Chemical Characterization of CMC and Its Relationship to Drilling-Mud Rheology and Fluid Loss

Abstract

Summary Four carboxymethyl cellulose (CMC) polymers were characterized by molecular weight, degree of substitution, and intrinsic viscosity. These polymers were used to make simple water-based muds with various polymer and bentonite contents. API fluid loss and high shear viscosity were determined for each mud. Fluid loss is independent of polymer molecular weight at low ionic strength. The high shear viscosity of muds and polymer solutions is related to the product of the intrinsic viscosity of the polymer and its concentration. Introduction Growing concern amongst government and environmental agencies over the environmental impact of oil-based drilling fluids1,2 has increased the drilling industry's reliance on water-based muds. An important aspect of water-based muds is the design and testing of water-soluble polymers to control the main mud functions: rheology, fluid loss, and shale stabilization. Both naturally occurring and synthetic polymers ranging from low-molecular-weight dispersants (e.g., lignosulphonates3,4) to high-molecular-weight polymers for shale stabilization (e.g., partially hydrolyzed polyacrylamides5–7) have been used extensively in water-based muds.3,8 Several recent papers9–11 describe the range of water-soluble polymers and their functions in water-based drilling fluids. The most common naturally occurring polymers are the polysaccharides, which include CMC, starches, xanthan gum, and guar gum. CMC polymers are probably the most common and are used routinely both to control fluid loss and to increase viscosity. Interest has increased in synthetic polymers that extend temperature and salinity/hardness limits of naturally occurring polymers. A recent example is the development of a sulfonated copolymer12 for fluid-loss control in drilling fluids subject to high temperatures and high calcium concentrations. Despite the growing potential and extensive use of polymers in water-based muds, they often are characterized poorly in terms of their basic compositions, average molecular weight, molecular-weight distribution, charge density (and charge distribution), and, particularly in the case of high-molecular-weight polymers, the frequency of long-chain branching. Polymers have long been used sucessfully in water-based muds; however, the influence of polymer Composition on mud properties often is unclear. For example, understanding of the mechanisms that allow shales to be stabilized by both high-5–7 and low-13 molecular-weight polymers has been inadequate. Another problem has been the paucity of reliable techniques to monitor polymer concentration and molecular-weight degradation in the field. The increasing use of polymers is expected to emphasize these problems. The objective of this paper is to establish the influence of molecular weight and charge density (degree of substitution) of a number of well-characterized CMC samples on the rheology and fluid loss of bentonite-based drilling fluids. CMC Characterization Composition and Impurities. Fig. 1 shows the chemical structure of cellulose and. CMC polymers. Cellulose is composed of repeating units of D-glucopyranosyl with a 1,4 glycoside linkage. Cellulose is modified to form the sodium salt of CMC by reaction with monochloroacetic acid in the presence of caustic soda 14:R(OH)3+ClCH2CO2H++2NaOHR(OH)2OCH2CO2Na+NaCl+H20 Each repeating D-glucopyranosyl unit contains three hydroxyl groups capable of etherification, to give a maximum charge density of three sodium ions per monomer unit (i.e., a degree of substitution of three). Ho and Klosiewicz15 used nuclear-magnetic-resonance (NMR) spectroscopy to demonstrate that the three hydroxide sites are not equally active in the etherification reaction; the order of reactivity is 2 > 6 > 3 (see Fig. 1). The addition of ionic groups to cellulose produces a water-soluble polymer that is used widely in many industries.16 CMC is water-soluble when the degree of substitution is greater than 0.414; the most common degree of substitution range for industrial CMC is 0.4 to 0.8.14,16 The most common form of CMC is the sodium form, where the carboxylate anion is balanced by a sodium counter-ion. A recent European patent application17 described the use of potassium CMC in drilling fluids. Table 1 shows the analysis of several CMC products, including a "polyanionic cellulose" (PAC). The CMC was dissolved in water (10 g/L) and acidified with an equal volume of 0.25 M sulfuric acid (providing a 10-fold excess of protons in solution over sodium from the polymers) to generate the acid form of the polymer and to release the sodium into solution. Some of the polymer precipitated and was. removed by centrifugation. The anion and cation content of the supernatant