Permeability Reduction by a Xanthan/Chromium (III) System in Porous Media
- Shahab Hejri (McCool, C.S.) | Jousset Francois (McCool, C.S.) | D.W. Green (McCool, C.S.) | G.P. Willhite (U. of Kansas)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1993
- Document Type
- Journal Paper
- 299 - 304
- 1993. Society of Petroleum Engineers
- 5.3.2 Multiphase Flow, 5.4.10 Microbial Methods, 2.4.3 Sand/Solids Control, 5.1.1 Exploration, Development, Structural Geology, 4.1.2 Separation and Treating
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This paper presents an experimental study on gelation of a xanthan/chromium(III) system in unconsolidated-sandpacks at frontal velocities between 3 and120 ft/D. High flow resistance developed at specific locations in the sandpacksin experiments conducted at velocities up to 35 ft/D; the locations correlatedwith velocity. No significant now resistance developed in the sandpacks atfrontal velocities of 83 and 118 ft/D. The effects of flow and shear rates andpermeability on development of high flow resistance in the sandpacks arediscussed. A conceptual model of the gelation process that incorporatesfiltration of gel aggregates is presented.
During a secondary or enhanced oil recovery process. a substantial amount ofoil may be bypassed by the injected fluid owing to reservoir heterogeneity.Fractures and permeability variations in the vertical direction are majorcontributors to preferential flow in the reservoir. Diversion of injected fluidinto unswept zones can be achieved with gelled-polymer treatments. In a typicaltreatment, a gel solution composed of a water-soluble polymer and a crosslinkeris injected into a depleted zone to reduce the permeability. permeability.Polysaccharides and polyacrylamides are used in gelled-polymer systems. Acommonly used crosslinker for these polymers is chromium (III). In thecrosslinking process, chromium (III) is either used directly in its trivalentstate or generated from chromium (VI) species by use of a reducing agent.Considerable attention has been given to use of xanthan/chromium (III) gelsprepared with chromium chloride. Gelation of these systems is controlled byseveral factors. Hester's rheological measurements showed the effects of pH andxanthan and chromium concentrations on the gelation process of axanthan/chromium (III) gel system. Increasing the gel solution pH in the rangeof 4.2 to 5.2 accelerated gelation. The gelation rate also increased withincreasing xanthan and chromium (III) concentrations. Dolan found that theinitial gelation rate for xanthan/chromium (Ill) solutions was inverselyproportional to the hydrogen-ion concentration between pH 4 and 6.Precipitation was observed at pH greater than 6, and gels would not form at pH7. When a gel solution is injected continuously into a porous medium, thepermeability is reduced as a result of interaction between the gel solution andthe porous matrix. McCool et al. and Marty et al. studied the mechanisms ofin-situ gelation for a polyacrylamide/chromium (VI)/thiourea gel system in4-ft-long polyacrylamide/chromium (VI)/thiourea gel system in 4-ft-longsandpacks. During the injection process, a localized high-flow-resistanceregion developed well behind the leading edge of the gel-solution bank andultimately caused the sandpack to plug. It was hypothesized that gel aggregateswere formed that were retained in the porous matrix. The existence of theseaggregates was established by experiments using equilibrium dialysis. McCool etal. postulated that ". . aggregates were filtered out of the flowingsolution by interactions with the porous matrix, by reacting with previouslyretained polymer, and by selective straining of the larger aggregates thatcould not pass through pore constrictions." Published information onin-situ gelation of xanthan/chromium (III) gels is limited. Hubbard et al.proposed a mechanism for in-situ gelation of a xanthan/chromium (III) gelsystem similar to that described by McCool et al. This paper summarizespreviously reported research. The primary objective was to study thepermeability reduction primary objective was to study the permeabilityreduction mechanisms of a xanthan/chromium (III) gel system in unconsolidatedsandpacks. Permeability reduction was investigated by flow experiments wherethe flow resistance was monitored as gel solution was displaced through thesandpacks. The displacement rate and porous-medium permeability were varied toinvestigate the effect porous-medium permeability were varied to investigatethe effect of insitu shear rate on gelation behavior.
The experimental program was based on continuous displacement of gelsolution in sandpacks at a constant rate. In-situ gelation, as indicated by thedevelopment of high flow resistance, was studied by monitoring pressure dropsalong the sandpack length and by determining effluent properties. Frontalvelocity and shear rate effects on in-situ gelation were studied by selectingflow rates and permeabilities. Studies were done in sandpacks because thegelation process was pH sensitive, and sandpacks could be conditioned tomaintain pH within a narrow interval, thus minimizing effects of pH changesresulting from the interaction between the injected fluids and the porousmedium.
Apparatus. Fig. 1 is a schematic of the apparatus used for flow experiments.The injection system consisted of pumps and transfer cylinders. Gauge oil orwater was pumped into the transfer cylinders, which contained xanthan andchromium solutions. The two solutions exiting the transfer cylinders werecombined in a static in-line mixer immediately before injection into asandpack. Two types of sandpack holders were used. One was fabricated from a4.5-ft-long, 1.44-in.-ID Lucite tube. Pressure ports were installed 10 in.apart along the tube. The ports segmented the holder into rive sections, with2-in.-long inlet and outlet sections. The tube was packed with sand with anautomatic sandpacker. A second holder was made of 0.295-in.-ID stainless-steeltubing. Five 3-ft-long sections were connected with tees that were used aspressure ports. The stainless-steel "slim tube" was packed withpressure ports. The stainless-steel "slim tube" was packed with sandmanually with a vibrator. Ottawa sand was used in all experiments. The sand waswashed with concentrated HCl to remove iron, rinsed with distilled water, anddried before packing. Teflon screens were placed at the end caps and pressureports to prevent sand movement. The sandpacks and injection system weremaintained at 77 degrees F with air and water baths. The sections of thesandpacks were labeled alphabetically from the inlet to the outlet. Pressuredrops across each section and the total length were monitored by transducersconnected to a computerbased data-collection system. Sandpack effluents werecollected in fractions.
Materials. The gel-solution concentration was 1,500 ppm xanthan, 50 ppmchromium (III), 1,500 ppm formaldehyde, and 5,000 ppm NaCl. Xanthan biopolymer(Flocon 4800MX, Lot 261-133) was supplied as a broth by Pfizer Inc. Thereported assay was 5.7 wt%, determined from a calibration curve based onsolution viscosity. Concentrations used in this paper were based on an assay of3.2 wt%. determined by precipitation with isopropanol. Extensive filtration wasnecessary to obtain polymer solutions with reproducible flow characteristics.Ref. 8 describes the filtration procedure. procedure. Reagent grade chromicchloride was the source of chromium (III).
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