Feasibility Evaluation of a New Approach of Seawater Flooding for Offshore Natural Gas Hydrate Exploitation

In this work, a new approach of seawater flooding was proposed for offshore natural gas hydrate (NGH) exploitation, whose feasibility was numerically evaluated based on a large-scale NGH reservoir situated in the Shenhu Area of the South China Sea. Then, two artificial means of well location rearrangement and hydraulic fracturing, as well as their combination, were proposed to be incorporated with seawater flooding to promote gas production from offshore NGH deposits. Simulation results indicated that seawater flooding had great advantages over depressurization and could substantially promote hydrate dissociation, effectively prevent secondary hydrate formation, and greatly facilitate gas production. Furthermore, a stable gas production period could be achieved for a long duration, which made the most significant contribution to gas production. The two artificial means of well location rearrangement and hydraulic fracturing could both promote hydrate dissociation and enhance gas recovery, but the promotion mechanisms were different. When these two methods were combined, hydrate dissociation in the whole hydrate-bearing layer could be further facilitated, and a favorable gas production rate could be obtained with a low water production and a high gas-water ratio, which demonstrated great superiority of the combined method over these two methods when used alone. Therefore, it is expected that the newly proposed approach of seawater flooding combined with well location rearrangement and hydraulic fracturing can be applied in the future commercial offshore NGH development.