The superior performance of invert emulsion fluids in challenging drilling operations such as HT/HP wells makes them the preferred fluid for such applications. However, there are instances such as drilling in environmentally sensitive areas, or where costs and logistics become prohibitive, where their use is undesirable. In such circumstances, a high-performance water-based drilling fluid is the only option available. A main challenge in developing a water-based fluid for such applications has been to maintain the stability of key fluid properties such as rheology and fluid loss at temperatures in excess of 130°C. Conventional additives, such as biopolymers or synthetic polymers, either become ineffective at such high temperatures or generate too high a rheology when used in quantities needed to curb fluid loss. This paper discusses a novel water-based drilling fluid system in which the synergy of the key fluid components produces a stable rheology and low fluid loss at temperatures approaching 200°C. Furthermore, by balancing the components of the novel system, fluid rheology can be controlled to meet the hydraulics and hole-cleaning demands of the drilling operation.1, 2 Introduction Today, an increasing number of drilling scenarios involve deeper and hotter reservoirs, complex well geometries and increasingly more stringent environmental requirements. These call for a new generation of high performance water-based drilling fluids, which can offer better drilling hydraulics (pressure losses and cuttings transport), high-temperature rheological stability, fluid loss control, shale stabilisation and lubrication. From a rheology point of view, the best fluid systems available to date are those that offer a high degree of shear-thinning to reduce friction losses and improve hole cleaning. These are based typically on biopolymers (e.g. xanthan and scleroglucan gum) or certain organic mixtures. While the most effective polymers for rheology control are those with high molecular weight (millions of Dalton), lower molecular weight polymers, frequently used for fluid loss control, can also have an effect on rheology. Thus, historically, control of rheology and fluid loss has required a fine balance between these two types of materials. At temperatures up to 130ºC biopolymers and organic mixtures are effective in controlling the rheology, but at higher temperatures stability problems set in and synthetic polymers must be used. Synthetic polymers, mainly polyelectrolytes, have good temperature stability but generate high plastic viscosities. This problem, which may be intensified by the high solids fraction of heavier drilling fluids, can lead to large pressure losses in narrower well sections. The conventional approach for generating and controlling drilling fluid rheology utilises various additives for the purpose of supporting and carrying the weight material and drill solids. To overcome the limits presented by such additives - biopolymers (temperature stability) and polyelectrolytes (poor shear-thinning and high plastic viscosity) - a different approach is needed. In the new approach described here, the polymers and the weight material are selected such that they both contribute to the generation and control of a highly shear-thinning, thermally stable rheology. Such improvement results from a synergistic interaction between the polymers and the weight material and requires polymers with: moderate molecular weight (to avoid excessive plastic viscosity), low charge density (charge on polymers increases backbone rigidity with impact on plastic viscosity; polyelectrolytes are more sensitive to salts, drilled solids and cement contamination) and stability at high temperatures (up to 180°C). Polymers that satisfy the above criteria produce relatively low viscosity if used on their own, which may not be adequate for suspending the weight material and for cuttings transport. However, when combined with a weight material with a specific surface charge, they produce highly shear-thinning aggregates with good suspending capacity. Figure 1 illustrates the conventional way of generating viscosity (1a) and the new approach (1b).

fax 01-972-952-9435. AbstractThe superior performance of invert emulsion fluids in challenging drilling operations such as HT/HP wells makes them the preferred fluid for such applications. However, there are instances such as drilling in environmentally sensitive areas, or where costs and logistics become prohibitive, where their use is undesirable. In such circumstances, a highperformance water-based drilling fluid is the only option available. A main challenge in developing a water-based fluid for such applications has been to maintain the stability of key fluid properties such as rheology and fluid loss at temperatures in excess of 130°C. Conventional additives, such as biopolymers or synthetic polymers, either become ineffective at such high temperatures or generate too high a rheology when used in quantities needed to curb fluid loss. This paper discusses a novel water-based drilling fluid system in which the synergy of the key fluid components produces a stable rheology and low fluid loss at temperatures approaching 200°C. Furthermore, by balancing the components of the novel system, fluid rheology can be controlled to meet the hydraulics and hole-cleaning demands of the drilling operation. 1, 2