Effect of High Calcium Concentrations on Anhydrite Solubility in High Temperature and High Pressure Conditions

Abstract The oil and gas industry encounters critical challenges in predicting and preventing scale, especially in unconventional reservoirs that exhibit extreme conditions like high calcium concentrations (>2 m), significant pressures (up to 1,250 bar), and elevated temperatures (up to 200 °C). This research enhances the ability to predict the scaling behavior of anhydrite under high-temperature, high-pressure (HTHP) conditions. A specially designed flow-through apparatus was developed to measure the solubility of anhydrite under controlled conditions, using feed solutions with varying concentrations of CaCl2 and NaCl. This apparatus enabled precise simulation of HTHP environments, where solutions were pumped through columns filled with solid anhydrite until equilibrium was achieved. The results give accurate data for developing a robust solubility prediction model based on Pitzer theory, which accommodates a wide range of brine compositions, making it applicable to various conditions involving different temperatures, pressures, and calcium concentrations. The study found that anhydrite solubility generally increases with ionic strengths, but noted that some ion interactions, particularly those involving divalent ions, remain poorly understood under HTHP conditions. By accurately quantifying interaction parameters for divalent ions (Ca2+, Ba2+, SO42−), the model offers reliable predictions of mineral solubility in brines with high calcium concentrations. This enhanced understanding of scale formation allows for more effective control of inhibitor dosages, leading to reduced operational costs and minimized environmental impacts.