Flow of polymer solutions through porous media

Abstract

These days leading oil and gas producing companies are investing increasing amounts of money into the development of non-fossil energy sources like wind-, solar-, biomass energy and forestry. On the other hand these companies are persisting in developing techniques to make energy recovery more efficient intending to stretch the life span of fossil resources. One of the more mature techniques to improve recovery efficiency is polymer flooding. Herein polymer solutions are injected in injector wells to pressurise producers thereby sweeping oil. The high viscosity attained by dissolving polymers in water makes oil displacement more efficient compared to conventional water injection. More generally, efficiency can be enhanced by reducing the water production, which is worldwide an increasing problem for operators. In mature oil fields, as oil depletion induces a rise in the oil-water contact, water breaks through in the production wells at a certain moment. Breakthrough can also result from stimulation by water injection. Depending on the specific situation water production can be decreased by shutting off the bottom of the well or plugging certain water producing layers by using polymer gels or cements, the so-called water shut-off treatments. Other shut-off treatments involve disproportionate permeability reducers (DPRs), which are usually polymer solutions. DPR working is based on the fact that adsorption of hydrophilic polymers can strongly decrease the permeability to water while having little effect on the permeability to oil. For an effective application of the above-mentioned technologies knowledge of polymer flow behaviour and retention behaviour, both in porous media, is indispensable. Apart from oil-related applications this field is important to many other industries and technologies, but nevertheless it is still not well understood. The main reason for this is the complex relationship between rheology and retention behaviour. Both are depending on porous medium structure and are clearly different from bulk behaviour. The aim in this work was to contribute to the understanding of this field, while focussing on systems based on aqueous polymer solutions. Chapter 1 is the introduction of this thesis. Chapter 2 provides an overview of current knowledge on the flow of Newtonian fluids and polymer solutions in porous media. Chapter 3 deals with the relation between permeability and packing structure in packed beds of monodisperse spheres. It shows that, even for Newtonian fluids in well-defined porous media, the prediction of dissipation is not straightforward at all as it is a complex function of wall- and internal friction, velocity gradients and tortuosity, all strongly related to flow path geometry. Chapters 4 and 5 deal with pressure drop predictions for laminar flow of Newtonian- and polymeric fluids through tubes with diameter variations in the axial direction. Modelling a porous medium by a bundle of these tubes is conceptually an improvement over the well-known capillary bundle model with straight tubes. The presence of diameter variations gives the possibility to account for elongational dissipation effects, which contribute to the measured permeability. Analytical predictions for different geometries and various fluid types have been derived by using the Lubrication Approximation Method (LAM). Our newly developed LAM involving spherical co-ordinates allows a good definition of deformation rates during flow. The resulting predictions are in good agreement with both measurements and numerical predictions and have been converted in capillary models, which do take into account elongational dissipation. Chapter 6 describes the results of coreflood studies on aqueous non-ionic Polyacrylamide (PAM) solutions in silicon carbide grain packs at low flow rates. PAM is the most frequently used system in polymer flooding- and water shut-off treatments. The experiments show that, above a certain concentration in the semi-dilute regime, the mobility reduction Rm does not stabilise during injection, as it does at lower concentrations in the dilute regime. This behaviour seems to be related to the formation of adsorption entangled multilayers whose thickness can grow during injection. The layer thickness is an important parameter as, together with the effective pore size, it determines the resulting permeability reduction and the magnitude of the so-called disproportionate permeability reduction (DPR) effect. In our studies on PAM, the DPR effect has been confirmed. Concerning PAM rheology in porous media, a full absence of the depleted layer effect was measured, in contrast with previous results on other water-soluble polymers. This absence is due to the fact that brine is only a mediocre-quality solvent for PAM resulting in an attractive force between the adsorbed layer and the free polymer molecules. Chapter 7 shows the results of coreflood studies on aqueous Cationic Polyacrylamide (CPAM) solutions in silicon carbide grain packs and Berea sandstones. The goal in these studies was to explore whether there is scope for a water shut-off process, called two-step gelant diversion, based on the following idea; In a first step a polymer solution is injected to create a protective skin on the oil-bearing zones. In a second step a main flush of gelant is injected to treat the water-bearing zone, while being diverted from oil-bearing zones. Finally the protective skin is destroyed by degradation or by a cleaning agent to restart oil production. As for PAM solutions, injection of CPAM solutions at high flow rates induces unsteady-state flow behaviour i.e. a plugging effect. It was found that plugging rate increases as permeability decreases which is important with respect to the selectivity to low permeability layers. Besides due to the higher adsorption energy CPAM adsorption level, adsorbed layer thickness and plugging rate were higher than for PAM. In all cases skin formation in the lab was easily obtained. Field trials should give conclusive information whether the gelant diversion process can work in practice. Based on experimental evidence a new retention mechanism, called flow-induced adsorption, has been proposed to explain unsteady state plugging behaviour at high flow rates. This mechanism, based on the fact that at high rates hydrodynamic forces become large enough to push extra polymer molecules over the osmotic energy barrier (due to the adsorbed layer) to adsorb, is consistent with all previous experimental results. Another conclusion obtained is that low flow rate polymer solution rheology in a porous medium is well described by Chauveteau's depletion model, provided that the medium is homogeneous and that the solution is under "good-solvent" conditions. Chapter 8 gives the results of rheological- and coreflood studies on aqueous Polyacrylic acid (PAA) solutions. Since PAA is a weak polyelectrolyte, both rheology and adsorption are strongly dependent on pH and ionic strength. Particularly interesting is the fact that after adsorption the permeability reduction can be regulated by changing pH or ionic strength of the injected fluid. The studies show that viscosity and induced permeability reduction show similar dependencies on pH and ionic strength and that the model based on OSF theory gives reasonable predictions. Chapter 9 describes the main conclusions of the thesis.