Investigation of Anhydrite Dissolution as a Potential Low Salinity Waterflooding Mechanism Using Carbonate Reservoir Rocks

Many mechanisms have been proposed for low salinity waterflooding enhanced oil recovery (EOR) in carbonate rocks over the last decade, and they are still in debate. One suggested mechanism is the dissolution of anhydrite (CaSO4) mineral from a rock material, which generates sulfate ions in-situ, and subsequently acts as a wettability modifier chemically. Another suggested mechanism is the increase in permeability due to mineral dissolution. Primary objective of this work was to verify whether dissolution of anhydrite could be the key low salinity waterflooding EOR mechanism. Spontaneous imbibition tests were conducted using six rock samples from two carbonate oil reservoirs. The first reservoir rock contains anhydrite, while the second reservoir does not contain anhydrite. If anhydrite dissolution is the key mechanism, then the amount of increased oil recovery due to low salinity brine should correlate with the amount of anhydrite dissolved from the rock. Our experimental results, however, did not suggest such a relationship. Hence, anhydrite dissolution was considered unlikely as the key mechanism of low salinity EOR for the crude-oil, brine and rock (COBR) system used in this study.