A new, environmentally safe water-based polymer system has been developed for drilling applications with temperatures up to 232°C (450°F) and high pressures. The system components are newly developed synthetic polymers that do not contain chromium or other environmentally harmful materials. These new synthetic polymers are designed to perform specific functions at high temperatures and the innovative designs of these thermally stable polymers allow for the use of a minimum number of products in the formulation of high-temperature fluids.
The new system consists of two basic polymeric components for rheology and filtration control at high temperatures. High-temperature fluid formulations are greatly simplified utilizing this new system, with only the two polymeric components being required, along with a pH control additive, weight material, and small amounts of clay for filter cake quality. This simplicity is a significant advantage over "traditional" high-temperature systems, which normally require the use of a large number of additives to control or limit the effects of thermal degradation.
The new system may be formulated with fresh water or sea water, providing flexibility for a variety of drilling environments. Excellent resistance to common contaminants, such as calcium and magnesium hardness and solids accumulation, is another important characteristic of this new system.
This paper will review the previous state of the art with respect to high-temperature, water-based muds and will generically discuss the unique chemistry of the newly developed polymer system components. System formulation and application will be discussed.
The application of high-temperature, water-based fluids is traditionally based on concerns associated with the use of oil-based fluids, such as environmental compliance, logistical problems in remote locations, gas solubility in the fluids, and anticipated lost circulation zones. Although during the past several decades high-temperature, water-based fluids composed of modified natural products have been proven to exhibit effective and predictable performance,1 today's environmental and drilling performance requirements have mandated an alternative to these materials.2
For over 20 years sulfonated polyacrylic chemistries have been available for fluid loss control additives and fluid deflocculants. Since their inception, most of these polymers contain one, two, or three starting materials selected from a relatively small group of common monomers. High-temperature, water-based drilling fluids prepared with these materials often require supplemental treatment with modified natural products to achieve their necessary performance properties. The utilization of new and economically feasible monomers, along with a different approach to polymerization, allows for the preparation of new materials for use in high-temperature, water-based drilling fluids. In turn, these new fluids do not require traditional products to obtain good rheological and fluid loss control properties.
Advances in high-temperature, high pressure (HTHP) fluid loss control have been realized with a new cross-linked copolymer prepared from acrylamide (monomer A), a sulfonated monomer (monomer S), and a cross-linking monomer (monomer X). The amount of cross-linking in the polymer's structure plays an important role in its solubility,3 which is related to the property of fluid loss control. Too much cross-linking will result in a polymer that is very rigid in physical structure and difficult to hydrate in water-based fluids. Too little cross-linking produces polymers having properties similar to those of the commonly used acrylamide copolymers, whose long and linear structures are known to have limited tolerances to contamination and shear.
The new cross-linked polymer is compact and globular in morphological structure, as Fig. 1 shows. It retains a somewhat compact spherical shape in aqueous solution when compared to the expanded form of non-cross-linked linear chain polymers. When comparing the properties of this polymer to traditional linear molecules having the same molecular weight, the cross-linked polymer has a much smaller hydrodynamic volume in aqueous solution. The unique structure of the cross-linked polymer results in it being sterically hindered, increasing its intrinsic hydrolytic stability. Also, cross-linking makes it less sensitive to solids and more resistant to shear. Consequently, both rheological and filtration control properties of water-based drilling fluids are improved and preserved with the use of this unique polymer.