Block is counted, and the main influencing factors are analyzed. The main factors of hydraulic fracturing-induced fault slip are (from strong to weak) parameters related to fluid injection volume, parameters related to segments and clusters, and parameters related to injection rate. Combined with physical experiments and numerical simulations, the fault activation law during fracturing has been studied. The degree of casing deformation and fault slip are linearly correlated. For hydraulic fractures to cross faults is very difficult, fault activation and casing deformation can only be mitigated as much as possible. We find that the number of clusters per segment and the injection rate are negatively correlated with the fault slip distance. Reducing the fluid injection volume can mitigate the fault slip distance. Therefore, low injection rates, low fluid volumes, and more clusters per segment are recommended for fracturing in high-risk segments. It is important to note that the scale and risk of fault activation induced by well-factory fracturing is much higher compared to single-well fracturing. In situations with extremely high risk, the injection volume should preferably not exceed 800 m³ to minimize the risk of geological and casing deformation.

Deep/ultra-deep oil and gas resources are abundant at vertical depths of more than 3,500 m, which is an important succeeding field for future oil and gas exploitation. However, a lack of understanding of the multi-scale mechanical behavior of deep reservoirs under in situ conditions, as well as an insufficiently accurate prediction of engineering sweet spots, restricts the effectiveness of hydraulic fracturing in deep shale gas exploitation. In this study, the application of cross-scale rock mechanics, digital rock core modeling, and machine learning in characterizing reservoir geomechanical properties and assessing engineering sweet spots was summarized. The challenges and future development directions of the above research elements were explored. To achieve efficient deep-resource exploitation, it’s essential to clarify the mechanical behavior of shales with different mineral compositions at micro- and macro-scales. Numerical models incorporating mineral spatial heterogeneity were developed to analyze the multifactorial synergistic mechanism influencing shale brittle failure. Finally, intelligent fracability prediction methods for deep shale were proposed to accurately identify engineering sweet spots. The research findings have identified the key research and development directions for deep-resources development from a rock mechanics perspective.

Although significant progress has been made in the development of shallow natural gas, the exploitation of deep shale gas continues to face numerous challenges. Therefore, conducting research on deep shale gas extraction is crucial. The efficient exploitation is contingent upon a comprehensive understanding of the mechanical properties, fracturing behaviors, and transformation processes of deep reservoir formations. This paper initially delineates the geo-mechanical characteristics and key development challenges associated with deep shale gas reservoirs. It subsequently reviews recent advancements in laboratory experiments, numerical simulations, and field technologies. Finally, suggestions and strategies are proposed to enhance the efficiency of deep shale gas development. The perspectives offered in this paper aim to provide new insights into optimizing exploration and production in deep and complex geological environments.

This study aims to investigate the potential factors affecting hydraulic fracturing of inter-salt oil shale reservoirs in the Qianjiang Depression, China. Using the inter-salt shale samples, the re-crystallization seepage tests, rock mechanical tests under high temperature and pressure, salt rock creep tests, and direct shear tests were conducted. The testing results suggest several major factors that affect hydraulic fracturing effects in the end. First, the seepage of reservoir and fracturing fluid through hydraulic fractures leads to salt dissolution and crystallization, reducing the effective seepage area of fractures. Second, the salt crystal may block the pore throats or micro fractures after brine invades the shale, decreasing the overall permeability. Third, the low strength and obvious plasticity of inter-salt shale and the strong creep characteristics of salt rock raise difficulties for proppant to effectively support fracture walls, thereby sharply narrowing the hydraulic fracture width. Lastly, the weak interfaces (bedding planes and lithology interfaces) in inter-salt oil shale reservoirs restrict the height of hydraulic fractures, resulting in the disconnection of seepage channels between multiple inter-salt shale reservoirs. Thus, several factors together reduce reservoir permeability, weaken the fluid flow capacity in the fracture, narrow the fracture width, and limit the effective stimulation volume, resulting in weaken the effect hydraulic fracturing.