The Effect of Sulfate-Rich Water Production on Surface Production Lines: A Lab Study

Calcium sulfate (CaSO4) scale precipitation can cause significant operating problems during oil production. Impermeable hard scale deposits of calcium sulfate can severely impair production in various locations, such as the formation matrix, well tubulars, and surface production lines. The development of calcium sulfate mitigation protocols is crucial to an effective flow assurance strategy, which should include prevention, characterization, and removal. This study shows an effective chemical formulation that has been proven to restore production and protect equipment in a time-efficient manner. The study illustrates three stages of scaling tendency study including precipitation and deposition, simulation, and removal. Two synthetic water samples were prepared to represent water from different sources, one with high calcium ions (CRW), while the other high in sulfate ions (SRW). Static bottle tests were conducted to replicate scale formation and deposition. XRD and ESEM analyses were conducted on the generated scale sample to confirm the composition. An extensive sensitivity and simulation analysis were conducted on many water samples with different ranges of calcium and sulfate ions to determine the ion concentration precipitation threshold. The effect of saturated gases, namely CO2, was also studied in terms of precipitated compounds. Finally, dissolution tests were conducted using variety of chemical fluid combinations and conditions to determine the optimal dissolution fluid. The results showed mixing samples associated with high calcium and sulfate ions can cause not only calcium sulfate but calcite precipitation as well. The calcium sulfate scale was found to adhere to test tube walls more strongly than the pure calcite or calcium sulfate. More importantly, the treatment of this scale is much more difficult, highlighting the necessity of applying mitigation measures when these scales are anticipated. The dissolution tests showed that manipulation of temperature, soaking time, and types of dissolvers are important factors for higher dissolution success. A proposed solution was prepared based on green chemicals to achieve around 90% dissolution capacity of calcite/calcium sulfate mixture. This paper presents the methodology used to determine the source of scale, factors that influence the formation of scale, and shows how calcite/CaSO4 scale can be removed using effective chemistry and removal protocols. Details of this study can be expanded to cater to many formation damage incidents faced in the oil and gas industry.