The prediction of frictional pressure loss in many oil wells drillings is of the utmost importance. Most studies in this area concerned fluid flow in pipes and annulus in order 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 drilling muds. (Tool-joint is the part in which two drill strings connect where the annulus has a gradual decrease and increase, respectively). The tool-joint plays an important role in extending the drill pipe, and its effect on pressure losses cannot be ignored. As of today, drilling muds must have certain qualities such as providing formation integrity, transporting the cuttings from bit to surface, drill 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, and the process of choosing appropriate drilling muds is of great importance. For this reason, oil-based viscoplastic drilling muds are being used in more drilling operations, but for these drilling muds, almost no studies were conducted to predict the effect of the tool-joints on flow behavior and pressure loss.

In this paper, the behavior of fluid flows and frictional pressure losses in weld-on tool-joints of different grades according to the IADC standard manual was studied using CFD simulations. The simulations were conducted using a drilling fluid with viscoplastic properties, following the Herschel–Bulkley model of Non-Newtonian fluids. And the characteristics of fluid flows in these tool-joints under different thermal boundary conditions were studied. The results show that tool-joints significantly affect the frictional pressure loss in the annulus. We also provided an accurate pressure loss prediction for the flow of oil-based viscoplastic muds passing through the tool-joint geometry. Furthermore, assuming the thixotropic behavior using the Moore-Cheng model for the drilling mud, the results showed that the flow behavior does not differ significantly from the Herschel-Bulkley drilling fluid in the range of study.

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