Effect of depressurization decomposition of combustible ice on wellbore stability considering gas-water two-phase hydro-mechanical coupling

Key words:  combustible ice  depressurization production  gas-water two-phase flow  physical parameters  wellbore stability

Abstract:

Marine combustible ice usually occurs in deep-water and shallow loose sediments under the condition of low temperature and high pressure, which mainly plays an effective cementation or skeleton support role in hydrate bearing sediments. The permeability of reservoir changes with the phase change during the combustible ice decomposition process, which influences the flow characteristics and the heat and mass transfer of gas-water two-phase fluid. This behavior restricts the continuous decomposition of combustible ice. The decomposition of combustible ice will further reduce the shear strength and bearing capacity of combustible ice bearing sediment reservoir, which reduces the wellbore stability. A coupled thermo-hydro-mechanical (T-H-M) model was built to describe the phase change of combustible ice depressurization production process. The evolution rules of gas-water two-phase flow, porosity, permeability and other physical parameters during the combustible ice depressurization production process were described, and the wellbore stability was studied. The results show that the methane gas saturation increases significantly compared with the water saturation, and the gas saturation around the well is significantly higher than the water saturation with the decomposition of marine combustible ice. After the plastic yield of the reservoir, the permeability and effective porosity in the plastic zone increase significantly, while the elastic modulus and cohesive force decrease greatly. The longer the production time, the greater the production pressure difference, the larger the plastic yield area of the reservoir, and the worse the wellbore stability.