Transient Effects of Time-Dependent Drilling Fluids on Wellbore Stability

ABSTRACT: Accurate prediction of dynamic pressures is essential for wellbore integrity analysis in deep water wells and wells with narrow operational windows. During the start-up of circulation, a higher circulating pressure is needed due to the gelation effect of drilling fluid. A coupled poro-elasto-dynamic model is developed to examine the effects of pump start-up on borehole stability. Firstly, a transient hydraulic model is developed to predict the temporal variation of pressure in flow conduits during the pump ramp-up periods. A constitutive equation is used to describe the time-dependent rheology of gelled fluid. The hydraulic model is then coupled with a wellbore stability model to study the effect of dynamic pressures on wellbore integrity. The method of characteristics is employed to solve the governing equations of the hydraulic model. Failure analysis is performed to evaluate the gelation effect of drilling fluid and pump ramp-up on borehole stability. The results of the hydraulic model are validated through comparison with field data and that of a published study. The results indicate that maximum circulating pressure is initially attained before approaching an equilibrium circulating pressure. A time-dependent failure can occur in the vicinity of the borehole depending on the magnitude of gel strength and pump ramp-up speed. The study concludes that by ignoring the time-dependent characteristics of drilling fluids the magnitude of circulating pressures in early times can be underestimated. Therefore, an integrated framework is suggested to consider the time-dependent effects of drilling fluids on circulating pressure and hence wellbore stability. The developed poro-elasto-dynamic model can be utilized to improve our understanding of the transient effects of drilling fluids on wellbore stability. 1. INTRODUCTION Drilling hydraulics is one of the most important factors to control the safety of drilling operations. It has received special attention in the past through downhole measurement tools to improve downhole pressure monitoring. However, it is not possible to deploy such tools in sensitive and risky areas. In addition, it is required to have a reliable hydraulic model to predict downhole pressure for design purposes. Several hydraulic models have been developed to estimate the frictional pressure losses during drilling fluid circulation under steady-state conditions (Reed and Pilehvari, 1993; Merlo et al. 1995; Mitchell and Miska, 2011). The major rheological models to describe the pressure losses in pipe and annuli include the Bingham-Plastic, Power-law, and Yield Power-law (Herschel-Bulkley) model. Drilling fluids generally exhibit thixotropic behavior due to their mixture composition (Bourgoyne et al. 1990). Nevertheless, the fluid gelation and thixotropic effect are not commonly considered in the majority of the abovementioned studies.