Plug & Perf Issues: The Preset Event

When meeting with customers, one of their main concerns about running a new composite frac plug is how many presets it has had or what has been done to limit the likelihood of a preset event. The concern is valid since a preset event can lead to NPT, require an intervention, and generally lead to higher costs of an operation. Based on these questions I thought it would be valuable to go over the different scenarios that can cause a preset, some of which will be the plug’s fault and others due to outside factors.

A preset event happens during the pump down operation, resulting in the plug becoming stuck in an undesired location. It helps to understand how the components of a plug work to recognize what causes such an event.

Frac Plug Construction

A frac plug becomes lodged into the casing when its slips are forced up the corresponding cone to contact the casing. The slips are designed to set in one direction and then limit the plug’s ability to move opposite direction once set. Most slips are designed to break apart as the cone is forced under them. The slip is either a solid ring of material that is designed to break or are individual segments that are held together with a shear-able band. If the slip experiences forces, either from an outside body or from the cone, greater than the designed break force it will become free to slide up its cone and contact the casing.

There are several things that can occur during the pump down operation to cause the slips to ride up their cone and lodge in the casing.

Setting Tool

The setting tool used to set the frac plug is designed to carry the plug to its desired location. Once ready, a signal is sent down the wireline to ignite a charge that actuates the setting tool and sets the plug. The charge burns to create a pressure against a piston, that then applies force to the plug setting it in place.

If pressure from the wellbore can act on the internal pistons, the setting tool will stroke prematurely. This is caused by failure of or missing o-rings in the tool. If the tool strokes, the plug will be come preset in the casing. In this case, the force from the wellbore won’t be enough to fully set and shear off the tool. The company will then fire the tool to complete the setting sequence. They will then have to remediate the well to remove the preset plug.

If the missing or failed o-rings are in the gas chamber of the setting tool. Firing the tool will be ineffective and the setting tool and frac plug will have to be fished from the well. Ensuring that your service company is properly redressing their setting tools each time will help to reduce the chances of these events.

This is the cause of most preset events. If you’d like to know more about how the setting tools work, I’ve written the following articles: Baker Style Setting Tool and Owen Style Setting tool.

Debris

When pumping a plug down there is a very small gap between the OD of the plug and the ID of the casing. For instance, in the ID of 5.5” 20# casing is 4.778”. The OD of most plugs is 4.375”. This leaves only a max of .2” on each side of the plug. If there is any debris in the well from perforations or even a sand bridge, this debris could become lodged between the slip of the plug and the casing. If this happens quickly, it could create enough force to break the slips and cause a preset event.

If this happens with the lower slip, there is a chance that the plug will be broken loose when the plug is pulled upwards.  In this case it is possible that the plug is retrievable.\

If this happens to the upper slip, when the wireline is pulled to free it from the event, the upper slip will keep the plug from moving upward and the lower slip will more than likely be broken and cause the plug to set harder.

Properly cleaning the wellbore prior to pump down operations will ensure a smooth plug and perf operation.

Flow Bypass

The pump down of a plug and perf wireline bottom hole assembly is not 100% efficient. Some of the fluid pumped is bypassing the BHA, which will be traveling at a slower rate than the fluid itself. This causes some fluid to pump around the bha and ultimately the plug. Because of the small gap between the plug and the casing, the fluid that bypasses the plug rapidly accelerates. The higher velocity fluid creates a low pressure zone around the plug.

For most plugs on the market, the seal is created by a rubber element. Due to the nature of rubber, its flexibility, the element can respond to this low pressure zone in a way that causes it to swell out toward the casing. If the element swells outward, the gap between the plug and the casing shrinks further increasing the velocity and compounding the problem. At high enough annular velocity around the plug the element will swell enough to touch the casing.

In this scenario, if the element touches the casing it could create enough drag to apply enough force to the upper slip to break it and create a preset event. 

If there is an event that abruptly stops the wireline BHA, the flow around the plug would accelerate quick enough to apply a high force from the element to the lower cone, possibly breaking the lower slip and creating the same result.

Most frac plug providers have graphs to communicate the safe combinations of pump rate and wireline feed rate that will reduce the chances of this happening. Maintaining these combinations of rate and speed will help to reduce the flow bypass mode of pump down events.

Frac Plug Quality

As with any manufactured product, there can be quality issues that arise. The quality issue that is most likely to cause a preset event has to do with the slip containment. Whether the slip is an entire ring or individual segments held together with a band, if the mechanism that holds the slips together fails it could cause a preset event. For most reputable composite plug manufacturers, this would be the least likely cause to a preset event.

The Next Step in Composite Technology: Higher Performance, Less Risk

The Rubicon RzrFRAC stands out as a different design from all the competition on the market today. Several of the design features for the RzrFRAC reduce the risk of a preset event during the pump down operation.

First, the slips do not rely on any shear media to keep them in place during pump down. They are held captive by the rings above and below them. Eliminating the shear mechanism, eliminates the chances of quality issues causing problems during deployment.

The RzrFRAC is set from the bottom of the plug, which places structure in the tool from the setting equipment at the bottom. This aids in events where the plug encounters something during pump down, transferring the load directly into the setting equipment rather than the slips and cones. This further reduces deployment risk.

Finally, the RzrFRAC utilizes a proprietary plastic seal ring to create isolation, rather than a rubber element. This further reduces the chance of the seal causing issues due to low pressure around the plug during deployment.

Understanding your composite plug construction along with the risks it can encounter during deployment will help in identifying root causes of issues and actions that can be taken to reduce the chances of a preset event.