Research on Hydraulic Fracture Propagation Patterns in Multilayered Gas Hydrate Reservoirs Using a Three-Dimensional XFEM-Based Cohesive Zone Method

Hydraulic fracturing is generally considered a promising stimulation technology for low-permeability gas hydrate reservoirs. Nevertheless, the fracturing behavior of multilayered "sweet-spot" reservoirs with coexisting hydrates and free gas is rarely studied. Herein, a three-dimensional (3D) multilayer fracturing model was built by combining the extended finite element and cohesive zone method, taking the reservoir containing a hydrate-bearing layer (HBL), three-phase layer (TPL), and free gas layer (FGL) located in the Shenhu Area as the geoengineering background. Particularly, the effects of key factors, such as the fracturing layer, hydrate saturation of the HBL, injection rate, and viscosity of fracturing fluids, on fracture initiation and propagation were investigated. The results show that the interlayer heterogeneity caused by differences in hydrate saturation results in various propagation patterns. Specifically, when fracturing in HBL, fractures tended to propagate upward under the influence of the geostress gradient, resulting in TPL and FGL not being fractured, whereas multilayers could be reconstructed simultaneously when fracturing in TPL, and proper fracturing parameters should be selected according to the hydrate saturation. For HBL with a hydrate saturation lower than 31%, fracture propagation was favored at a low injection rate and low-viscosity fracturing fluid, whereas for HBL with 43% hydrate saturation, it played a "shielding" role against fracture penetration due to the high mechanical strength and low effective permeability. In this case, fracture penetration was encouraged by increasing the fracturing fluid viscosity. Therefore, multilayer fracturing can be realized by fracturing in TPL and optimizing fracturing plans, which is of great significance for enhancing the development potential of challenging hydrates.