Multi-salinity core flooding study in clay-bearing sandstones, a contribution to geothermal reservoir characterisation

Abstract. Porosity and permeability measurements aid the characterisation of geothermal reservoirs as they improve understanding of the impact of rock–fluid interactions during the life cycle of wells. Core flooding experiments can help us comprehend the rock–brine electrochemical system as critical parameters like salinity, pH, temperature, or pressure change. If the clay mineral content is significant it can reduce permeability and porosity since these particles can block the pore throat network connectivity through clay migration or swelling. A multi-salinity experiment was conducted in three tight clay-bearing (kaolinite, chlorite, and glauconite) sandstones to study the impact of clay on their petrophysical properties. The experiment consisted of core-flooding brines with salinities of 75 000–200 000 and 0–50 000 ppm NaCl at very low flow rates. Electrical resistivity, the differential pressure across the sample, outlet brine electrical conductivity, and brine permeability were measured. Pore size distribution was acquired by measuring nuclear magnetic resonance (NMR) T2 relaxation time. Cation-exchange capacity (CEC) was derived using the Waxman and Smits (1968) approach. The derived CECs were 71.5, 4.7, and 3.6 meq per 100 g for the kaolinite, chlorite, and glauconite sandstones, respectively. Kaolinite was the least water-sensitive as its permeability decreased uniformly. Chlorite and glauconite were more water-sensitive as in the low salinity range; their permeability increased, and both displayed a bimodal NMR T2 distribution and pore size rearrangement towards the mesoporosity and macroporosity range, indicating that the cation-exchange site prevailed within the pore space. This investigation highlights the importance of ensuring that appropriate fluid chemistry is used on brines flowing in clay-bearing geothermal reservoirs.