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. IntroductionPolymer flooding is an EOR method for improving the sweep efficiency. Polymers increase the water viscosity and reduce the water permeability. The most frequently used EOR polymers are HPAM which can be produced in large volumes and transported to the field as dry powder; biopolymers, such as Xanthan, have also been applied. The EOR polymers are pseudoplastic fluids, with their viscosity decreasing when the shear rate increases; this behaviour is very important in terms of injectivity.Polyacrylamide polymers are known to be sensitive to shear degradation and shear thickening at high shear rates. An overview was given by Heemskerk et al. (1984) and in a recent work by Seright et al. (2009). In this work we relate the shear degradation to the rheological properties of the polymer. When a po...

Reduction of the effective shear viscosity in polymer solutions due to crossflow migration in microchannels: Effective viscosity models based on DPD simulations

“…show an increased effective viscosity only at estimated shear rates of about 100 s −1 and higher [3]. Thus, we expect that extensional thickening can happen near well bores, but not often further into the reservoir.…”

“…The exponent n varies with concentration and molecular mass [3], and the average velocity in a channel without slip is…”

“…Polymer solutions are used in many applications, from drag reducers in turbulent pipe flow [1], to viscosity boosters in injected water during oil recovery [2,3,4]. A transition from capillary fingering to a stable invading water front can be achieved in an oil reservoir when the viscosity of the injected water is increased by adding polymer [5].…”

“…Migration and depletion is a fundamental mechanism that is likely to occur also in the complex geometry of reservoir rocks [2,3,4], although modifications (e.g., due to more complex steam lines) will be present. It is therefore expected that water breakthrough may occur with higher probability than anticipated, since the effective viscosity is reduced compared to a polymer solution with homogeneously distributed polymer in the pore structure.…”

Sweep enhancers for oil recovery

“…For a pressure gradient of 1.0 bar/m, the resistance factor of the high permeability core (4400 mD) is over five times higher compared to the low permeability core (160 mD). In order to compare the shear viscosity with the measured resistance factor, typical shear rates in porous flow can be calculated which are proportional to flow velocity v (Stavland et al 2010):…”