Summary. Procedures that involve the use of size exclusion chromatography (SEC) for the measurement of concentration and weight-averaged molecular weight, k Mw, of some EOR polymers were developed and found to give improved detectability, accuracy, and/or efficiency. The separation of polymer from low-molecular-weight impurities by size allows unambiguous polymer from low-molecular-weight impurities by size allows unambiguous detection of polymer properties such as concentration and M,. A combination of an SEC column of a pore size small enough to exclude the polymer totally and a mobile phase of ionic strength of 1.5 was found polymer totally and a mobile phase of ionic strength of 1.5 was found suitable for the separation of polyacrylamide, partially hydrolyzed polyacrylamide, cationic polyacrylamide derivative, and xanthan polyacrylamide, cationic polyacrylamide derivative, and xanthan polysaccharide from impurities. Concentration detection of the separated polysaccharide from impurities. Concentration detection of the separated polymer sample with a variable-wavelength ultraviolet (UV) detector was polymer sample with a variable-wavelength ultraviolet (UV) detector was found to give superior detectability over detection by refractive index difference. A wavelength of 214 nm [2,140 k] was used for the detection of these polymers on the basis of the spectra of samples purified by dialysis. With the active polymer assay determined by reprecipitation into a nonsolvent, the detection limit by UV was determined to be less than 0.1 ttg/cm3 for polyacrylamide and a cationic polyacrylamide derivative, less than0.2 tLg/cm3 for partially hydrolyzed polyacrylamide, and less than 0.7 ug/cm3 for a xanthan polysaccharide. The linear calibration range was up to 500 ug/cm3. The precision of the concentration measurement was better than 4% for polyacrylamide and its derivative and 5% for polysaccharide at a 95% confidence level. On-line measurement of Mw, of four different types of EOR polymers was made by connecting a low-angle laser-light-scattering (LALLS) detector to the SEC system and interfacing the detectors to a computer. The polymer sample was dispersed in the SEC column to give a concentration profile while being separated from the impurities. Continuous measurement of concentration and Rayleigh ratio of the dispersed slug by UV and LALLS detectors allows the calculation of the Mw for the whole sample. The precision of this measurement is expected to be better than +/-5% at a 95% precision of this measurement is expected to be better than +/-5% at a 95% confidence level. The Mw, determined for a polyacrylamide molecular-weight standard was in good agreement with the reported value (0.98 million compared to 1 million). Such a procedure substantially minimizes the interference by dust and requires only a single injection, thereby improving both the accuracy and efficiency of the M, measurement. Introduction The application of high-molecular-weight, water-soluble polymers in waterflooding operations for mobility control and profile modification has been demonstrated to improve reservoir sweep and displacement efficiency and thereby recover additional crude oil during a displacement process. The addition of polymers in low concentration to injection waters and the crosslinking of these polymers by metal ions in situ are two effective techniques that divert displacement fluids from higher-permeability zones into the lower permeability and unswept zones of heterogeneous and/or fractured permeability and unswept zones of heterogeneous and/or fractured reservoirs. important properties that influence the injectivity of these polymer solutions and the extent of permeability reduction in the polymer solutions and the extent of permeability reduction in the contacted zones of the reservoir are solution viscosity, shear stabdity, and loss or retention of polymer in the porous medium. These properties are directly influenced by the size or molecular weight properties are directly influenced by the size or molecular weight of the polymer molecules and their concentration in solution. Therefore, an accurate description of polymer concentration and molecular weight is needed for the study and design of polymer flooding processes. processes. The concentration of polymer solutions can be measured analytically by such techniques as turbidimetry, differential refractometry, and flocculation. The application of these methods to dilute polymer solutions is often restricted by limited detectability and, in many cases, by interference to the concentration measurement caused by Unpurities usually present in solutions of