In offshore operations where seawater is commonly used to prepare hydrochloric acid, calcium sulfate precipitation, the potential of which can greatly reduce the effectiveness of these treatments. This is because high concentreation of calcium produced in spent acid mixed with high level of sulfate in seawater. However, a few studies have provided evidence for this problem and the effect of calcium sulfate precipitation on acid treatments has not been fully examined.
In this work, core flood experiments at 0.5, 1, and 5 cm3/min flow rates were performed at 25°C to investigate formation damage due to calcium sulfate precipitation during matrix acidizing treatment. Austin Chalk cores (6 in. length and 1.5 in. diameter) with a permeability of 10 md and synthetic seawater were used. The core permeability before and after acid treatment, pressure drop response, calcium ion, sulfate ion, and pH values in the core effluent samples were measured. Solids collected in the core effluent samples were analyzed using XPS technique. Both acid prepared in seawater and in deionized water were examined.
Results showed that calcium sulfate precipitation occurred when seawater was used in any stage during matrix acidizing including preflush, post-flush, or in the main stage. Injection rate was the most important parameter that affected calcium sulfate precipitation; permeability reduction was significant at low flow rates, while at high rates wormhole breakthrough reduced the severity of the problem. This work confirms the damaging effect of preparing hydrochloric acid using seawater for acid treatments.