An Experimental Study of CO2 Exsolution and Relative Permeability Measurements During CO2 Saturated Water Depressurization

Advisor

Sally M. Benson

Abstract

Dissolution of CO₂ into brine is an important and favorable trapping mechanism for geologic storage of CO₂. There are scenarios, however, where dissolved CO₂ may migrate out of the storage reservoir. Under these conditions, CO₂ will exsolve from solution during depressurization of the brine, leading to the formation of separate CO₂ phase. For example, a CO₂ sequestration system with a brine-permeable caprock may be favoured to allow for pressure relief in the sequestration reservoir. In this case, CO₂-rich brine may be transported upwards along a pressure gradient caused by CO₂ injection. Here we conduct an experimental study of CO₂ exsolution to observe the behaviour of exsolved gas under a wide range of depressurization.

Exsolution experiments in highly permeable Berea sandstones and low permeability Mount Simon sandstones are presented. Using X-ray CT scanning, the evolution of gas phase CO₂ and its spatial distribution is observed. Additionally, we measure relative permeability curves for exsolved CO₂ and water based on mass balances and continuous observation of the pressure drop across the core from 12.41MPa to 2.76MPa.

The results show that the minimum CO₂ saturation at which CO₂ bubbles mobilize is from 11.7%~15.5%. Exsolved CO₂ is distributed uniformly in homogeneous rock samples with no statistical correlation between porosity and CO₂ saturation observed. No gravitational redistribution of exsolved CO₂ is observed after depressurization. Also, significant differences exist between the relative permeability of exsolved CO₂ and water and the relative permeability derived from steady-state core flooding experiments. Specifically, low relative permeabilities of both CO₂ and water are measured, even when the CO₂ saturation is as high as 40%. The large mobility reduction is considered the result of disconnected gas bubbles in this two-phase flow system. This feature is also thought to be favorable for storage security after injection.