Re-Formation of Xanthan/Chromium Gels After Shear Degradation

Abstract

Summary Studies were conducted on the re-formability of xanthan-gum/chromium gelsthat have been degraded by shear. The regelation study of xanthan/chromium gelsshowed that gels that have been sheared take longer to re-form than to gelinitially and that the original gel strength often was not recovered aftershearing. The weaker the gel at the time of shearing, the more likely it isthat it will obtain the ultimate strength of an unsheared gel of the sameformulation. Introduction Polymer gels are frequently used-in attempts to reduce permeability Polymergels are frequently used-in attempts to reduce permeability selectively inhigh-permeability zones to improve vertical sweep efficiency. Numerous chemicalsystems and treatment schemes have been used in such treatments. Xanthan/Cr(III) gels are being used increasingly to improve reservoirconformance by reducing the permeability of high-permeability zones. In some ofthese permeability of high-permeability zones. In some of these applications, the gel is formed on the surface, broken by the high-shear conditions duringflow down the tubing and through the perforations, and then reformed after thegel fluid is placed away from perforations, and then reformed after the gelfluid is placed away from the wellbore where the shear conditions have beenreduced. The success of such a treatment depends on the ability of the gel toreform after having been degraded by high-shear conditions. The reformabilityof some polymer gels has been reported. The reformability of xanthan/chromiumgel is attributed to the rod-like helical structure of xanthan polymer and itsweak crosslinking bond. However, neither the mechanism of degradation nor thebehavior of the regelation has been well characterized. How the re-formabilityof a gel is affected by various shearing conditions and to what extent the gelwill reform are two important factors in this type of treatment. Therefore, theobjectives of this study were to characterize the behavior of a gel before, while, and after being sheared and to identify the effect of shear on there-formability of a gel. The laboratory experiments were carried out in three phases:the initialgelation phase, simulating the solution gelling in the tank at the surface;the shearing phase, simulating a mechanical degradation period through thepumping system; andthe regelation phase, simulating a regelation processoccurring in the formation. phase, simulating a regelation process occurring inthe formation. The behavior of polymer gels in the initial gelation andregelation phases was characterized by the dynamic viscosity obtained fromphases was characterized by the dynamic viscosity obtained from dynamicoscillatory measurements. The mechanism of shear in the shearing phase wasaccomplished by various shearing devices in which shear intensity and sheartime were adjustable. The effect of shear conditions on the re-formability ofthe gels was established by a comparison of the dynamic viscosity of unshearedgels defined in the initial gelation phase with the final gel strength afterthe regelation phase. Experimental Procedures A commercial-grade biopolymer (Flocon 4800 TM) was used for the entirestudy. This polymer, consisting of D-glucose, D-mannose, and D-glucuronic acidin a repeating pentasaccharide unit, is an extracellular polysaccharideproduced by the microorganism Xanthomonas campestris. It was supplied by themanufacturer in the form of an aqueous broth having 5.0% solids and preservedwith 2,500 ppm formaldehyde. Its molecular weight was reported as at least1,000,000. A 5,000-ppm-polymer stock solution was prepared by dispersing the broth indistilled water with a blender at 8,000 rev/min for 6 minutes. The polymerconcentration was calculated on the basis of an assay of 5.0 wt %. The stocksolution was further diluted in a 4% NaC] solution and stirred with a magneticstirring bar for 2 minutes to reach the final working solutionconcentration. The crosslinking agent, CR(III), was obtained from a solution ofreagent-grade chromium (III) chloride hexahydrate (CrCI3 - 6H2O). A2,000-ppm-chromium stock solution was prepared and aged for at least 2 days toallow the chromic chloride to hydrolyze. The various concentrations of chromiumsolution were prepared by diluting the aged stock solution. The pH value of thestock solution at 25 degrees C before further dilution was approximately 2.7, which is close to the estimated value in a hydrolysis study of Cr(III). The final concentration of gelling solution was prepared by mixing therequired amount of polymer working solution with an equal weight of chromiumsolution and stirring with a magnetic stirring bar for 2 minutes. The technique applied in the field for profile control using a xanthan gelwas simulated in the laboratory through three phases:the initial gelationphase, simulating the solution gelling in the tank at the surface;theshearing phase, simulating a. mechanical degra-dation period through thepumping system; andthe regelation phase, simulating a regelation processin the formation. To carry phase, simulating a regelation process in theformation. To carry out these experiments, the whole study was divided intothree stages: gelation tests, shearing experiments, and regelation studies. Most of the studies were conducted at 50 degrees C, with some exceptions thatwill be mentioned. A series of gelation tests was first conducted to select a gelling solutionthat has an appropriate gel strength and lack of syneresis for subsequentregelation studies. The gel strength, in terms of the dynamic viscosity, wasdetermined by dynamic oscillatory measurements performed with a Rheometrics Fluid Spectrometer Model 840OTM (RFS-8400) with a parallel-plate test fixture. A volume of 2.0 cm3 of gelling solution was loaded inside the test fixture andcovered by 3.0 cm3 of mineral oil to prevent evaporation. The evolution of thegel structure was characterized by the dynamic viscosity as a function of time. The measurements were conducted at a frequency of 10 rad/s, a strain amplitudeof 1%, and a gap of 1 mm. Syneresis was determined by observing the phaseseparation of water and gel in vials. A study was also made of coneand-plateand parallel-plate geometries of two concentrations of gelling solution toinvestigate the effect of rheometer geometry on the reproducibility of themeasurements. Three techniques were used to shear the gels. The first method consisted ofshearing the gel vigorously in a blender at various speeds for 6 minutes. Theshearing was performed at ambient temperature. A controllable flow developed inthe rheometer was used as a second shearing environment. The initial gel wasformed inside the rheometer sample holder. Both cone-and-plate andparallel-plate geometries were used for shearing the gel. The shear schemeparallel-plate geometries were used for shearing the gel. The shear schemeshown in Fig. 1 was implemented by use of a thixotropic loop. The shear ratefirst increased linearly from zero to a maximum in 3 minutes, was kept constantat the maximum rate for 6 minutes, and ended in a linearly decreasing rate fromthe maximum to zero in another 3 minutes. The maximum shear rates used were100, 1,000, 2,000, and 4, 000 seconds. The dynamic viscosities of gels beforeand after shearing were compared to determine the extent of mechanicaldegradation.