Conceptual design and methodology for rheological control of water-based drilling fluids in ultra-high temperature and ultra-high pressure drilling applications

Abstract

A new concept for the design and development of ultra-high temperature and ultra-high pressure water-based drilling fluid (ultra-HTHP WBDF), based on particle dense packing theory and suitable for ultra-HT (205–260 °C) and ultra-HP (138–241 MPa) drilling applications is proposed in this study. The concept for the rheological control of ultra-HTHP WBDF can be concluded as that the drilling fluid would have the highest possible density and the best fluidity when the mix of weighting agent particles of variable density, size and size distribution form the closest packing. An experimental study was conducted to verify the new concept by investigating how the weighting agent particle density, size and size distribution affect the rheological properties, filtration loss and sagging characteristics of WBDF designed for ultra-HTHP drilling applications. The ultra-high density (ultra-HD) WBDFs were formulated according to this concept by using the optimum mixtures of the weighting agents, which include industrial ferroferric oxide, barite and ultrafine barite. The laser particle size analysis technique was used to determine particle size and size distribution in different drilling fluid samples. The sagging characteristics of the drilling fluids were determined by using the Static Sagging Test method. Test results of the fluids formulated according to particles dense packing theory confirmed that rheological properties, filtration loss and sagging characteristics and density limit of the drilling fluid were all significantly influenced by the variation of the particle size, size distribution and the packing of the weighting agents, which indicated that the non-structural viscosity caused by the interactions among the weighting agent particles such as friction and collision dominantly controlled the rheological properties of ultra-HTHP WBDF. Finally, a new methodology for rheological control of ultra-HTHP WBDF was proposed, which can be summarized as using all means and methodologies to drive the dispersion state of solid weighting agent particles in ultra-HTHP WBDF to be at a more discrete state. By proper selection of the weighting agent particle size and size distribution, it was possible to form a thin, dense mud cake, which was conducive to decrease the filtration loss. Moreover, results have also shown that rheological properties, filtration loss and sagging characteristics of ultra-HT-HP WBDF can be effectively controlled by using three or even more different types of weighting agents with varying density, particle size and size distribution.