Multiphase flow model coupled with fine particle migration for applications in offshore gas hydrate extraction

Abstract

Sand production, which causes clogging of pore space and reduces the gas hydrate production efficiency, is a crucial problem that prevents the successful extraction of offshore gas hydrate. The key physical processes behind sand production, including the detachment, migration and settling down of fine particles in pore space, were not well characterized and coupled in current mathematical models. In this work, a mathematical model is developed to characterize the fine particles migration and clogging for multiphase flow in the gas hydrate reservoir. Each fine particle considers the Stokes drag force, frictional force and buoyancy force in saturated pore space. The capillary force is further considered as an extra hydrodynamic force for fine particles migration in unsaturated pore space. The impacts of gas hydrate phase transition on pore space saturation and capillary pressure are quantified, and then the stress equilibrium analysis on each fine particle is re-evaluated. Fine particles can be either in static, rolling or suffusion status in the simulation. Based on this mathematical model, the impacts of fluid flow rate, fine particle concentration and bulk skeleton grain size on fine particle breakthrough curve and permeability reduction are investigated in this work. To capture fine particles migration and clogging processes in gas hydrate reservoir, the critical pore clogging ratio of 6 is quantified in this work. At last, the proposed model is applied to an in-situ gas hydrate test at Nankai Trough, and both calibration and the long-term production prediction are made.