Improved Scleroglucan for Polymer Flooding Under Harsh Reservoir Conditions

Abstract

Summary Polymer flooding is commonly used to improve water sweep efficiency in oilreservoirs. Successful application of this method, however, has been restrictedto low-temperature reservoirs because suitable polymers are not available forharsh conditions. Scleroglucan, a polymer polymers are not available for harshconditions. Scleroglucan, a polymer produced by fermentation that showspromising properties, forms solutions produced by fermentation that showspromising properties, forms solutions that are very viscous and highlyresistant to shear. Viscosity is insensitive to both salts and pH and onlyslightly affected by temperature. To facilitate industrial development ofscleroglucan polymer, improvements in the filterability of the solutions andbetter knowledge of their behavior in porous media are necessary. Ahigh-quality scleroglucan was obtained by eliminating the impuritiesresponsible for polymer aggregation. This allowed evaluation of the intrinsicproperties of the polymer molecules. Complete elimination of impurities fromthe polymer solution led to a scleroglucan without any aggregation tendency andwith a good filterability, particularly at high temperatures. The performanceof this improved scleroglucan in porous media was evaluated by corefloodexperiments in Berea cores at temperatures ranging from 30 to 90 degrees C. Results showed low permeability reduction and a mobility reduction close torelative viscosity. Injection of successive slugs into a Berea core underanaerobic conditions indicated low polymer retention at high temperatures (30g/g at 90 degrees C). Introduction Until recently, polymer flooding generally has not been feasible inreservoirs with high temperatures and high-salinity brines be-causecommercially available polymers do not meet the quality requirements necessaryfor successful application under harsh reservoir conditions. Interest insemirigid polysaccharides (xanthan and scleroglucan) has been high because theyare excellent viscosifiers, even in the presence of high salt concentrations. Their excellent salt tolerance results from the rigidity of the rod-likemolecules formed in solution. In contrast, partially hydrolyzed polyacrylamides(the most widely used mobility-control partially hydrolyzed polyacrylamides(the most widely used mobility-control polymers) provide low viscosities owingto weak expansion of polymers) provide low viscosities owing to weak expansionof polyelectrolyte coils in high-ionic-strength solutions. polyelectrolytecoils in high-ionic-strength solutions. Davison and Mentzer investigated morethan 140 polymers for viscosity retention and mobility reduction in porousmedia at high temperature (90 degrees C), salinity, and pressure. Theirevaluation showed scleroglucan to have the best potential. Other authors alsomentioned the promising properties of scleroglucan as a water viscosifier.properties of scleroglucan as a water viscosifier. Scleroglucan is a nonionicpolysaccharide produced by fermentation of a plant pathogen fungus of the genus Sclerotium that is now an industrial plant pathogen fungus of the genus Sclerotium that is now an industrial product. The backbone of the moleculeconsists of linearly linked -1, product. The backbone of the molecule consistsof linearly linked -1, 3-D-glucose residues. A -1, 6-D-glucose side chain isattached to every third main-chain residue in the backbone. Because of thetotally nonionic character of the molecule, the viscosity of scleroglucansolutions is insensitive to salt. Scleroglucan was shown to exist as rod-liketriple-helix chains, and thus behaves as semirigid molecules in aqueoussolutions. This structure explains the very high viscosifying properties andresistance to shear. Scleroglucan has the same chemical structure asschizophyllan, another triple-helix polysaccharide that is stable up to 130degrees C. Thus, this polymer is expected to be a good water viscosifier up toabout this temperature. Scleroglucan could extend the use of biopolymers asmobility-control agents to a temperature and salinity domain where xanthansexperience transition from rod-like conformation to random coils. Thetriple-helix conformation also prevails over a wide pH range up to 12. Literature reports on the thermal stability of scleroglucan in saline solutionsdo not concur. Akstinat found that scleroglucan remained stable at 80 degrees Cfor 3 months in highly saline conditions. Davison and Mentzer observed highviscosity retention for scleroglucan in seawater at 90 degrees C over 500 days. Recently, Kalpack et al. observed that scleroglucan solutions with and withoutadditives exhibited a better thermal stability than xanthan over 700 days. Incontrast, Ryles found that scleroglucan degraded after only 3 months at 90degrees C. A major drawback noted by a number of authors has been the pluggingtendency of scleroglucan samples. Filterability improvement is necessary tofacilitate the development of scleroglucan as a mobility-control polymer. Significant progress recently has been made in this respect. Modifications inthe biosynthesis process and in post-treatments have led to a product withimproved filterability. Purification and additional treatments were performedin the laboratory to eliminate residual impurities present in the industrialproduct. This allowed evaluation of the intrinsic properties of our improvedscleroglucan. The results confirmed excellent rheological characteristics forthis product, particularly insensitivity to salt and stability at hightemperatures. In this paper, we first review results obtained previously withpurified samples. Then, we evaluate an industrially manufactured scleroglucanof improved quality. This evaluation is based on viscosity and filterabilitymeasurements and flow properties in porous media. Experimental Preparation of Polymer Solutions. The scleroglucan used for this study Preparation of Polymer Solutions. The scleroglucan used for this study wasmanufactured by Sanofi Bio Industries and was supplied as a concentrated broth. Solutions of two different grades were prepared. In Type A, the concentratedbroth was diluted to 600 ppm polymer. This solution was subjected to extensiveultrafiltration with a 20-g/L NaCl solution. The solution was subsequentlymaintained at 90 degrees C for 48 hours to decrease the degree of polymerassociation. In Type B, the concentrated broth was diluted to 600 ppm polymer, but ultrafiltration was not performed. A dissociation treatment consisted ofmaintaining the solution at 90 degrees C for 24 hours and then shearing it byinjection through a 1.2- m filter at high velocity. NaN3 was incorporated intothe solution at a concentration of 400 ppm as a biocide. Type A scleroglucan iswell suited to characterization of intrinsic properties. Type B, being closerto an industrial product, is better suited for flow experiments. For long-termstability tests, oxygen was eliminated by bubbling oxygen-free nitrogen throughthe solutions. The final tent, measured by a dissolved-oxygen test kit(CHEMetrics(TM)) inside an anaerobic chamber, was less than 5 ppb. Measurement Techniques in Solution. Polymer concentrations were determinedwith a Dohrmann carbon analyzer. Effluents collected during tests in porousmedia were analyzed by gel permeation chromatography. Below 50 degrees C, viscosities were measured at very low shear rates with a Contraves low-shearviscometer. Above 50 degrees C, a capillary viscometer was used to measureviscosity in high-temperature, low-shear-rate conditions.