The Impact of CCUS for Improved Oil Recovery on CaCO3 Scaling Potential of Produced Fluids

Abstract

Unlike other CCUS technologies, CO2 EOR has been widely implemented at a commercial level and on an industrial scale. In CO2 EOR, CO2 can be injected on its own or alternated with water in CO2 WAG (water-alternating-gas). Both applications have a direct impact on produced fluid compositions influencing GOR, water cut, CO2 concentration and consequently Ca2+, alkalinity and pH. The variation of fluid compositions has an inevitable impact on the scaling potential of produced fluids and on the resulting level of scale formation and its mitigation strategy. The aim of this work is to investigate the scaling potential changes for a wide range of CO2 WAG scenarios in a high salinity carbonate reservoir in the Middle East using input data from reservoir modelling simulations and running multiple sensitivity studies. The main scale formed in this reservoir is calcium carbonate (CaCO3). The equilibrium reservoir water, the produced water chemistry profiles from downhole to stock tank and the scaling risk profiles are modelled using a commercial integrated PVT and aqueous phase software. A rigorous scale prediction procedure previously published by the authors is applied to accurately calculate scale risk trends for variable production scenarios. As CO2 increases in the WAG cycle, reservoir pH drops but the equilibrium with CaCO3 rock causes an increase in alkalinity. This results in more CaCO3 precipitation in the production system where pressure drops and CO2 flashes off solution. Hence, these results show unequivocal detrimental impact of CO2 WAG on the calcium carbonate scaling potential of produced fluids. This leads to a need for operational and/or chemical adjustments to the scale management program when this technology is deployed. Whilst in this field some CaCO3 scale is predicted to form downhole, but this is not a severe problem although it may need to be addressed. The separator is operated at a sufficiently high pressure that calcium carbonate is not expected to form there. Changing operating pressures and CO2 and H2S concentrations can shift some of the problem to the separator, but if this remains at high pressure there will be no scale precipitation here. However, the calcium carbonate scale will predominantly precipitate at stock tank conditions. Implementing green technologies such CCUS is fundamental to achieving net zero goals and this work clearly shows that actions need to be taken to manage the associated CaCO3 scale problems in the produced fluids to make this application successful.