Wellbore Integrity During Production and Drilling Risk Analysis in Gas Hydrate Bearing Sediments by Coupled THMD Model

Abstract

Abstract Wellbore integrity in a gas hydrate bearing formation during drilling and production is a great challenge in the energy industry since a complex thermal-hydraulic-mechanical interaction with decomposed solid hydrate process is involved. A wellbore temperature increase and/or depressurization process may induce additional stresses and thermal and fluid flows, which may trigger wellbore instability, sand production, and undesired fracturing. Production-related wellbore integrity issues such as solid production during wellbore depressurization or heating are also widely observed and studied. To understand the thermal-hydraulic-mechanical behavior with hydrate decomposition (THMD) process and to simulate the THM responses of hydrate gas bearing sediments to wellbore pressure and temperature variations under a given in-situ stress regime during drilling and production, a fully coupled THMD model is presented in this paper. The conceptual THMD model with simplified semi-analytical solutions for the induced stress, pore pressure, and temperature at the wellbore and inside the formation is discussed. A linear Mohr-Coulomb criterion is utilized to define the onset of the wellbore instability or plastic yielding when sand production defined by effective plastic strain (EPS) is considered with a cohesion dependent on hydrate saturation. Numerical method with Laplace transformation is used to solve the transformed homogeneous PDE. We conclude that thermally- and hydration-induced stresses will affect wellbore integrity during hydrate gas production due to wellbore pressure reduction and temperature increase. Unlike the stress perturbation in the conventional gas reservoir, critical temperature and pressure for the equilibrium phase change must be surpassed to induce additional incremental stresses due to the hydrate decomposition. Additional fluid mass and energy transfer may take place with induced temperature and pore pressure because of the hydrate decomposition/recomposition. In addition, the hydrate saturation changes due to the typical drilling strategy applied may reduce the hydrate formation significantly, which will affect the design of production pressure control and management. Thus, pressure optimization is crucial for both maximum production and wellbore integrity.