Understanding of the Co-Deposition of Calcium Sulphate and Calcium Carbonate Deposition in ESP

Electric Submersible Pump (ESP) in oilfield is prone to scale deposition, largely due to temperature increases by motor heating, and turbulent flow and high shear inside of pump. In this work, the formation of calcium sulphate and calcium carbonate were studied under a range of conditions that could possibly be encountered in ESPs. The produced water chemistry changes from 100% formation water to mixed formation water and injected seawater over time. The scaling tendency and potential mass deposition are simulated using a thermodynamic prediction model. Dynamic scale loop tests were carried out to study the calcium carbonate, calcium sulphate and co-deposition under field conditions. The results indicate that the risk of calcium carbonate scale at the initial stage of production in both downhole and ESP. With the breakthrough of seawater, the risk of calcium sulphate scale increases, and harsh calcium sulphate scale could form in ESP. This study also suggests that the interference between CaSO₄ and CaCO₃ formation is minor under the test conditions. These findings will be used to develop the scale mitigation strategies.

Scale formation is one of the flow assurance problems encountered in the oil and gas industry. It can deposit from reservoir, downhole tubing to topside facilities. Once formed, it could have a significant impact on production, including tubing and valve blockage, interference of well intervention, and even well abundance.^{1, 2}

Calcium carbonate and calcium sulphate are two common scales formed in oilfields. Both scales could form when the saturation of scaling water increases to a certain level due to the change of temperature, pressure, pH, etc. Calcium sulphate also could precipitate when mixing of incompatible waters, such as the injected seawater which contains high concentration of sulphate ion mixed with and calcium-rich formation water. Calcium sulphate can form in the reservoir, near wellbore area, production tubulars, and topside facilities.³ Calcium sulphate dihydrate (gypsum, CaSO₄.2H₂O) is commonly precipitated at low temperature and calcium sulphate (anhydrite, CaSO₄) often precipitated at elevated temperatures and pressures.⁴