Numerical simulations of strata subsidence induced by marine hydrates exploitation via depressurization

Large-scale exploitation of methane hydrates in deep waters may result in severe strata subsidence, jeopardizing operations of foundations or pipelines laid on the seabed. The exploration of hydrate is essentially a process with coupled thermal-hydrodynamic-chemical-mechanical fields. A new scheme is developed, with the former three fields reproduced using the finite difference approach and the force balances solved using the finite element approach. Data are mapped between the two approaches at each time step, and mapping accuracy is verified by comparisons with the previous numerical results. Based on geological data from the Shenhu area in the South China Sea, exploitations of a multilayer hydrate reservoir are simulated to elucidate the triggering mechanisms of subsidence and its effects on the gas productivity. Compared to the depressurization through the single well, greater strata subsidence is caused given that a dual-horizontal well system is employed, although the secondary hydrate is prevented. After three-year depressurization, the strata subsidence occurs predominantly in the clayey soils overlying the hydrate layer, which is contributed to soil consolidation. The vertical displacements are minimal in the hydrate-bearing layers due to the existence of residual hydrates. The strata subsidence within the first year takes over half of the total value over three years. Subsidence around the wellbore resulted in compression of pore volumes, causing more reduction on gas productivity in the upper wellbore.