Starch-Acrylamide Graft Copolymers For Use In Enhanced Oil Recovery

Abstract

Abstract 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.