A critical review of the possible effects of physical and chemical properties of subcritical water on the performance of water-based drilling fluids designed for ultra-high temperature and ultra-high pressure drilling applications

The design and development of ultra-high temperature and ultra-high pressure water-based drilling fluids (ultra-HTHP WBDF) has become one of the key technologies to unlock deep and the ultra-deep reservoirs, as well as the geothermal resources due to the bottomhole conditions. In this paper, we reviewed the technical problems associated with controlling the stability of ultra-HTHP WBDF rheological properties, particularly for the mechanisms of how the properties of ultra-HTHP WBDF are affected by the subcritical water. A comprehensive review has shown that the physical and chemical properties of subcritical water are not conducive to the performance of drilling fluid additives. Water in subcritical state is inhibitive for additives to perform their function properly. Water at subcritical state would cause a phenomenon of thermally induced phase separation, the polarity change of subcritical water would cause some of the additives to be extracted, and the solubility of low-polar organic compounds would increase, etc. Finally, based on the analyses of the physicochemical properties of the subcritical water and its influence on the performance of ultra-HTHP water-based drilling fluids, the ideas and methodologies for molecular structure design of ultra-high temperature resistant polymers were proposed. By making the drilling fluid additives to have better compatibility in subcritical water, the new methodology would also help to develop more stable drilling fluids that can be used for the ultra-HTHP drilling fluid applications.