Abstract
Circulation subs are downhole tools designed to create an additional flowpath from the pipe to the annulus. The percentage of fluid that exits the string instead of traveling down to the bit or shoe depends not only on the size of the circulation sub ports, but also upon the density and rheology of the fluid, the flow rate, and the sub's position in the string.
This paper takes a thorough look at the variables that affect the percentage of fluid travelling the two flowpaths when using a circulation sub, along with their impact on pump pressure and equivalent circulating density (ECD). A description of the various common operations and uses of circulation subs are also discussed. The flow rate, circulation sub's position in the string, fluid density, fluid rheology, and total flow area out of the circulating sub were analyzed to determine the degree of flow split, pump pressure, and ECD changes.
Graphical representations of outputs are presented to illustrate the results of changing variables using circulating subs. The sensitivity of the flow split between fluid traveling down the string vs. into the annulus when the total flow area, depth of the circulation sub, flow rate, fluid viscosity, and density are discussed. These variables are also used to evaluate the impact on each other, with a focus on the resulting pump pressure and bottom hole ECD.
This paper seeks to disseminate information on the impact of an available tool on common downhole operations, variables which need to be understood to properly plan for these tools, and the appropriate methods for modeling using computer aided numerical analysis.
