Rheology of drilling fluids is usually evaluated without taking wellbore pressure and temperature effects into consideration. In most instances these evaluations produce fairly accurate results for shallow wells owing to the usually very small difference between surface and subsurface bottom-hole temperatures in shallow wells. As such, the rheology of the drilling fluid does not change substantially while drilling wells of these types. However, in wellbores where bottom-hole pressures and temperatures are well above the average pressure and temperature, especially in deep and ultra-deep wells, significant changes in drilling fluid rheology may result. A classic example of this scenario is common in HPHT drilling environments. In these difficult and challenging environments pore pressure gradient could be in excess of 0.8psi/ft and bottom hole temperature (BHT) in the range of 150°C to 300°C or even greater.

Invert emulsion drilling fluids have reportedly been used to drill HPHT wells with temperature of up to 260°C with little problems or challenges. Nevertheless, recent HPHT activities have presented more difficult challenges with bottom hole temperature approaching 316°C and pore pressure in the neighbourhood of 20000 psi. These challenges have in turn presented the need to adopt a further innovative approach to drilling fluid management under this extreme condition.

This paper presents a numerical investigation of application potentials of molybdenum oxide as a rheology control additive in drilling fluids for more challenging HPHT well drilling. Simulations were carried out using Ansys Fluent CFD™ software whilst analysis and interpretation of results was carried out with the aid of Tecplot Visualisation™ Software.

The Simulations replicated the real life flow scenarios for both concentric and eccentric annuli under bottom-hole conditions of HPHT; and rotating (dynamic) and non-rotating (static) drill pipes.

Application of Molybdenum as a drilling additive for all the simulated scenarios resulted in improved effective drilling mud viscosity, supporting the conceptual propposition that molybdenum oxide can serve as a good additive for drilling fluid rheology control and maintenance in HPHT environments.

You can access this article if you purchase or spend a download.