In Europe where environmental regulations prohibit the use of oil-based mud, high-temperature wells are drilled with HPHT water-based fluids. Chrome-lignosulfonates are a common component in high-temperature water-based fluids, acting as effective dispersants and conferring excellent fluid-loss control and rheological properties. However, tightening of regulations signals the emergence of restrictions on the use of chrome-based products in water-based fluids. Elsewhere in land operations where currently oil-based fluids are used for high-temperature applications, a move towards HPHT water-based fluids, and ultimately to chromefree fluid systems is thought to be inevitable. Thus, alternative chrome-free products are desired that will perform at least as well as the current products in high-temperature, high-density water-based fluid systems.Fluid-loss control in high-temperature environments can become problematic due to the degradation of many polymers at high temperature. A main challenge is to use a high-temperature synthetic polymer that can control fluid loss without a significant impact on rheology. High-temperature gelling is another problem that may confront high-density fluid systems and lead to significant problems during the drilling operation. This must be prevented by using effective, temperature-stable dispersants.This paper describes the development of a new chrome-free, high-density HPHT water-based fluid system. The new fluid uses a combination of clay and synthetic polymers to provide excellent fluid-loss control and to generate thermally stable rheology. The paper presents the results of extensive testing to show that the use of highly efficient dispersants prevents high-temperature gelling and improves fluid resistance to drill solids contamination. The authors will also present results from an initial field testing of the fluid system. IntroductionEnvironmental considerations have led to increasing interest in the use of water-based drilling fluids (WBM) in applications where oil-based fluids have previously been preferred. In Europe, for example, where environmental regulations prohibit the use of oilbased mud, high-temperature wells are drilled with HPHT water-based fluids. The least expensive and most widely used waterbased fluids for such applications are dispersed muds made up with bentonite clay. The low-colloid version of such fluids uses small amounts of clay for filtercake quality. To improve the rheological stability and fluid-loss properties of these fluids at elevated temperatures, chromium-containing thinners and fluid-loss additives have been used. These additives, however, are increasingly subject to environmental regulation.The environmental constraints have created the need for a chrome-free drilling fluid additive with cost and performance characteristics similar to those of chromium-based additives. One option has been to prepare lignosulfonate additives complexed with other metal ions, e.g., Fe, Ti and Zr. Park (1988) reported that the mixed titanium/zirconium lignosulfona...