Posted onAugust 21, 2018
Composite Plug Innovation: Sealing Matters
Innovation of Plug & Perf completion technology over the past 10 years has been focused on reducing or eliminating the difficulty of removing the plugs after the completion. So much so, operators are judging composite frac plugs by how fast they mill up or the efficiency of the milling operation. While this focus has resulted in significant improvements in milling, it has neglected the real reason for running a composite plug.
When a completion engineer is designing the treatment for a well, they make decisions about how much proppant to pump per foot along with the fluid required to carry it to the reservoir. To ensure the entire wellbore is treated properly, they break the well into individual stages and treat them separately. These individual stages are created with the use of a composite frac plug. The plug is designed to anchor and seal within the well such that all of the treatment is directed into the perfs above the plug. If the seal or anchor is compromised, some of this treatment will bypass the plug and “treat” the stage below. This compromises the design, with unknown amounts of treatment per stage.
The seal on the plug is critical to making sure the reservoir is treated as designed.
Traditional Composite Plugs
Over 70 years ago, RC Baker patented a hydraulically set packer that will remain in autonomously in the well. The innovation in the design is centered around a rubber element that is held compressed between two anchoring systems (called slips). When compressed the element “squishes” between the mandrel of the packer and the ID of the casing creating a seal. The slips above and below the element ensure that this compression and resulting seal is maintained.
This has proven to be very successful and really the basis of most packers currently used in the industry today, including composite frac plugs. If you look at the first patent for a composite plug filed by Halliburton in 1993, anchor and element relationship remains largely unchanged.
Elastomer Element Design
When designing an element system to create a seal between the mandrel of the plug and the ID of the casing, a delicate balance must be struck. The element must be flexible enough to compress and seal, but strong enough that when compressed and seeing pressure the rubber element doesn’t just tear or stretch in a way that would compromise the seal.
Historically, to ensure the seal is maintained, packer designers will add elaborate back up systems around the elastomer seal to keep it in place when seeing pressure. This can be seen in the Halliburton patent. The three piece element design with backup rings allows the center element to create the seal and the outside elements to keep the center element from moving when seeing the pressure.
This design method has proven very effective at creating and maintaining a seal. The difficulty is that this design requires a long assembly with multiple components that doesn’t lend itself to easy mill up of a composite frac plug. As such, most composite plug providers have shifted to single element designs, and in some cases, even removed all backup systems from the design. The result is a more compact assembly for ease of milling but how much has the seal been compromised?
Seal Innovation: Liner Top Packers
The most significant innovation is wellbore isolation in the past 30 years has been focused on upgrading liner top packers. These packers act as a barrier when connecting two casing strings and are critical for maintaining well control. The seals must accommodate a high volume of water pumped passed it and then create a high integrity seal. Since this isolation is critical, providers have focused on creating the best seal possible. Advances have come from seals that are created by removing as much elastomer and replacing it with expanded metal. These seals utilize a metal carrier that expands with the elastomer, which eliminates reliance on the elastomer to maintain the seal by placing the elastomer on an expanded metal carrier.
Another benefit of this design is that it creates a highly repeatable sealing mechanism. When the metallic carrier is expanding up the cone it does so in the same way each time. With a rubber element that is compressed to create a seal; wellbore conditions, tolerances, and variences in the rubber all contribute to the element folding and flexing in different ways each time it is set. This brings some variance to the performance and repeatability of the element.
Composite Frac Plug Innovation
Rubicon understands how critical the sealing provided by the composite plug is for achieving the desired completion design. To achieve superior zonal isolation, the engineering team at Rubicon worked to develop a solution that looks more like the innovation achieved on liner top packers rather than the legacy hydraulic packer designs. The result is the RzrFRAC Composite Plug’s proprietary plastic seal ring. This ring expands up the cone of the RzrFRAC and creates a robust seal between the ID of the casing and the OD of the cone. With this design we know that the entire length of the ring creates the seal on the casing in the same way every time. Again, eliminating variance in the sealing performance from one plug to the next.
Using this proprietary plastic material removes the need for specific back-ups. Since the material is tougher than a traditional elastomeric seal, the slips create the only back-up necessary to maintain the seal under high pressure situations.
The plastic seal also helps during pump down. It has proven to be unresponsive to fluid bypass during the pump down phase of the completion. This eliminates risk of swabbing or swelling traditional elements have when experiencing high bypass rates, that could affect the pump down.
Finally, the proprietary seal ring design is proven to provide superior isolation. In a recent monitored test well one of our customers ran several provider’s plugs in the same well. The monitoring was designed to determine which plugs were providing the best seal. According to the operator the RzrFRAC exceeded the sealing performance of the other options tested in their well. Another operator used micro-seismic to monitor how well the plugs were sealing in the well and they communicated that there was no seismic activity below the plug, indicating how well the plugs sealed. They said that this was different than what they’ve seen in the past with other plugs.
If you care about how well your completion design is being executed, make sure to understand how the sealing mechanism works on your composite frac plug. Consider utilizing the RzrFRAC Composite Frac Plug to achieve the best isolation on the market today.
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