Petrophysical and petrochemical effects of long-term CO2 -exposure experiments on brine-saturated reservoir sandstone

Abstract

Sandstone samples of the Stuttgart Formation at Ketzin have been experimentally treated with CO2 and synthetic reservoir brine in high-quality steel autoclaves at simulated in situ P-T conditions of 5.5 MPa and 40 °C. In order to observe mineralogical changes induced by CO2, untreated samples are compared to CO2-treated ones. Samples show a comparable petrography of mainly quartz and plagioclase. Heterogeneities are related to minor mineral phases, such as K-feldspar, hematite, muscovite, biotite, illite, chlorite, and opaque phase(s). This variability of the Stuttgart Formation is primarily attributed to the sandstone sedimentation in a meandering, fluvial environment. The samples are weakly consolidated. Analcime, anhydrite and dolomite are only found as cement phases. SEM micrographs of CO2-treated samples show corrosion textures on mineral surfaces of anorthitic plagioclase and K-feldspar, as well as euhedral albite crystals. Overall, the data indicate preferred dissolution of calcium-rich plagioclase, K-feldspar and anhydrite, and stabilization or precipitation of albite. The petrophysical properties of the sandstone samples also show slight changes. Nuclear magnetic resonance (NMR) and mercury injection measurements indicate a slightly increased porosity and a shift to larger pore sizes; both suggesting dissolution processes.