Exploration of production capacity-geomechanical evaluation and CO2 reinjection repair strategy in natural gas hydrate production by multilateral horizontal wells

Multilateral horizontal wells (MHW) can enhance gas production by expanding the natural gas hydrate (NGH) decomposition area, but inevitably leading to larger changes in reservoir pressure and rock stresses, causing subsidence or disasters. Therefore, this study established a THMC model that evaluated the production effect and geomechanical response of MHW in Class-I reservoirs with different permeability and proposed a CO2 reinjection repair strategy to achieve subsidence recovery and carbon storage. Results indicate that MHW with vertical branch arrangements performs best in short-term production for low-permeability reservoirs, while the temperature and pressure evolution in high-permeability reservoirs is more drastic than in low-permeability reservoirs, producing many unfavorable factors. Higher capacity also results in larger strata subsidence and stress changes, affecting the reservoir stability and wellbore safety: (i) subsidence occurs mainly in the NGH sediment and the upper zone of the wellbore; (ii) normal stresses around the wellbore increase most significantly in the vertical direction, while shear stresses generally decrease. The repair indicators of subsidence recovery rate, energy recovery rate, and hydrate conversion rate suggest that reinjecting CO2 into depleted NGH reservoirs can improve the mechanical properties of the strata and achieve carbon storage via the hydrate method as well.