Gas hydrate exploitation using CO2/H2 mixture gas by semi-continuous injection-production mode

Highlights: • A novel NGH exploitation method by CO₂/H₂ semi-continuous injection-production was proposed. • CH₄ recovery and concentration were enhanced dramatically when injecting low CO₂ concentration. • Fast CH₄ hydrate decomposition stage can be retained separately in the replacement process. • A circulating injection-production strategy was beneficial to CO₂ storage with high CH₄ recovery. -- Abstract: The CO₂ replacement technique is considered as a promising approach for both gas hydrate recovery and CO₂ sequestration. Based on this technique, the continuous CO₂/H₂ injection-production mode had been studied in our previous work. However, there are limitations on this continuous mode, such as lower CH₄ recovery and CO₂ sequestration ratios, lower CH₄ concentration in produced gas and higher injection-production ratio caused by the fast breakthrough of injected gas. Here we proposed a so called semi-continuous injection-production mode, in which continuous injection-production process is interrupted periodically by stopping injection and production operations for letting injected gas diffuse sufficiently, delaying its breakthrough and increasing its effective sweep region. A series of experimental simulations were performed with respect to this mode in a three-dimensional simulator with a volume of 10.6 L. The results indicated that for the injected gas with low CO₂ concentration, the CH₄ recovery and concentration in the produced gas could be enhanced dramatically by the semi-continuous gas injection method. Additionally, the fast decomposition stage of CH₄ hydrate could be retained separately in the replacement process, thereby effectively improving production efficiency. However, the corresponding CH₄ concentration in the produced gas decreased and the injection production ratio increased. Notably, the process combining the CH₄ steam reforming with cyclic injection-production was first simulated. The results showed an excellent CO₂ storage capability and CH₄ hydrate recovery ratio, which was mainly controlled by the gas composition of the re-injected gas. Through systematically comparing and analyzing the CH₄ concentration, recovery ratio, production efficiency and injection-production ratio, it is demonstrated that the semi-continuous injection-production mode is superior to the continuous one. The results obtained in this work are of significance for guiding future NGHs exploitation.