Study of Thixotropic Behavior of Non-Newtonian Fluids in Tool-Joints of Oil Wells

Abstract

Abstract The prediction of frictional pressure loss in many oil wells drillings is of the utmost importance. Most studies in this area are concerned with fluid flow in pipes and annulus to predict friction losses. However, studies on the tool-joint effect in frictional pressure estimations were limited to only a few experimental and theoretical studies on water-based power-law and Herschel-Bulkley drilling muds. Tool-joint is the piece that connects two drill strings. The tool joint plays an important role in extending the drill pipe, and its effect on pressure losses cannot be ignored. In addition, the drilling muds must have certain qualities such as providing formation integrity, transporting the cuttings from bit to surface, drilling pipe lubrication and heat transfer, and low-pressure loss to perform as a qualified and effective drilling fluid. Therefore, drilling muds have become more complex and expensive. For this reason, oil-based viscoplastic drilling muds are being used in more drilling operations. These drilling muds almost always also show a thixotropic behavior which is the key point of this study. This paper studied the behavior of fluid flows and frictional pressure losses in weld-on tool-joints of different grades using CFD simulations. The simulations were conducted for a drilling fluid with viscoplastic (similar to the Herschel-Bulkley model) and thixotropic behavior. The characteristics of fluid flows in tool joints under different thermal boundary conditions were studied. Moreover, the different thixotropic behavior models and various thixotropic boundary conditions were compared to select the most fitting model. The results showed that tool joints significantly affect the frictional pressure loss in the annulus. The study also provided an accurate pressure loss prediction for the flow of oil-based viscoplastic muds passing through the tool-joint geometry. Furthermore, using the thixotropic behavior of the Moore-Cheng model for the drilling mud, the transient processes of the beginning of the drilling operations were investigated.