Experimental investigation of the effect of polymers on residual oil saturation
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The main objective of this research was to better understand the effect of polymer flooding on the remaining oil saturation by conducting experiments and interpreting these experimental data in terms of measured polymer and rock characteristics. This is because one of the most important factors in chemical enhanced oil recovery (EOR) is mobility control, for which partially hydrolyzed polyacrylamide (HPAM) and other polymers are extensively used. Rheological properties of the EOR polymer solutions depend on the various factors such as a polymer’s molecular properties and concentration, salinity, hardness, shear rate and temperature. Therefore, rheological measurements with commonly employed EOR polymers under various conditions were made and the effect of these factors on the polymer’s viscosity and mobility was quantified. In addition to the steady shear viscosities, the oscillatory rheological properties were measured to better define the polymer's viscoelastic behavior during flow in porous media. Commonly used partially hydrolyzed polyacrylamides (HPAM) have been successfully used in the field for decades, but they hydrolyze at high temperature and eventually precipitate in the presence of high concentrations of divalent cations. New polymers that are stable in harsh environments (high salinity/hardness and high temperature) are in high demand because of the need for chemical EOR in oil reservoirs with these conditions. Both scleroglucan and NVP co- or ter-polymers show good filterability and transport properties in sandstone and carbonate cores at high temperature and in brine with high salinity and hardness. Therefore, both polymers are promising candidates for polymer flooding, surfactant-polymer flooding and alkali-surfactant-polymer flooding in hard brine at high temperature, and their rheological properties were also evaluated for some representative reservoir conditions. Several polymer coreflood experiments have been carried out using both sandpacks and reservoir cores, starting at different water cuts to measure the effect of polymer on the remaining oil saturation. In order to interpret the polymer corefloods, fractional flow theory that incorporated non-Newtonian rheology was developed and applied. The much higher oil recovery from polymer flooding compared to water flooding observed in numerous coreflood experiments is deemed to be mainly due to the improved microscopic or displacement sweep efficiency of the polymer. There is no clear evidence from these experiments that polymer floods reduce the residual oil saturation substantially when the experiments are done with low pressure gradients typical of the pressure gradients that are feasible under field conditions.