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Although there is disagreement concerning the role of microgel in mobility control, there is little doubt that appreciable amounts of crosslinked microgel exist in commercial preparations of partially hydrolyzed polyacrylamide, and in Dow Pusher chemical the crosslinks are to some extent anhydride bridges. Other hydrolyzable crosslinks, such as imides, may also be present, but since for our purposes here their behavior is similar to that of anhydrides, equations for only the latter will be written. The purpose of this note is to report some reactions this crosslinked material undergoes with various additives, particularly with compounds containing two or more hydroxyl groups. These reactions may be useful in mobility control technology since, as with the reaction between Pusher and formaldehyde, they are a means of Pusher and formaldehyde, they are a means of controlling the effective molecular weight of the polymer in solution.When polymer molecules are crosslinked, the size of the aggregates formed can vary over wide limits. Small particles composed of relatively few polymer molecules crosslinked together behave like true molecules in solution and are called microgel. Larger particles of crosslinked material form colloidal particles of crosslinked material form colloidal dispersions, and still larger particles become visible to the naked eye and tend to settle from the solution on standing.Uncrosslinked partially hydrolyzed polyacrylamide is composed of large chainlike molecules of high molecular weight having CONH2 groups, COONa groups and COOH groups randomly attached to every other carbon atom on a carbon backbone chain. In the preparation of these polymers, particularly in the drying operation, there is a tendency for water to split out of two COOH groups and form anhydride bridges and thus crosslink polymer molecules together into larger aggregates. An amide group and a carboxyl group call react in a similar manner to form imide crosslinks. The number of crosslinks that form, and consequently the size of the microgel, is determined by a number of factors such as the ratio of CONH2 to COONa and the temperature of drying. The pH of the solution from which the polymer was dried is also of utmost importance' since this largely determines the number of COONa groups compared with the number of COOH groups, and it is the latter, as pointed out above, that are actually capable of forming anhydride and imide bridges.When microgel crosslinked by hydrolyzable bridges is redispersed in water, there is a tendency for the crosslinks to rehydrate, with a resulting disintegration of the microgel. However not all crosslinked aggregates are equally stable. Some pairs of molecules are bound together by more crosslinks than others. Furthermore the stability of the microgel network would be determined not only by the number of crosslinks but by their arrangement as well. Thus, when two crosslinks exist between a pair of polymer molecules, the size of the ring structure formed would affect the stability, the smaller rings being the more stable.The disintegration of microgel due to rehydration of hydrolyzable crosslinks is a reaction that by its very nature is governed by the pH of the system. As the pH of the system is increased, the rate of crosslink pH of the system is increased, the rate of crosslink disintegration also increases. Owing to the presence of Na2CO3 in Pusher-type polyacrylamides, solutions prepared for field use are alkaline. prepared for field use are alkaline. JPT P. 545

Abstract With the use of polymer solutions in secondary recovery operations, the need has developed to understand the mobility control mechanism. This study investigated mobility control by considering both permeability and rheological effects. Experiments used a high molecular weight, partially hydrolyzed polyacrylamide polymer. Flow studies took place in reservoir and Berea cores having zero oil saturation. Effective size of the polymer flow unit was inferred from Nuclepore filter tests. Clay studies indicated the particle size capable of decreasing the core permeability. Flushed permeabilities measured the approximate core permeabilities with flowing polymer solutions. These permeabilities were considerably lower than original values. With mobility data and the flushed permeability, maximum effective viscosities were determined for polymer solution flow in a core. Effective viscosities showed that rheological properties play an important part in mobility control with polymer solutions. The study showed that permeabilities decrease and stabilize with polymer flow. At the lower permeabilities, high shear rates exist in the cores. Because of the pseudoplastic character of the polymer solution, the high shear rates caused low effective viscosities. This condition pointed to the inefficient use of the potentially high viscosity of the polymer solution at low shear rates. Introduction In the oil industry, a great deal of interest is being shown in the use of polymer solutions for secondary recovery and a number of polymer floods are being performed in the United States. Some of these floods have become commercial while others have been reported as failures. A number of floods are still in progress and remain to be evaluated. With the advent of polymer flooding, the need developed to understand the mobility control mechanism in porous media.

