determine the anchoring force required to hold plug

Setting ForceĬomposite 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.

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

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. 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. Frac Plug Anchoring Systemĭuring 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.

As with the preset event, there are several things that cause or contribute to this issue. 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.

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.

Determine the anchoring force required to hold plug