Optimizing fracturing techniques for enhanced hydrate dissociation in low-permeability reservoirs: Insights from numerical simulation

Abstract

Natural gas hydrate represents an environmentally friendly alternative energy. However, the low-permeability hydrate reservoirs pose significant challenges to commercial production. To optimize hydrate production, reservoir stimulation methods like hydraulic fracturing, gas fracturing, and explosive fracturing are potential. In the investigation of the fracturing effect on hydrate dissociation behavior, we developed a three-dimensional model that comprehensively incorporates the arrangement of fracture planes, encompassing fracture permeability, spacing, and height. The model was implemented using a self-developed hydrate simulator which was validated at multiple scales. We firstly conducted a comparative analysis, contrasting the conventional depressurization method in horizontal wells with a combined depressurization and fracturing method. The comparison suggested that fracturing brings significant promotion (113.5%) on hydrate dissociation. Sensitivity analysis revealed fracture permeability and fracture height evidently exhibit an optimal value that facilitates the dissociation behavior. The optimal value of fracture permeability is dependent on the geological background. The optimal value of fracture height occurs just before penetrating the hydrate-bearing sediment layer and the overlying/underlying formations, i.e., near the thickness of the hydrate-bearing sediment layer. The impact of spacing on gas volume is limited while decreasing the spacing leads to more evident localized dissociation and differential subsidence of hydrate reservoir. It is necessary to balance production and geological risks when selecting the fracture spacing. The conclusions suggest that the fracturing operation need to be optimized by controlling the injection rate and the fracturing interval. It is not advisable to blindly pursue the effectiveness of reservoir stimulation but rather to consider the geological conditions. Additionally, we have found that it is preferable to establish barriers between the reservoir and overlying/underlying formations near the hydraulic fracturing section before reservoir stimulation, to prevent excessive early water production. In conclusion, this study would provide valuable insights for the optimization of fracturing techniques in stimulation of hydrate reservoir.