Geoelectrical characterization of carbonate and silicate porous media in the presence of supercritical CO2–water flow

Abstract

The relative permittivity (er) and the electrical conductivity (σ) of porous media are known to be functions of water saturation (S). As such, their measurements can be useful in effective characterisations and monitoring of geological carbon sequestration using geoelectrical measurement techniques. In this work, the effects of pressure, temperature and salt concentration on bulk er–S and σ–S relationships were investigated for carbonate (limestone) and silicate porous media (both unconsolidated domains) under dynamic and quasi-static supercritical CO2 (scCO2)-brine/water flow. In the silica sand sample, the bulk er (eb) for scCO2–water decreases as the temperature increases. On the contrary, slight increase was seen in the eb with temperature in the carbonate sample for the scCO2-water system. These trends are more conspicuous at high water saturation. The eb–S curves for the scCO2–water flow in the silica sand also show clear dependency on the domain pressure, where eb increases as the domain pressure increases. Furthermore, the bulk σ (σb), at any particular saturation for the scCO2-brine system rises as the temperature increases with more significant increase found at very high water saturation. Both eb and σb values are found to be greater in the limestone than silica sand porous samples for similar porosity values. Based on different injection rates investigated, we do not find significant dynamic effects in the eb–S and σb–S relationships for the scCO2-brine/water system. As such, geoelectrical characteristics can be taken as reliable in the monitoring of two-phase flow system in the porous media. It can be inferred from the results that the geoelectrical techniques are highly dependent on water saturation. This dependence is more conspicuous at higher water saturation. Different mathematical models examined show their reliability at different water saturation ranges. The polynomial fit developed in this work takes into consideration the fluid pressure in the system as well as the initial bulk relative permittivity prior to the injection of CO2. The polynomial fit shows a good reliability in the prediction of the geo-electrical properties of the CO2–water–porous media system, especially at higher water saturation. In comparison, the mixing model from the literature shows more reliability in the prediction of similar property at lower water saturation.