An Analytical Model and Its Application in Reliability Analysis of Formation Failure during Hydrate Production in Deep-sea Areas

Abstract

Methane hydrates (MHs) have potential economic and environmental significance. However, due to the sharp reduction in the mechanical properties of methane hydrate-bearing sediments (MHBS) caused by hydrate dissociation, the risk of formation failure in marine MHs exploitation is higher than that in conventional oil and gas exploitation. Formation failure in marine hydrate production probably leads to serious sand production or subsequent formation instability. A simplified and efficient analytical model is proposed at first for prediction of formation failure as well as the displacement around the wellbore during hydrate production, and then the reliability analysis for formation failure is performed by the Advanced First Order Second Moment Method based on the analytical solutions. This model considers the key factors affecting the formation failure, including 1) the partial coupling of multiple fields (the influence of pore pressure on mechanical field and the influence of hydrate dissociation on pore pressure, temperature and mechanical field), 2) the effect of gas/water or heat absorption caused by hydrate dissociation on pore pressure or temperature, 3) the change of mechanical properties of formation induced by hydrate dissociation and 4) the casing-formation interaction. The analytical solutions are verified and validated by numerical models. According to the analysis, the casing is sufficiently safe in the cross section due to its high stiffness, while the formation would be failure at the wellbore or dissociated front due to the stiffness/strength deterioration induced by hydrate dissociation. Hydrate dissociation leads to an increase in formation failure probability by approximately 20%. The probability of formation failure at the wellbore is about 30% higher than that at the dissociated front. The uncertainties in the internal friction angle and elastic modulus ratio have the greatest effect on formation failure probability.