| Abstract |
The anhydrite mineral deposition inside the wellbores of Cawayan wells is investigated in order to predict its occurrence and come up with possible remedies. The chemistry of the fluids is thoroughly evaluated and some geochemical indicators of deposition such as Ca-, SO,--. Mg-, SiO2s, and CI- are established. The behavior of various calcium and sulfate species during deposition, and the effects of, temperature and pH are determined using the SOLVEQ and WATCH 2.1 programs. Results show that at higher temperatures. sulfate and calcium species decrease in activity and this tends to increase super saturation. At lower pH. Sulfate activity is significantly reduced by the formation of bisulfate ions, and under saturation is attained; at higher pH, under saturation also occurs due to decrease in calcium activity. However, calcium supersaturates at higher pH. The CHILLER program is utilized to simulate fluid-fluid mixing, and determine the effects of NaCl addition, Na, HPO, addition, acid treatment and CaCl, injection on anhydrite deposition. Results show that NaCI, Na2HPO, and HC1 are possible inhibitors of anhydrite deposition. Increasing the salinity of production fluid before mixing with high-sulfate waters tends to retard deposition and increase the amount of mixing fluid necessary to initiate formation of anhydrite. Na2HPO, prevents anhydrire deposition by significantly reducing activity of calcium through association with phosphate ions. However, the solution tends to become super saturated with respect to aparite. CaCI, addition can induce deposition in the acid zones but this further lowers the pH of the hi&-sulfate acid fluid as bisulfate dissociates into sulfare ions and releases hydrogen ions. |