Analytical approach for the fast estimation of time-dependent wellbore stability during drilling in methane hydrate-bearing sediment

Abstract

Methane hydrates (MHs) have been widely considered to be clean energy resources that are stored in great quantities in the ocean. During wellbore drilling in methane hydrate-bearing sediment (MHBS), the coupling effect of the mass/heat transfer, MH dissociation and creep response of MHBS makes wellbore stability a complex time-dependent problem.Aimed at efficiently estimating the time-dependent wellbore stability when drilling in MHBS, this study provides an efficient analytical approach (solution) for quickly predicting the plastic zone and displacement around the wellbore, precisely considering the rheological properties (i.e.., viscoelastic-brittle-plastic) of the sediments, the effect of the unsteady seepage/temperature fields and the sharp deterioration of the mechanical properties of the sediments after hydrate dissociation. Because of the time-dependent hydrate dissociation caused by the unsteady seepage/temperature field, a multiregion problem with time-varying boundaries is deduced, and the Laplace transformation is employed when solving the differential equations under various working conditions. The analytical solutions agree well with the results from the numerical method under the same assumptions, and the boundary and compatibility conditions of the analytical solutions are satisfactorily verified. Furthermore, the analytical predictions of the dissociation front radius and the distribution of the pressure and effective stress near the wellbore are essentially consistent with the results from complex numerical simulations, which validates the applicability of the analytical solutions. A parametric investigation on the specific effect of the rheological properties and drilling fluid pressure on the wellbore stability during drilling in MHBS is finally performed. The results show that both the plastic radius and displacement increase sharply at the beginning of drilling. When the plastic zone is smaller than the dissociated region, the slower the creep rate of MHBS is, the slower the plastic zone expansion in the dissociated region is, and the slower the displacement increase is over time.