Thermo-hydro-chemical modeling and analysis of methane extraction from fractured gas hydrate-bearing sediments

Abstract

Hydraulic fracturing is widely used in enhancing hydrocarbon recovery efficiency of unconventional reservoirs including gas hydrate-bearing sediment (GHBS). However, the underlying mechanisms governing the influences of fractures on the intricate multi-field coupled processes during methane extraction remain poorly understood. This study develops a fully coupled thermo-hydro-chemical (THC) model for methane extraction from fractured GHBS under combined heat injection and depressurization operations. Novel dimensionless numbers are proposed to characterize the underlying heat transfer, fluid flow, and hydrate decomposition processes. The results reveal that depressurization induced fluid flow dominates methane extraction in the early stages, while heat injection induced thermal processes become the primary controlling factor in the intermediate and long-term stages. The flow field primarily governs the global-scale hydrate decomposition process, whereas the thermal field dictates the local-scale evolution of the pre-decomposition front. We find that increasing injection temperature and decreasing production pressure could improve gas recovery efficiency, although economic consideration may constrain these production strategies. Particularly, increasing injection temperature expands local hydrate decomposition zone and boosts gas production, while depressurization at the production well reduces the overall gas hydrate saturation and leads to extra gas production. Variation in well spacing show little effect on the gas recovery efficiency, but the total gas production changes for the change of reservoir scale. This study provides insights into the mechanisms of coupled thermo-hydro-chemical behaviors during methane extraction from fractured GHBS, and offers a fundamental model for the optimization of both extraction efficiency and economic viability of gas hydrate recovery.