was determined by ion chromatography, and the ion concentrations were reported as moles per gram of polymer. The technical oilfield-grade CMC samples generally were characterized by high (up to 20 wt %) NaCl content. The NaCl, a byproduct of manufacturing, therefore has been only partially removed. In contrast, the laboratory-grade samples are almost free of NaCl. The pH's of the various CMC solutions (Table 1) indicated that residual acids and alkalis also may have been contaminants; again, the technical oilfield-grade solutions showed the greatest degree of contamination. Also, CMC is hygroscopic, and a significant fraction of its weight may be water. Measurement of weight loss after drying indicated that the water content ranged from 5 wt % to 16 wt %; equilibrium moisture content from 20 wt% to 30 wt% for CMC polymers with degrees of substitution of 0.7 to 1.2 at 80% relative humidity have been reported.16 The measured degree of substitution for the oilfield CMC products ranged from 0.80 to 0.96, compared with reported values ranging from 0.7 to 0.917,18; our PAC sample, with a 1.00 degree of substitution, was in agreement with the range of 0.9 to 1.5 that has been reported.17–19 The laboratory reagent-grade samples generally have lower degrees of substitution (0.71 to 0.83) than the oilfield grade samples but contain lower impurity levels. Two samples of potassium CMC, prepared as in Ref. 17, also were analyzed; the measured degree of substitution for these polymers was in good agreement with published values.18 Composition and Impurities. Fig. 1 shows the chemical structure of cellulose and. CMC polymers. Cellulose is composed of repeating units of D-glucopyranosyl with a 1,4 glycoside linkage. Cellulose is modified to form the sodium salt of CMC by reaction with monochloroacetic acid in the presence of caustic soda 14:R(OH)3+ClCH2CO2H++2NaOHR(OH)2OCH2CO2Na+NaCl+H20 Each repeating D-glucopyranosyl unit contains three hydroxyl groups capable of etherification, to give a maximum charge density of three sodium ions per monomer unit (i.e., a degree of substitution of three). Ho and Klosiewicz15 used nuclear-magnetic-resonance (NMR) spectroscopy to demonstrate that the three hydroxide sites are not equally active in the etherification reaction; the order of reactivity is 2 > 6 > 3 (see Fig. 1). The addition of ionic groups to cellulose produces a water-soluble polymer that is used widely in many industries.16 CMC is water-soluble when the degree of substitution is greater than 0.414; the most common degree of substitution range for industrial CMC is 0.4 to 0.8.14,16 The most common form of CMC is the sodium form, where the carboxylate anion is balanced by a sodium counter-ion. A recent European patent application17 described the use of potassium CMC in drilling fluids. Table 1 shows the analysis of several CMC products, including a "polyanionic cellulose" (PAC). The CMC was dissolved in water (10 g/L) and acidified with an equal volume of 0.25 M sulfuric acid (providing a 10-fold excess of protons in solution over sodium from the polymers) to generate the acid form of the polymer and to release the sodium into solution. Some of the polymer precipitated and was. removed by centrifugation. The anion and cation content of the supernatant was determined by ion chromatography, and the ion concentrations were reported as moles per gram of polymer. The technical oilfield-grade CMC samples generally were characterized by high (up to 20 wt %) NaCl content. The NaCl, a byproduct of manufacturing, therefore has been only partially removed. In contrast, the laboratory-grade samples are almost free of NaCl. The pH's of the various CMC solutions (Table 1) indicated that residual acids and alkalis also may have been contaminants; again, the technical oilfield-grade solutions showed the greatest degree of contamination. Also, CMC is hygroscopic, and a significant fraction of its weight may be water. Measurement of weight loss after drying indicated that the water content ranged from 5 wt % to 16 wt %; equilibrium moisture content from 20 wt% to 30 wt% for CMC polymers with degrees of substitution of 0.7 to 1.2 at 80% relative humidity have been reported.16 The measured degree of substitution for the oilfield CMC products ranged from 0.80 to 0.96, compared with reported values ranging from 0.7 to 0.917,18; our PAC sample, with a 1.00 degree of substitution, was in agreement with the range of 0.9 to 1.5 that has been reported.17–19 The laboratory reagent-grade samples generally have lower degrees of substitution (0.71 to 0.83) than the oilfield grade samples but contain lower impurity levels. Two samples of potassium CMC, prepared as in Ref. 17, also were analyzed; the measured degree of substitution for these polymers was in good agreement with published values.18