(35b) Modeling of Phase Transitions during Supercritical Carbon Dioxide Migrating in a Leaky Pathway

Carbon dioxide sequestration in geological formations has emerged as one of the key technologies for reducing greenhouse gas emissions. Nevertheless carbon dioxide leakage from storage sites will lead to disastrous consequences for human beings. Owing to compressibility and buoyancy effects, carbon dioxide would possibly migrate upwards from these storage reservoirs to the atmosphere along potential leakage pathways, which are probably abandoned wells or geological faults. During leakage carbon dioxide transforms from the supercritical state to normal states under certain conditions that are influenced by the pressure and temperature of the surrounding formation. In the process of the phase transition, the dramatic change of properties of carbon dioxide can exert strong impacts over the carbon dioxide leakage process. Hence predicting phase change behaviors during carbon dioxide leakage along leaky pathways is of significant importance in the evaluation of potential risks of carbon dioxide storage in subsurface formations.

In this paper we established a two-dimensional mathematic model to describe flow and heat transfer of carbon dioxide migrating along a leaky pathway. The major objective is to investigate phase change behaviors and relative impact factors during carbon dioxide leakage through a vertical porous channel. In the mathematic model, on the basis of the features of carbon dioxide saturation line, we proposed a piecewise function to identify whether phase change occurs or not in the course of supercritical carbon dioxide flow in the porous channel. In the mass balance equation, a correction to the source term is adopted to describe the distribution of the phases. In the energy balance equation, the equivalent heat capacity method is employed to describe the energy change during phase change, in which the density, viscosity, thermal conductivity and other properties of carbon dioxide are expressed as properties of mixture of gas and liquid phases by piecewise functions.

The proposed model was solved by using a finite element method-based commercial software package COMSOL Multiphysics. The results indicate that phase change of carbon dioxide is mainly dependent upon the coupling effects of heat exchange between the surroundings and the leaky pathway and the temperature variation caused by carbon dioxide expansion process. The heat exchange depends on interactions between carbon dioxide flow and geological conditions, and the depressurization process of carbon dioxide determines the action of Joule-Thomson effect. Under different conditions of the formation pressure and temperature, the relevant properties of carbon dioxide would change dramatically due to phase change that carbon dioxide crosses over the saturation line. Consequently, accurately predicting the properties of carbon dioxide, such as density and viscosity, is the key to understand the behaviors of phase change process during carbon dioxide leakage. The model proposed in this paper can predict the phase change behaviors during carbon dioxide leakage along a leaky pathway, which will provide an analytical tool for the prediction of characteristics of carbon dioxide leakage and the assessment of potential leakage risks.