Nonequilibrium Modeling of Hydrate Dynamics in Reservoir

摘要

Gas hydrates in reservoirs are generally not in thermodynamic equilibrium., and there may be several competing phase transitions involving hydrate. Calculations of hydrate phase transitions based on a modified statistical—mechanical model for hydrate (Kvamme, B.; Tanaka, H. J. Phys. Chem. 1995, 99, 7114—7119) are used to illustrate differences in properties of hydrates formed from different phases. In more general terms, hydrates in porous media are discussed in terms of the Gibbs phase rule. It is argued that phase transitions involving hydrates in porous media can rarely reach any state of equilibrium due to situations of over specified systems with reference to requirements for equilibrium. As a consequence of this, a strategy for nonequiiibrium description of hydrates in reservoirs is proposed. This involves the formulation of kinetic expressions for all possible hydrate formations and dissociations as competing pseudoreactions. This involves hydrate formation on a water/carbon dioxide interface, from water solution and from carbon dioxide adsorbed on mineral surfaces, as well as all different possible hydrate dissociation possibilities. The basic idea is that the direction of free energy minimum, under constraints of mass and heat transport, will control the progress of phase transitions in nonequiiibrium systems. A new hydrate reservoir simulator based on this concept is introduced in this study. The reservoir simulator is developed from a platform previously developed for carbon dioxide storage in aquifers, RetrasoCodeBright (RGB). The main tools for generating kinetic models have been phase field theory simulations, with thermodynamic properties derived from molecular modeling. The detailed results from these types of simulations provides information on the relative impact of mass transport, heat transport, and thermodynamics of the phase transition, which enable qualified simplifications for implementation into RGB, The primary step was to study the effect of hydrate growth or dissociation with a single kinetic rate on the mechanical properties of the reservoir. Details of the simulator and numerical algorithms are discussed, and relevant examples are shown.