In my previous article, here, I highlighted presets as the number one operational issue that customers are concerned about when selecting a composite frac plug. A preset during the deployment of a frac plug can have a major impact on the schedule for delivering a well to production. The second concern that most operators are curious about is slipping, or for the purposes of this article skid. A skid happens after a plug is set, the stage is perforated, and the frac begins. As the plug sees pressure something happens to cause it to fail and slide downhole. This event is typically indicated on the frac charts during the frac and then confirmed during mill up. The concern for the operator is that the frac they've designed for a certain zone is now being pumped, at least partially, into the zone below. As with the preset event, there are several things that cause or contribute to this issue.

Frac Plug Anchoring System

During the frac, a composite plug must perform two functions: Anchor & Seal. The component on the plug that creates the anchor is called, ironically, the slips. The slips bite into the casing to hold the plug in place. The seal is created by a compressed element. A plug skid event is caused when the anchoring system is compromised in some way.

To understand how the anchoring system can be compromised, let's take a detailed look at how they work. During the setting sequence, the setting tool applies a force to the plug causing the element to compress and then the slips to ramp up their respective cone until the hardened material on the slip is forced to bite into the casing. Once the plug becomes locked into the casing, the setting tool force will increase, ultimately exceeding the shear value of the plug’s shear media. This will cause the shear media (shear screws, shear thread, or shear ring) connecting the plug to the setting tool to fail, releasing the setting tool from the plug. For more information on how the setting tools work you can read a few articles here and here. Each slip, upper & lower, are designed to hold pressure in opposite directions. For a traditional frac plug, the lower slip prevents the plug from moving down the well during the frac. The upper slip maintains the setting pressure into the element system prior to the frac and keeps the plug from moving up-hole.

Anchor

The slip system, whether a ring of material or individual segments held together are designed to anchor inside the inside diameter (ID) of a casing tubular. The goal when the slips set into the casing is to distribute the load evenly through each slip individually around the outside diameter (OD) of the plug. The slips have hardened components designed to penetrate the ID of the casing and lock the slip in place. The slips must bite into the casing 0.03-0.05”, which is equivalent to the thickness of only 10 sheets of paper. This bite is critical to the plug’s ability to stay put while seeing high pressures. When a plug skids, it is due to a failure of the plug to generate a bite or its inability to maintain the bite.

Generating a Bite

Several factors can affect the plug’s ability to generate a bite into the casing wall.

Setting Force

Composite plugs are largely set using a wireline setting tool. These tools utilize a power charge to generate pressure through combustion. This pressure is then converted to mechanical energy to set the plug. While these tools set the plug, each plug has some sort of shear media that controls the amount of force exerted on the plug. This shear media is designed or shear at a specific force. These will be designed to shear at a value lower than the max force exerted by the setting tool, but high enough to fully set the plug. A fully set plug will have a compressed element and slips that have bit into the casing .03”-.05”. If the setting force is too low, it will limit the anchor system’s ability to bite into the casing. Setting force too low can be caused by shear media that doesn’t meet the specification, causing it to break too early. Another, cause of setting too low is due to installation issues. If the plug is installed with the incorrect number of shear media it can limit the plugs ability to anchor as well.

Casing Roundness

If the load is not distributed evenly, this can increase stresses on the slip or cones in a way that could compromise the performance of the plug. Another cause of uneven force distribution through the slips is setting the plug in out of round or oval casing. If the casing is out of round by only .06”, or just 1%, the slips in the minor ID will become fully set while the slips in the major ID could be just barely touching the casing.

Casing Hardness

The slips have hardened components designed to penetrate the ID of the casing and lock the slip in place. Casing generally has a hardness rating of 28-36 HRC depending on the material and yield strength. Though, hardness is not a property that is outlined in the API spec requirements for casing. This makes finding the exact hardness of different casings difficult. Most slip designs have hardened areas with 60+ HRC to ensure that the slip is able to penetrate that of the casing. For metal slips, this hardness is achieved through a heat-treatment process. For slips with other materials, the material specifications provide the hardness. If the casing happens to be too hard or a manufacturing issue causes the material of the slip to be softer, the anchoring system can be compromised.

