Simulating Wellbore Integrity During Drilling in Gas Hydrate Bearing Sediments by Coupled THMD Model
Authors: Wang Y. 
Published: September 2022

ABSTRACT:
Wellbore integrity in a gas hydrate bearing formation during drilling remains great challenge as a complex solid-fluid and decomposed hydrate process interaction is involved. A wellbore temperature increase and/or depressurization processes my induce additional stresses and extra fluid flow, which in turn may trigger wellbore instability and unexpected fracturing. In order to understand and quantify the thermal-hydraulic-mechanical behavior with hydrate decomposition (THMD) process and the wellbore responses to the pressure and temperature variation under a given in-situ stress regime, a fully coupled THMD model is proposed and developed in this paper. A simplified semi-analytical solution for the induced stresses, pore pressure, and temperature change at the wellbore and inside the formation is presented. A linear Mohr-Coulomb criterion is utilized to define the onset of the wellbore instability in which a hydrate saturation depending cohesion is also incorporated. Laplace transformation is applied and a numerical method is used to solve the transformed homogeneous PDE. We conclude that thermally and hydration induced stresses may affect wellbore integrity during hydrate gas production due to wellbore pressure reduction and temperature increase. Unlike those stress perturbation in the conventional gas reservoir, critical temperature and pressure must be surpassed and additional incremental stresses may be induced due to the hydrate hydrate decomposition. Also additional fluid mass and energy transfer may take place by which both temperature and pore pressure may be induced due to the hydrate decomposition/recomposition. In additional the hydrate saturation change due to the typical drilling strategy applied may reduce the hydrate formation significantly,and this may also affect the production pressure control and management. Thus managing pressure for both maximum production and wellbore integrity must be optimized accordingly.

1. INTRODUCTION
Wellbore stability has been a long-observed and widely studied problem during drilling and production in petroleum industries. Stresses around a created borehole due to primarily the in-situ stress from the far-field may increased during drilling and production when the wellbore pressure is reduced. Once such an induced stress state exceeds a strength, measured or estimated, a borehole stability issue, i.e. the borehole lost its integrity, will be brought up. Other than affected by the intact formation strength and initial in-situ stresses, wellbore orientation, formation hydraulic and thermal conditions, and drilling mud chemistry are all critical factors in stability analysis [Bradley, 1979; Santarilli et al., 1986, Detournay and Cheng, 1988; Ewy and Cook, 1990; Wang et al., 1996; Gassemi and Liu, 2005; Neyan, 2010]. Producing hydrate gas either by depressurization or/and increasing wellbore temperature has been proposed for gas exploitation in gas hydrate-bearing sediments (GHBS)[Masuda et al., 1997; Moridis, 2003; Moridis et al., 2007]. Wellbore stability has been reported due to a much more complex mechanical responses maybe provoked near a borehole to the exploiting procedure mentioned [Hyoto et al., 2002; 2008; 2013; 2014; Li et al., 2019,Wang, 2019]. The most economical approach in producing hydrate gas is to change the ice-like hydrate into fluid by reducing the pressure or increase the temperature so that a critical equilibrium condition is exceeded [Kim, 1987; Holder and Hand; 1982]. Such a procedure on the other hand will dissociate the hydrate so that newly created fluid (water and /or gas) mass may alter the conventional equilibrium condition. The new dynamic equilibrium system due the hydrate phase change may affect the mechanical response of the entire formation and particularly in the vicinity of the borehole, where phase change shall take place first and persist until the solid hydrate is decomposed completely. Furthermore the formation strength may be reduced significantly after hydrate dissociation [Masui et al., 2005; 2008; Hyoto et al., 2002; 2008; 2013, 2014; Li et al., 2019; Priest et al.; 2019]. A coupled thermal-hydraulic-mechanical model with hydrate dissociation (THMD) is proposed and numerical methods are typically utilized, i.e. Uchida et al., [2013]; Freij-Ayoub et al., 2017]. Although analytical solutions are convenient and a tool easy to use, but difficult to be tractable, a semi-analytical solution is developed in this paper for borehole mechanical responses under depressurization. A analytical solution and fully coupled THMD model are required for drilling and well integrity design during drilling and production; A semi-analytical solution is developed by incorporating THMD model including phase change and hydrate saturation-dependent cohesion. Specifically the solution is applicable specifically for the situations when the critical temperature/pressure are surpassed, i.e. the solid hydrate dissociation initiates and is decomposed into water and gas phase. Unlike those in the conventional reservoirs, the cohesion in GHBS is weakened significantly with hydrate saturation reduction [Koyno et al.,2002; 2008; 2013; 2014; Li et al., 2019]. gas and water saturation are balanced between fluid extraction at a producing well and supply from either the adjacent formation and those decomposed in each pore volume. The porosities based on the pore volume in the solid phase from the porous skeleton and combined solid hydrate+skeleton should be defined, as