Research on natural gas hydrate exploitation by mixed gas sweeping-replacement from dual horizontal wells

Author of the article:LI Zhong1,2, PANG Weixin1,2

Author's Workplace：1. State Key Laboratory of Offshore Natural Gas Hydrates, Beijing, 100028, China; 2. CNOOC Research Institute Co., Ltd., Beijing, 100028, China

Key Words：Natural gas hydrate exploitation; Sweeping-replacement; Mixed gas injection; Horizontal well; CH4 recovery rate; CO2 sequestration rate

Abstract：The injection of CO2-containing gas mixture has emerged as one of the main research directions in hydrate exploitation due to its triple advantages of high methane recovery, long-term CO2 storage and reservoir geo-mechanical stability, showing tremendous potential for future application. This study, for the first time, utilized a medium-sized three-dimensional reactor to develop a production simulation scenario for gas injection exploitation of hydrates via dual horizontal wells, to examine the CH4 recovery and CO2 sequestration effects of CO2/N2 and CO2/H2 injection under the dual principles of sweeping and replacement. The results indicate that reducing the CO2 content in the injected gas can increase CH4 recovery rate, while simultaneously decreasing CO2 sequestration rate. When the CO2 concentration exceeds 54% mole, compared to CO2/N2 system, using CO2/H2 can increase CH4 recovery rate by 12%~16%, but decrease CO2 sequestration rate by 8%~17%. When the CO2 concentration in the injected gas is 33% mole, there is a significant difference in the gas diffusion paths for CO2/H2 and CO2/N2. CO2/H2 tends to diffuse upward, posing a greater leakage risk and resulting in a lower CH4 recovery rate and concentration in produced gas during the early production stage. Additionally, an economic analysis for the two mixed gas systems is conducted based on the net CH4 recovery, providing reference for natural gas hydrates exploitation using replacement method.