Starch-acrylamide, graft copolymers are effective but shear-sensitive viscosifiers which, when synthesized with 18 or less grafts per starch molecule, duplicate the rheology of acrylamide homopolymer of 1 to 3 times higher molecular weight. Solutions of copolymer must be prepared at shear rates below 100 sec(-1) and lose up to 40 percent of their viscosity when filtered through 0.45 mu pores. Copolymers with 18 or less grafts per starch molecule have higher intrinsic viscosities, higher solution viscosities, and higher solution screen factors than equal molecular weight homopolymers and all three properties of the copolymer increase with increasing molecular weight or decreasing number of grafts. The viscosity of copolymer solutions drops by less than 3 percent when sodium chloride concentration varies from 0 to 10,000 ppm or calcium ion concentration varies from 0 to 1,000 ppm. Solutions of copolymer may lose up to 60 percent of original viscosity from 100 shearings at a rate of 4,300 sec(-1). Sensitivity to shear increases with increasing molecular weight and, at constant molecular weight, increases with decreasing number of grafts. Loss of solution viscosity under successive shearings can be expressed with a modified Williams-Watts equation. Introduction Most of the polymers currently used in oil reservoirs are linear, synthetic copolymers* prepared for other markets but applied to oil recovery. This paper is the first in a series of studies of the paper is the first in a series of studies of the rheology of polymers designed and synthesized to meet the requirements of enhanced oil recovery. The tested material is a graft copolymer prepared by polymerizing acrylamide side chains on a starch backbone. The starch is a series of dehydroglucose units -linked at the positions and yields, when reacted with acrylamide, a bonding structure based on the formula of Figure 1. A distinct bond between backbone (anhydroglucose) and side chain (acrylamide) is not shown in Figure 1 because there are three possible sites of attachment. The copolymers were tested for applicability to oil recovery by measuring intrinsic viscosity, solution viscosity after low- or high-shear mixing, viscosity after filtration, variation of viscosity and screen factor as a function of copolymer concentration, Huggins constant, Schulz-Balishka constant, Kraemer constant, variation of viscosity as a function of sodium chloride or calcium ion concentration, loss of viscosity when the solution is sheared at a rate of 4,300 sec(-1), shear decay constants, solution viscosity loss with time, and Ostwald-deWaele exponent. Most viscosities were measured with Cannon-Fenske viscometers at a shear rate of 53 sec(-1). The viscosities used in the calculation of the Ostwald-deWaele exponent were measured in a low-shear, capillary viscometer with a variable pressure head. All viscosity tests were performed at 30 degrees C. Solutions were gravity filtered through Millipore filters under a maximum pressure head of 5 kilopascals. The orifice mixer is described in Appendix A. SYNTHESIS AND PROPERTIES OF THE COPOLYMERS Synthesis Starch-acrylamide graft copolymers are formed in aqueous solution by ceric ion initiated, radical polymerization of acrylamide on starch. Polymerization polymerization of acrylamide on starch. Polymerization is conducted under an inert atmosphere. The average number of chains, Ng, of acrylamide added to each starch molecule is calculated from Equation 1, Moles of Ceric Ion Initiator Added To The Reaciton Mixture N = ..........(1)g Moles of Starch Placed in The Reaction Mixture The calculated degree of polymerization, Dp, is the average number of monomer units contained in each grafted chain on the starch molecule.

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Surface charge and accessible surface area control the retention of polyacrylamide by berea sandstone, baker dolomite, and sodium kaolinite. Hydrolysis of polyacrylamide increases retention by materials with polyacrylamide increases retention by materials with a positively charged surface, such as dolomite, and decreases retention, when surface area remains constant, on materials with negatively charged surfaces, such as sandstone. Additives which form a more "theta" or less effective solvent promote retention. Water becomes a less effective solvent for polyacrylamide when its salt content is increased or when nonsolvents, such as alcohols, are added to it. Berea sandstone cores retained over 50 percent of polyacrylamide injected under all salinities, polyacrylamide polyacrylamide injected under all salinities, polyacrylamide concentrations, and degrees of hydrolysis tested and sodium kaolinite retained over 90 percent of all injected polyacrylamide under the same conditions. In contrast, baker dolomite, a high purity dolomite, retains only 17.9 percent of the unhydrolyzed polyacrylamide injected. Since berea sandstone contains polyacrylamide injected. Since berea sandstone contains 7.5 percent kaolinite, these data suggest that clay content has a significant effect on total polymer retention. polymer retention. Flocculating sodium kaolinite, which reduces its accessible surface area by over 80 percent, reduces retention by 70 percent, from 1,217 ug/g to 360 ug/g. With other variables held constant, reduction in accessible surface area will reduce retention of polyacrylamide by a porous medium as long as adsorption polyacrylamide by a porous medium as long as adsorption dominates the retention process. Introduction Over fifty oil recovery projects using dissolved polymers have been initiated in the United States. polymers have been initiated in the United States. Since 60 percent of the oil in place in the U.S. is in sandstone reservoirs, most of the reservoirs being flooded are sandstone. However, polymer floods in other lithologies are becoming common and already sand-conglomerate, sand-shale, and limestone reservoirs are being flooded. To aid in the design of polymer floods for dolomite, limestone, and conglomerate reservoirs, a study of the effect of the rock comprising the porous media on retention of polymer was undertaken. Three substrates used in this study were berea sandstone, baker dolomite, and highly crystalline, georgia kaolinite. These three substrates were flooded with hydrolyzed and unhydrolyzed polyacrylamide. The polyacrylamide solutions were made at two different concentrations in two brines of differing sodium chloride content. All flooding experiments were run under operating conditions representative of those produced in a reservoir. No oil was present or injected into the substrates. MATERIALS Polymer Synthesis Polymer Synthesis Polyacrylamide of molecular weight 5.4 million was synthesized by the method of Tanaka. Potassium peroxydisulfate and 2-propanol were used as redox peroxydisulfate and 2-propanol were used as redox initiators to conduct a radical polymerization of an aqueous acrylamide solution maintained at 30 deg. C under a nitrogen blanket. After 24 hours, the reaction mixture was precipitated into 5 times its volume of 2-propanone, filtered, washed with 2-propanone, ground for 2 minutes in a high speed blender, and dried under vacuum to constant weight. Hydrolyzed polyacrylamide was prepared by adding the amount of sodium hydroxide needed to hydrolyze the desired fraction of amide units to a 1 percent by weight solution of polyacrylamide and maintaining the solution at 60 deg. C for 2 hours. The reaction mixture was precipitated into 5 times its volume of 2-propanone, filtered, the precipitate washed with 2-propanone, ground in a blender for 2 minutes, and dried under vacuum to constant weight. Properties of the two polymers are given in Table 1. Equations for calculating the properties of Table 1 are described in Appendix A. The radioactive carbon-14 in the polymer is the number 1 carbon atom of acrylamide, CH2 = CHC14ONH2, and the 500 microcuries of radiotracer constitute a 1.78 × 10-3 mole fraction of the polyacrylamide. P. 61

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