Vinyl Sulfonate Copolymers for High-Temperature Filtration Control of Water-Based Muds

Abstract

Summary This paper describes the properties of two synthetic high-molecular-weight vinyl sulfonate copolymers and their use in water-based muds for controlling high-temperature filtration properties. Vinyl sulfonate copolymers are more effective at high temperatures because they are not depolymerized by the hydrolytic and oxidative environments of the drilling mud, as are derivatives of polysaccharides, and they do not form insoluble salts in the presence of electrolytes. Laboratory tests show that these copolymers are effective filtration-control agents for water-based muds over a wide range of electrolyte concentrations and that they remain effective after aging at 350deg.F [177deg.C]. The effectiveness of these copolymers has been demonstrated in field tests. Introduction Oil-based muds have received considerable attention and enjoyed increasing popularity for drilling oil and gas wells because they offer the user a fluid with stable rheological and filtration properties at elevated temperatures. The oil-based fluid enhances borehole stability, corrosion control, and lubricity. The combination of the polycrystalline diamond bit and the relaxed filtration muds has allowed one to drill as fast with these fluids, particularly in shales, as with water-based systems. The introduction of the low-aromatic mineral seal oil as a replacement for diesel oil has reduced the ecological concerns associated with accidental spills. Wider use of oil-based fluids, however, is curtailed by certain factors, which include the various environmental restraints placed on them. In offshore and inland waters, oil muds may be used but not discharged into the water regardless of the oil type. Cuttings may be discharged provided they have been cleaned of oil; however, provided they have been cleaned of oil; however, cuttings-wash systems are expensive and sometimes ineffective, which can lead to costly seabed cleanup operations. Nance states that oil muds should be avoided in areas prone to lost-circulation problems if the mud density must prone to lost-circulation problems if the mud density must be maintained near the fracture pressure of the exposed formation. In directional holes larger than 12 in. [30 cm] in diameter in which a fast penetration rate is expected, he recommends not using the oil mud if other options exist. The industry must have available other fluid systems if it is to drill the deep, expensive wells of the future. No one product is necessarily the answer to improved fluids. A total-systems concept consisting of several products must be integrated to meet the changing demands on the fluid during the course of drilling the well. These systems will require a series of new materials, each with a particular function, to be used when needed. particular function, to be used when needed. Researchers have looked continuously at improving the thermal stability of water-based muds. In 1973, a patent was granted that covered the application of a low-molecular-weight sodium polyacrylate for stabilizing the rheology of aqueous muds at elevated temperatures. Earlier that year, Perricone and Youngs were granted a patent covering the use of a sulfonated styrene/maleic anhydride copolymer for stabilizing the rheology of waterbased muds at high temperatures. The primary advantage of the latter copolymer was its increased resistance to cement and calcium ion contaminations. Improved mechanical equipment and greater emphasis on solids removal have contributed to better fluid systems. A reduction in the quantity of bentonite added to a system has aided in the control of high-temperature flow properties. Filtration control of water-based systems, particularly above 300F [149C], has received the attention of many investigators who attempted to develop new products to improve control of this property. The natural polymers commonly used as filtration-control agents are not effective at polymers commonly used as filtration-control agents are not effective at the higher bottomhole temperatures (BHT's). The starches and cellulose derivatives are thermally degraded by oxidation and hydrolysis reactions, producing lower-molecular-weight byproducts that cause a loss of viscosity producing lower-molecular-weight byproducts that cause a loss of viscosity and filtration-control effectiveness. The rate of degradation depends on the fluid, dissolved oxygen, pH of the mud system, and time of exposure to the elevated temperature. Thomas recommends that starches not be used routinely above 225F [107C] nor carboxymethyl-cellulose-based products above 300F [149C]. Lignitic products are used for filtration control in high-density water-based muds. Upon neutralization with caustic soda, lignite produces a water-soluble salt with some remaining caustic-insoluble residue. SPEDE p. 358