Injectivity Improvement of Xanthan Gums by Enzymes: Process Design and Performance Evaluation

Abstract

Summary. Injectability and filterability of xanthan gum dispersions, especially in hard brines, can be considerably improved by successive use of cellulose and alkaline protease enzyme treatments. A thorough optimization of the different parameters controlling enzymatic activity has led to an original clarification process. Improvements observed in flow behavior of treated xanthan gum solutions through reservoir rocks is the result of almost complete elimination of both insoluble bacterial cells and microgels. Introduction Aqueous xanthan gum dispersions prepared either by dilution of raw fermentation broths or by dissolution of xanthan powders still contain insoluble particles and microgels. Different treatments, including, diatomaceous earth filtration or the action of protease enzynes, have been proposed to improve the limpidity and thereby the injectability of diluted xanthan gum solutions. These clarification procedures were shown to be unable to reduce the plugging, capacity of shear-deformable microgets at a very short distance from the injection well. The result is a poor sweeping efficiency of reservoir rocks by imperfectly clarified polysaccharide solutions. Plugging by microgels can be reduced by subjecting xanthan gum dispersions to the action of polysaccharase enzymes. This treatment is considered satisfactory it applied to xanthan powder dispersions containing, a limited amount of residual dead bacterial cells. The higher biomass content of raw xanthan broths renders the cellulose treatment less efficient. A synergetic effect in elimination of both microgels and insoluble particles is obtained by application of polysaccharase and protease treatments either successively or simultaneously. This paper describes the process resulting, from previous studies and enables these treatments to be performed previous studies and enables these treatments to be performed either in the field or at the factory site at the end of the biopolymer fermentation process. Performance evaluation of the enzymatic treatments on xanthan broths and Powders, respectively, was carried out by injection of the resultant solutions at high and low flow rates through both calibrated filters and natural cores. Fundamentals of Enzymatic Treatments Recent publications have shown that it is easy to hydrolyze the main chain of the xanthan molecule by polysaccharase enzymes, commonly called celluloses. As a polysaccharase enzymes, commonly called celluloses. As a consequence, glucose or reducing sugars are released in solution, and both molecular weight and viscosity diminish. Nevertheless. hydrolysis through cellulases occurs only if the xanthan molecule is in an unordered conformation. This conformation prevails at room temperature in low-salinity brines and at higher temperatures if salinity is high. Similar treatments applied to raw xanthan gum dispersions in brines with an overall salinity greater than 5 × 10 mol/L and at temperatures lower than 65 degrees C [149 degrees F] were shown to contribute to the clarification of these solutions and to the progressive microgel elimination. With this treatment. the main xanthan chain remains unaltered, the viscosity of the polymer solution is unchanged, and the xanthan gum' s injectability is improved. Treatments by cellulases performed on xanthan gum dispersions in brines exhibiting greater hardness were nevertheless found to be unsatisfactory. A synergetic treatment was therefore developed consisting of two successive enzymatic treatments, one at pH 4.5 and 50 degrees C [122 degree F] with a cellulase and the second at pH 9 and 50 degrees C [122 degrees F] with an alkaline protease. This process allows almost total microgel elimination and improvement in the filterability of the resultant xanthan gum solutions. Experimental Polymers. The main characteristics of the xanthan gums Polymers. The main characteristics of the xanthan gums tested throughout this study are listed in Table 1. Both fermentation broths and powdered-form products of various origins were used. Enzymatic treatments were applied to xanthan gum dispersions with concentrations ranging from 1 to 9 wt%. For laboratory tests. no special equipment was used to disperse the polymer; gentle stirring in the hydrolysis unit for about 2 hours before enzymes were added was found to be sufficient. For pilot tests (1 m [264 gal]), a special dispersion device was pilot tests (1 m [264 gal]), a special dispersion device was used for the powder to ensure complete solubilization of the polymer in less than 2 hours. Enzymes. Two types of enzymatic preparations that are commercially available in powder form were used throughout this study: JPT P. 835