Gas hydrates are being considered as a future-alternate energy resource due to enormous quantities of natural gas deposition in hydrate form in the permafrost and offshore regions of the world. This study introduces an analytical numerical model for simulation of gas production by hydrate decomposition in porous media through depressurization process. Developments of several analytical and numerical models have been reported in the literature, however, mainly focused on decomposition processes. We coupled intrinsic kinetics of hydrate decomposition and heat transfer to analyze the production process. A commercial reservoir simulator was then used to construct a field scale model. A well is completed in the gas zone to follow the effect of depressurization in porous media and its effect on hydrate decomposition. Using the constructed analytical-numerical model a production strategy for a hydrate reservoir was designed. By use of this model the influence of different parameters related to formation properties, operating conditions, etc, on gas production may be predicted and evaluated.
Natural gas hydrates in permafrost regions and offshore environments represents a potentially enormous supply of natural gas. This resource is expected to be an energy resource of the future. A portion of the gas hydrate reserves could help eliminate concerns of domestic energy shortage in the 21st century.
Different estimations presented about the total energy reserves trapped in methane hydrates. The United States geological survey (kvenvoldsen, 1993)1 estimated that the total amount of methane in gas hydrates around the world is approximately 7*105 trillion cubic feet (ft3). According to Sassen2 the resource potential of methane in hydrate exceeds the combined worldwide reserves of conventional oil and gas reservoirs, coal and oil shale.
From technical view, the gas could be produced from the hydrate disociation by depressuriztion, thermal stimulation, inhibitor injection or a combination of these methods. The potential for gas production through different methods is still uncertain and under investigation.
Developed models can be divided into two broad categories: analytical and numerical models. Analytical models are used for mechanistic studies and improve understanding of the process. Numerical models have smaller number of simplifying assumption, so they are more comprehensive.
In this study, we focused on depressurizing method, where the pressure is decreased below the thermodynamic equilibrium pressure of hydrate. As previous studies mentioned, the heat transfer to the decomposing zone, intrinsic hydrate decomposition, and gas-water two phase flows are the three mechanisms involved in hydrate decomposition in porous media.
Previous studies3 showed the two first mechanisms are rate controlling in hydrate decomposition. So we used the analytical model to describe the heat transfer to the decomposing zone and intrinsic hydrate decomposition.
Hoe and when after the production starts, the pressure decreases below the equilibrium pressure is a subject that studied more in this work.
Previous studies3 showed that the rate of decomposition in controlled by the intrinsic kinetics of decomposition and the heat transfer to decomposing zone. So in this study these two processes are coupled to model the decomposition step.
