Abstract
The stability of hydrate-bearing near-wellbore reservoirs is one of the key issues in gas hydrate exploitation. In most previous investigations, the damage evolution process of the sediment structure and its effect on near-wellbore reservoir stability have been neglected. Therefore, the damage variable is introduced into a multi-field coupled model based on continuous damage theory and multi-field coupling theory. A thermo-hydro-mechanical-chemical (THMC) multi-field coupling mathematical model considering damage of hydrate-bearing sediments is established. The effects of damage of hydrate-bearing sediments on the thermal field, seepage field, and mechanical field are considered. Finally, the distributions of hydrate saturation, pore pressure, damage variable, and effective stress of a near-wellbore reservoir in gas hydrate exploitation by depressurization are calculated, and the stability of a hydrate-bearing near-wellbore reservoir is analyzed using the model. Through calculation and analysis, it is found that structural damage of hydrate-bearing sediments has an adverse effect on the stability of hydrate-bearing near-wellbore reservoirs. The closer to the wellbore, the worse the reservoir stability, and the near-wellbore reservoir stability is the worst in the direction of minimum horizontal ground stress.
Highlights
As a new type of clean energy, natural gas hydrate has the advantages of large reserves, wide distribution, high energy density, and non-pollution [1]
Freij-Ayoub et al [4] established a hydrate-bearing wellbore stability analysis model considering the coupling of porous media deformation, heat transfer, fluid transport, and hydrate dissociation, but in this model the fluid was regarded as a single-phase flow without taking into account the effect of hydrate dissociation on reservoir permeability
The results showed that the change in pore pressure and the decrease of the mechanical strength of the stratum caused by hydrate dissociation were the key factors affecting the stability of the wellbore reservoir
Summary
As a new type of clean energy, natural gas hydrate has the advantages of large reserves, wide distribution, high energy density, and non-pollution [1]. Rutqvist et al [5] proposed a multi-field coupled mathematical model considering thermal, fluid-flow, and geomechanical responses during hydrate dissociation, and the Mohr–Coulomb strength criterion was used to judge the shear failure of a wellbore reservoir under two gas hydrate exploitation schemes of a vertical well and a horizontal well. Zhou et al [11] used a fully coupled THM numerical simulator to examine the stability of a hydrate-bearing reservoir during gas production, and the critical state constitutive model for hydrate-bearing sediments was used to evaluate the mechanical response of the formation. A damage statistical constitutive model of hydrate-bearing sediments considering the effects of damage threshold and residual strength is established by introducing a three-parameter Weibull distribution and residual strength correction coefficient, and the damage constitutive model is introduced into the multi-field coupled model.
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