Viscous Oil Recovery by Polymer Injection; Impact of In-Situ PolymerRheology on Water Front Stabilization

摘要

Polymer injection for viscous oil displacement has proven effective and gained interest in the recent years.The two general types of EOR polymers available for field applications,synthetic and biological,displaydifferent rheological properties during flow in porous media.In this paper,the impact of rheology on viscousoil displacement efficiency and front stability is investigated in laboratory flow experiments monitored byX-ray.Displacement experiments of crude oil(~500cP)were performed on large Bentheimer rock slab samples(30×30cm)by secondary injection of viscous solutions with different rheological properties.Specifically,stabilization of the aqueous front by Newtonian(glycerol and shear degraded HPAM)relative to shear thinning(Xanthan)and shear thickening(HPAM)fluids was investigated.An X-ray scanner monitored the displacement processes,providing 2D information about fluidsaturations and distributions.The experiments followed near identical procedures and conditions in termsof rock properties,fluxes,pressure gradients,oil viscosity and wettability.Secondary mode injections of HPAM,shear-degraded HPAM,xanthan and glycerol solutions showedsignificant differences in displacement stability and recovery efficiency.It should be noted thatconcentrations of the chemicals were adjusted to yield comparable viscosity at a typical average floodvelocity and shear rate.The viscoelastic HPAM injection provided the most stable and efficient displacement of the viscous crudeoil.However,when the viscoelastic shear-thickening properties were reduced by pre-shearing the polymer,the displacement was more unstable and comparable to the behavior of the Newtonian glycerol solution.Contrary to the synthetic HPAM,xanthan exhibits shear thinning behavior in porous media.Displacementby xanthan solution showed pronounced viscous fingering with a correspondingly early water breakthrough.These findings show that at adverse mobility ratio,rheological properties in terms of flux dependentviscosity lead to significant differences in stabilization of displacement fronts.Different effective viscositiesshould arise from the flux contrasts in an unstable front.The observed favorable "viscoelastic effect",i.e.highest efficiency for the viscoelastic HPAM solution,is not linked to reduction in the local Sor.We rather propose that it stems from increased effective fluidviscosity,i.e.shear thickening,in the high flux paths.This study demonstrates that rheological properties,i.e.shear thinning,shear thickening and Newtonianbehavior largely impact front stability at adverse mobility ratio in laboratory scale experiments.Shearthickening fluids were shown to stabilize fronts more effectively than the other fluids.X-ray visualizationprovides an understanding of oil recovery at these conditions revealing information not obtained by pressureor production data.