Long-timescale interaction of CO_2 storage with reservoir and seal: Miller and Brae natural analogue fields North Sea

摘要

The Miller oilfield may form the first UK offshore CO_2 storage site. We have examined sandstones from the Miller reservoir and mudrocks, which form the vertical seal, using techniques adapted from investigations of oilfield reservoir quality. The present-day distribution of natural CO_2 suggests emplacement up a fault to the west, bounding the basin. A scoping study of equilibrium geochemical modelling, with enforced reducing chemistry, predicts that CO_2 addition makes little change to the assemblage of minerals present, but could produce changes of mineral ratios. Carbonate cement is common in the reservoir. Stable isotope analyses provide a fingerprint of C and O origins, and show no difference from standard burial cementation trends. The physical distribution of calcite, however, shows an increase from north to south - possibly due to CO_2 filling, and a horizontal alignment that we interpret as a paleo gas-water contact. Fluid inclusions fossilise samples of ancient fluid, these suggest an input of high salinity water when the reservoir was at cool to moderate temperatures (ie shallow to medium burial depths). Mudrocks above the reservoir show enrichment in kaolin compared to the same mudrock distant from CO_2. Stable isotope analyses of mudrock carbonates, including a low CO_2 comparison well, show a trend similar to the sandstones. Prediction of C, O isotope values in calcites formed from current CO_2 is unlike these measured values. We interpret that there is no direct evidence of abundant ancient CO_2 leakage vertically, although reduced reservoir pressures show that leakage has occurred - possibly laterally. CO_2 seems to have been retained for about 120 million years, with little mineral trapping in the reservoir or seal.