Many waterflood projects now experience significant amount of water cut, with more water than hydrocarbon flowing between the injectors and producers. In addition to the impact on water viscosity and density from using different injection water sources during a field’s life, water chemistry itself may impact oil recovery as demonstrated by recent research on low salinity water injection schemes. It is also known that water chemistry has profound impact on various chemical EOR processes. Moreover, the effectiveness and viability of such EOR schemes is strongly dependent on reservoir brine and injection water compositions. In particular, the presence of divalent cations such as Ca+2 and Mg+2 has a significant adverse effect for chemical EORs. Utilising new developments in reservoir simulation, this paper outlines a method to account rigorously for water chemistry with only a small incremental computational cost compared to the same simulation model without geochemical reactions. The new modelling capability considers chemical reactions triggered by injection water and/or injected reactive gases such as CO2 and H2S, including mineral dissolution and precipitation, cation exchange and surface complexation.

For waterflood performance assessment, this paper demonstrates that rigorous modelling of water chemistry can significantly impact simulation results. In addition, interpretation of saturation logs requires inputs about brine composition. Thus, the new modelling capability makes possible a more optimum evaluation of petrophysical logs for well intervals where injection water invasion is suspected. By modelling transport of individual species in the aqueous phase from injector to producers, reservoir characterization can also be improved through the use of these natural tracers, provided the compositions of the actual produced water are used in the history matching. The simulated water compositions in producers can also be used by production chemists to assess scaling and corrosion risks. For CO2 EOR studies, we illustrate chemical changes inside a reservoir and in the produced water before and after CO2 breakthrough, and discuss geochemical monitoring as a potential surveillance tool. Alkaline flood induced water chemical changes and calcite precipitation is presented to illustrate applicability for chemical EOR with the new simulation capability.

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