Effects of stepwise depressurisation rate on methane hydrate dissociation dynamics at pore scale using microfluidic experiments

Methane hydrate, recognized as a potential alternative energy source, faces challenges in achieving efficient production. Stepwise depressurisation has emerged as a viable technique for enhancing productivity, yet optimizing the depressurisation rate remains a complex issue. This study employs pore-scale experiments using microfluidic chips to examine the dissociation characteristics of methane hydrate under varying stepwise depressurisation rates. At a high depressurisation rate of 0.4 MPa/15 min, the dissociation process exhibits a three-stage pattern: stabilisation, rapid dissociation, and slow dissociation. During these stages, the mass transfer limitation in the water phase significantly impedes the dissociation rate. The gas-water migration triggered by depressurisation can mitigate this limitation. As the depressurisation rate is reduced to 0.2 MPa/15 min, the rapid dissociation stage splits into two due to a decrease in gas-water migration intensity. The dissociation rate decreases by 45% compared to the 0.4 MPa/15 min case. This results from an insufficient driving force for dissociation, necessitating another depressurisation step. Further reduction of the depressurisation rate to 0.1 MPa/15 min leads to a less pronounced gas-water migration, which is inadequate to significantly counteract the mass transfer limitation. As a result, the rapid dissociation phase occurring at a depressurisation rate of 0.4 MPa/15 min, which exhibits an average dissociation rate of 0.2%/s, subsequently transitions into a more uniform and slower dissociation stage, where the average rate of dissociation declines to below 0.07%/s. The experimental results offer valuable insights for guiding hydrate exploration strategies during the stepwise depressurisation process by adjusting the depressurisation rate to regulate production.