Simulation code development for T-H-M coupled processes and behaviors of sand - clay layered hydrate reservoirs during depressurization

Abstract

In this study, a thermo, hydro and also geomechanically coupled simulation code was developed to analyze the behaviors of hydrate reservoirs. Explicit Euler scheme- based finite volume method was used to construct the simulation code. The explicit (two-way) coupling scheme was utilized to couple geomechanics and flow – thermal problem. Two verification problems were tackled to secure the reliability of the developed code, proving that the code shows satisfying degree of conformity with analytical solutions and also good match with results from other numerical software or from actual laboratory experiments. Once verified, the code is used to perform a 50-day natural gas production simulation on a multi-layered, offshore hydrate reservoir. The major aim of this numerical study was to identify a multi-layered hydrate reservoir system’s unique geomechanical behaviors during depressurization-induced gas production. Important findings from the 50-day simulation are as follows: First, gas hydrate selectively and rapidly dissociates along the clay – sand layer interfaces. Second, the presence of the layer interfaces causes faster sediment consolidation (compaction) within the sediment near the layer interfaces. Third, zones of excessive shear develop along the layer interfaces. And lastly, fourth, a conjecture was made that there would be an “asymptotic” maximum shear induced by depressurization. Presumably, the asymptotic value might be proportional to the magnitude of depressurization. In addition, two hydrate reservoir models with different layering structure were compared in terms of their production and mechanical behaviors. The comparison revealed that, although a “densely” layered reservoir shows more favorable gas production behaviors, it also exhibits more intense geomechanical responses against depressurization. Noting that not only production behaviors but also geomechanical stability of hydrate reservoirs are predominantly affected by their sediment layering structure, it appears that accurate sediment profiling should be engaged in numerical hydrate reservoir simulations to reliably predict T-H-M responses of the hydrate reservoir.