Alternative alkalis for ASP flooding in anhydrite containing oil reservoirs

The presence of gypsum or anhydrite in oil reservoirs limits the application of alkali-surfactant-polymer (ASP) flooding using conventional alkalis such as sodium carbonate (Na2CO3) because these alkalis precipitate calcium carbonate (CaCO3) in the presence of gypsum leading to high alkalinity loss and permeability damage. Sodium metaborate (NaBO2) and ammonium hydroxide (NH4OH) were investigated as possible alternatives to Na2CO3. Batch mixing experiments were performed with these alkalis in the presence of excess gypsum, single phase brine-alkali transport experiments were performed in sandstone and carbonate cores containing gypsum, and ASP corefloods were performed in outcrop and reservoirs cores (containing gypsum). Effluent pH and ion concentrations including boron, calcium and sulfate were measured using an ion chromatograph (IC) and inductively coupled plasma (ICP). NaBO2 and NH4OH were found to maintain and propagate a high pH of more than 10 in the batch and transport experiments without significant permeability changes. Gypsum dissolution was not increased by NH4OH (over that of the brine itself) in batch or transport experiments. It, however, increased during NaBO2 injection depending on the flow rate. High oil recoveries and low surfactant retentions were obtained in the ASP corefloods with both sodium metaborate and ammonium hydroxide performed in cores containing gypsum.