Numerical study on the critical pressure point of ice formation in the processing of methane hydrate dissociation by depressurization method

The depressurization-induced decomposition behaviors of methane hydrate around the freezing point in porous media (quartz sand) are investigated in this numerical study. The critical pressure point which makes the ice start to generate was captured with the TOUGH+HYDRATE code in an experimental scale physical model under constant temperature boundary condition. The values of recovery pressure at wellhead decline from 3.0MPa to 1.4MPa with interval of 0.2MPa. The temperature profile, the pressure profile, the hydrate saturation profile, and the ice saturation profile are analyzed. The results indicate that the lowest point of reservoir temperature is around zero when the recovery pressure decreases down to 2.6 MPa. The pressure keeps at the value of 2.6MPa for a short time before secondary repaid reduction. The hydrate decomposition rate under 2.6MPa of recovery pressure is obviously much higher than that above 2.6MPa, because the additional latent heat of ice transition is provided to promote the hydrate decomposition. Ice starts to generate if the recovery pressure is down to 2.6MPa and the ice saturation increase rapidly when the recovery pressure decrease from 2.6MPa to 2.4MPa. Therefore, the critical pressure point which makes the ice start to generate can be supposed at 2.6MPa under the boundary and initial condition of porous media in our paper.