Optimization of CO-WAG and Calcite Scale Management in Pre-Salt Carbonate Reservoirs

Abstract

CO2-WAG (Water-Alternating-Gas) has been applied in offshore Brazilian oilfields to improve recovery rates and mitigate the environmental impact that venting produced CO2 would bring. Although CO2 is highly miscible in oil under these reservoirs conditions, this gas is also extremely mobile, and its speciation in the aqueous phase drives reactions with carbonates that can cause severe inorganic scaling problems in production systems. It is crucial, therefore, to effectively design CO2-WAG operations for mobility control and, consequently, enhance reservoir performance, CO2 utilization and flow assurance. This paper addresses the design optimization of coupled CO2-EOR and storage operations applied to the Brazilian Pre-salt offshore context (reservoir properties, infrastructure, regulatory framework and economic characteristics), examining the trade-offs of project profitability, CO2 utilization and calcite scale risk. Several compositional simulations of miscible WAG scenarios were performed and key design parameters were optimized using statistical sampling and evolutionary algorithms. Aqueous and mineral reactions were included in the calculations, allowing us to quantify the calcite mass that can potentially deposit in the perforations and production system. The results showed how optimizing WAG operations can significantly improve the economics and the scale management of oil production from carbonate reservoirs. The optimal WAG design greatly increased incremental NPV per volume of CO2 stored and reduced calcite scale risk by simply rearranging the WAG slugs in a tapered manner. Here we demonstrate that this methodology can be used to determine how to recycle CO2 in a given field for better economics and lower carbon footprint, doing so without triggering calcite mineral deposition to the point of permanent jeopardy of production wells and facilities operability. Therefore, the workflow integrates critical challenges that are correlated, yet often addressed independently, supporting the complex decision-making of CO2-EOR operational design in carbonate reservoirs.