Permeability Reduction Mechanisms Involved in In-Situ Gelation of a Polyacrylamide/ Chromium (VI)/Thiourea System

Abstract

Summary An investigation of the in-situ gelation of apolyacrylamide/chromium(VI)/thiourea system under flow conditions inunconsolidated sandpacks is presented. High flow resistance was observed in alocalized section of the sandpacks that was consistent with deep-bed filtrationmechanisms. A method to measure the size of aggregates in a gel solution isalso presented. Results from this method showed that the gel aggregates growwith reaction time and distance traveled through the sandpack, supporting theconclusion that filtration is a dominant factor in the in-situ gelation processunder flow conditions. Introduction Poor volumetric sweep efficiencies are encountered in many waterfloods andother displacement processes because of a permeability variation betweendifferent reservoir layers or zones. Injected fluids preferentially flowthrough the zones of relatively high permeability, bypassing oil in theless-permeable zones. A practical method to increase volumetric sweep is theapplication of gelled polymer treatments for in-situ permeabilitymodification. One such treatment consists of injecting a mobile gel solution into thehigh-permeability zones. The gel solution reacts to produce, after some time, arelatively immobile gel that increases the flow resistance. When a displacementprocess is resumed after a treatment, injected fluid is diverted to theunswept, less-permeable zones of the reservoir, resulting in increased oilrecovery. Studies of several gel polymer systems have been reported, but most of the work has focused on the gelation process of "bulk" gelsolutions. These studies indicate that rather complex chemical and physicalmechanisms are involved in the gelation process. The process is complicatedfurther when gelation occurs in situ in a porous medium. Previous research on achromium(VI)/ thiourea/polyacrylamide system showed that the rate of gelation of solution flowing through a porous medium (as indicated by the development of high flow resistance) is affected by the in-situ shear rate imposed by theflow. Polymer retention as the gel solution flowed through the medium wasidentified as a significant factor in the development of the high flowresistance. In a study of a chromium(III)/xanthan system, it was proposed thatthe initial stage of the gelation reaction involved a coagulation process thatresulted in the formation of gel aggregates. It was speculated that the gelaggregates were adsorbed/retained as the gel solution flowed through a porousmedium. It was also suggested that a "secondary" stripping processoccurred where flowing species (gel aggregates) were retained by reaction withpreviously retained gel aggregates. The study reported here was undertaken toinvestigate the in-situ gelation process for athiourea/chromium(VI)/polyacrylamidecopolymer system when it flows through anunconsolidated sandpack. A primary objective was to identify the importance of the various mechanisms that contribute to the development of increased flowresistance as the gel solution flows through a porous medium. Experimental Materials, Equipment, and Procedures Gel System. The gel system used in the study consists of CatAn-160 polymerand a chromium/thiourea redox system. The redox system generates chromium(III)in solution that reacts with the polymer and forms a gel over a period of time. CatAn-160 is a cationic polyacrylanmide copolymer that is reported by thedistributor to have an average molecular weight (MW) of 5 million and acationic content of 20 wt%. The cationic group(s) is unknown. Reagent grades of sodium dichromate dihydrate, thiourea, and NaCl were used as obtained. Asequence of steps was followed to prepare a gel solution. Fig. 1 shows thefinal mixing step of Solutions A and B and chemical compositions. This mixingstep was done either manually in ajar or by pumping the solutions through anin-line mixer for displacement experiments. Solutions A and B remained stablefor at least 3 weeks; i.e., when they were mixed within this period, theresulting gel solution had reproducible properties, which is important whendisplacements are conducted over many days. Displacement Experiment Apparatus. Displacement experiments were conductedwith the equipment shown schematically in Fig. 2. Red gauge oil pumped fromsyringe-type pumps displaced solutions A and B from glass transfer cylindersinto a static in-line mixer. The gel solution exiting the mixer was flowed atconstant rate through a sandpack that was initially saturated with brine. Pressure drops were measured across the sandpack and between pressure portsthat were placed along the length of the sandpack holder. Pressures were sensedby a transducer/demodulator system. Pressure data were read every 10 seconds, averaged over 30 data points, and recorded every 5 minutes by a computerequipped with an analog-to-digital converter. Effluent fractions from thesandpack were collected by an automatic collector or collected in jars thatwere changed manually. Sandpacks. Sandpack holders were constructed of Lucite tubing (ID=3.8 cm). Lucite endplates and teflon screens confined the sand in the holder. Pressureports placed along the length of a sandpack demarcated individual sections(labeled A, B, etc.) over which pressure drops were measured. The length of each section was 10 cm except Section A (inlet, 6.7 cm) and Section M (outlet, 6.0 cm). Grade F-140 Ottawa sand was used as the porous medium. The sand waswashed with acid before use to remove fine black particles and to neutralizethe inherent basicity. Sand was packed into a holder by an automated packingdevice. During the packing procedure, a 1-cm length of relatively coarse sandwas packed at the inlet and outlet faces to provide a buffer between the F-140sand and screens placed at the ends. After packing, sandpacks were evacuatedand saturated with brine. Porosity was determined from the holder volume, andweights were measured before and after brine saturation. Permeabilities of eachsection of the sandpack and for the total sandpack were determined frompressure-drop and flow-rate measurements with brine. Just before a gel-solutiondisplacement experiment, the pH of the brine in the sandpack was adjusted to 4.7, the initial pH of a gel solution, by flowing several PV's of brine throughthe sandpack until the pH of the effluent stabilized at 4.7, the pH of theinjected brine. SPERE P. 77^