For heavy oil recovery applications, mobility control is more important than interfacial tension (IFT) reduction and therefore, importance should be given to the recovery of remaining mobile oil by enhanced sweep efficiency. While the relative role of polymer's viscosity and elasticity on capillary-trapped residual light oil recovery has been studied extensively, their role on the sweeping the mobile viscous oil has not been explored. Injectivity is vital for heavy oil recovery applications and polymer selection criteria are done solely based on shear rheology. In this paper, the influence of viscous (shear) resistance and elastic (extensional) resistance of viscoelastic polymer on the mobile heavy oil recovery and injectivity is investigated through the combination of bulk shear/extensional rheology and single phase, and multiphase core flood experiments at typical reservoir flooding rate of 1 ft/day.
Two polymer solutions with different concentration and salinity are selected such that low molecular weight (Mw) polymer (HPAM 3130) provides higher shear resistance than high Mw polymer (HPAM 3630). Extensional characterization of these two polymer solutions performed using capillary breakup extensional rheometer revealed that HPAM 3630 provided higher extensional resistance than HPAM 3130. The results show that the behavior of polymers in extension and shear is completely different. Two multiphase and two single-phase experiments are conducted at low flux rate to investigate the role of extensional viscosity on mobile heavy oil recovery and high flux rates on injectivity. After 1 PV of polymer injections, higher concentration and lower Mw HPAM 3130 contributes to ~17% higher incremental recovery factor over lower concentration and higher Mw HPAM 3630. The core scale pressure drop generated by HPAM 3130 is more than twice the pressure drop generated by HPAM 3630. Under low flux rate conditions at the core scale, shear forces dominate and displacing fluid with higher shear viscosity contribute to better sweep. HPAM 3630 exhibits shear thickening phenomenon and possess the apparent viscosity of ~ 90 cP at the flux rate of ~90 ft/day. Whereas HPAM 3130 continued showing shear thinning and has the apparent viscosity of around ~70 cP at ~ 90 ft/day. This signifies the role of extension rheology on the injectivity at higher flux rates.
Results revealed that while extensional rheological role towards sweeping the mobile heavy oil recovery at low flux is lesser when compared to shear role, its negative role on the polymer injectivity is very significant. Polymer selection criteria for heavy oil recovery applications should incorporate extensional rheological parameters.