A novel technique for obtaining representative water samples during CO2 core-flooding experiments on chalk at reservoir conditions

There is a huge potential for using CO 2 gas to recover additional oil after water flooding in reservoir chalk. However, the injection of CO2 into chalk reservoirs will disturb the chemical equilibrium between formation water, injection water and chalk. A proper understanding of these CO2induced interactions and the resulting changes in the physical properties at representative reservoir conditions is required. Unfortunately, reliable chemical data are rare because data cannot be acquired directly at reservoir conditions with present-day techniques. In published experiments, water samples are in many cases obtained at atmospheric conditions with the aid of a back-pressure regulator. Thus, water samples are not representative of in situ reservoir conditions and if proper care is not taken, the collected data cannot be used to judge the magnitude of the chemical reactions taking place at reservoir conditions. However, in some cases water obtained at laboratory conditions can give information on in situ reservoir conditions by using geochemical speciation models to account for dissolved gases that are lost from the effluents during sampling (Bachu & Adams 2003). The objective of this study was to develop and test a new technique for obtaining water samples during CO2-brine– rock interactions in reservoir chalk under representative reservoir conditions and gain a better understanding of the chemical interactions which occur during the injection of CO2. The experiments were per formed by injecting CO2-saturated seawater at supercritical CO2 conditions, at a pressure of 17.24 MPa (2500 psig; pounds per square inch of gauge pressure) and a temperature of 75°C. These values are typical of a region in a chalk field where the rock is exposed to long term reactions with flow of CO 2-bearing water. In addition, the numerical code PHREEQC-3 (Parkhurst & Appelo 2013) was used to reproduce the experiments by assuming equilibrium between calcite and injected fluids. In this way, we can validate the sampling pro cedure and investigate how the measured parameters (pH, CO2 pressure, calcium (Ca) and bicarbonate (HCO3 – ) concentrations) compared with the calculated parameters.