Increasing the viscosity of injected water for more efficient oil recovery has been of interest to the petroleum industry since 1900. In recent years, certain high molecular weight water soluble organic polymers have been developed which, in small concentrations, can increase the viscosity of water substantially and, under certain conditions, recover additional oil economically. The rheological properties, adsorption, transport characteristics, and oil recovery efficiency of several polymer solutions are presented. The viscosity of the polymer solutions depends largely on the flow rate, the type of polymer, and the solvent. Oil recovery depends primarily on the permeability distribution of the porous model. Adsorption varied from 30 to 225 μg/g. Because the process utilizes a polymer solution slug of small volume, the stability of the slug was considered. Following a polymer slug with water led to viscous instabilities and slug breakdown. If the polymer concentration in the slug was reduced to zero asymptotically before injecting water, the viscous instabilities were eliminated. L'accroissement de la viscosite de l'eau injectee pour assurer une recuperation plus efficace de l'huile est un sujet qui interesse l'industrie du perole depuis 1900. On a mis au point recemment certains polymeres organiques qui sont solubles dans l'eau et possedent un poids moleculaire eleve; ils peuvent, en faibles concentrations, augmenter la viscosite de l'eau d'une maniere appreciable et permettent, dans certaines conditions, de recuperer plus d'huile economiquement. On indique, pour plusieurs solutions de polymeres les proprietes rheologiques, l'adsorption, les caracteristiques relatives au transport et l'efficacite de la recuperation d'huile. La viscosite des solutions de polymeres varie beaucoup avec le debit, le genre de polymere et le solvant. La recuperation d'huile depend principalement de la repartition de la permeabilite que presente le modele poreux. L'adsorption varie de 30 a 225 μ g/g. Vu que, dans le procede en question, on utilise un bouchon de solution de polymere, on considere sa stabilite. L'introduction d'eau apres le bouchon de polymere a produit une instabilite de la viscosite et la rupture du bouchon; en reduisant a zero (d'une maniere asymptotique) la concentration du polymere dans le bouchon avant l'injection d'eau, la dite instabilite est disparue.

The most important decision made in the early stages of this miscible recovery project was to hang loose. The original plan of operation allowed for injecting all gas, all water, or any combination, depending upon what new information was gained as the project went along. Radioactive tracers, new logging techniques, numeric modeling, and sophisticated down-hole equipment provided that information. Introduction The Fairway (James Lime) field is about 30 miles south of Tyler, Tex. The discovery well was drilled in July, 1960. near the eastern edge of the field, which eventually proved up over 400 million bbl of oil in place under about 23,000 acres. The 10,000-ft James Lime reservoir is a reef development at Fairway, quite different from the regional character of the formation. Three major zones exist in the reservoir as a result of differing conditions of reef growth environment. From the start, this complex, highly stratified, variable-permeability, reservoir offered quite a challenge to the operators. The James Lime oil is an undersaturated 48 degrees API-gravity crude containing from 1,350 to 1,600 cu ft of solution gas per barrel. Saturation pressure varied with depth and ranged from 3,950 to 4,350 psia. The reservoir fluid was determined to be miscible, with lean hydrocarbon gas or flue gas at about 4,800 psia - about 400 psi below the initial reservoir pressure. Connate water saturation ranged from 10 to 30 percent, and the porosity averaged about 12.5 percent. Unitization studies showed that incremental economics favored an alternate gas-water high-pressure miscible recovery project over waterflooding or immiscible gas injection. The operators recognized that many miscible projects had experienced poor recovery and even lost projects had experienced poor recovery and even lost money. Alternating gas and water injection represented the "outer limits" of miscible recovery technology available at the time and involved a high degree of risk. The decision was made to go ahead with the alternating approach, but with a program designed to have a large degree of operational flexibility. In this manner, necessary changes and adjustments could be made along the way. Additional recovery from the alternating gas-water high-pressure miscible recovery technique in use at Fairway is significant. Production exceeded the estimated primary reserves of 65 million STB of oil in June, 1971 (see Fig. 1). Currently the Unit is producing the top allowable of 40,500 BOPD. producing the top allowable of 40,500 BOPD. Injection capacity determines the maximum production rate at Fairway. The average reservoir production rate at Fairway. The average reservoir pressure has been maintained essentially constant at pressure has been maintained essentially constant at 4,500 psi since early 1970. An expansion program to increase gas injection capacity from 70 to 100 MMcf/D and water injection from 60,000 to 90,000 BWPD is currently in progress. With the increased injection capabilities and 13 new wells, now drilling. the Unit will be capable of producing at 100-percent market demand factor, about 53,000 BOPD for about 2 years before it declines as a result of an increasing voidage of injected fluids. Mobility Control The use of water injection to restrict the mobility of the injected gas is the key factor in the improved recovery of the Fairway project. A reduction of viscous fingering of the gas and improved sweep efficiency through water injection is very important in the over-all control of the Fairway project. Many of the early indications of fingering were in the north end of the field (Fig. 2) where gas injection was concentrated initially. Fig. 3 illustrates the typical relationship between producing trends and injection cycles. producing trends and injection cycles. JPT P. 354

