Understanding the effects of salinity on bitumen-calcite interactions

Recovery of heavy oil from carbonate oil ores is always a challenge by water-flooding process which is highly dependent on the water chemistry. Herein, experimental tests (by atomic force microscopy (AFM), quartz crystal microbalance with dissipation (QCM-D)) and molecular dynamic (MD) simulation have been conducted to understand the exact role of salinity (cations and anions) in influencing the bitumen-CaCO3 interactions. It is found that the addition of K+ and Ca2+ cations (up to 10 mM) into the solution would decrease the repulsive force strength (range from 20 nm to less than 5 nm), and even converts the repulsion force into adhesion force. However, the SO42- anions are observed to be able to strengthen the bitumen-CaCO3 repulsion force. This is different from that for processing quartz oil sands. In QCM-D measurement, additional ions inhibit the bitumen calcite adsorption behavior but the effect is influenced by the ionic types and strength. Based on the zeta potential measurement and MD simulation, it is found that the Ca2+ cations are more preferred to adsorb on both the CaCO3 surface and bitumen. The adsorbed Ca2+ cations perform as ion bridges linking the oil and CaCO3 surface. Therefore, the accumulation of Ca2+ cations in the solution will deteriorate the oil recovery from carbonate oil reservoirs. However, SO42- anions are more inclined to only adsorb on the CaCO3 surface and prevent the adsorption between bitumen and calcite surface. Therefore, rejecting Ca2+ or increasing SO42- in the low salt water would be helpful for the EOR of carbonate oil reservoir