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...

Microbial enhanced oil recovery (MEOR) represents a possible cost-effective tertiary oil recovery method. Although the idea of MEOR has been around for more than 75 years, even now little is known of the mechanisms involved. In this study, Draugen and Ekofisk enrichment cultures, along with Pseudomonas spp. were utilized to study the selected MEOR mechanisms. Substrates which could potentially stimulate the microorganisms were examined, and l-fructose, d-galacturonic acid, turnose, pyruvic acid and pyruvic acid methyl ester were found to be the best utilized by the Ekofisk fermentative enrichment culture. Modelling results indicated that a mechanism likely to be important for enhanced oil recovery is biofilm formation, as it required a lower in situ cell concentration compared with some of the other MEOR mechanisms. The bacterial cells themselves were found to play an important role in the formation of emulsions. Bulk coreflood and flow cell experiments were performed to examine MEOR mechanisms, and microbial growth was found to lead to possible alterations in wettability. This was observed as a change in wettability from oil wet (contact angle 154 • ) to water wet (0 • ) due to the formation of biofilms on the polycarbonate coupons.

This paper describes a new method for water control by the use of bullhead injection. A water based gelant is emulsified in oil and injected into the formation. The emulsion is designed to separate into a water phase and an oil phase at static conditions in the formation. Upon reaction in the formation the water phase gels up while the oil phase remains mobile. It has been found that the controlling parameter for disproportionate permeability reduction (DPR) is to control the fraction of gel occupying the porous media. The water fraction in the emulsion controls the reduction in relative oil and water permeabilities. A program was undertaken to verify this DPR method in a field test, using a commercial blocking gel system. The first treatment was performed in well 30/3 A 16 T2 at the Statoil operated Veslefrikk field offshore Norway. Results show that water production was reduced by 30% after the pilot test, while maintaining the oil rate. As expected, total well productivity was reduced by more than 80%. The treatment consisted of 124 m3 emulsion, bullheaded from surface. Step rate testing and ion water analysis were combined to study the relative change in flow contribution between the 6 perforated intervals. Introduction The increasing number of high water-cut producing wells requires simple and cost effective water control technologies. One option is bullhead injection of chemicals capable of reducing the water permeability more than the oil permeability, normally described as Disproportionate Permeability Reduction (DPR) or Relative Permeability Modification (RPM). DPR will be effective in multilayered reservoirs without crossflow and with some zones producing clean oil or in treating coning problems.[1–3] In such situations DPR treatment will reduce the water cut and may result in increased oil production. The existence of DPR fluids is well known.[4–8] DPR fluids may be classified as polymer systems, weakly crosslinked gel systems or rigid gel systems. The polymer systems are the most frequently used. The DPR mechanism has been explained by polymer adsorption on a water-wet rock surface.[9,10] This will reduce the effective pore space for flowing water. The ratio between the thickness of the adsorbed layer, e, and the pore radius, r, is used to express the permeability reduction, RRF (defined as the ratio between permeability before and after treatment), using Poiseuille flow in a bundle of capillary tubes. ………………………………………..(1) The oil will flow more or less un-restricted in the middle of the pores. Consequently the oil permeability reduction, RRFo, will be lower than the water permeability reduction, RRFw. From Eq. 1 the following can easily be derived:RRF will decrease with increasing pore radius, i.e. increasing the permeability.RRF will increase by increasing the thickness of the adsorbed layer. The latter one can be obtained by using polymers with larger molecular weight and higher affinity for adsorption. Alternatively weak gels or aggregates can be formed in-situ by injection of polymer and crosslinker.[5,11–12] Often, the polymer and the weakly crosslinked gel systems involve additional retention mechanisms, such as pore trapping. The use of preformed gel particles will also increase the effective adsorption thickness.[13] Rigid crosslinked gel is normally used for total blocking. Blocking gels have DPR properties[7,14–17] but for practical purposes in matrix treatment the oil permeability reduction has been recognized far too high for bullheading. However, crosslinked gels could be promising alternatives if the permeability reduction, RRF, can be controlled. This is mainly because crosslinked gels show (i) better temperature stability, (ii) DPR properties regardless of wettability and (iii) high RRF also in high permeability zones. A crosslinked gel will probably also be more resistant towards the high shear rates close to the wellbore. Consequently the volume of active DPR fluid can be reduced. It has been demonstrated that RRF depends on the fraction of gel occupying the pore space.[15–16,18]

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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.