Different polymers produce decreased mobility in porous media by different mechanisms, which involve polymer-matrix interactions and solution rheology. Surprisingly, the mobility decreases do not correlate with adsorption of the polymers. Viscoelastic behavior provides a basis for a convenient measure of the mobility control activity of some polymers. Introduction The increasing use of high molecular weight polymers to improve waterflood efficiency has naturally resulted in increased interest on the part of potential users, polymer manufacturers, and universities in the polymer manufacturers, and universities in the mechanisms by which these polymers exert their mobility control effects. This interest has resulted in a number of excellent publications on polymer behavior (Ref. 1 is a recent one). There is a temptation to include in an additional study material that merely supplements or extends information already published. We have tried to resist this temptation and to present new information on the following subjects:the relationship of the rheological properties of different kinds of polymers to their behavior under reservoir flow conditions,the nature of the mobility-reducing interaction between certain polymers and a porous system, anda correlation, with practical implications, between the effects produced by certain polymer solutions in slow flow through porous media polymer solutions in slow flow through porous media and viscoelastic properties of the solutions, which are evident only at high flow rates. Partly tar reasons of convenience, and partly from long-standing habit, we shall use the following nomenclature and definitions to describe some of the effects produced by polymer solutions in porous media:The resistance factor describes the decrease in mobility of a polymer solution in comparison with the flow of the water or brine in which it is prepared. .................(1) The residual resistance factor is used to indicate the decrease in mobility of water that follows a polymer solution relative to water flow before the flow polymer solution relative to water flow before the flow of the polymer solution. ..............(2) We hope that the relationship between these designations and those used by others to describe the same effects will be readily apparent. Experimental Materials and Methods Polymers Polymers The various polymers employed in the experiments are described in Table 1. The abbreviations listed in the table will be used throughout the text to designate the different types of polymers. The experiments to be described were conducted over a period of years, which has caused something of a problem, since the quality of some of the polymers has improved considerably during this time, and polymers has improved considerably during this time, and it was natural to use the best available material in a particular experiment. The result has been a somewhat particular experiment. The result has been a somewhat regrettable lack of correlation from experiment to experiment, although the conclusions drawn from one set of data are generally valid. JPT P. 391

Abstract A new tool has become available to the water flood engineer as a result of recent discoveries in polymer chemistry. It is now possible to treat injection brines with appropriate polymeric materials to reduce the mobility of the brine solution in the formation. This permits the adjustment of the water oil mobility ratio to the most advantageous point for efficient oil recovery. Standard calculation procedures of reservoir engineering can be applied to estimate maximum recovery improvement through the use of polymer solutions, however, the economic optimum point is not calculable by prior techniques. The ability to predict recovery improvement, however, makes it possible to eliminate fields where mobility control is not required and also to decide on an appropriate value of mobility ratio to use on the field in question. The equipment used for a mobility-controlled flood is primarily standard oil field equipment. Capital expenditures for such equipment are negligible. The polymers used present no special problems of toxicity or corrosiveness. INTRODUCTION Recently announced discoveries in water flooding have yielded new techniques of potentially great benefit to the petroleum industry. It has been shown that certain water soluble polymeric materials may be dissolved in water flood brine to give a flooding medium whose mobility in porous media is greatly reduced. The extent of the mobility reduction may be varied at will over a considerable range depending On the field conditions. Practical reductions of mobility up to twenty fold have been disclosed. This reduction in mobility 1s important because by properly controlling the mobility ratio between the oil and the displacing phase important improvements in recovery can be achieved thus potentially improving the economics and hence extending the useful range of the water flooding process. The present paper discusses the techniques and apparatus necessary in the field use of such solutions.

Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract Polymer solutions used to increase oil recovery by waterflood are prepared with automated equipment which dissolves dry polymer powder in flood brine. Proper equipment design, powder in flood brine. Proper equipment design, compatible brine and correct polymer selection are essential to providing a polymer solution which exhibits good mobility control at lowest cost and which retains mobility control throughout the life of the flood. Brine requirements, equipment arrangements and additives for a variety of field conditions are discussed. Introduction High molecular weight polymers are increasingly used to improve sweep efficiency in waterfloods. Improved sweep is obtained because the polymers reduce the mobility of the water polymers reduce the mobility of the water Polymer is used during a substantial part of the Polymer is used during a substantial part of the flood life, a period of several years, so that efficient, economical means of preparing the polymer solutions are required. polymer solutions are required. Preparation of the polymer solution require the blending of dry polymer powder with brine to form a slurry, this slurry is given sufficient dissolving time and finally the solution is injected into the formation. This paper will discuss the properties of the polymers used, required properties of the brine, mechanical equipment needed and the methods of solution quality control used. POLYMER PROPERTIES POLYMER PROPERTIES Prior to the start of the field operation, the proper polymer is selected on the basis of laboratory studies using cores and fluids from the field in question. The mobility of the crude oil, of the brine used, and of the polymer solutions at various strengths is measured in the field cores. From these measurements, a flood play is developed for the most profitable operation. This plan specifies the amount and use rate of polymer and the laboratory measurements define the properties of the solution which must be handled in the plant. Specifically the properties to be discussed in this paper refer to PUSHER chemicals. These are a family of partially hydrolyzed polyacrylamides especially manufactured for waterflood use. polyacrylamides especially manufactured for waterflood use. As furnished, PUSHER chemical is a finely ground, moderately hygroscopic powder supplied in 50 pound sacks, 250 pound drums or as bulk material pound sacks, 250 pound drums or as bulk material in hopper cars or trucks. Bulk density of PUSHER chemical is 42 pounds per cubic-foot. PUSHER chemical is 42 pounds per cubic-foot.

The efficient use of aqueous polymer solutions as mobility control agents demands an understanding of the mechanisms of mobility reduction. Although much work has been directed toward this goal, there have been very few physical insights or conclusive explanations. This paper presents experimental results that imply a reasonably consistent physical interpretation and demonstrate that the amount of polymer retained increases with flow rate. polymer retained increases with flow rate. In Fig. 1, effluent concentration and mobility reduction profiles are plotted vs pore volumes injected for a 500-ppm polysaccharide solution (XC biopolymer, Xanco, Div. of Kelco Co.) with 2 percent NaCl in a 121-md Berea core 1 in. in diameter and 6 in. long. Effluent concentration fractions were determined by measuring efflux times for effluent and inlet samples in a constant-head capillary viscometer. At Position A, the flow was stopped for 16 hours and then resumed at the same pressure drop. This resulted in sharp increases in effluent concentration and mobility reduction relative to previous steady-state conditions. The result can be interpreted as follows. Under a positive pressure gradient, some polymer molecules become trapped and deformed polymer molecules become trapped and deformed within the porous structure. Cessation of flow eliminates hydrodynamic drag and permits the molecules to assume relaxed, random-coil configurations. This facilitates migration to larger flow channels and permits transport when flow is resumed. Further, if flow permits transport when flow is resumed. Further, if flow is stopped for a sufficiently long time, concentration gradients favor the diffusion of polymer molecules into less constricted regions of the porous matrix. When flow is resumed, the resulting increase in the concentration of flowing polymer increases viscosity and, hence, mobility reduction. Permeability may also increase, but evidently this effect is overwhelmed by the attendant viscosity increase. Subsequently, polymer trapping recurs and decreases the effluent concentration below the steady-state value. This, in turn, lowers the in-situ solution viscosity and also the mobility reduction. When all trapping sites are once again saturated, the system returns to its initial steady state. Positions B and C indicate points at which the pressure drop across the core was increased without pressure drop across the core was increased without interrupting the flood. In these cases additional polymer is immediately retained, lowering both the polymer is immediately retained, lowering both the effluent concentration and the mobility reduction. The minima and asymptotic approaches to steady state with continued injection are as described above, except that a lower equilibrium mobility