Plug & Perf: Alternative Plug Designs
In the quest to build a better mousetrap, several companies have introduced new technology designed to reduce the cost, reduce the material, and increase the efficiency of the overall plug and perf operation. While most of the design upgrades have come to the components of a traditional frac plug, maintaining the same design discussed previously, some designs differ from the traditional recipe. For the most part, these new designs shorten the overall assembly by eliminating the upper slip.
Body Lock Ring
Many downhole oilfield tools utilize what is called a body lock ring (BLR). This design enables a tool component to move in a certain direction, but then will not move in the opposite direction. This is achieved by adding a ratcheting profile on the mandrel. The BLR has a matching ratcheting profile and can expand to move up the profile. There is then a BLR housing that goes over the ring and captures in in place. The result is a component that can move downward during the setting process and then, due to the ring and ratcheting profile, cannot move back up the mandrel. In these plug innovations, this is what is used in lieu of the upper slip.
The benefit of this design is you eliminate quite a bit of length by eliminating the upper slip and replacing the load ring/upper cone with the BLR and BLR housing. This results in less material in the well and less cost to manufacture. With the BLR design there is no mandrel shift during the frac, this results in a smaller stump during mill out and could reduce mill times.
The challenge with this design is making the components strong enough to handle the forces exerted by the frac and flow back. The traditional design uses an upper slip that bites into the casing and then has interference with the upper cone as well as back pressure from the element to stay set. For this design to fail, either the hardened bite into the casing must shear or the force through the slip body must shear the slip. The first failure mode is not likely because the hardened surface of the slip and the casing itself are relatively strong compared to the composite components. The second failure mode is not likely because the total area the slip would have to shear is comparatively large.
For the BLR design to fail either the mandrel or the BLR would have to shear. The shear area on these ratcheting profiles (or wickers) is small compared to that of the upper slip. This is especially case if either of the components are made from wound composite. Wound composites are built up in layers, creating weaker areas between the layers. A failure of these layers is called interlaminar shear.
These components see force during the frac, as the pressure from frac is trying to force the mandrel downward. If the BLR does fail during the frac, there would not be an immediate cause for concern unless the mandrel fully pumped through the components. If the mandrel was designed to have a stop at the top and kept the mandrel from going through the components the frac could continue as required. The trouble would arise during any type of flowback because there wouldn’t anything holding the plug in the well when seeing pressure from below. As a result of these challenges, I have heard of providers limiting the recommended flow back rate through these plugs. This is an effort to reduce the chances of unsetting, as well as an indication that they have experienced plugs failing downhole. Many providers have abandoned this design due to these issues.
Another innovation with the BLR design is to move the BLR to the bottom. Some providers have moved the BLR and the BLR housing are to the bottom of the plug. In this configuration the plug will be set from the bottom by shifting the mandrel and components downward through the BLR. The benefit of this design is it removes the forces from the frac on the BLR. This should help with maintaining the set during flowback, because the BLR is less likely to be compromised.
Single Slip, Small Cone Angle
A few providers have eliminated the upper slip, and the traditional mandrel, but adopting a single slip and cone configuration. This design overcomes the need of an upper slip or BLR by using the slight cone angle to create a self-holding taper between the cone and the slips once set in the casing. Once set, this geometry works very well for creating an anchor in the well. There are however other challenges with this design including pump down and drill up.
A Better Mouse Trap?
Just like with mouse traps, new designs have not been able to unseat the incumbent for Frac Plug Designs. Most plugs being run follow the traditional design of upper slip, element, and lower slip recipe.
Composite Frac Plug Ebook
This information was initially published in my comprehensive composite plug Ebook. If you are interested in learning more please download it here: