Weatherford’s global product line manager for composites, Matthew Crump, tells the story behind the development of the company’s TruFrac® composite plug
Efficiencies in the fracing market are based on the need for speed. Speeds of setting, perforating, pumping, milling and producing, among other improvements in precision, have ground down completion times year on year, month on month. Staying on top of this is proving a welcome challenge for some of the industry’s biggest innovators.
Economics have also changed what the market wants. Whereas last year saw plug-and-perf and sliding sleeve completions split roughly 75:25 respectively, improvements in technology and the fall in prices have prompted a slide further in favour of plug and perf. Weatherford global product line manager for composites, Matthew Crump, reckons this now accounts for around 85% of completions in the US.
“They’ve discovered they can get better production if they run cased-hole wireline logs to pinpoint the frac to certain pieces of the reservoir in a more educated manner than how they did in the past,” he explains to InnovOil by phone from Houston. In order to do this, he says, operators want the flexibility to run wireline logging systems once their casing is installed, something that sliding sleeve completions do not offer.
With this shift in the market, Weatherford saw an opportunity to refine its frac plugs to enhance the efficiency and time taken to run plug-and-perf completions.
Faster by design
The TruFrac® composite plug offers some interesting advances both in composite moulding, and in overall system design. Suitable for vertical, deviated or horizontal wellbores, the use of a 97% composite blend allows the plug to be run in and milled out faster than most other plugs on the market.
Crump is keen to stress the back-to-basics approach the team took when examining how they could enhance the product: “In 2013, we realised we needed to make a step-change in our composite technology. From then until the beginning of 2014 we did product development, design and lab testing.” He outlines the three main considerations which informed their approach.
“The first thing when designing a composite plug is to ensure it’s robust enough to get where it’s going to perform its duty,” he points out. The plug needs to reach the desired depth in the well without snagging on the well casing or being damaged by pump fluid. Once in place, it needs to seal securely enough to stay in place, and to make sure the frac fluid reaches the desired part of the formation.
“Third and most important,” he continues, “operators want to remove the plugs to produce [by milling], so customers are interested in how fast they can mill out all the plugs, and how big are the pieces which come up when they do. On average, milling time in the Eagle Ford is somewhere between 20 and 25 minutes. Doing that over and over in a 35-stage completion can add days to your operation, before the well even produces.”
“In the past, in much older technology with cast-iron plugs, etc., the pieces that came off the milling would be big, and when they’re big you have a higher chance of your milling assembly being stuck downhole,” Crump says. It is in addressing this problem that the inherent advantages of composite materials become more apparent.
“The crux of the design was creating a composite lower slip system with the least amount of metal as possible,” Crump explains. Although drawing from a number of years of experience – Weatherford has been in the market since 2002, when it partnered with a manufacturer to develop the first generation of composite plugs – the complex moulding of a new design for the lower slip led it to partner with another outside manufacturer.
It involved a great deal of fundamental testing over 2013-14, in order to create an easily millable material which would stand up to the 10,000 psi and 150°C (300°F) downhole conditions. After working with the supplier to develop and test a number of different moulds and materials, Crump says, the team “settled on a segmented design which holds four small metallic inserts – these bite into the casing and create an anchor. They are then are held together with composite bands that break once the plug is set, allowing the slips to bite into the casing.”
The result is a plug which, on average, mills out in around 10.5 minutes.
Test your metal
Gains were made in other parts of the assembly too. By reducing the amount of metal in the overall system – the upper slip and element included – the TruFrac could be made smaller and faster to deploy and remove.
“Since a frac plug is only holding pressure from above,” Crump says, “the lower slip is what does all the work and holds all the pressure. When you look at most of the plugs on the market, everyone works really hard on the design of the lower slip to make sure it can hold 10,000 psi at 300°F – then they just reverse that and throw it on the top.”
“The result is an upper slip that has a lot of metal in it, and a lot of high-strength composites, and is really just over-designed for the frac plug. So we designed a fit-for-purpose upper slip – filament-wound with small powdered metal buttons – that is easily millable and has the least amount of metal as possible, just to hold the setting force of the plug,” he continues.
The element system, the section of the plug which seals and holds the well pressure during a frac, was also redesigned. Traditionally, it poses the most concerns when being pumped downhole. The rubber section, vital to providing a robust seal, can flare out or become snagged on the casing. In order to ensure the element stays put during the frac most plugs employ an exposed backup system too – but these also can create extra lips and profiles which can become snagged in the wellbore.
Crump elaborates: “Historically, Weatherford’s plugs had an exposed system, but when we were looking at how to pump it down efficiently, we really wanted a system that was completely smooth on the OD [outer diameter] but still had the composite backup system to hold the seal in place.”
The solution was to incorporate the best features of the existing design into a new rubber element. “We took our composite backup system which had proven to be reliable in our previous designs, and designed an element where that backup was moulded directly into the rubber of the element itself,” he says. This creates a completely smooth OD on the element, offering what the company believes is “a superior seal” compared to other plugs.
At a total length of 23.44 inches (595 mm) the plug is small in comparison to its rivals, again contributing to a much reduced mill-out time. Over a 35-stage frack, this adds up. “That can be up to 18 extra feet of material you have to mill out!” Crump says.
Results with TruFrac have been positive so far. Its testing phase saw 16,000 runs with 99.9% reliability – a key factor, Crump says, in qualifying its abilities to customers.
With further cost reduction and efficiencies on the minds of customers, Weatherford is now looking at incorporating newer composites into its design, and shortening its plugs even further. “We’re trying to push the bounds of composite strength and design,” Crump affirms, adding: “We’re in later stages of testing for dissolvable technology, which is the next step in plug-and-perf.”