Plug Pump Down Fluid Dynamics

Frac Plugs and any wireline BHA are being pumped down with less than ¼” clearance on each side of the equipment. Considering the dynamics of the fluid velocity around the plug, they can experience some extreme forces. Because the bypass around the plug is so much smaller than that of the casing, the velocity of the fluid around the plug is going to accelerate. The calculators above show how the velocity around the plug changes with different fall speed, in the vertical, as well as wireline speed in the horizontal. I hope this can be a resource for troubleshooting and writing procedures for your next operation. The Vertical and Horizontal situations have different considerations. Here is a little more detail on how these calculations work.

Vertical Section

As the BHA is lowered into the well, the plug encounters the first challenge. During this phase, the plug is moving downward in stagnant water.  Typically, the operator is not pumping during the vertical section of the “pump down” operation. The result is fluid moving upward against the plug.

As the wireline BHA moves through the fluid, water with the equivalent volume of the plug must move between the plug and the casing.  For example, if we assume the plug is 20” long and has an OD of 4.375”, it has a volume of .17 ft3.

If the plug moves downward 20”, .17ft3 of water will have to move around the plug/casing annulus. If we assume this is 20# casing with a nominal ID, 4.778”, the volume of the plug/casing annulus is as follows:

The annulus around the plug is 1/5 the volume of the plug. As the plug moves down through the fluid, the velocity of the water around the plug is 5x the velocity of the plug.

 At a 350 ft/min fall rate the velocity of the fluid around the plug will be 1,750 ft/min or 30 ft/s. Depending on the design of the plug, this could be a concern for the components if the fall rate is increased drastically. You can see this by choosing these same variables 5.5” 20-lb casing and a 4.375” plug. If you highlight the nominal casing size at 350 ft/min it shows a 30 ft/s speed around the plug.

Horizontal Section

As the well becomes more horizontal the pumps will start to pump the wireline BHA along the horizontal section of the well to the specified location. 

Let us consider, for example, the dimensions of a 5.5” 20# pump down operation:

If you run the numbers, pumping 15 bbl/min into 5.5” 20# casing will result in a fluid velocity of 675 ft/min. If, at this flow rate, the plug is moving at 400 ft/min, the velocity passing the plug is 1,711 ft/min or 28.51 ft/s. As you can see, the velocity of the fluid past the plug is much higher because the bypass is only ~10% of that in the entire casing ID. If the pump rate holds steady and the speed of the plug slows by 1 ft/min the velocity passed the plug increases by over 6 ft/min. These higher velocities will create a low pressure around the plug. With lower pressures on the OD of the plug if there are any material, such as the rubber or elastomers used in the seal, they can respond to the lower pressure causing them to swab or swell. The swelling of the elements can cause them to contact the casing and cause a snag in the operation.

This becomes a larger concern as operators want to push the boundaries of plug speed. As plugs are traveling faster with higher flow rates, relative changes in plug velocity have a bigger impact on fluid velocity around the plug. The result is greater risk of issues as you increase the velocity of the plug.

 Understanding the dynamics of the operation and how different plug designs can affect pump down efficiencies, risk level, speed, and overall success is important for making the right decisions for your well.