CO2 Brine-rock Interactions in Reservoir Chalk Rock – An Experimental Evaluation at Reservoir Conditions

CO2 flooding can improve oil recovery in chalk plugs with low matrix permeability. However, several coupled physical and chemical processes may occur. These phenomena can either increase and/or decrease the injectivity. A series of dynamic laboratory experiments have been used to investigate CO2-brine-rock interactions during CO2 injection in a reservoir chalk rock. The experiments were designed to represent the various situations in the field – near well-bore or far field regions. The experiments consisted in the injection of CO2-WAG at reservoir conditions and CO2 saturated water at supercritical CO2 conditions. The experiments were carried out in plugs with either Swi= 100% and/or Swi= 15%. A novel sampling technique was used to sample produced fluids under pressure without disrupting the injection process. Results based on chemical analyses of produced fluids lead to a comprehensive understanding of the coupled mechanisms taking place. Based on the measured Ca-concentration and alkalinity, the presence of oil was observed to slightly delay the calcite dissolution process. Wormholes evident of calcite dissolution were seen on the inlet sides of the plugs after the flooding process. Pre and post-floods petrophysical analyses of the plugs also showed some porosity and permeability increase as a result of flooding.