Developments on Calcium Sulfate Scaling Prediction and Control in Oil and Gas Production

Abstract Calcium Sulfate (CaSO4) is precipitated due to super saturation of Ca2+ and SO42-ions that can be created due to change pH, pressure, temperature or due to mixing of the brines. Among the CaSO4 species, CaSO4.2H2O or gypsum is stable in solid phase at lower temperature below 50 °C which has been observed frequently in wells treated with CO2 flooded-EOR technique. As a result, mineral scaling, especially calcium sulfate (gypsum) deposition has been a serious flow assurance problem in oil and gas production leading to production shut down because of formation damage or equipment failure. Use of scale inhibitors may be the efficient method to prevent the scale formation. However, there is a knowledge gap on types of scale inhibitor that might be efficient in controlling the calcium sulfate scaling under varied field conditions. Additionally, there is a need of an efficient method to evaluate the performance of scale inhibitor and simulate the dynamic field condition. This study provides information on several options of inhibitors to control CaSO4 at various conditions as established with modified laser-light based Kinetic Turbidity Test (mKTT) method that can assess several samples over a range of concentrations simultaneously. Other newly developed modified Continuous Stirred Tank Reactor (mCSTR) method showed to be an efficient tool to understand scale inhibitor performances, as it can overcome limitations of dynamic scale loop (DSL) with low residence time and low volume capacity of capillary tubing. Similarly, as minimum effective dosage (MED) needed can be easily established in one setting with mCSTR method. It showed much better efficiency compared to that of standard bottle test, which suffers lack of dynamics and multiple bottle tests needed to establish MED. This study provides more effective test methodologies for scale inhibitors performance testing. The mCSTR apparatus can be used for various mineral scales such as barium sulfate, halite, calcite, etc. Additionally, this study will provide a mathematical model (scaling risk-inhibitor dosage recommendation) via mCSTR method which can be used to predict scaling tendency and inhibitor need under desired conditions.