The Hydraulic Effect of Tool-Joint on Annular Pressure Loss

Abstract

For a successful drilling operation, downhole pressure or equivalent circulating density (ECD) control is very critical. Conventional drilling techniques require maintaining the bottomhole pressure between pore and fracture pressures. Specially, in deepwater wells, the margin between these pressures is very narrow. As a result, the bottomhole pressure and ECD must be predicted accurately and maintained within the narrow margin to avoid kicks and circulation losses. In the past years, a number of wellbore hydraulic studies have been conducted to predict the annular pressure losses. The effects of drillpipe rotation speed, borehole geometry, and pipe eccentricity on annular pressure loss have been investigated earlier. However, very limited studies have been carried to investigate the effect of tool-joint on the hydraulics. The presence of a tool-joint changes the annulus geometry between the drillpipe and casing/hole resulting in strong turbulence and fluid acceleration that generate additional viscous dissipation and pressure losses. This article presents the results of theoretical and experimental studies conducted to examine the hydraulic effects of rotating and non-rotating tool-joints. Two different tool-joint geometries were considered in the investigation. Tests were performed with water based fluids that have different rheological properties under both laminar and turbulent flow conditions. Results show substantial increase in pressure loss gradient around the tool-joints. The increase in pressure loss depends on fluid properties and flow geometries. The rotation of the pipe tends to slightly affect the pressure loss. Using dimensional analysis in conjunction with theoretical methods, new theoretical and semi-empirical models have been developed to account for the contribution of tool-joints to the total annular pressure loss. Models predictions show good agreement with previously reported laboratory experiments. The new models are very useful to predict downhole pressure in deepwater and extended reach wells where accuracy in hydraulic calculations is very important.