Mechanistic Understanding of Sand Production in Marine Hydrate Exploitation: Insights from Field Tests and THMC Simulations

The cessation of marine hydrate exploitation in the Nankai Trough resulted from severe sand production challenges. Currently, the mechanisms underlying sand production during hydrate reservoir exploitation lack comprehensive understanding. To address this knowledge gap, we developed a COMSOL-based computer model to predict gas and sand production for hydrate exploitation, which efficiently integrated a sand production model with a convectional thermo-hydro-mechanical coupling model. In the sand production model, the detachment of skeletal particles from sediments is regulated by both fluid erosion and shear deformation. Specifically, when hydraulic gradient exceeds a certain critical value and meanwhile shear deformation also occurs, skeletal particles will detach and migrate with the flowing fluids. Based on the developed computer model, we numerically simulated gas and sand production in the 2013 Nankai field test, and conducted a systematic analysis about sand production mechanism. Our numerical simulations showed: (1) For weakly cemented sand-dominated sediments, a moderate hydraulic gradient (>5) could potentially trigger particle detachment. Within the 6-day test duration, cumulative gas production volume reached 1.17 × 105 m3 with 26.9 m3 of sand generated, exhibiting a striking alignment with measured data, with a mere 3% deviation; (2) Due to weak mechanical strength, the overburden experienced more obvious deformation than other areas, which, together with its high hydraulic gradient, led to the heightened particle detachment. Thus, the overburden provided the main source of sand particles for sand production; (3) Both small depressurization amplitude and low permeability tend to moderate shear deformation and hydraulic gradient, and accordingly constitute unfavorable conditions for sand production. The negative correlation of permeability with sand production well explains why severe sand production occurred in the Nankai Trough rather than the Shenhu area where sediments have extremely low permeability.