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 CO2 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 CO2-induced 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).