[H-SC04] CCUS (Carbon Dioxide Capture, Utilization, and Storage) for Climate Mitigation

[HSC04-05] Simulation of methane hydrate enhanced recovery using the heat of CO2 hydrate formation

Kosuke Ito¹, Yosuke Sawano², Kazuhiro Tada², *Toru Sato¹ (1.University of Tokyo, 2.Geosphere Environmental Technology Corp.)

Keywords:Carbon dioxide capture and storage, Methane htdrate, CO2 haydrate, Enhanced recovery

Methane hydrate exists abundantly under the seafloor of Japan's exclusive economic zone, and has attracted attention as a domestic natural gas resource. Depressurisation is one of the methods for producing methane gas from methane hydrate under the seafloor, in which methane hydrate is decomposed into water and methane gas by pumping up water and reducing the surrounding pressure. However, the production rate decreases after a certain period of time because the stratum is cooled by the endothermic decomposition of methane hydrate. Therefore, an enhanced recovery method has been proposed: namely, CO2 is injected into the stratum to generate CO2 hydrate, and the surrounding area is heated by the exothermic formation of CO2 hydrate to promote the decomposition of methane hydrate. CO2 hydrate storage and enhanced recovery of methane hydrate by the hydrate formation heat are attracting attention as a possibility of achieving both efficient methane gas production and reduction of greenhouse gas emissions. In this study, CO2 is injected into the subseafloor stratum where methane hydrate exists, with the purpose of evaluating the performance of the enhanced recovery of methane hydrate using the heat of CO2 hydrate formation and investigating the factors affecting production efficiency. We developed a numerical method and performed simulations using it. First, the equation of state and models of enthalpy, viscosity, heat conduction, and methane hydrate decomposition were newly implemented in the existing simulation software GETFLOWS. Next, we performed simulations of CO2 hydrate formation and methane hydrate decomposition in the sand-mud alternating layers. As a result, in selecting the injection depth of CO2, it is suggested that it is important to secure a CO2 hydrate domain by increasing the vertical distance between the mud layer that caps CO2 and the BSR of CO2 hydrate as much as possible. In addition, increasing the CO2 injection rate may increase the methane gas production rate. Moreover, even if CO2 injection is stopped or the CO2 injection rate is reduced, the effect of promoting decomposition of methane hydrate continues for a certain period of time, suggesting the possibility of continuing to depressurise the stratum and recover methane gas. In addition, It has been suggested that cooling the injected CO2 is not effective for enhanced recovery because the effect of directly cooling the methane hydrate may exceed the effect of warming by promoting the CO2 hydrate formation, depending on the CO2 injection rate.