Experimental investigations on energy recovery from water-saturated hydrate bearing sediments via depressurization approach

Abstract

A huge amount of natural gas hydrates remains untapped in permafrost and continental margin. While several short term field production tests have been carried out, the underlying challenges during hydrate dissociation in porous media, such as the interdependent production behavior of gas and water, is still not well understood. In this work, we employed depressurization technique to recover natural gas from a water saturated hydrate bearing sediment (40% SH, 50% SA and 10% SG) at 281.5K surrounding temperature. During depressurization, the bottom hole pressure (BHP) was maintained at constant pressures of 5.0, 4.0, 3.0 and 2.1MPa respectively to evaluate its effect on gas and water production. As expected, a higher BHP (corresponding to a lower dissociation driving force) resulted in a slower gas and water production. At a BHP of 2.1MPa, thermal buffering was observed below ice point (272.7K), accompanied by enhanced gas production. By lowering BHP from 5.0MPa to 2.1MPa, the percentage methane produced increased from 45.5% to 83.0%; whereas the cumulative water production decreased from 217mL to 157mL. The difference in gas and water production was attributed to the preferential production of aqueous phase at higher BHPs (5.0 and 4.0MPa) nearing the end of hydrate dissociation whereby less hydrates were present.