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An experimental study of shear stability of several high-molecular-weight polymers used as mobility control agents in EOR projects has been performed in well-controlled conditions. The shearing device was made of a capillary tube with ID of 125 μm, through which polymer solution was injected at controlled rate. The set-up enables a precise measurement of the shear rate to which the polymer macromolecule is submitted. The degradation rate was measured by the viscosity loss induced by the passage into the capillary tube. The shear rate was gradually increased up to 106 sec-1 while checking degradation rate at each stage. Different commercial EOR polymer products were submitted to the test with polyacrylamide backbone and different substitution monomer groups. All macromolecules behave as flexible coils in solution. The parameters investigated were: Molecular weight (between 6 and 20x106)Nature of substitution group (Acrylate, ATBS/sulfonate, nVP/Vinyl-Pyrrolidone)Salinity Polymer shear degradation increases with molecular weight and salinity, but decreases with the presence of Acrylate, ATBS and nVP. All results can be interpreted in terms of chain flexibility. The highly flexible polyacrylamide homopolymer is the most sensitive to shear degradation. Introduction of acrylate groups in the polymer chain induces some stability because of the rigidity provided by charge repulsion, which vanishes in the presence of high salinity (due to the screening of acrylate negative charges). ATBS and VP groups, which are larger in size, provide significant chain rigidity thus better shear stability. It is also shown that some very-high molecular-weight polymers, after passing the shearing device, attain a final viscosity lower than lower-molecular-weight products with the same chemical composition. This factor has to be taken into account in the final choice of a polymer for a given field application. As a comparison, although less popular today than two decades ago, xanthan gum, which behaves like a semi-rigid rod, is shown to be much less sensitive to the shear degradation test than the coiled polyacrylamides.

Summary An experimental study of shear stability of several high-molecular-weight polymers used as mobility-control agents in EOR projects has been performed in well-controlled conditions. The shearing device was made of a capillary tube with an internal diameter (ID) of 125 μm, through which polymer solution was injected at a controlled rate. The setup enables a precise measurement of the shear rate to which the polymer macromolecule is submitted. The degradation rate was measured by the viscosity loss induced by the passage into the capillary tube. The shear rate was gradually increased up to 106 sec–1 while checking degradation rate at each stage. Different commercial EOR polymer products were submitted to the test with polyacrylamide backbone and different substitution monomer groups. All macromolecules behave as flexible coils in solution. The parameters investigated were Molecular weight (between 6 and 20×106)Nature of substitution group (acrylate, ATBS/sulfonate, nVP/ vinyl-pyrrolidone)Salinity Polymer shear degradation increases with molecular weight and salinity, but decreases with the presence of acrylate, ATBS, and nVP. All results can be interpreted in terms of chain flexibility. The highly flexible polyacrylamide homopolymer is the most sensitive to shear degradation. Introduction of acrylate groups in the polymer chain induces some stability because of the rigidity provided by charge repulsion, which vanishes in the presence of high salinity because of the screening of acrylate negative charges. ATBS and VP groups, which are larger in size, provide significant chain rigidity, and thus better shear stability. It is also shown that some very-high-molecular-weight polymers, after passing the shearing device, attain a final viscosity lower than lower-molecular-weight products with the same chemical composition. This factor has to be taken into account in the final choice of a polymer for a given field application. As a comparison, although less popular today than 2 decades ago, xanthan gum (XG), which behaves like a semirigid rod, is shown to be much less sensitive to the shear-degradation test than the coiled polyacrylamides (Sorbie 1991).

High water production is one of the major problems faced by the petroleum industry. One method of controlling water production that has been used with some success is to inject polymer gels into the near-well bore formation. However, polymer gel injections are not always successful, in part because the exact mechanisms by which they reduce water permeability more so than oil permeability are not understood. To elucidate the fundamental mechanisms involved in disproportionate permeability reduction (DPR), we have conducted a series of experiments on flow of water and oil through bulk polymer gels and through polymer-filled glass micromodels. Flow experiments through weak polyacrylamide-based gels have been performed to obtain the gel permeabilities by using different test conditions: constant pressure, constant flow rate, and the falling head method. Gel permeability was found to vary with flowrate, according to a power-law that holds over several orders of magnitude of velocity. This was true for both water and oil flow, although with different pre-factors and exponents. Water flows through the gel matrix as if flowing through a porous medium, whereas the oil pushes its way through in the form of immiscible drops or filaments. Mathematical models to quantify these flows are currently being developed.

Polymers in the form of either solutions or gels are being used to control water production, especially when oil bearing and water zones cannot be isolated. Results of field treatments have varied widely, but often no obvious reason can be given for the success or the failure of the treatment. The lack of understanding of the basic mechanisms by which polymers influence the flow of water and oil hinders a wider application of this process. The achievement of a disproportionate permeability reduction (DPR - reducing the water permeability while producing minimum reduction in the oil permeability) is crucial, and several mechanisms have been proposed to explain this effect. Among those mechanisms, polymer adsorption and lubrication effect are being thought as the main reason for DPR when polymers (without cross-linker) are used. This paper presents a mechanistic study of the effect of polymer injection on single and two-phase flow. Experiments have been performed in glass micromodels, under both water-wet and oil-wet conditions. The oil and water end-point permeabilities have been obtained, before and after the injection of a cationic polyacrylamide, while observing the fluid distribution and flow patterns. During polymer injection, under water-wet conditions, polymer layers were seen to form and build-up on the crevices between the grains. This mechanism is known as adsorption-entanglement. Data on this phenomenon are scarce, and this paper presents the first visualization study of this phenomenon in porous media. In water-wet models there was a significant permeability reduction to water, while there was no significant reduction in oil permeability. The entanglement of polymer reduces the area available for water to flow, whereas the oil, occupying the centre of the larger pores, was nearly unaffected by the polymer entanglement. In oil-wet models, where polymer layers were not observed, the oil and water effective permeability before and after polymer injection remained unchanged. Our results confirm that adsorption-entanglement for water-wet media, which produces the build-up of the polymer layers on the grain crevices, is a basic mechanism by which polymers can modify the flow characteristics and thus preferentially reduce the water relative permeability. Introduction Several methods aimed at reducing water inflow into production wells have been proposed and tested in the field. These methods generally consist of placing a barrier across the water-producing zone. The barrier consists of, for example, cements, resins, suspensions of solid particles, or paraffin waxes. Unfortunately, these methods have the disadvantage of blocking the oil or gas flow as well as the water inflow. More recently, polymers have been successfully used to reduce the water flow1. The success of polymer treatments is based on reducing the water permeability while producing minimum reduction in the oil permeability, this phenomenon is known as disproportionate permeability reduction (DPR). However, until now, there is no consensus on the mechanisms by which the polymer produces the DPR.