Calcium Oxide Thermal Stimulation for Sustainable Methane Hydrate Production: Effects of Reservoir Temperature on Gas Recovery and Water Control

Efficient, low-water, and energy-conscious production is essential for the green and sustainable development of natural gas hydrate resources. Natural gas hydrate production by depressurization is often constrained by insufficient reservoir heat supply, high water production, and unstable gas production. This study evaluates calcium oxide heat-generation-powder thermal stimulation (HPTS), used alone and combined with depressurization, as an in situ thermal and water-control method for methane hydrate production under different reservoir temperatures. Laboratory hydrate-production experiments were conducted at 8 and 14 °C to compare pure HPTS and HPTS-assisted depressurization in terms of gas and water production, temperature–pressure response, and energy performance. At 8 °C, the HPTS-assisted depressurization stage contributed approximately 56–79% of total gas production, compared with approximately 38% in the near-equilibrium HPTS reference case. Both temperature conditions maintained low water–gas ratios, with maximum values of 0.156 g/L at 8 °C and 0.619 g/L at 14 °C. The local thermal response was more variable among the 14 °C cases. The nominal methane-energy ratio and apparent dissociation-heat ratio of the HPTS stage reached 13.25 and 0.81 at 8 °C, higher than 10.44 and 0.64 at 14 °C. These results demonstrate the gas-production and water-production characteristics of CaO-based HPTS-assisted depressurization under the tested laboratory conditions, providing a basis for further evaluation of this method. However, hydration kinetics and cementation must be controlled to preserve gas-flow pathways and sustained production.