Experimental investigation of competitive mechanisms of precipitation and dissolution due to seawater and low salinity water injection in carbonate reservoirs

Abstract

The dissolution and precipitation mechanisms are very effective in the water injection process, especially at low salinity and smart water injection. In this research, the focus is on competition between dissolution and precipitation mechanisms in the presence of calcite rock samples at reservoir conditions (100 °c and 2500 psi). Static experiments were designed in two parts; a) mixing of incompatible brines for investigation of precipitation process and b) repeating the process in presence of reservoir rock samples for precipitation/dissolution competition. Incompatible real formation and sea water brines were used to determine maximum scale formation ratio and dominate scale types. Based on high pressure static experimental results, mixing ratio of 50% of the formation water, as the maximum-precipitate-producing ratio, has been considered as the desired ratio for the experiments with rock samples in this study. The synthetic water compositions were designed to have the dominant scales as those were found in initial experiments of oil field water samples. In the next stage, the compatibility experiments were carried out for the mixture of 20-times diluted seawater and formation water. This was followed by mixing experiments in presence of the reservoir rock in the reservoir condition. The results were interpreted with ICP, SEM and EDX. The results of water mixing experiments with rock sample showed that most of the precipitations were sulfate scales types as strontium and calcium sulfates precipitations were significant in the ratio of 50% of formation water. Both precipitation and dissolution were detected in all experiments based on change in ions, rock surface chemical analysis and scale properties. The concentration of Ca2+, Sr2+ and SO42− ions determines the dominant mechanism. For 20-times diluted sea water, dissolution was dominant mechanism despite the presence of sulfate precipitations. However, this mechanism was indicated in sea water with less impact. In the compatibility tests, Mg2+ ion did not contribute to the precipitation process. In the process of mixing in presence of reservoir rock, the Mg2+ concentration was reduced in the end of experiment. Increase in calcium ion and decrease in magnesium ion indicated ion exchange at calcite rock surface.