Pegasus Vertex Inc. (PVI) is pleased to introduce the first of four blogs related to circulating subs. PVI will also present this topic at the 2017 ATCE conference as SPE-187151-MS. Please follow along today as we introduce the definition of a circulating sub, general uses in the oilfield, and the numberical simulation study related to key variables affecting circulating subs’ performance downhole.
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.
These four blog articles will take a thorough look at the above-mentioned 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). We will describe the various common operations and uses of circulation subs. 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 will be shared to illustrate the results of changing variables when using circulating subs. The sensitivity of the flow split between fluid traveling down the string vs. into the annulus when changes occur in 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.
What Is a Circulation Sub?
A circulation sub is a downhole tool designed to control flow between the pipe and annulus. Once activated, the downward flow through pipe at the circulation sub location will be split. Some of the circulating fluid will flow through the sideway ports to the annulus between the wellbore and pipe, either downward or upward, depending on the operation.
One of the common ways to activate circulation through the circulating sub is to drop a ball in the drill string and pump it down. After the ball lands in the pre-designed seat in the sub, pump pressure is applied. A pin will be sheared that consequently allows the sleeve to shift and hence open the circulation ports to allow fluid flow sideway into the annulus.
The second blog article will introduce the common uses of circulating subs in the oilfield, including:
- Spotting Remediation Fluids
- Wellbore Cleanup
- Blowout Preventer (BOP) Stack Jetting
- Surge Pressure Reduction