The potential for nonaqueous phase liquid (NAPL) mobilization is one of the most important considerations in the development and implementation of surfactantbased remediation technologies. Column experiments were performed to investigate the onset and extent of tetrachloroethylene (PCE) mobilization during surfactant flushing. To induce mobilization, the interfacial tension between residual PCE and the aqueous phase was reduced from 47.8 to 0.09 dyn/ cm by flushing with different surfactant solutions. The resulting PCE desaturation curves are expressed in terms of a total trapping number (N T ), which relates viscous and buoyancy forces to the capillary forces acting to retain organic liquids within a porous medium. The critical value of N T required to initiate PCE mobilization fell within the range of 2 × 10 -5 to 5 × 10 -5 , while complete displacement of PCE was observed as N T approached 1 × 10 -3 . The interplay of viscous and buoyancy forces during PCE mobilization is illustrated in horizontal column experiments, in which angled banks of PCE were displaced through the columns. These results demonstrate the potential contribution of buoyancy forces to PCE mobilization and provide a novel approach for predicting NAPL displacement during surfactant flushing.

In this paper we present a partitioning interwell tracer test (PITT) technique for the detection, estimation, and remediation performance assessment of the subsurface contaminated by nonaqueous phase liquids (NAPLs). We demonstrate the effectiveness of this technique by examples of experimental and simulation results. The experimental results are from partitioning tracer experiments in columns packed with Ottawa sand. Both the method of moments and inverse modeling techniques for estimating NAPL saturation in the sand packs are demonstrated. In the simulation examples we use UTCHEM, a comprehensive three‐dimensional, chemical flood compositional simulator developed at the University of Texas, to simulate a hypothetical two‐dimensional aquifer with properties similar to the Borden site contaminated by tetrachloroethylene (PCE), and we show how partitioning interwell tracer tests can be used to estimate the amount of PCE contaminant before remedial action and as the remediation process proceeds. Tracer tests results from different stages of remediation are compared to determine the quantity of PCE removed and the amount remaining. Both the experimental (small‐scale) and simulation (large‐scale) results demonstrate that PITT can be used as an innovative and effective technique to detect and estimate the amount of residual NAPL and for remediation performance assessment in subsurface formations.

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There are increasing laboratory and field evidences that the viscoelastic characteristics of polymer solutions help improve polymer-flood efficiency. Extensive rheological measurements and laboratory corefloods with partially hydrolyzed polyacrylamide polymers with very high molecular weight were carried out to delineate the role of their viscoelastic behavior in improving oil recovery from polymer flood. Different polymer solution's elastic contribution is modeled in the polymer's apparent viscosity in porous media, which is implemented in UTCHEM simulator for quantification of improved reservoir sweep.As the application range of polymer flood is extended to recover more viscous oils with use of polymers at high concentrations and with very high molecular weights, a mechanistic understanding of polymer rheology in porous media and accurate numerical modeling are essential for successful field implementation of polymer flood.Oscillatory and shear viscosity measurements and polymer flow coreflood experiments were carried out for different shear rates (and flow velocities and permeabilities in core), polymer concentrations, and molecular weights. The polymer's shear-thickening characteristic was correlated with the Deborah number via its molecular relaxation time, which is in turn determined from the rheological data. An apparent viscosity model that accounts for both shear-thinning and shearthickening behavior of polymer in porous media was developed, which fit the laboratory data well. The model was then implemented in a compositional chemical flooding simulator and successfully history-matched published coreflood oil recovery experiment.Through systematic rheological measurements and corefloods, and their use in the apparent viscosity model for simulation, the elastic contribution of different polymers in improving polymer-flood efficiency is quantified. Specifically, a polymer solution's shear-thickening behavior is characterized in terms of the molecular relaxation time determined from bulk rheology measurements.

Introduction Fractional flow theory has been applied by various authors to waterflooding, polymer flooding, carbonated waterflooding, alcohol flooding, miscible flooding, steamflooding, and various types of surfactant flooding. Many of the assumptions made by these authors are the same and are necessary for obtaining simple analytical or graphical solutions to the continuity equations. Typically, the major assumptions, which are sometimes not stated explicitly, are:one dimensional flow in a homogeneous, isotropic, isothermal porous medium,at most, two phases are flowing,at most, three components are flowing,local equilibrium exists,the fluids are incompressible,for sorbing components, the adsorption isotherm depends only on one component and has negative curvature,dispersion is negligible,gravity and capillarity are negligible,no fingering occurs,Darcy's law applies,the initial distribution of fluids is uniform, anda continuous injection of constant composition is injected, starting at time zero. Several of these assumptions are relaxed easily. One of the most useful to relax is Assumption 12, continuous injection. The principles of chromatography can be applied to analyze the more interesting case of injecting one or more slugs. Most of these processes require slug injection of chemical or solvent to be economical. In fact, a lower bound on the slug size necessary to prevent slug breakdown can be obtained from a simple extension of fractional flow theory. In this and other extensions the common new feature is the need to evaluate more than one characteristic velocity. A second example of this is the extension of fractional flow theory from simultaneous immiscible two-phase flow (the classical Buckley-Leverett waterflood problem) to simultaneous immiscible three-phase flow (the classical oil/water/gas flow problem). A third example is the extension to nonisothermal cases. Here we need to consider the energy balance, mass balance, and velocity of a front of constant temperature. A fourth example is when one or more components are partitioning between phases. In all cases, mathematically, the extension is analogous to the generalization from the one-component adsorption problems (or two-component ion exchange problems with a stoichiometric constraint) to multicomponent sorption problems. The latter theory has been worked out in a very general way for many component systems using the concept of coherence. Pope et al. recently have applied this theory to reservoir engineering involving sorption problems. SPEJ P. 191＾

