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

Abstract

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-CO 2 processes. But the effect of fines in EOR during LSWAG-CO 2 has not been previously studied. This study explores the possibility of fines formation during CO 2 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 CO 2 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-CO 2 processes were correlated to the described oil recovery. It is observed that significant amounts of fines formation can take place during CO 2 injection in limestone reservoirs. At reservoir conditions, a considerable fraction of the injected CO 2 becomes soluble in sea water. This dissolved CO 2 causes dissolution of CaCO 3 from the mineral surface and releases Ca 2ϩ ions into the pore space. Excess Ca 2ϩ ions form anhydrite fines with the available SO 4 2ions. The salinity and composition of brines present in pore space shows direct correlation with the amount of fines produced during CO 2 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. This study suggests that CO 2 injection in carbonate reservoir can have two distinct effects.1. CO 2 miscible with oil can decrease oil viscosity, thus increase oil displacement. 2. CO 2 dissolved in water causes fines formation following dissolution of CaCO 3 . These fines can alter sweep efficiency by blocking pore throats and also increase oil displacement through its emulsification.Injection of CO 2 leads to fines formation at most reservoir conditions. These anhydrite fines formations have been neglected in previous studies. The amount of fines formation taking place shows significant correlation to EOR obtained from LSWAG injection. The Extended UNIQUAC model can be used to precisely calculate both the amount of mineral dissolution and the amounts of fines formation taking place during CO 2 injection over varied pressure and temperature conditions.