Reservoir Simulation of Low Salinity Impact on Polymer Flooding and Evaluation of Electrodialysis Reversal Benefits

Abstract

Summary The objective of polymer flooding is to improve macroscopic sweep efficiency and oil recovery by increasing the viscosity of the displacing water thereby decreasing the water/oil mobility ratio. One major parameter that determines polymer flood performance is therefore the in-situ polymer solution viscosity, which is dependent on several factors such as formation temperature, polymer concentration, shear rate and salinity (salt concentration). Rheological polymer properties such as bulk viscosity variation with polymer concentration for a given water salinity have been derived from laboratory measurements. Generally, a higher solution salinity yields less viscosity for a given polymer concentration. Such relationship between salinity and polymer solution viscosity needs therefore to be implemented in dynamic simulations to consider the mixing effect of different water salinities (injected, connate, aquifer, etc.) in order to obtain representative in-situ polymer viscosities for polymer flood evaluation. This paper describes the implementation of salt-dependent polymer viscosity functions in reservoir models and the evaluation of simulation results in order to provide answers related to the impact of salinity variations on polymer flood performance. Evaluation of results shows variation of low salinity and polymer flood fronts within the reservoir when a lower salinity (e.g. <1 g/l) viscous solution encounters the in-situ formation water at higher salinity (5 – 15 g/l), and how this leads to improved macroscopic sweep efficiency and oil recovery. Overall, low salinity viscous fluid injection resulted in a higher oil recovery (up to 6% incremental OOIP) given the same polymer concentration, or a similar recovery with lower polymer concentration (up to 50% less polymer consumption). The above results proved useful in evaluating the business case of using low salinity water (desalting using Electrodialysis Reversal (EDR) technology) for polymer solution preparation. Chemical savings and polymer concentration reduction are the main advantages for a polymer flooding project. Preliminary evaluations show that significant OPEX and CAPEX savings could be achieved by using EDR on an onshore field with moderate reservoir salinity, which is linked to increased operational efficiency and reduction of chemicals consumption.