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Summary

Studies were conducted on the re-formability of xanthan-gum/ chromium gels that have been degraded by shear. The regelation study of xanthan/chromium gels showed that gels that have been sheared take longer to re-form than to gel initially and that the original gel strength often was not recovered after shearing. The weaker the gel at the time of shearing, the more likely it is that it will obtain the ultimate strength of an unsheared gel of the same formulation.

Introduction

Polymer gels are frequently used-in attempts to reduce permeability Polymer gels are frequently used-in attempts to reduce permeability selectively in high-permeability zones to improve vertical sweep efficiency. Numerous chemical systems and treatment schemes have been used in such treatments. Xanthan/Cr(III) gels are being used increasingly to improve reservoir conformance by reducing the permeability of high-permeability zones. In some of these permeability of high-permeability zones. In some of these applications, the gel is formed on the surface, broken by the high-shear conditions during flow down the tubing and through the perforations, and then reformed after the gel fluid is placed away from perforations, and then reformed after the gel fluid is placed away from the wellbore where the shear conditions have been reduced. The success of such a treatment depends on the ability of the gel to reform after having been degraded by high-shear conditions. The reformability of some polymer gels has been reported. The reformability of xanthan/chromium gel is attributed to the rod-like helical structure of xanthan polymer and its weak crosslinking bond. However, neither the mechanism of degradation nor the behavior of the regelation has been well characterized. How the re-formability of a gel is affected by various shearing conditions and to what extent the gel will reform are two important factors in this type of treatment. Therefore, the objectives 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 the re-formability of a gel.

The laboratory experiments were carried out in three phases: (1) the initial gelation phase, simulating the solution gelling in the tank at the surface; (2) the shearing phase, simulating a mechanical degradation period through the pumping system; and (3) the regelation phase, simulating a regelation process occurring in the formation. phase, simulating a regelation process occurring in the formation. The behavior of polymer gels in the initial gelation and regelation phases was characterized by the dynamic viscosity obtained from phases was characterized by the dynamic viscosity obtained from dynamic oscillatory measurements. The mechanism of shear in the shearing phase was accomplished by various shearing devices in which shear intensity and shear time were adjustable. The effect of shear conditions on the re-formability of the gels was established by a comparison of the dynamic viscosity of unsheared gels defined in the initial gelation phase with the final gel strength after the regelation phase.

Experimental Procedures

A commercial-grade biopolymer (Flocon 4800 TM) was used for the entire study. This polymer, consisting of D-glucose, D-mannose, and D-glucuronic acid in a repeating pentasaccharide unit, is an extracellular polysaccharide produced by the microorganism Xanthomonas campestris. It was supplied by the manufacturer in the form of an aqueous broth having 5.0% solids and preserved with 2,500 ppm formaldehyde. Its molecular weight was reported as at least 1,000,000.

A 5,000-ppm-polymer stock solution was prepared by dispersing the broth in distilled water with a blender at 8,000 rev/min for 6 minutes. The polymer concentration was calculated on the basis of an assay of 5.0 wt %. The stock solution was further diluted in a 4% NaC] solution and stirred with a magnetic stirring bar for 2 minutes to reach the final working solution concentration.

The crosslinking agent, CR(III), was obtained from a solution of reagent-grade chromium (III) chloride hexahydrate (CrCI3 - 6H2O). A 2,000-ppm-chromium stock solution was prepared and aged for at least 2 days to allow the chromic chloride to hydrolyze. The various concentrations of chromium solution were prepared by diluting the aged stock solution. The pH value of the stock 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 the required amount of polymer working solution with an equal weight of chromium solution and stirring with a magnetic stirring bar for 2 minutes.

The technique applied in the field for profile control using a xanthan gel was simulated in the laboratory through three phases: (1) the initial gelation phase, simulating the solution gelling in the tank at the surface; (2) the shearing phase, simulating a. mechanical degra-dation period through the pumping system; and (3) the regelation phase, simulating a regelation process in the formation. To carry phase, simulating a regelation process in the formation. To carry out these experiments, the whole study was divided into three stages: gelation tests, shearing experiments, and regelation studies. Most of the studies were conducted at 50 degrees C, with some exceptions that will be mentioned.

A series of gelation tests was first conducted to select a gelling solution that has an appropriate gel strength and lack of syneresis for subsequent regelation studies. The gel strength, in terms of the dynamic viscosity, was determined 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 and covered by 3.0 cm3 of mineral oil to prevent evaporation. The evolution of the gel 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 amplitude of 1%, and a gap of 1 mm. Syneresis was determined by observing the phase separation of water and gel in vials. A study was also made of coneand-plate and parallel-plate geometries of two concentrations of gelling solution to investigate the effect of rheometer geometry on the reproducibility of the measurements.

Three techniques were used to shear the gels. The first method consisted of shearing the gel vigorously in a blender at various speeds for 6 minutes. The shearing was performed at ambient temperature. A controllable flow developed in the rheometer was used as a second shearing environment. The initial gel was formed inside the rheometer sample holder. Both cone-and-plate and parallel-plate geometries were used for shearing the gel. The shear scheme parallel-plate geometries were used for shearing the gel. The shear scheme shown in Fig. 1 was implemented by use of a thixotropic loop. The shear rate first increased linearly from zero to a maximum in 3 minutes, was kept constant at the maximum rate for 6 minutes, and ended in a linearly decreasing rate from the maximum to zero in another 3 minutes. The maximum shear rates used were 100, 1,000, 2,000, and 4, 000 seconds. The dynamic viscosities of gels before and after shearing were compared to determine the extent of mechanical degradation.