Mathematical Modeling and Numerical Simulation of Methane Production in a Hydrate Reservoir

Abstract

Methane hydrate, a potential future energy resource, is known to occur naturally in vast quantities beneath the ocean floor and in permafrost regions. It is important to evaluate how much methane is recoverable from these hydrate reserves. This article introduces the theoretical background of HydrateResSim, the National Energy Technology Laboratory (NETL) methane production simulator for hydrate-containing reservoirs, originally developed for NETL by Lawrence Berkeley National Laboratory (LBNL). It describes the mathematical model that governs the dissociation of methane hydrate by depressurization or thermal stimulation of the system, including the transport of multiple temperature-dependent components in multiple phases through a porous medium. The model equations are obtained by incorporating the multiphase Darcy's law for gas and liquid into both the mass component balances and the energy conservation equations. Two submodels in HydrateResSim for hydrate dissociation are also considered: a kinetic model and a pure thermodynamic model. Contrary to more traditional reservoir simulations, the set of model unknowns or primary variables in HydrateResSim changes throughout the simulation as a result of the formation or dissociation of ice and hydrate phases during the simulation. The primary variable switch method (PVSM) is used to effectively track these phase changes. The equations are solved by utilizing the implicit time finite-difference method on the grid system, which can properly describe phase appearance or disappearance as well as the boundary conditions. The Newton-Raphson method is used to solve the linear equations after discretization and setup of the Jacobian matrix. We report here the application of HydrateResSim to a three-component, four-phase flow system in order to predict the methane produced from a laboratory-scale reservoir. The first results of HydrateResSim code in a peer-reviewed publication are presented in this article. The numerical solution was verified against the state-of-the art simulator TOUGH+Hydrate. The model was then used to compare two dissociation theories: kinetic and pure equilibrium. Generally, the kinetic model revealed a lower dissociation rate than the equilibrium model. The hydrate dissociation patterns differed significantly when the thermal boundary condition was shifted from adiabatic to constant-temperature. The surface area factor was found to have an important effect on the rate of hydrate dissociation for the kinetic model. The deviation between the kinetic and equilibrium models was found to increase with decreasing surface area factor.

Review of natural gas hydrates as an energy resource: Prospects and challenges

“…Many literature works have focussed in gas recovery via depressurization by developing numerical models to simulate natural gas hydrate recovery [115][116][117][118][119][120][121][122][123][124][125]. Sung et al [116] simulated a threedimensional, multiphase (gas-water-hydrate) numerical model to estimate recovery performances implementing a kinetic model proposed earlier [115].…”

Numerical Simulation of Methane Production from Hydrates Induced by Different Depressurizing Approaches

“…These information are all described in the above mathematical equations, which constitute a system of four coupled partial differential equations which are non-linearity and cannot be solved analytically (exactly). Several numerical examples have been tested to solve these equations [10,14,15,19,[31][32][33][34][35]. In this work, the fully implicit simultaneous solution method combined with Newton's iterative method is used to solve this model.…”

“…They discussed the effects of the assumption of a stationary water phase on moving front speed and the gas flow rate. Then, Gamwo and Liu [19] adopted the HydrateResSim numerical simulator to predict the methane production from a laboratory-scale reservoir. They compared the numerical results calculated by using kinetic and equilibrium models of hydrate dissociation theories.…”

Evolution of Hydrate Dissociation by Warm Brine Stimulation Combined Depressurization in the South China Sea

“…The discretization in the core of the circle is very fine, but it gets coarser in the direction of the radius. Supposing the hydrate dissociation with inhibitors in equilibrium state [29], the number of the coupled equations in this simulation is 48,032, which is from multiplying by 12,008 (active cells) and 4 (the equations of per cell include the mass balance equations of gas, water and salt, as well as the energy balance equation [31,32]). …”