The impacts of scaled capillary pressure combined with coupled flow and geomechanics on gas hydrate deposits

Abstract

In this study, we numerically analyze the effect of capillary pressure on gas hydrate deposits through coupled flow and geomechanics simulation, with a focus on the scaled capillary pressure. The scaled effect is predicated on sediment pore-size variations resulting from hydrate dissociation or formation, leading to non-monotonic capillary pressure curves influenced by two primary factors: alterations in pore space and gas saturation. Specifically, hydrate dissociation may increase pore space, thereby reducing capillary pressure. Conversely, enhanced gas saturation owing to dissociation can elevate capillary pressure. We employ a scaled capillary pressure model, accounting for porosity fluctuations caused by hydrate formation or dissociation. Additionally, equivalent pore pressure is utilized to ensure the numerical stability and accuracy in scenarios of strong capillarity. The numerical experiments incorporate two distinct methodologies for hydrate dissociation: heat injection and depressurization. In the heat injection scenario, sensitivity analyses are conducted using a range of model parameters, exhibiting characteristic non-monotonic capillary pressure behaviors attributable to the aforementioned competing factors. Regarding the depressurization approach, the UBGH2-6 site in the Ulleung Basin, East Sea, South Korea, is selected as a real-world field case. Over a 30-day gas production simulation, we observe notable enhancements in hydrate dissociation, signifying improved productivity, and distinctive geomechanical responses, under the influence of the scaled model. This investigation demonstrates that the scaled capillary pressure model, upon the hydrate or ice (i.e., solid) phase change, with coupled flow and geomechanics is crucial for accurate modeling of gas hydrate deposits.