Evaluation of the scaling potential during field appraisal or initial development allows for a better estimation of project costs and reduces uncertainties (e.g. desulphation or scale inhibitor injection, frequency of well workover operations, etc.). When CO2-WAG is injected into a carbonate reservoir, several interactions between the injected fluid, the formation water and the carbonate rock may occur. These interactions may also modify the scaling potential of the produced water. Nevertheless, the prediction of these changes is challenging because it requires complex thermodynamic and kinetic modelling.
The present study outlines a methodology where the geochemical modelling of CO2-WAG injection is used in predicting the scaling potential in carbonate reservoirs. For this, a geochemical simulation was developed, and several cases with different injection strategies were applied to obtain the produced water compositions as a function of time, and to calculate the scaling potential. The recovery method which is least susceptible to inorganic scale deposition in the studied field can be identified. The methodology was applied to data from two Brazilian carbonate fields with different petrophysical and compositional characteristics.
When compared to the standard thermodynamic modelling used to calculate scaling risk, this approach takes account of the impact of the several chemical reactions that occur in situ in the reservoir, and addresses the tendency to overestimate scaling risk when reservoir reactions are not accounted for. A fully quantitative evaluation is not developed in this study since several simplifications were assumed, and many uncertainties exist in the geochemical modelling input parameters (e.g. constants related to reaction kinetics and equilibria). However, the proposed methodology provides a powerful tool for informing project decisions and field injection design. In this study, for example, CO2-WAG injection was found to reduce the risk of precipitation of various scales compared to seawater or Low Sulphate Water (LSW) injection.