Thermal Stability of Starch- and Carboxymethyl Cellulose-Based Polymers Used in Drilling Fluids

Starch- and cellulose-based polymers have been used to control water loss for many years. Thermal degradation of the polymers is the most important problem with their use. Representative starch and cellulose fluid loss reducers were tested for their thermal stability in mud systems. The thermal decomposition was found to be dependent on both exposure time and temperature. The rate of decomposition can be predicted using first-order reaction rate kinetics and the decomposition activation energy estimated for both polymer types. This technique allows the calculation of a polymer's usable lifetime at a given temperature. A table of half-lives (time for fluid loss to double) vs. exposure temperature is presented for both starch- and cellulose-based polymers. This paper discusses the results of the calculations and the method used to obtain the data. The method is generally applicable to any material used in drilling fluids that is subject to thermal degradation. Introduction Starch, carboxymethyl cellulose (CMC), and their derivatives frequently are used in drilling fluids as viscosifiers and fluid-loss reducers. Their general properties are well known because they have been used for properties are well known because they have been used for many years. One important area that has been neglected somewhat is the effect of exposure to various temperatures for varying lengths of time on fluid-loss reduction. Vendor literature quotes maximum temperature limits for starch from 200 to 250 degrees F (93 to 121 degrees C). This information is useful but is not sufficient for precise work. The length of exposure to a certain temperature bears strongly on a polymer's stability. For example, a standard pregelatinized starch might have an API fluid loss of 20 cm3 after exposure at 250 degrees F (121 degrees C) for 4 hours, while after 24 hours its fluid loss is greater than 80 cm3 and after 48 hours is 240 cm3. Some data may show that starch gave an acceptable high-temperature high-pressure (HTHP) fluid loss at 275 or 300 degrees F (135 or 149 degrees C). These data can be misleading because a HTHP fluid-loss test can be completed in an hour, while long-term aging at the same temperature will destroy the polymer. Similar comments can be made about cellulosic polymers except that the temperatures stated are about 50 degrees F (28 degrees C) higher.Starch- and cellulose-based polymers degrade thermally by the same mechanism. The polymer chains are broken, and the glucopyranose units are converted to other compounds. The decomposition rate can be determined by use of chemical kinetics methods. This paper describes experiments that determined the stability of these polymers at various temperatures using kinetic methods. Starch Chemistry Starch, as used in drilling fluids, is a powder that disperses readily in water to give a low-viscosity fluid that can be used to seal microfractures and prevent fluid loss. This starch has been processed after separation from corn, wheat, rice, or potatoes. "Pregelatinization" is a cooking process that ruptures the starch granules to release the constituent starch polymer molecules. Cooking at 140 to 212 degrees F (60 to 100 degrees C) destroys the outer structure of the granule, yielding a thick slurry, much like thickened gravy. This slurry is dried and milled, giving the product used in drilling fluids. This gelatinization process was done at the rig in early applications of starch to drilling fluids. Cooking of starch at the rig ended in the late 1930's to early 1940's with the availability of pregelatinized starches. There has been some recent interest in ungelatinized starches to provide a "time-release" source of starch for fluid-loss control. Such materials would be limited to relatively hot wells [about 200 degrees F (93 degrees C)] because the march granules must be broken down to release the starch molecule for fluid-loss control. SPEJ P. 171