EXPERIMENTAL DETERMINATION OF THE REACTIONS OF CLAY MINERALS IN CLASTIC RESERVOIR ROCK RESULTING FROM THE INJECTION OF SUPERCRITICAL CO2 INTO THE CRETACEOUS FRONTIER FORMATION

The Cretaceous Frontier Formation has been considered as a potential reservoir for anthropogenic CO₂. The reservoir zones are composed of fine-grained quartz and potassium feldspar rich sandstones cemented with clay minerals (kaolinite and interlayered illite and montmorillonite). The purpose of these experiments was to determine whether susceptible minerals such as illite, montmorillonite, and potassium feldspar would undergo in-situ “weathering” reactions when exposed to the high concentrations of carbonic acid generated by the dissolution of supercritical CO₂ in formation water. The transformation of these minerals has the potential to alter the reservoir properties by opening up pore space through dissolution, reducing pore space and/or closing pore throats by precipitating new minerals, or causing little change if the reactions take place slowly. Core samples were obtained from the USGS Core Repository in Denver, CO. The rocks were measured for initial and final porosity and permeability. Pore space distribution was analyzed by CT scan. Mineralogy was explored by thin section analysis, X-Ray diffraction, and Scanning Electron Microscope. Bulk chemical composition was measured by XRF. The cores were reacted with CO2 saturated brine in a flow-through reactor designed to simulate the injection of supercritical CO2 into a reservoir. Two, ten day experiments were carried out at 100C and 15MPa. Samples were withdrawn at regular intervals and concentrations of dissolved species measured by ion chromatograph, and spectrophotometry. Mineral reactions were not significant enough to be discerned by XRD, but their presence was indicated by changes to the chemistry of the water that had reacted with the sample. The experiments also depleted the samples of calcium and iron as measured by XRF bulk elemental analysis.