An investigation of the effect of CO2–brine–rock interaction on determination of Archie's saturation exponent for carbon dioxide evaluation in carbonate reservoirs

Abstract

Accurate evaluation of reservoir fluid in place using electrical resistivity logs depends on accurate laboratory measurements of Archie parameters. Sensitivity of resistivity-based saturation models on these parameters, and their determination in non-Archie rocks have been extensively discussed in numerous technical papers. Conventional estimation of saturation exponent involves either steady state or porous plate displacement methods whereby a non-wetting phase displaces the wetting phase. When applying this technique in carbon dioxide sequestration projects in order to estimate and monitor carbon dioxide (CO2) accumulation in underground saline carbonate aquifer, it becomes imperative to consider the effect of CO2–brine–rock interaction (CBRI) since carbonic acid formed after dissolution of CO2 in formation brine will dissolve some soluble carbonate grains thereby altering rock pore structure and possibly change saturation exponent as CO2 sequestration lasts. In this paper, we investigated the effect of CBRI on Archie’s electrical parameters. Systematic laboratory procedures were designed to study saturation exponent and cementation factor for samples with and without CO2 sequestration taking into account the effect of hysteresis and measurement errors. State of the art characterization techniques like nuclear magnetic resonance (NMR) and mercury injection capillary pressure (MICP) were used to characterize and select core plugs of most similar pore character from a set of plugs obtained from a homogeneous Indiana limestone core. Saturation exponents and cementation factors of samples in which CO2 were sequestered were found to be different from other samples with the same pore structure but without CO2 sequestration. Saturation exponent for the sample in which CO2 was not stored was 1.46, while for those samples in which CO2 was stored, saturation exponent was found to be in the range of 1.65–5.68 with a corresponding porosity increase also in the range of 5–10%. Cementation factors were also found to decrease slightly from 1.44 to between 1.29 and 1.38. The extent of the variation in saturation exponent depended on the time of storage and the degree of CBRI in each sample. Post CO2 sequestration NMR and MICP analysis also confirmed altered pore character in the storage samples. Some of the dissolved calcite minerals were also seen as precipitates in brine effluents after CO2 storage.