Reactive transport modelling of hydrate phase transition dynamics in porous media
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Gas hydrate is believed to be so abundant in nature that some estimates suggest it contains more than two times equivalent energy of the whole fossil fuels on Earth. Besides, reservoir hydrates have other important aspects such as environmental impacts and CO2 storage potentials. These characteristics make it an important topic of research within academia and industry.
Taking into account the impacts of solid mineral surfaces and corresponding adsorbed phases during hydrate processes in reservoirs reveal that these systems cannot reach equilibrium as explained by Gibbs phase rule. Therefore, water and hydrate formers will distribute among possible phases, including hydrate, under the minimum free energy criteria according to the combined first and second laws of thermodynamics.
Complexities of hydrate behaviour in the reservoir require a coupling between different mechanisms ranging from nano-scale to macro scale. For this reason, in this study a reactive transport reservoir simulator named RetrasoCodeBright (RCB) has been used to develop a new hydrate simulator, which is capable of incorporating the kinetics of hydrate phase transitions due to temperature, pressure and concentration super-saturation and under-saturation. Kinetics of hydrate phase transitions is incorporated in the model through two different methods. The first one is a multiscale approach in which, phase field theory as the core element has been used for estimating kinetic rates of different possible phase transitions. The results from these advanced theories are simplified and implemented into RCB for different hydrate phase transitions.
Second approach has been based on the use of non-equilibrium thermodynamics. In this method, the change in free energy of the system due to phase transition from one hydrate phase to any other phase is calculated to identify the impossible phase transition scenarios as well as the unlikely ones and prioritize the likely scenarios according to minimization of Gibbs free energy.
Another important aspect of the platform used in this study is the implicit geomechanical module, which allows analysis of stress changes in the reservoir structure as a result of hydrate processes.
The new simulation tool and the explained features are presented in this study and different applications of this simulator are illustrated through example cases and comparisons published in the attached papers. It gives the possibility to study methane production processes from hydrate reservoirs as well as CO2 storage potentials. The approach used in this study is a step forward towards a more realistic description of reservoir hydrates processes in simulation studies.