Geoelectrical characterisation of CO₂–water systems in porous media: application to carbon sequestration

Carbon sequestration is a promising method for the reduction of carbon dioxide (CO₂) emissions as it permits the storage of compressed CO₂ in the subsurface. The carbon sequestration sites must be monitored to detect potential leaks; one possible method involves the monitoring of geoelectrical properties such as electrical conductivity (σ_b) and dielectric constant (ε_b). This investigation focuses on using a time domain reflectometry (TDR) sensor to determine the influence of different factors on the measurements of the electrical conductivity (σ_b) and dielectric constant (ε_b) of a porous rock reservoir in relation to the soil water saturation (S_w). The factors investigated were presence of surfactant, salt concentration, pH and rock type which are unique to a given storage site. A number of dynamic two-phase flow experiments were performed using gaseous CO₂. It was found that salt concentration, rock type and presence of a surfactant had a notable effect on the σ_b–S_w and ε_b–S_w relationships. Higher salt concentrations were found to give higher values for σ_b and ε_b for given S_w values. Limestone was found to result in the highest values of both σ_b and ε_b for any given S_w, followed by silica and basalt samples. The presence of a surfactant resulted in higher values for σ_b at higher S_w values and lower values for σ_b at lower S_w values compared to the case when no surfactant was present. Surfactant presence also resulted in lower values for ε_b at given S_w values. Initial pH values (with silica sand) were found to have no significant effect on the σ_b–S_w and ε_b–S_w relationships. The measurements of σ_b and ε_b indicate that the use of TDR presents a viable monitoring option. Furthermore, statistical analysis using non-linear regression was carried out on the experimental results and the model shows a good reliability in the prediction of the monitoring process in geological carbon sequestration.