Evaluation of Thermal Effects on Drillability in HPHT Wells in Niger Delta

Abstract Overcoming wellbore drilling instability presents one of the major challenges to drilling engineers mostly in high pressure and high temperature wells. These are wells with reservoir pressure in excess of 10,000 psi and bottom-hole temperature exceeding 300°F, the temperature gradient between the drilling mud and the formation leads to induced thermal stresses and pore pressure responses. The aim of this work is to design an optimum mud weight at different depths and investigate the effect of temperature on wellbore stresses as it affects drillability performance. Improper mud design is one of the major causes of instability problems and also increasing temperature of the mud leads to increase in wellbore stresses which enhances the likelihood of breakouts and inhibits tensile fracture formation, and on the other hand, cooling the mud inhibits breakouts and which increases the likelihood of development of tensile wall fractures. A model was developed in this work, which was used to analyze the thermal effects on wellbore stresses and the optimum mud weight at different depth as it affect drillabilty performance. Analyses reveal that at bottom hole temperature of 300°F, the tangential stress was 7,378psi at the depth of 10,000ft. when the temperature increased to 438°F, the tangential stress was 10,025psi at the depth of 16,000ft for shale formation.