Optimized gas and water production from water-saturated hydrate-bearing sediment through step-wise depressurization combined with thermal stimulation

There have been several trial production tests carried out from marine natural gas hydrate reservoir recently, showing its great potential as an alternative source of energy. Yet an unsustainable production with low productivity and short duration is generally encountered. The marine hydrate reservoirs are mostly highly water-saturated; the resulting water production behaviors remain largely unclear. In this work, the gas and specially water production from a water-saturated reservoir were investigated. The role of water acting as a diffusion barrier of gas was determined: a higher water yield will significantly improve the following gas production. The step-wise depressurization was found to help relieve the initial water production compared with the straightforward depressurization scenario. A high-water-production stage was for the first time identified, accounting for ~47% of the total water production. A further controlled depressurization with finer steps in this stage could enhance the gas productivity by at most 31%; yet its effect on controlling water production was limited. The cumulative water yield depended much on the overall degree of depressurization, regardless of the number of steps. In order for an enhanced gas production under a regulated water yield, thermal stimulation is introduced in the high-water-production stage. This is found to effectively contribute to an optimized water producing process and an at least 30% decline of water yield under comparable gas productivity. The proposed combination method could be applied in the field tests from water-saturated marine reservoir to achieve a high gas-water ratio and a thereby improved energy and economic efficiency.