Key factors analyses for prediction of accurate gas production rate in hydrate reservoirs during model construction

Decomposition characteristics of methane hydrate in porous media under continuous and intermittent microwave heating

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. 

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

Gas recovery from silty hydrate reservoirs by using vertical and horizontal well patterns in the South China Sea: Effect of well spacing and its optimization

Depressurization combined with thermal stimulation is a promising method for gas hydrate production. Owing to its advantages of rapid uniform heating, microwave radiation is an efficient source of heat. However, the mechanisms of hydrate decomposition under depressurization combined microwave stimulation are currently unclear. In this study, methane hydrate was synthesized under 6 MPa and 2 °C, with hydrate saturation of approximately 42% in natural quartz sand. We then compared hydrate decomposition via rapid depressurization and piecewise depressurization, with or without microwave stimulation at 2.45 GHz and 400 W. When using the depressurization method alone, the hydrate decomposition rate increased with the depressurization amplitude. However, in the last stage of hydrate decomposition, the external flow of gas was hindered by the Jamin effect, especially under larger depressurization amplitudes; therefore, extremely low production pressure is not justified. When combined with microwave stimulation, both depressurization methods resulted in increased reservoir temperature within a few seconds, and microwave heating provided an extra driving force for hydrate decomposition. Furthermore, microwave heating was more effective when larger amounts of undecomposed hydrate remained after depressurization. When depressurization was combined with microwave stimulation, the average gas production rate at 100% gas production was 0.269–0.601 L/min, which was significantly higher than that for depressurization alone. However, the energy efficiency ratio was approximately 1, which has no practical value. Conversely, the average gas production rate at 90% gas production was 0.452–2.945 L/min and the energy efficiency ratio was 2.9–17.6. Under the combined method, gas hydrate decomposition at a production pressure of 1.9 MPa achieved the subtle balance between the average gas generation rate and energy efficiency. Thus, optimizing the gas production pressure and microwave stimulation time can improve the average gas production rate and energy efficiency ratio according to the reservoir hydrate conditions. 

Yunkai Ji

Papers

Energy recovery behavior of low-frequency electric heating assisted depressurization in Class 1 hydrate deposits