Numerical Studies on Two-Way Coupled Fluid Flow and Geomechanics in Hydrate Deposits

Abstract

Coupled flow and geomechanics play an important role in the analysis of gas hydrate reservoirs under production. The stiffness of the rock skeleton and the deformation of the reservoir, as well as porosity and permeability, are directly influenced by (and interrelated with) changes in pressure, temperature and fluid (water and gas) and solid (hydrate and ice) phase saturations. Fluid and solid phases may coexist, which, coupled with steep temperature and pressure gradients, result in strong nonlinearities in the coupled flow and mechanics processes, making the description of system behavior in dissociating hydrate deposits exceptionally complicated.In previous studies, the geological stability of hydrate-bearing sediments was investigated using one-way coupled analysis, in which the changes in fluid properties affect mechanics within the gas hydrate reservoirs, but with no feedback from geomechanics to fluid flow. In this paper, we develop and test a rigorous two-way coupling between fluid flow and geomechanics, in which the solutions from mechanics are reflected in the solution of the flow problem through the adjustment of affected hydraulic properties. We employ the fixed-stress split method, which results in a convergent sequential implicit scheme.In this study of several hydrate reservoir cases, we find noticeable differences between the results based on one-and twoway couplings. The nature of the elliptic boundary value problem of quasi-static mechanics results in instantaneous compaction or dilation over the domain, through loading from reservoir fluid production. This induces a pressure rise or drop at early times (low pressure diffusion), and consequently changes the effective stress instantaneously, possibly causing geological instability. Additionally, the pressure and temperature regime affects the various phase saturations, the rock stiffness, porosity, and permeability, thus affecting the fluid flow regime. These changes are not captured accurately by the simpler one-way coupling. The tightly coupled sequential approach we propose provides a rigorous, two-way coupling model that captures the interrelationship between geomechanical and flow properties and processes, accurately describes the system behavior, and can be readily applied to large-scale problems of hydrate behavior in geologic media. IntroductionBackground. The interrelationship between fluid flow through porous media and the geomechanical status of the system is controlled by the properties of the specifics of flow, various phases present in the pores (e.g., gas, aqueous, oil/organic, ice, etc.) and of the solid system (i.e., the individual grains of the geologic medium and the reservoir skeleton). Under certain conditions, the interrelationship is strong, and coupling of the flow processes with geomechanics is necessary to accurately describe the system behavior. In such cases, changes in pressure brought about by flow (e.g., in the process of fluid production from reservoirs) alter the stress fields, resulting in changes in por...

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“…To this end, we consider variations in not only permeability but also pore volume, which lead to dynamic permeability and porosity of the reservoirs, respectively. In particular, compaction in undrained condition such as low permeability induces the increase of pore-pressure, which can change effective stress fields significantly and may result in geological failure (Kim et al, 2012a). Compaction-induced shear failure by fluid production was also studied by Dusseault et al (2001) and Bagheri and Settari (2008).…”