Optimization of Scale Treatment Program of a CO2 Water-Alternating-Gas System for Enhanced Oil and Gas Production

Abstract

Abstract The use of a water-alternating-gas (WAG) system for enhanced oil and gas production can introduce challenges from scaling issues. At a CO2-WAG facility in the western Rockies of the USA, scale solids were observed to accumulate in the produced fluid gathering and water injection systems. In addition to carbonate scales, suspended solids such as formation fines, corrosion products and bacteria were typically present in the production and injection waters. Although scale inhibitor applications are the preferred approach to prevent scale formation, the suspended solids can limit the performance or availability of these inhibitors by providing large surface areas for their adsorption. To investigate the effects of pre-existing solids in diminishing performance of scale inhibitors, three types of solids, CaCO3, Fe2O3 and FeS were used in static bottle tests to mimic the impact on scale inhibitor performance in the operating system. A variety of scale inhibitor chemistries were screened, including various types of phosphonate-based and polymer-based scale inhibitors. Test bottles containing simulated field brine were spiked with 100, 1,000 or 10,000 mg/L of each solid and dosed with different scale inhibitors at various concentrations. Results showed that a decrease in scale inhibitor efficiency accompanies increasing amounts of added solids. With CaCO3 and Fe2O3 solids, equivalent amounts of solids limited scale inhibitor performance comparably. FeS solids had the greatest adverse impacts on scale inhibitor performance, possibly because of FeS oxidation or stronger adsorption of scale inhibitors onto FeS oxidation products. In the final phase of this study, scale inhibitor applications were conducted in the CO2 WAG system. Optimization was achieved based on monitoring and system performance. These applications are carefully documented and presented to provide a perspective for effective management of scale in WAG systems. In conclusion, laboratory results on scale inhibitor performance in the presence of pre-existing solids provided insight into the influence of suspended solids on the efficacy of the scale treatment mechanism. It also established a guideline for inhibitor dosages during field applications to successfully manage scale in this CO2 WAG system in the western Rockies.