In any of these scenarios the slip will not bite deep enough into the casing. This will reduce the size of the shear plane through the slip and casing, increasing the chance that the anchor will fail. As you can see in the image below, when a slip bites into the casing deeper it creates a larger shear plane through the slip. This larger shear plane increases the anchoring force of the slip by increasing the amount of material that would have to break to cause a failure. The deeper the bite, the larger the shear plane, and the stronger the anchor.

Maintaining the Bite

Lower Cone

The slips must be supported by the casing by biting into its surface. On the other side, the slip must be supported by the cones to lock them into the casing. If the cones become compromised, eliminating this support, the bite into the casing won’t matter. The failure of the cones is mostly caused by a collapse. When the high pressure is applied from the frac, the forces are distributed into the lower slips and cones. For 5.5” plugs at 10,000 psi differential this force is equivalent to ~200,000 lbs. This weight, or force, is distributed around the slips into the casing. And, with the force into the casing there is an equal and opposite force directed into the cones. The cones have to be designed to hold up to these forces without collapsing.

The Mandrel

While the cone supports the slips, the mandrel supports the cones. If the cone isn’t strong enough to support the full pressure 100% then the mandrel must be able to make up the difference for the plug to perform. One way the mandrel can fail is to break in a way that allows the mandrel to move through the components. If the pressure becomes too high or the plug is compromised in some way it can cause the mandrel to be pushed through the ID of the components. Another cause of the mandrel being pushed through the components is when an operator lands a ball at too high of a rate. The recommendation is 10-15 bbls/min, however if the ball lands sooner or the operator allows it to land at higher rates it can cause a water hammer affect that spikes the forces on the mandrel and causes a failure. This can cause a slipping event as well.

One of the largest challenges for the mandrel is the pressure and forces exerted by the element. When the plug is set, the seal is created by compressing the rubber element between the ID of the casing and the mandrel. This compression exerts a force onto the mandrel before the plug sees any pressure. Once the pressure is applied the differential pressure boosts this force increasing the force on the mandrel above just the hydraulic forces caused by the differential pressure. If this higher-pressure zone causes the mandrel to collapse it can compromise the entire mandrel, causing a slip event.

The Seal

The failure modes discussed above deal with a mechanical failure caused by forces. The seal can also play a role in a plug slip failure. If, during the frac, the seal becomes compromised allowing frac fluid and sand to bypass the plug, this bypass can cause erosion. The erosion past the seal and slips could compromise the slips ability to hold pressure and cause an issue.

Slip Failure

If the load is not distributed evenly, this can increase stresses on the slip or cones in a way that could compromise the performance of the plug. Another cause of uneven force distribution through the slips is setting the plug in out of round or oval casing. If the casing is out of round by only .06”, or just 1%, the slips in the minor ID will become fully set while the slips in the major ID could be just barely touching the casing.

The slips have hardened components designed to penetrate the ID of the casing and lock the slip in place. Casing generally has a hardness rating of 28-36 HRC depending on the material and yield strength. Though, hardness is not a property that is outlined in the API spec requirements for casing. This makes finding the exact hardness of different casings difficult. Most slip designs have hardened areas with 60+ HRC to ensure that the slip is able to penetrate that of the casing. For metal slips this hardness is achieved through a heat treatment process. For slips with other materials the material specifications provide the hardness. If the casing happens to be too hard or a manufacturing issue causes the slips material to be softer, the anchoring system can be compromised.

Once set, the anchoring system relies on the slip itself to lock in the casing, the lower cone to support the slip and maintain this lock, and the mandrel to provide further support. Each one of these components is important to maintaining the anchor during a high pressure frac. If any of these components or the composites that make up the component fail it will cause a skid event.

Plug Selection

Understanding the operation of and the failure modes associated with the plug you select for your well can go a long way towards diagnosing any issues encountered in the field. If there is anything I can do to help you understand completion tools and how they work, please do not hesitate to reach out.

Plug & Perf Issues: Plug Slipping