Impact of gas/liquid phase change of CO2 during injection for sequestration

CO₂ sequestration in deep saline formations is an effective and important process to control the rapid rise in CO₂ emissions. The process of injecting CO₂ requires reliable predictions of the stress in the formation and the fluid pressure distributions – particularly since monitoring of the CO₂ migration is difficult – to mitigate leakage, prevent induced seismicity, and analyze wellbore stability. A key aspect of CO₂ is the gas–liquid phase transition at the temperatures and pressures of relevance to leakage and sequestration, which has been recognized as being critical for accurate predictions but has been challenging to model without ad hoc empiricisms.

This paper presents a robust multiphase thermodynamics-based poromechanics model to capture the complex phase transition behavior of CO₂ and predict the stress and pressure distribution under super- and sub- critical conditions during the injection process. A finite element implementation of the model is applied to analyze the behavior of a multiphase porous system with CO₂ as it displaces the fluid brine phase. We find that if CO₂ undergoes a phase transition in the geologic reservoir, the spatial variation of the density is significantly affected, and the migration mobility of CO₂ decreases in the reservoir. A key feature of our approach is that we do not a priori assume the location of the CO₂ gas/liquid interface – or even if it occurs at all – but rather, this is a prediction of the model, along with the spatial variation of the phase of CO₂ and the change of the saturation profile due to the phase change.