Numerical simulations of hydraulic fracturing in methane hydrate reservoirs based on the coupled thermo-hydrologic-mechanical-damage (THMD) model

Hydraulic fracturing (HF) has been proved to be a promising technology to achieve economic production of hydrate. However, the research on HF of hydrate reservoirs is at its initial stage with limited information being available. In particular, the mechanism of fracture propagation in hydrate reservoirs is not well understood and requires further investigations. In this study, a new coupled thermo-hydrologic-mechanical-damage (THMD) model for HF simulations in hydrate reservoirs is proposed. Damage mechanics theory is used as the criterion of hydraulic fracture initiation and propagation, and the variation of hydrate properties due to hydrate phase transformation is considered in this THMD coupled model. The influence of hydrate saturation, reservoir permeability, fracturing fluid viscosity and fluid injection rate on HF were analyzed, and the mechanism of fracture initiation and propagation during HF in hydrate reservoir was revealed for the first time. The results showed that fracturing fluid destroys phase equilibria and causes hydrate dissociation, which in turn, releases the pore spaces occupied by hydrate, resulting in the increase of reservoir permeability near fracture surface. Besides, the hydrate dissociation reduces its cementation on sediment particles, causing the decrease of cohesion near the fracture surface. The fracture initiates and propagates perpendicularly to the direction of minimum principle field stress. Hydrate dissociation leads to heterogeneity of reservoir physico-mechanical properties, resulting in a irregular fracture surface morphology. Enhancing the fracturing fluid viscosity and injection rate promotes to improve the reservoir pore pressure and inhibit methane hydrate dissociation, which is beneficial to increase the fracture length. The hydrate reservoir with high methane hydrate saturation and low permeability is conductive to form long fractures during HF.