Research of the pulsating flow of drilling fluid in the drill string

Abstract

The object of research is the pulsating flow of drilling fluid in the drill string. One of the most problematic places is pressure loss due to friction forces distributed along the length of the flow and concentrated in its nodes (threaded joints and pipe bends).

In the course of the study, transformation methods were used that allow the drill string to be represented in the form of straight pipes – elements with distributed parameters connected by different inhomogeneities. This makes it possible to reduce the characteristics of the pulsating flow of the drilling fluid to the determination of the lumped parameters of the inclusions, the limiting conditions at the beginning and end of the drill pipes, as homogeneous sections of the drill string. In turn, pressure losses in the drill string during rotary drilling were divided into two types of losses. These are losses along the entire length of the column (flow) and local pressure losses, which are obtained only in certain places of the liquid flow (for example, tool joints, etc.), due to the fact that the flow suffers local deformation.

It has been found that from a technological point of view, the most favorable well diameter is the one at which the flow resistance in the pipes is equal to the resistance in the annulus. This is due to the fact that during the flow of the drilling fluid, the speed of the turbulent flow decreases only at the walls of the pipe. Therefore, under the action of centrifugal forces on pipe bends, as in heterogeneities, when local pressure losses occur due to separation of the transit flow, the pipe diameter narrows due to the accumulation of solid particles in whirlpool zones and flow velocities. With a smooth turn of the pipe, the specified separation may be absent. In this case, local pressure losses are largely due to the occurrence of a «steam vortex» at the turn (a helical movement caused by the action of inertial forces). Therefore, a necessary condition for rotary drilling is the continuous circulation of the flushing solution, the complete or partial cessation of which makes further drilling impossible. In this case, the drilling process slows down or leads to an accident. This is due to the accumulation of the hard phase in the places where whirlpools appear.

The research results will be useful to scientists and specialists in the oil and gas industry in the physical modeling of well flushing processes in the process of drilling and designing technological flushing processes.