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

Numerical Studies on Two-way Coupled Fluid Flow and Geomechanics in Hydrate Deposits J. Kim, SPE; G. J. Moridis, SPE; Lawrence Berkeley National Laboratory; D. Yang, SPE, Texas AM J. Rutqvist, SPE, Lawrence Berkeley National Laboratory 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 geome- chanics 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 two- way 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 perme- ability, 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. Introduction Background. 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 sys- tem 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 porosity and permeability, and potentially leading to yielding, failures and fracture evolution or closures; these processes can in turn affect the flow behavior of the entire system. Reservoir engineering is replete with examples of such strongly coupled flow-geomechanics processes with a significant impact on production and economic consequences: stability of borehole and surface facilities, hydraulic fracturing for fluid production from low-permeability reser- voirs, reservoir compaction (especially in highly compressible systems) and land surface subsidence, sand production during reservoir fluid production from unconsolidated or unstable formation, system responses during geologic CO 2 sequestration, gas production from hydrate accumulations, etc. - see Bagheri and Settari (2008), Merle et al. (1976), Lewis and Schrefler (1998), Kosloff et al. (1980), Freeman et al. (2009), Rutqvist et al. (2010b), and Rutqvist and Moridis (2009). Hydrate reservoirs are considered as potentially substantial future energy resources (Moridis, 2003; Moridis et al., 2009a, 2011) because of the vast quantities of hydrocarbon gas (mainly CH 4 ) they trap (Sloan and Koh, 2008). Hydrate deposits that are desirable gas production targets almost invariably involve coarse, unlithified, unconsolidated media (such as sands and gravels). In such deposits, it is the hydrate that imparts mechanical strength to the medium, and hydrate dissociation for gas production drastically changes the geomechanical status of the system. Additionally, the system undergoes significant temperature changes because dissociation is a strongly endothermic reaction. Thus, fluid flow, heat transport and geomechanics are inexorably intertwined and need to be considered together as strongly coupled processes in hydrate accumulations under production because the inevitable significant changes in pressure P , temperature T and saturations S J of the various phases J (aqueous, gas, hydrate, ice) during dissociation directly affect the stiffness of the solid skeleton and the stress and strain fields, resulting in deformation of the reservoirs and potentially large changes in the porosity and permeability (Rutqvist and Moridis, 2009; Rutqvist et al., 2009). Thus, in the study of gas production from hydrate deposits, the geomechanical stability and integrity of both the hydrate-bearing sediments (HBS) and the wellbore need to be considered. Gas hydrates are solid crystalline compounds in which gas molecules are trapped within the lattice of ice crystals (Moridis, 2003). Trapped gases and the ice crystals are called guests and hosts, respectively. Given the availability of appropriate gas sources, hydrates evolve according to the exothermic equation G + N H H 2 O = G · N H H 2 O + Q H ,