Permeability Reduction by a Xanthan/Chromium (III) System in Porous Media

Abstract

Summary 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 sandpacks in experiments conducted at velocities up to 35 ft/D; the locations correlated with velocity. No significant now resistance developed in the sandpacks at frontal velocities of 83 and 118 ft/D. The effects of flow and shear rates and permeability on development of high flow resistance in the sandpacks are discussed. A conceptual model of the gelation process that incorporates filtration of gel aggregates is presented. Introduction During a secondary or enhanced oil recovery process. a substantial amount of oil may be bypassed by the injected fluid owing to reservoir heterogeneity. Fractures and permeability variations in the vertical direction are major contributors to preferential flow in the reservoir. Diversion of injected fluid into unswept zones can be achieved with gelled-polymer treatments. In a typical treatment, a gel solution composed of a water-soluble polymer and a crosslinker is injected into a depleted zone to reduce the permeability. permeability. Polysaccharides and polyacrylamides are used in gelled-polymer systems. A commonly used crosslinker for these polymers is chromium (III). In the crosslinking process, chromium (III) is either used directly in its trivalent state or generated from chromium (VI) species by use of a reducing agent. Considerable attention has been given to use of xanthan/chromium (III) gels prepared with chromium chloride. Gelation of these systems is controlled by several factors. Hester's rheological measurements showed the effects of pH and xanthan and chromium concentrations on the gelation process of a xanthan/chromium (III) gel system. Increasing the gel solution pH in the range of 4.2 to 5.2 accelerated gelation. The gelation rate also increased with increasing xanthan and chromium (III) concentrations. Dolan found that the initial gelation rate for xanthan/chromium (Ill) solutions was inversely proportional 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, the permeability is reduced as a result of interaction between the gel solution and the porous matrix. McCool et al. and Marty et al. studied the mechanisms of in-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-resistance region developed well behind the leading edge of the gel-solution bank and ultimately caused the sandpack to plug. It was hypothesized that gel aggregates were formed that were retained in the porous matrix. The existence of these aggregates was established by experiments using equilibrium dialysis. McCool et al. postulated that ". . aggregates were filtered out of the flowing solution by interactions with the porous matrix, by reacting with previously retained polymer, and by selective straining of the larger aggregates that could not pass through pore constrictions." Published information on in-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 summarizes previously reported research. The primary objective was to study the permeability reduction primary objective was to study the permeability reduction mechanisms of a xanthan/chromium (III) gel system in unconsolidated sandpacks. Permeability reduction was investigated by flow experiments where the flow resistance was monitored as gel solution was displaced through the sandpacks. The displacement rate and porous-medium permeability were varied to investigate the effect porous-medium permeability were varied to investigate the effect of insitu shear rate on gelation behavior. Experimental The experimental program was based on continuous displacement of gelsolution in sandpacks at a constant rate. In-situ gelation, as indicated by the development of high flow resistance, was studied by monitoring pressure drops along the sandpack length and by determining effluent properties. Frontal velocity and shear rate effects on in-situ gelation were studied by selecting flow rates and permeabilities. Studies were done in sandpacks because the gelation process was pH sensitive, and sandpacks could be conditioned to maintain pH within a narrow interval, thus minimizing effects of pH changes resulting from the interaction between the injected fluids and the porous medium. Apparatus. Fig. 1 is a schematic of the apparatus used for flow experiments. The injection system consisted of pumps and transfer cylinders. Gauge oil or water was pumped into the transfer cylinders, which contained xanthan and chromium solutions. The two solutions exiting the transfer cylinders were combined in a static in-line mixer immediately before injection into a sandpack. 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 an automatic 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 as pressure ports. The stainless-steel "slim tube" was packed with pressure ports. The stainless-steel "slim tube" was packed with sand manually with a vibrator. Ottawa sand was used in all experiments. The sand was washed with concentrated HCl to remove iron, rinsed with distilled water, and dried before packing. Teflon screens were placed at the end caps and pressure ports to prevent sand movement. The sandpacks and injection system were maintained at 77 degrees F with air and water baths. The sections of the sandpacks were labeled alphabetically from the inlet to the outlet. Pressure drops across each section and the total length were monitored by transducers connected to a computer based data-collection system. Sandpack effluents were collected in fractions. Materials. The gel-solution concentration was 1,500 ppm xanthan, 50 ppm chromium (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. The reported assay was 5.7 wt%, determined from a calibration curve based on solution viscosity. Concentrations used in this paper were based on an assay of3.2 wt%. determined by precipitation with isopropanol. Extensive filtration was necessary to obtain polymer solutions with reproducible flow characteristics. Ref. 8 describes the filtration procedure. procedure. Reagent grade chromicchloride was the source of chromium (III). SPERE P. 299