A dynamic model of well hydraulics in deepwater riserless drilling operations using synthetic-based drilling fluids

Abstract

The petroleum industry is contemplating drilling in water depths of 10,000 ft and possibly 13,000 ft at the turn of the century in the U.S. Gulf of Mexico, Brazil and West Africa. The current offshore technology for deepwater drilling operations requiring the use of a 21-in marine riser seems to have reached its limit at water depths about 7,500 ft. Riserless Drilling (RD) appears to be the solution for the challenge that faces the industry today. The RD concept involves utilizing a bare drillstring and a separated nonconcentric return line. A rotating Blowout Preventer (RBOP) caps the return drilling fluid and forces it to circulate through the return line to the surface. A subsea pump is designed to maintain a constant inlet pressure equivalent to the hydrostatic pressure of seawater at the sea floor. Since the use of synthetic-based muds (SBM’s) has increased over the last decade in offshore drilling operations because of their lower environmental impact, they were selected for the present investigation. The particular problem experienced when designing hydraulic programs with SBM’s is the strong pressure and temperature dependence of SBM density and viscosity. In a RD system the problem tends to be worse following the combination of high pressures with low temperatures at the sea floor. Also, gas is highly soluble in the oil phase of this type of mud. Therefore, the dissolution of gas after a gas kick is taken in a SBM may mask the surface responses and the kick shall travel undetected up to near the surface. Dangerous amounts of gas may be released over a short period of time catching the drilling crew virtually by surprise. Therefore, a hydraulic simulator was developed to generate the temperature and pressure profile during the steady-state flow of a synthetic-based mud in a riserless configuration. Also, an EOS model was calibrated with PVT data to calculate the swelling of the synthetic-based mud containing dissolved gas.