ASP Process for 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 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 perfomed 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). The residence times in injection experiments were increased up to 15 days to study the effect of reaction kinetics. 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. High oil recoveries were obtained in the ASP corefloods along with low surfactant retention due to the high pH propagation.