Stability of the wellbore wall during marine hydrate depressurization production based on a modified Drucker-Prager model

Abstract

Different from oil and gas production, hydrate reservoirs are shallow and unconsolidated, whose mechanical properties deteriorate with hydrate decomposition, so there is a risk of wellbore wall failure during depressurization production. However, the existing researches of wellbore stability focuses on the drilling process, and the stress model used does not taking the effect of casing into account. Therefore, this paper aims to propose an analytical approach suitable for the analysis of wellbore wall stability during production. We proposed a stability analysis criterion based on a modified Drucker-Prager model, derived an effective stress model considering casing-reservoir interaction, established a multi-field coupling model reflecting the dynamic effect of hydrate decomposition, and developed a solver. An experimental system was designed to verify the models. A case study found that the stability is worst near the casing in the direction of maximum horizontal in-situ stress, relatively better in the direction of minimum horizontal in-situ stress, and progressively increase away from the casing. The stability decreases with the decrease of hydrate saturation, and the lower the saturation, the faster the stability decreases. The inhomogeneous of in-situ stress has great influence on the stability, and if it is too high, the wellbore wall will be damaged.