Polymer Flooding - Flow Properties in Porous Media versus Rheological Parameters

Abstract

Mobility reduction is one of the critical parameters in polymer flooding. The EOR polymers have shear thinning bulk rheology, while core flood experiments with hydrolysed polyacrylamide (HPAM) in this paper show four different viscosity regimes. Through a systematic work, in which the molecular weight and degree of hydrolysis as well as the permeability and brine salinity were varied, the apparent viscosity was well-matched with theoretical models. The following four viscosity regimes were identified: (i) At low shear rates, the apparent Newtonian viscosity is less than the bulk viscosity; this effect is because of the inaccessible pore volume (IPV) with the polymer not entering the entire pore space. (ii) Shear thinning behaviour which is controlled by the polymers relaxation time, λ 1 . (iii) At high shear rates, the apparent viscosity increases by increasing the shear rate caused by elongation whose onset is controlled by a critical shear rate which depends on the relaxation time. (iv) At very high shear rates, the apparent viscosity decreases by increasing the shear rate because of mechanical polymer degradation caused by polymer rupture.The controlling parameter is the bulk relaxation time for the polymer. The critical shear rates for elongation and shear degradation increase when the effective molecular weight decreases. Typical injection shear rates in offshore matrix reservoirs exceed the critical shear rate for elongation and shear degradation. Consequently, high molecular weight HPAM will either have poor injectivity or cause fracturing. For injection into fractured wells, the shear rate is substantially reduced and the shear degradation can be avoided. Acrylamido-Propyl-Sulfonate (AMPS) co-polymers have similar apparent viscosity versus shear rate as the HPAM. However, the AMPS co-polymers seem to tolerate higher shear rates before degradation sets in.For comparison, core flood experiments with Xanthan have been performed. This polymer was shear stable and the shear thinning apparent viscosity was similar to the bulk viscosity.The polymers reduced the permeability and the permeability reduction is understood by the polymer size, i.e., the permeability reduction increases by increasing the molecular weight.

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