Seafloor subsidence induced by gas recovery from a hydrate-bearing sediment using multiple well system

The response behavior of the methane exploitation from natural gas hydrate (NGH) using multiple well system is complex and needs to be investigated, as gas production from a single vertical well generally cannot meet commercial demand. This study numerically investigates the production performance and geomechanical response of an unconfined hydrate deposit in Shenhu area, South China Sea, under single and multiple vertical well conditions. For a single vertical well with a mild constant bottom-hole pressure, gas production is relatively stable. However, seafloor subsidence exhibits an initial rapid drop and a subsequent mild drop stage. The vertical displacement is highly developed at the top and bottom of the production zone. The results from doublet and triplet vertical wells indicate that both the gas and water production and seafloor subsidence increase with the increase in number of production wells. The superimposition of subsidence leads to a deterioration in subsidence and the change in location for the largest subsidence, which may affect the risk location of well instability. The interference of pore pressure and subsidence increases with the decrease in well spacing. However, gas production decreases and water production changes insignificantly. Furthermore, a same subsidence at seafloor cannot indicate the same evolution of subsidence in the vertical and lateral direction. The results presented in this study help in balancing the production and subsidence of the NGH exploitation in complex well configurations.