A new reservoir simulator for studying hydrate dynamics in reservoir

Abstract

Natural gas hydrates in sediments are generally, not in thermodynamic equilibrium, due to Gibbs phase rule. This is when the impacts of solid mineral surfaces and corresponding adsorbed phases are taken into account. As a consequence, the distribution of water and hydrate formers over possible phases, including hydrate is governed by minimum free energy according to the combined first and second laws of thermodynamics. In this work, we propose the use of a reactive transport reservoir simulator as a new platform for dynamic modelling of hydrates in porous media. Each hydrate phase transition (formation and dissociation) is modelled as a pseudo reaction, with corresponding changes in free energies as the driving forces for the phase transition itself and dynamically coupled to transport of mass and heat. This simulator is different from any of the current platforms developed by industry and academia. The main purpose of this paper is therefore to describe the simulator, integration algorithms as well as approaches for modelling non-equilibrium thermodynamics and kinetics. More specifically, a multi-scale approach, with phase field theory as the core, is used for estimating kinetic rates of different possible phase transitions. Kinetic results from these advanced theories for the different hydrate phase transitions are simplified and implemented into the reservoir simulator (RetrasoCodeBright) in a similar fashion as for reaction kinetic models for mineral/fluid reactions. Another advantage of this specific platform is that it contains implicit geo mechanics. Representative examples are used to illustrate the simulator.