This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper IPTC 17026, ’Massively Parallel Simulation of Production From Oceanic- Gas-Hydrate Deposits,’ by Matthew T. Reagan, SPE, George J. Moridis, SPE, Craig M. Freeman, SPE, Katie L. Boyle, and Noel D. Keen, Lawrence Berkeley National Laboratory, prepared for the 2013 International Petroleum Technology Conference, Beijing, 26-28 March. The paper has not been peer reviewed.

Large volumes of gas can be produced at high rates with conventional horizontal- or vertical-well configurations for long periods of time from some methane-hydrate accumulations by means of depressurization-induced dissociation. However, most assessments of hydrate production use simplified or reduced-scale 3D or 2D production simulations. This study used a message-passing-interface parallel code to make the first field-scale assessment of a large deep-ocean hydrate reservoir. Systems of up to 2.5 million gridblocks, running on thousands of supercomputing nodes, were required to simulate such a large system 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. The main difficulties are water production and achieving depressurization.

Introduction

Gas hydrates are solid crystalline com-pounds in which gas molecules occupy the lattices of ice-like crystal structures called hosts. Gas hydrates occur in the permafrost and in deep-ocean sediments, where the necessary conditions of low temperature and high pressure exist for their formation and stability. Most naturally occurring gas hydrates contain methane in abundance. Not all hydrates are desirable targets for production. Of the three possible methods of hydrate dissociation for gas production—depressurization, thermal stimulation, and use of inhibitors—depressurization appears to be the most efficient. Recent studies have indicated that, under certain conditions, gas can be produced from natural hydrate deposits at high rates over long periods of time by use of conventional technology. Earlier work focused on production from vertical wells, but more-recent studies show that horizontal wells are more productive and are easier to manage, if the technology is available.

Study

The objective of this study was to use real geophysical data at the field scale to simulate a realistic 3D gas-hydrate reservoir. Previous studies focused on simple 2D modeling, 2D modeling with artificial heterogeneity, or extremely simplified (coarse-scale) 3D modeling. Through collaboration, Statoil provided real data on the geometry and geology of a known oceanic-hydrate system. The deposit is a layered system, approximately 7×5.5 km and 350 m thick. The hydrate is arranged in a series of high-permeability channels bounded by slightly-lower-permeability levees. The system is probably impermeable at the top and bottom boundaries, but may connect to an aquifer along the x–z face.

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