Depressurization induced gas production from hydrate deposits with low gas saturation in a pilot-scale hydrate simulator

• Hydrate samples with low gas saturations are synthesized in a 3-D simulator.
• Hydrate is dissociated using depressurization method in a single vertical well.
• Production behaviors predicted by the kinetic and equilibrium models are compared.
• The equilibrium model is found to be more preferred than the kinetic model.

The kinetic behaviors of methane hydrate dissociation under depressurization in porous media are investigated through experimental and numerical simulations. Hydrate samples with low gas saturations (SG ⩽ 0.10) are synthesized in the pilot-scale hydrate simulator (PHS), a novel three-dimensional pressure vessel with effective inner volume of 117.8 L. Three experimental runs with different production pressure at the central vertical well have been carried out. The intrinsic dissociation rate constant k0 is fitted to be approximately 4578 kg/(m2 Pa s) using the experimental data of run 1, and it is used for the kinetic simulation in all the three runs. The whole production process can be divided into two stages: the free gas and mixed gas production (stage I) and the gas production from hydrate dissociation (stage II). Both the experimental and numerical simulation results show that the gas production rate increases with the decrease of the production pressure, while the water extraction rate will rise much higher if the wellbore pressure is dropped extremely low. The free gas saturation is found to be a key factor that affects the overall production behaviors of marine hydrate deposits. In addition, the comparisons of the kinetic and equilibrium models indicate that the kinetic limitations are very small in the PHS. The hydrate dissociation under depressurization in the PHS is mainly controlled by the mass and heat transfer processes.