Investigation on the effects of ultra-high pressure and temperature on the rheological properties of oil-based drilling fluids

摘要

Designing a fit-for-purpose drilling fluid for high-pressure, high-temperature (HP/HT)operations is one of the greatest technological challenges facing the oil and gas industrytoday. Typically, a drilling fluid is subjected to increasing temperature and pressure withdepth. While higher temperature decreases the drilling fluid?s viscosity due to thermalexpansion, increased pressure increases its viscosity by compression. Under theseextreme conditions, well control issues become more complicated and can easily bemasked by methane and hydrogen sulfide solubility in oil-base fluids frequently used inHP/HT operations. Also current logging tools are at best not reliable since theanticipated bottom-hole temperature is often well above their operating limit. TheLiterature shows limited experimental data on drilling fluid properties beyond 350?F and20,000 psig. The practice of extrapolation of fluid properties at some moderate level toextreme-HP/HT (XHP/HT) conditions is obsolete and could result in significantinaccuracies in hydraulics models.This research is focused on developing a methodology for testing drilling fluids atXHP/HT conditions using an automated viscometer. This state-of-the-art viscometer iscapable of accurately measuring drilling fluids properties up to 600?F and 40,000 psig. Aseries of factorial experiments were performed on typical XHP/HT oil-based drillingfluids to investigate their change in rheology at these extreme conditions (200 to 600?F and 15,000 to 40,000 psig). Detailed statistical analyses involving: analysis of variance,hypothesis testing, evaluation of residuals and multiple linear regression areimplemented using data from the laboratory experiments.I have developed the FluidStats program as an effective statistical tool for characterizingdrilling fluids at XHP/HT conditions using factorial experiments. Results from theexperiments show that different drilling fluids disintegrate at different temperaturesdepending on their composition (i.e. weighting agent, additives, oil/water ratio etc). Thecombined pressure-temperature effect on viscosity is complex. At high thresholds, thetemperature effect is observed to be more dominant while the pressure effect is morepronounced at low temperatures.This research is vital because statistics show that well control incident rates for non-HP/HT wells range between 4% to 5% whereas for HP/HT wells, it is as high as 100%to 200%. It is pertinent to note that over 50% of the world?s proven oil and gas reserveslie below 14,000 ft subsea according to the Minerals Management Service (MMS). Thusdrilling in HP/HT environment is fast becoming a common place especially in the Gulfof Mexico (GOM) where HP/HT resistant drilling fluids are increasingly being used toensure safe and successful operations.