Mineral scale prediction modelling : precipitation of CaCO3 scale in CO2-water alternating gas production systems
Silva, Duarte Jorge Alves de Carvalho e
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The injection of CO2 in oil reservoirs for tertiary oil recovery is one of the main Enhanced Oil Recovery (EOR) processes and it is widely used in the oil and gas industry. To prevent early gas breakthrough, CO2 is commonly injected in alternated slugs with water, in a process known as CO2 Water Alternating Gas (CO2 WAG). When such processes are carried out in carbonate reservoirs, there is the potential for calcite (CaCO3) dissolution in the reservoir and its subsequent re-precipitation in production systems, thus posing a flow assurance risk that must be carefully managed. A new thermodynamic model that addresses all of the major steps involved in the precipitation of CaCO3 scale in CO2 WAG production systems is proposed, including: i) the dissolution of injected CO2 into the reservoir brine; ii) the rock-brine interactions and the dissolution of CaCO3 rock; iii) the reactive flow and transport of aqueous components in the reservoir; iv) the partition of components between the liquid, vapour, and water phases; and v) the precipitation of CaCO3 scale as a function of decreasing pressure (and temperature) in CO2 WAG production systems (i.e. in the well and in topside equipment). Thus, an aqueous electrolyte model has been implemented and coupled with a Vapour-Liquid Equilibrium (VLE) model, a multiphase flash model, and a reactive transport model. The non-ideal behaviour of the aqueous and hydrocarbon phases (vapour and liquid) has been modelled by using respectively the Pitzer equations and an Equation of State (EOS) (Soave-Redlich-Kwong, SRK, and Peng-Robinson, PR, EOS, have been used, among others). The implementation of these models has been validated by comparing results with experimental data and/or with results obtained by using industry standard software. In addition, the impact of VLE, multiphase flash, and reactive transport calculations on the precipitation of CaCO3 scale has been investigated, by considering commonly available production data. Also, a procedure had been devised to address each step involved in the precipitation of CaCO3 scale in CO2 WAG production wells individually, and together in an integrated approach. In fact, this work focuses on building the boundaries for the CaCO3 scaling system, thus allowing to define, and work on, worst case scenarios. This gives the required information – both qualitatively and quantitatively – to manage CaCO3 scale in CO2 WAG production wells.