MECHANICAL STABILITY ANALYSIS OF STRATA AND WELLBORE ASSOCIATED WITH GAS PRODUCTION FROM OCEANIC HYDRATE-BEARING SEDIMENTS BY DEPRESSURIZATION

Gas production from hydrate-bearing sediment by depressurization will result in the changes of its porosity, permeability, pore pressure, effective stress and cementing strength, which will reduce the shear strength and carrying capacity of the hydrate-bearing sediments. As a consequence, the possible geohazards could trigger, such as wellbore instability, submarine landslide, and seafloor subsidence. Given this, based on the extended 3D Biot consolidation theory, the coupled thermo-hydro- mechanical (THM) model was built in the framework of TOUGH+Hydrate. The new THM model considers the coupling processes, including hydrate dissociation and formation, heat conduction, convection, and mechanical behavior of hydrate-bearing sediments. Taking the geologic data obtained from the Shenhu area SH2 site of South China Sea as an example, the THM coupling model for analyzing the mechanical stability of strata and wellbore using vertical well by depressurization was constructed. The evolution rules of reservoir temperature, pore pressure, stress, and hydrate dissociation zone during depressurization were predicted. In addition, the dominant sand producing region and seabed subsidence trend were revealed. The results show that the drawdown of reservoir pressure controls the increase in effective stress and strata subsidence around the production well. The subsidence mainly occurs in the early stage of depressurization, and the maximum subsidence is located around the production interval. The dissociation of hydrate results in significant decrease in the mechanical strength of sediments, which aggravates the subsidence. Wellbore depressurization results in the stress concentration in the perforation section significantly, which causes the potential wellbore damage. What's more, the stress concentration region is the key area for the prevention and control of sand production associated with hydrate production.