Methane hydrate structures are increasingly recognized as potential energy resources due to the significant amount of fuel energy tied up in them Kvenvolden (1993). Furthermore, they might have a significant contribution to the methane balance in the atmosphere, thus affecting global climate and buffering its change. Therefore, knowledge of the reservoir properties of methane hydrate, such as the amount of hydrate present, its mobility and recoverability, is essential. The means of seismic detection and characterization is provided by bottom simulating reflectors (BSRs), which mark the bottom of the stability zone of methane hydrates. Since hydrates are stable only in a certain pressure-temperature regime, limited information is available from deep sea drilling Dallimore and Collett (1995); Kvenvolden and Barnard (1983); MacKay et al. (1994) and most information regarding the elastic properties is inferred directly from the seismic reflection data Ecker and Lumley (1994); Katzman et al. (1994); Singh et al. (1993). Since the recognition of hydrate structures in seismic data is primarily dependent on the presence of BSRs, the understanding of their formation, stability and breakdown is essential.
As the base of the hydrate stability zone, BSRs are highly dependent on the pressure-temperature regime in shallow marine sediments. Changes in sea-level, mass slides, diapirs, etc. can have a considerable impact on these regimes, possibly causing hydrate decomposition or growth, and thus resulting in possible breakdown or reformation of the BSRs Dillon et al. (1993). This could explain the deformation and discontinuities in the BSRs often visible in seismic sections. Fluid-flow simulation of the change of hydrate structures under certain pressure/temperature perturbation can give a good first-order insight into the BSR behavior coupled with it.
Information about kinetic hydrate decomposition and growth is available from several laboratory measurements Englezos et al. (1987); Kim et al. (1987); Yousif et al. (1991). Furthermore, several 1-D simulations have been conducted on the dissociation of methane hydrate to investigate possible schemes of gas recovery from the hydrate structures Bondarev et al. (1989); Jamaluddin et al. (1989); Verigin et al. (1980); Yousif et al. (1991). Little work, however, has been done in directly trying to simulate the changes in BSR occurrence coupled with pressure/temperature perturbations and, subsequently, relating it to real seismic observations.
In this paper, I present the theory and development of a three-dimensional finite-difference fluid-flow simulator for hydrate growth and decomposition in porous media. I show the fluid equations for the 3-phase/2-component gas-water-hydrate system based on the conservation of mass and discuss the code development based on those equations. In this first phase of the simulation attempt, the system is considered to be completely isothermal and the thermodynamic effects are not yet been taken into account. A more realistic flow system based on heat transfer can be formulated using an equation of energy balance and can be added once the isothermal model has been sufficiently tested. The code is still in a testing phase and does not work properly for all required cases. I show the preliminary results of some code testing based on simple gas-water flow. The hydrate simulation itself requires more work in order to obtain a working program. Finally, I discuss how the code could be extended to include the thermodynamics of the system.