Long-Term Performance and Security of Gas Production for Horizontal-Well Depressurization Exploitation: Insights from a Coupled Thermo-Hydro-Mechanical-Chemical Model for the Shenhu Hydrate Reservoir

Horizontal-well depressurization for marine hydrate exploitation started in 2020. Short-term gas production stability (similar to 30 days) contrasts with ongoing uncertainties about long-term performance and security. To address these concerns, we used the 2020 Shenhu field test as a case study and created a fully coupled thermo-hydro-mechanical-chemical model. Employing this model, cumulative gas production closely matched field test data, with a deviation below 1.5%. Our numerical simulations showed the following: (1) Following depressurization initiation, the rapid increase in effective principal stress of the hydrate reservoir stabilized near the well after 30 days and at the seafloor after 2 years, without reaching failure conditions. (2) Variable effective principal stress led to complex temporal and spatial formation deformation. During the 30-day test, subsidence mostly occurred near the well (<10 cm). However, after approximately 250 days, measurable seafloor subsidence (similar to 3 cm) began, reaching 21 cm after 4 years. (3) Formation deformation hindered gas production. Simulations indicated reductions of 33.6 and 15.2% in cumulative gas release and production volumes, respectively, over 5 years. Our findings shed light on the interplay among depressurization, formation deformation, and gas production in marine hydrate exploitation, contributing to a better understanding of the long-term performance and dynamics of marine hydrate exploitation.