Abstract: At present, most studies on hydrate reservoir deformation do not consider the role of capillary force, and the hydrate pressure and fluid pressure in the reservoir are considered to be consistent. However, in the South China Sea, the hydrate reservoirs are mostly clayey silty with small pore size, thus the capillary effect is obvious. Based on poromechanics and hydrate phase equilibrium theory, a reservoir deformation model was established. In this model, the material in the pores is divided into two phases, i.e. the solid phase of the hydrate and the equivalent fluid phase consisting of discontinuous bubbles. The model focuses on the pressure difference between the hydrate solid and the equivalent fluid, known as capillary pressure. Finally, according to the data of Shenhu area in the South China Sea, the depressurization method and the thermal stimulation method of hydrate exploitation are analyzed in detail. The results show that the capillary effect has a great influence on the phase equilibrium condition of the hydrate and the reservoir deformation of the clayey silty reservoir in the South China Sea. For a given pressure, the melting point of the hydrate will decrease with the decrease of the aperture, which makes the phase equilibrium condition of the hydrate appear as a region rather than a curve in the pressure-temperature, and if the capillary effect is ignored, the reservoir deformation will be seriously underestimated.

Key words: natural gas hydrate, phase transition, capillary effect, poroelasticity