Displacement of oil by polymer solution has several unique characteristics that are not present in normal waterflooding. These include nonNewtonian effects, permeability reduction, and polymer adsorption. The rheological behavior of the flow of polymer solution through porous media could be Newtonian at low flow rates, pseudoplastic at intermediate flow rates, and dilatant at high flow rates. The pseudoplastic behavior is modeled with the BlakeKozeny model for power-law model fluids. The dilatant behavior is modeled with the viscoelastic properties of the polymer solution.The reduction in permeability is postulated to be due to an adsorbed layer of polymer molecular coils that reduces the effective size of the pores. A dimensionless number has been formulated to correlate the permeability reduction factor with the polymer, brine, and rock properties. This dimensionless number represents the ratio of the size of the polymer molecular coil to an effective pore radius of the porous medium.A model has been developed to represent adsorption as a function of polymer, brine, and rock properties. The model assumes that the polymer is adsorbed on the surface of the porous medium as a monolayer of molecular coils that have a segment density greater than the molecular coil in dilute solution.

A number of commercially available polymers have been tested for enhanced oil recovery based upon viscosity, filterability, and surfactant compatibility, and chemical and thermal stability testing has been carried out with some of these as well. Several high molecular weight polymers exhibited high viscosities at salinities up to 170,000 ppm NaCl and with greater than 17,000 ppm CaCl2 present. Polyacrylamide polymers hydrolyze at high temperatures and beyond a certain point are subject to precipitation by calcium. If calcium concentrations can be kept below about 200 ppm, the use of polyacrylamide polymers is feasible up to reservoir temperatures of at least 100 °C. For higher concentrations of calcium, copolymers including AMPS moieties should be considered. Calcium tolerance can be improved with sodium metaborate or by using copolymers of acrylamide and sodium 2-acrylamido-2-methylpropane sulfonate (AMPS). The stability problems at elevated temperatures in the presence of iron can be mitigated by the use of chemicals such as sodium dithionite and sodium carbonate. The polymers tested did not lose viscosity after 220 days of aging at 100 °C with dithionite present. Introduction Following Muller's (1981) terminology, we will use "chemical degradation" when referring to the hydrolysis of polymer functional groups and "thermal degradation" when describing the free radical induced breakdown of the acrylic backbone, resulting in molecular weight reduction. Chemical degradation leads to a higher degree of hydrolysis and can only be prevented by the inclusion of more chemically stable monomers, but does not necessarily limit application, and in fact, often results in higher viscosity. Thermal degradation results in a reduction of molecular weight and a loss in viscosity, but this degradation can be prevented in most situations. Chemical Degradation; Hydrolysis and Precipitation of Polymers. Solutions of non-hydrolyzed polyacrylamide (PAM) are nonionic, and hence the viscosity is essentially insensitive to salinity. At elevated temperature and/or pH, the amide moiety undergoes hydrolysis, resulting in a acrylate moiety and the evolution of ammonium ion, as illustrated in Figure 1. The anionic charges of the acrylate moieties results in intramolecular repulsions that increase the hydrodynamic radius of the polymer molecules and hence the solution viscosity. Because of this benefit, commercial polyacrylamide for EOR is usually either post-hydrolyzed by addition of alkali or produced as a copolymer of acrylamide (AM) and acrylic acid or its salt (AA). In either case, the molar fraction of the acrylate moiety is referred to as the degree of hydrolysis (t), and is typically between 0.15 and 0.40 for commercial hydrolyzed polyacrylamide (HPAM) polymers used for enhanced oil recovery. During its residence in the reservoir at elevated temperature and/or pH, t of polyacrylamide polymers increases. At high salinity, the acrylate moieties on HPAM are strongly associated with cations, and the viscosity approaches that of non-hydrolyzed polyacrylamide. Multivalent cations have a much stronger effect than monovalent cations. If t exceeds approximately 0.33 (Zatouin and Potie, 1983), then precipitation is possible if excessive amounts of multivalent cations are present. The critical amount of calcium necessary to precipitate hydrolyzed polyacrylamide decreases with temperature, t, and decreasing monovalent cation concentration. At a high degree of hydrolysis, this has been described as a site fixation phenomenon, and occurs at close to the stochiometric equivalence point between acrylate moieties and cations (around 200 ppm of calcium for a 1000 ppm polymer solution). At lower t, the precipitation phenomenon is due to poor solvation (theta-type precipitation) (Ikagami, 1962).