Modeling of gas production from hydrates in porous media
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AbstractGas hydrates are considered to be an alternative energy resource of the future, as they exist in enormous quantities in permafrost and offshore environments. However, due to the severe environmental conditions of gas hydrate reservoirs and their solid nonflowing form, extensive technological development and expertise are required before commercial gas production becomes possible. In this work, both analytical and numerical mathematical models are developed to perform simulation studies of gas production from methane-hydrate by the depressurization method. The three primary mechanisms involved in hydrate decomposition in porous media are considered to be the heat transfer to the decomposing zone, the kinetics of hydrate decomposition and the gas-water two-phase flow. The relative importance of these mechanisms is compared over a realistic range of physical properties. Considering the rate-controlling mechanisms, a new analytical model is developed to predict the performance of hydrate decomposition in porous media. In a second part of the work, a two-dimensional numerical model for gas production from hydrate reservoirs is developed. The model includes equations of conductive and convective heat transfer, kinetics of hydrate decomposition, and gaswater two-phase flow. A single-well case is considered to model a hydrate reservoir where the hydrate-bearing layer overlies a free gas zone. The importance of the different mechanisms is examined by investigating the effect of various parameters, including formation properties, operating conditions and kinetic parameters, on gas production behaviour. The results suggest that the rate of heat transfer plays an important role in the process. Furthermore, the rate of gas generation from hydrate decomposition would not be affected unless the reaction rate constants in porous media are smaller than those measured in stirred reactors by many orders of magnitude. Under operating condition of constant bottom-hole-pressure, the rate of gas generation increases significantly with an increase in the rock permeability. It is estimated that the gas production rate from hydrate decomposition can be as high as 2-4 MMSCF/D in a drainage area of one- section (1600m x 1600m) with 10 m hydrate at top.
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