reduction results for each increase in pressure drop. This can be attributed to lower polymer solution viscosities at high shear rates (pseudoplastic, non-Newtonian behavior). Here again, the possible lower permeability caused by added polymer retention, which permeability caused by added polymer retention, which opposes the effect of viscosity on mobility reduction, is dominated by the viscosity contribution to the mobility reduction. These results are consistent with data reported by Desremaux et al. Similar behavior has been observed for polyacrylamide solutions (Pusher 700, Dow Chemical U.S.A.) polyacrylamide solutions (Pusher 700, Dow Chemical U.S.A.) in Berea sandstone in a different type of experiment. A 6-in. core, which had been saturated with a filtered 2,500-ppm solution of polyacrylamide in 2 percent NaCl, was flushed with 2 percent NaCl at a constant pressure drop of 20 psi until the flow rate stabilized pressure drop of 20 psi until the flow rate stabilized and a residual permeability was determined. Without interrupting the flow, the pressure was lowered to 15 psi. A detectable quantity of polymer was found in psi. A detectable quantity of polymer was found in the effluent by monitoring capillary viscometer efflux times. Furthermore, the stabilized residual permeability reduction, which is the ratio of brine permeability reduction, which is the ratio of brine permeability before the addition of polymer to brine permeability before the addition of polymer to brine permeability after the addition of polymer, dropped 25 permeability after the addition of polymer, dropped 25 percent. These findings were repeated after another percent. These findings were repeated after another reduction in pressure drop. Further increases in permeability were caused by eliminating the pressure permeability were caused by eliminating the pressure gradient for periods as short as 10 minutes and then resuming flow. However, the presence of small quantities of polymer in the effluent ceased to be detectable. The residual permeability reduction decreased from its first stabilized value of 19.4 to 3.3 after 146 PV of brine had been injected with intervening PV of brine had been injected with intervening periods of no flow. There was no indication that the periods of no flow. There was no indication that the trend would change when experimentation was suspended at this point. It must be concluded that solutions of polysaccharide and polyacrylamide will lose more molecules polysaccharide and polyacrylamide will lose more molecules through interaction with porous rock at larger flow rates, and the interaction is somewhat reversible. P. 1307

Polyacrylamide solutions have been used recently in Polyacrylamide solutions have been used recently in secondary recovery processes as mobility control agents. The literature presents several papers dealing with the behavior of these solutions in porous media and viscometers. Various investigators have presented theories describing the shapes and sizes of presented theories describing the shapes and sizes of polyacrylamides in water; however most of these polyacrylamides in water; however most of these investigators have based their conclusions on the measurement of apparent viscosities and resistance factors. But no one has made direct microscopic observations. This note is based upon results of microscopic studies' regarding the shape and size of Pusher 500* and Pusher 700* polyacrylamide polymer solutions after water had been evaporated. It was observed that the structures of these polymers are not linear but rather branched, and their shape and size can be modified by changing the pH or the NaCl concentration, or both. The structures observed have distances between their extremes several times greater than 0.45u; however, they can pass through filters of 0.45u. Fig. 1 shows portions of drops of Pusher 500 water-evaporated solutions as photographed through a conventional microscope. The pH of the solutions was 3.6, 8.0, and 10.8, reading from left to right. As can be seen from this figure, the shape of the polymer structures varies with the pH of the solution. The structures present more branches at an intermediate pH and larger, degraded structures at a low pH. This pH and larger, degraded structures at a low pH. This behavior explains the effect of pH on the apparent viscosities of polyacrylamide solutions as presented by Patton. Patton indicated that these solutions exhibit maximum apparent viscosities at intermediate pH. As we stated earlier, however, the structures of pH. As we stated earlier, however, the structures of polyacrylamides observed are more branched at this polyacrylamides observed are more branched at this pH. Consequently, the probability of interference pH. Consequently, the probability of interference between structures is greater, hence there is more resistance to flow. A photograph of a drop of Pusher 500 water-evaporated solution is shown in Fig. 2a. The pH of the solution was increased from an initial value of 3.6 to 10.6. It can be seen that branches regenerate; however, they seem to be larger than those shown in Fig. 1c, where the final pH was 10.6 but the initial pH was 8. This suggests that there is no permanent pH was 8. This suggests that there is no permanent change of structure of the Pusher 500 upon changing the Ph and that the original shapes can be nearly restored by