Formation of Anhydrite due to Interaction Between Water Soluble CO₂ (aq) and Calcite Mineral During Enhanced Oil Recovery

In the Low Salinity based EOR method, formation and migration of fines have proved to have profound effect on the displacement efficiency of residual oil. Salinity variations of injected brines have also been shown to affect oil recovery for WAG-CO₂ processes. But the effect of fines in EOR during LSWAG-CO₂ has not been previously studied. This study explores the possibility of fines formation during CO₂ injection and attempts to understand its implication in EOR. In this study we use the Extended UNIQUAC model to calculate the possibility of fines formation during CO₂ injection. Detailed simulations were conducted over a temperature range of 50°C to 250°C and a pressure range of 5 bars to 500 bars. The amounts of fines formation taking place for different LSWAG-CO₂ processes were correlated to the described oil recovery. It is observed that significant amounts of fines formation can take place during CO₂ injection in limestone reservoirs. At reservoir conditions, a considerable fraction of the injected CO₂ becomes soluble in sea water. This dissolved CO₂ causes dissolution of CaCO₃ from the mineral surface and releases Ca²⁺ ions into the pore space. Excess Ca²⁺ ions form anhydrite fines with the available SO₄^2- ions. The salinity and composition of brines present in pore space shows direct correlation with the amount of fines produced during CO₂ injection. With increase in temperature and pressure, the amount fines formation increased significantly. The described oil recovery for different LSWAG injections showed a consistent correlation with the amounts of fines formation taking place in the pore space. The amount of mineral dissolution taking place was also precisely calculated using the Extended UNIQUAC model. Good correlation was also observed between calculated amounts of mineral dissolution and the observed increase in permeability. Copyright: 2015. Society of Petroleum Engineers