Modelling CaCO₃ scale in CO₂ water alternating gas CO₂-WAG processes

CO₂ Water Alternating Gas (CO₂-WAG) is one of the main Enhanced Oil Recovery (EOR) techniques which is currently being implemented in the emerging pre-salt projects offshore Brazil. CO₂-WAG consists of the alternated injection of CO₂ and water in the reservoir for tertiary oil recovery. However, this process may lead to the enhanced deposition of CaCO₃ in production wells. This may occur since, in a CO₂-WAG scheme, CO₂ dissolves in the water slug causing a decrease in the pH. At lower pH levels, carbonate rock is dissolved causing an increase in the carbon and calcium content in the water slug. As fluids are later produced, the operating pressure is reduced and dissolved CO₂ is evolved from solution, causing an increase in the pH. At these less acidic pH levels, CaCO₃ may become oversaturated and precipitate. In order to address each process involved in CaCO₃ formation, an integrated modelling approach between aqueous scale prediction modelling, Vapour-Liquid Equilibria (VLE) modelling and reservoir modelling is proposed. In particular, acid equilibria and precipitation reactions coupled with the Pitzer equations are used to build the scale prediction model. The facility to calculate CaCO₃ co-precipitation with other minerals, such as BaSO₄, FeCO₃, FeS, etc., is also introduced. Five different equations of state (namely SRK, PR, PRSV, PT and VPT) are used in VLE calculations to model the solubility of CO₂ (or a mixture of CO₂, H₂S and CH₄) in brine. To model the reactive flows in the reservoir, the advection-diffusion equation is coupled with the scale precipitation equations. The reactive transport model addresses carbonate rock-brine interactions and fluid flow through a porous medium. Once integrated in one single model, these equations have been shown to address all steps in CaCO₃ formation relevant for CO₂-WAG, i.e., the geochemical processes taking place in the reservoir and at the production wells, thus capturing the full dimension of the scaling problem. In addition, the proposed model has been successfully validated with experimental data on the injection of seawater adjusted to various pH levels into a CaCO₃ packed column. The measured effluent pH levels and calcium concentrations were used to validate the model.