commercial EOR polymers. polymers. Two analytical procedures were recently reported in which the impurities were separated before the polymer concentration was detected. Both procedures involve the use of SEC for the separation. With a properly selected column, SEC is a technique that can separate these impurities from the polymer molecules because of the large difference in the size of the molecules. In one procedure, a differential refractive index (DRI) detector was used to procedure, a differential refractive index (DRI) detector was used to measure the concentration of xanthan polysaccharide purified by SEC. Because of the relatively low sensitivity of DRI, the detectability was somewhat limited. In the other procedure, the concentration of polyacrylamide was measured by a variable-wavelength UV detector. Detectability of this procedure was found to be good, but was determined for only one nonionic polyacrylamide sample. The determination of the molecular weight of highly water-soluble polymers is a difficult task. Although reported molecular weights polymers is a difficult task. Although reported molecular weights of commercial polymers based on their intrinsic viscosities are generally poorly defined, molecular-weight data based on the measurement of well-defined molecular properties are scarce in the literature. Commonly used methods for measuring polymer molecular weight, such as osmometry, centrifugation, SEC, and light scattering, are often found unapplicable to EOR polymers for various reasons. The use of osmometry and ultracentrifugation is restricted by detectability and instrument capability, while SEC alone fails to give well-defined average molecular weights for large EOR polymers because of a lack of proper packing material and calibration polymers because of a lack of proper packing material and calibration standards. SPERE P. 835

Summary This paper presents experimental data on the gelation of apolyacrylamide/thiourea/Cr(VI) gel system in unconsolidated sandpackspolyacrylamide/thiourea/Cr(VI) gel system in unconsolidated sandpacks at flowrates typical of those encountered beyond the immediate vicinity of a wellbore. At these low rates, in-situ gelation during flow was characterized by an abruptincrease m flow resistance that occurred over a short distance at a specific location along the sand pack. The location of this region was a function of now raw, and its distance from the inlet increased as flow rate increased. Filtration of gel aggregates appears to be the mechanism causing this behavior. Introduction In waterflooding, reservoir heterogeneity is often the primary reason forpoor volumetric sweep efficiency. When there are large differences inpermeability, the injected fluid tends to flow in the higher-permeabilityzones, bypassing the oil in the lower-permeability zones. One way to alter thepermeability to modify the movement of injected fluids and to force them tocontact the oil in the lower-permeability regions is to use gelled polymers. One gelation process involves the reduction of CR(VI) to CR(III) at arelatively slow rate by a reducing agent in the presence of poly-acrylamide. The CR(III) is believed to react with the polymer to poly-acrylamide. The CR(III) is believed to react with the polymer to form a 3D gel structure thatis resistant to flow. This redox/polymer gel is of interest because gelationtime can be controlled by selection of process parameters such as polymer andmetal ion concentration. The gelation reaction is a strong function oftemperature, as expected from chemical kinetics, and is affected by shear rate. The exact nature of the gelling process is unknown and probably consists of anumber of reacitons. A model of the crosslinking reaction based oninterpretation of rheological measurements during the gelation process has beenproposed. The kinetics of the uptake of CR(III) by polyacrylamide have beendetermined. The in-situ gelation behavior of these gelling systems is different fromthat expected from the study of the gelling system in quiescent beakers orbottles. Huang et al. studied the gelation of a cationic polyacrylamide (Water Cut 160)/redox gel system in an unconsolidated polyacrylamide (Water Cut160)/redox gel system in an unconsolidated sandpack. They found that thegelation rate was a function of the shear imposed by the flow. Retention of thegelling solution appeared to be a significant factor in the gelation process. In subsequent experiments with the same gelling system, McCool et al. used anin-line mixing technique to study the displacement of the gel system through a4-ft unconsolidated sandpack at a constant injection rate. The in-situ gellingprocess was characterized by a highly localized increase in the flow resistancein the interior sections of the sandpack. This behavior is consistent with thehypothesis that in-situ gelation occurs when gel aggregates formed by thereaction of polymer and CR(III) are retained by reaction with the previouslydeposited molecules and/or become too large to pass through the pore throats.pore throats. The purpose of this work was to obtain experimental data on theeffect of flow rate on the gelation of a polyacrylamide/redox system in anunconsolidated sandpack. Experimental Materials, Equipment, and Procedures The experimental approach is described by McCool et at, and a brief summaryis presented here. Displacement experiments were conducted by injectingpolyacrylamide/thiourea and chromium solutions into an in-line mixer connectedto a 4-ft-long unconsolidated sandpack. The in-situ gelation process wasmonitored through pressure taps along the sandpack that were connected topressure pressure taps along the sandpack that were connected to pressuretransducers. Transducers were monitored continuously to determine pressurehistory during the gelation process. Apparent viscosities pressure historyduring the gelation process. Apparent viscosities of the fluid flowing in eachsection were determined and interpreted to infer in-situ gelation mechanisms. Fig. 1 is a schematic of the experimental equipment. The polymer used was Aldrich polyacrylamide (Lot #12) that was found to beabout 2.9 % hydrolyzed. The chromium ions were introduced into the solution assodium dichromate dihydrate (Na2Cr2O7.2H2O) and the reducing agent as thiourea(NH2CSNH2). Both materials were reagent grade and used as received. Theprocedure described by McCool et al. was used to prepare the polymer solution. The reducing agent solution (11,250 prepare the polymer solution. The reducingagent solution (11,250 ppm thiourea) and the chromium solution (1,200 ppmsodium ppm thiourea) and the chromium solution (1,200 ppm sodium dichromate)were prepared by addition of the dry solid to distilled water and filtrationthrough a 0.22- m membrane. A polymer/ thiourea solution was prepared by mixingthe polymer with the reducing agent solutions at a 4:1 weight ratio. The pH wasadjusted to 4.47 by adding diluted drops of HCI solution. The-gel solutioneventually was obtained by mixing the polymer/thiourea and the chromiumsolutions at a 2:1 ratio, giving an initial pH of about 4.5. This last mixingwas done manually (hand mixed) or by the pumping system through the in-linemixer for a displacement pumping system through the in-line mixer for adisplacement experiment. Experiments were willed to begin 7 days after theinitial dissolution of the polymer to eliminate the effect of aging on theproperties of the polymer. properties of the polymer. Before each displacement, about 70 mL of gel solution was taken from the outlet of the mixer. The initialviscosity, pH, and flow rate were checked. If these three parameters wereconsistent with what was expected, the mixer was connected to the sandpack. Measurement of the pH and viscosity of this solution continued for 3 days. After the displacement, another 70 mL of solution was taken from the outlet ofthe mixer and the same measurements were performed. Viscosity and pHmeasurements made on the in-line mixer performed. Viscosity and pH measurementsmade on the in-line mixer samples were comparable with measurements made on agel solution that was hand-prepared by weighing the components added to thesolution. This ensured that the gel solution injected was uniform during anexperiment. The sandpack holder (120 cm long × 3.65 cm ID) was constructed from acrylicpipe. Pressure ports were installed at intervals of about 4 in. to create 12measurement sections (A to L) to study in-situ gelation. The sandpack was finedwith acid-washed Ottawa sand (F-140-mesh size), a clean unconsolidated sand. Itwas then flushed with CO2 and saturated with brine. Porosity was determined byweighing before and after saturation. Permeability distribution along eachsandpack was determined by flowing brine at several flow rates. Five sandpackswith the properties presented in Table 1 were prepared. The porosity was about35 % for all the sandpacks. Table 2 gives permeability distributions for eachsandpack. Average permeabilities varied from 3.2 to 4.5 darcies. Because the pHof the gel solution affects its gelation time, a preflush of about 20 PV ofbrine (pH adjusted to 4.5) was injected to stabilize the effluent at theinitial pH of the gel. Injection of the gelling solution began immediatelyafter the preflush.

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