Massively Parallel Simulation of Production from Oceanic Gas Hydrate Deposits
Authors: Reagan Matthew T. , Moridis George J , Freeman Craig M. , Boyle Katie L. , Keen Noel D. 
Published: March 2015

Abstract
The quantity of hydrocarbon gases trapped in natural hydrate accumulations is enormous, leading to significant interest in the evaluation of their potential as an energy source. It is known that large volumes of gas can be readily produced at high rates for long times from some types of methane hydrate accumulations by means of depressurization-induced dissociation with conventional horizontal or vertical well configurations. However, most assessments of hydrate production use simplified or reduced-scale 3D or 2D production simulations. In this study, we use the MPI-parallel TOUGH+HYDRATE code (pT+H) to make the first field-scale assessment of a large, deep-ocean hydrate reservoir. Systems of up to 2.5M gridblocks, running on thousands of supercomputing nodes, are required to simulate such large systems at the highest level of detail. The simulations begin to reveal the challenges inherent in producing from deep, relatively cold systems with extensive water-bearing channels and connectivity to large aquifers, mainly the difficulty of achieving depressurization and the problem of water production. Also highlighted are new frontiers in large-scale reservoir simulation of coupled flow, transport, thermodynamics, and phase behavior, including the construction of large meshes, the computational scaling of larger systems, and the complexity and resource-intensiveness of large-scale volume visualization of unstructured meshes.

Introduction
Gas hydrates are solid crystalline compounds in which gas molecules occupy the lattices of ice-like crystal structures called hosts (Sloan and Koh, 2008). They may occur in two distinctly different geographic settings, in the permafrost and in deep ocean sediments, where the necessary conditions of low T and high P exist for their formation and stability. The majority of these naturally occurring hydrates contain CH4 in overwhelming abundance. Interest in hydrates is enhanced by ever-increasing global energy demand and the environmental desirability of natural gas. Although there has been limited work mapping and evaluating this resource on a global scale (Moridis et al., 2009), current estimates of in-place volumes vary widely (ranging between 1015 to 1017 ST m3), but the consensus is that the worldwide quantity of hydrocarbon gas hydrates is vast (Milkov, 2004; Burwicz et al., 2011). Even if only a small fraction of the most conservative estimate is recoverable, the sheer size of the resource is so large that it demands evaluation as a potential energy source.
However, not all hydrates are desirable targets for production (Moridis et al., 2011). Of the three possible methods of hydrate dissociation (Makogan, 1997) for gas production— depressurization, thermal stimulation, and use of inhibitors—depressurization appears to be the most efficient (Moridis et al., 2009). Recent studies (Moridis and Reagan, 2007a; b) have indicated that, under certain conditions, gas can be produced from natural hydrate deposits at high rates over long periods using conventional technology. Earlier work focused on production from vertical wells, but more recent studies (Moridis et al., 2008) show that horizontal wells are more productive, and easier to manage, that vertical wells, if the technology is available.