Shale oil and gas resources are considered one of the most important strategic resources globally today. However, the matrix permeability of oil shale reservoirs is extremely low, it requires modification through hydraulic fracturing technology to realize their economic and effective development. Against this background, we focused on the oil shale in the Xunyi area of the Ordos Basin, employing the ABAQUS platform to simulate and investigate the characteristics of hydraulic fracturing fracture propagation in fracture‐developed oil shale reservoirs and the principal influencing factors. The results of the study show that the larger the angle between the natural fracture and the hydraulic fracture, the easier it is for the hydraulic fracture to pass through the natural fracture; the larger the elastic modulus of the matrix, the stronger the ability of the fracture to penetrate through the stratum, and the fracture morphology tends to be more narrow and long. The direction of fracture propagation tends to be in the direction of the geostress difference, and with the increase of the geostress difference, the degree of convergence of the direction of fracture propagation and penetration is greater. The higher the viscosity of the fracturing fluid, the wider and shorter the fracture tends to be, but it has minimal impact on the direction of fracture propagation. Increasing the fracturing fluid displacement can increase the fracture width and length and enhance the effect of the hydraulic fracturing modification. The research results are significant for the optimized fracturing design in oil shale reservoirs.

Methane hydrates as a clean and abundant energy source, have attracted the attention of the world. However, the instability of hydrate reservoirs makes it difficult to drill production wells. After drilling, the creep properties of hydrate reservoir would cause wellbore shrinkage, which will affect the drilling safety. This study is to analysis law of wellbore shrinkage after a hydrate well drilled. The results demonstrate that wellbore shrinkage mainly results from plastic deformation and creep deformation. The largest and smallest values are separately found in the directions of the maximum and minimum horizontal in‐situ stress. With the increase of initial in‐situ stress ratio, wellbore shrinkage linearly rise. Under the condition of low horizontal in‐situ stress ratio, wellbore shrinkage is mainly determined by creep deformation. Under the condition of high horizontal in‐situ stress ratio, wellbore shrinkage commonly depends on plastic deformation and creep deformation. With deepening methane hydrate reservoirs, wellbore shrinkage shows a nonlinear increase. For formations with higher hydrate saturation, larger wellbore shrinkage would occur. The research results can provide a theoretical basis for the selection of drilling method while drilling in hydrate formations.

As we all know, development and utilization of clean energy is the only way for society to achieve its sustainable development. Although natural gas hydrates is a new type of clean energy, uncontrollable hydrate dissociation and accompanying methane leakage in drilling operation threaten drilling safety, as well as marine environment. However, the dissociation range of natural gas hydrates around wellbore cannot be reasonably determined in previous investigations, which may lead to the inaccurate estimation of borehole collapse and methane leakage. Then, the marine environment will be greatly damaged or affected. The purpose of the present work is to experimentally explore the dissociation characteristics of gas hydrates around wellbore in drilling operation and analyze the influence law and mechanism of various factors (such as hydrate saturation) on hydrate dissociation. It is expected to provide reference for exploring effective engineering measures to avoid the uncontrolled hydrate dissociation, borehole collapse and accompanying methane leakage. The experimental results demonstrate that acoustic velocity of hydrate-bearing sediment can be accurately expressed as quadratic polynomial of hydrate saturation, which is the theoretical basis for determination of hydrate saturation in subsequent experiments. Owing to the fact that hydrate dissociation is an endothermic reaction, hydrate dissociation gradually slows down in experiment. Throughout the experiment, the maximum dissociation rate at the beginning of the experiment is 8.69 times that at the end of the experiment. In addition, sensitivity analysis found that the increase in the stabilizer concentration in drilling fluid can inhibit hydrate dissociation more effectively than the increase in the hydrate saturation. Hydrate dissociation was completely inhibited when the concentration of soybean lecithin exceeds 0.60wt%, but hydrate dissociation definitely occurs in the near-wellbore region no matter what hydrate saturation is. In this way, based on the requirements of drilling safety and/or environment protection, hydrate dissociation and accompanying methane leakage can be controlled by designing and adjusting the stabilizer concentration in drilling fluid.