changing the pH to its original value. The effect of NaCl in polyacrylamide solutions can be deduced from Fig. 2b. This figure shows a portion of a drop of 500 ppm water-evaporated solution of Pusher 500 with 0.1 percent NaCl and a pH of 8.0. Pusher 500 with 0.1 percent NaCl and a pH of 8.0. The structure shown in Fig. 2b is comparable with that of the low-ph pusher solution shown in Fig. la. This explains the behavior of ionizable polymers when there is an increase in NaCl concentration as reported by Mungan et al. The effect of filtration of polyacrylamide Pusher 500 solutions was studied by polyacrylamide Pusher 500 solutions was studied by passing the solution through filters of 0.45u. It was passing the solution through filters of 0.45u. It was observed that polymers having, certain structures were able to pass through these filters. Figs. lb and 2c are photographs of solutions of the same NaCl photographs of solutions of the same NaCl concentration and pH, before and after filtration, respectively. The structures are markedly similar. This suggest that they are highly flexible because they can pass through filters of 0.45u even when the maximum pass through filters of 0.45u even when the maximum distance between the extremes is several times greater than the openings in these filters. Microscopic observations on some nonionizable polymers indicate that their structures look like polymers indicate that their structures look like poly-acrylamides in the presence of NaCl or when the pH poly-acrylamides in the presence of NaCl or when the pH is low. P. 80

The April, 1969, JPT Forum contained an article by Burcik in which it was suggested that "microgels" were the principal, if not the sole agent responsible for the mobility control that is obtained with polyacrylamide solutions. These gels were described as polyacrylamide solutions. These gels were described as chemically linked aggregations of polymer molecules. Although we agree with much of the data that he presents, having observed these phenomena also, we presents, having observed these phenomena also, we cannot agree with the conclusions, both real and implied, that are drawn therefrom. In particular, we feel that the presence of microgels is not only unnecessary, but actually detrimental to the performance of a polyacrylamide solution. polyacrylamide solution. A linear polymer molecule in solution is usually considered to be a series of segments, each moving freely and randomly, subject to the restrictions of other segments. Conversely, a nonlinear, multi-molecular system may have structures resulting from entanglement, branching, hydrogen bonding, and/or chemical crosslinking by anhydride, imide, or methylene bisamide formation. The resistance factor of a polymer solution, as measured with a short core, can polymer solution, as measured with a short core, can be made very high by introducing the proper amount of nonlinearity into the molecules. The true nature of this polymer is revealed when a long core is used, however, for then it is discovered that the nonlinear material is permanently removed from solution by filtration through the first inch or two of rock, There may be no evident gel buildup on the core face. Because Burcik used discs only 1/8-in. thick for measuring resistance factor, he was not able to observe this effect. To translate this into practical terms, a lightly cross-linked polymer will limit the injection rate of a well, and will do little for mobility control in the center of the field. For this reason, suppression of chemical cross-linking is of constant concern in the manufacture of Pusher brand polymer. As discussed in another paper, Burcik calculated the root-mean-square end-to-end distance to be 500for a polyacrylamide molecule having a molecular weight of a 3 x 10(6). He also found that some polymer could be strained from a solution of this type of polymer could be strained from a solution of this type of material with a Millipores filter having 0.45 mu (4,500 ) pore openings. His logical conclusion was that the polymer must exist in solution as aggregates of many polymer must exist in solution as aggregates of many molecules. The error in this approach lies in the calculation of molecular size, since the equation which was used is valid only for a polymer molecule that is a non-electrolyte in a Theta solvent. Partially hydrolyzed polyacrylamides dissolved in water or dilute salt solutions do not satisfy these requirements. A more reasonable calculation of polymer size may be made from intrinsic viscosity, [eta], which can be estimated for unhydrolyzed polyacrylamide from the following correlation: (1) where Mw is the weight average molecular weight. For Mw = 3 x 10(6), [eta] = 7.7. The mean square end-to-end distance square root of r can then be calculated from the relation given by Flory: (2) Measurements on partially hydrolyzed polymer in 3 percent salt solutions indicate that repulsion between charged groups on the polymer backbone creates a viscosity at least double that of an unhydrolyzed polymer. Therefore, the molecular size would be at polymer. Therefore, the molecular size would be at least 2,870 for partially hydrolyzed material. Polymerization does not produce molecules of uniform size. A typical vinyl-type polymerization yields a significant fraction of material having a molecular weight that is 2 1/2 times the average. P. 1247