Surfactant-Mediated Spontaneous Imbibition in Carbonate Rocks at Harsh Reservoir Conditions

The goal of this work was to pursue strategies to improve oil recovery in highly fractured carbonate reservoirs by altering the wettability from oil-wet to preferentially water-wet at high temperature (100 degrees C or above), high salinity, and especially in high hardness environments. Cationic surfactants and anionic surfactants were investigated for their compatibility with hard brine at a high temperature. Sequestration agents were added to improve aqueous solubility. The performance of surfactant formulations was evaluated by measuring contact angles on calcite plates and spontaneous imbibition in originally oil-wet dolomite cores. Cationic surfactants altered the wettability of oil-aged calcite plates toward a more water-wet state in the presence of hard brines; oil recovery by spontaneous imbibition from dolomite cores was 50 to 65% of original oil in place (OOIP). Anionic surfactant formulations changed the carbonate wettability to strongly water-wet only when the brine salinity and divalent-ion concentration were reduced. The wettability could be altered in hard brines if a sequestration agent [e.g., ethylene diamine tetraacetic acid (EDTA)] is added to anionic surfactant formulations; up to 45% of OOIP was recovered by spontaneous imbibition. EDTA provides alkalinity, saponification, chelation of divalent ions, and dissolution of dolomite, which may contribute to the increase of imbibition rate and ultimate oil recovery in fractured carbonates.