Limited Entry Fracturing
Plug and perf (or limited entry) is the preferred and most common fracturing method for unconventional wells. Often employed in wells with cemented liners, it involves pumping down a bridge plug on wireline with perforating guns. The plug is set at a horizontal location near the well toe and the zone is then perforated. Next, the tools are removed from the well and the fracture stimulation treatment gets pumped in. A ball-activated plug diverts fracture fluids through the perforations and into the formation. As the “plug and perf” process is repeated for each stage, the downhole tools move from the end of the wellbore back to the beginning until each stage has been fraced.
Limited entry operations are predictable and repeatable—two things operators love. However, limited entry comes with its limitations. One disadvantage is that multiple permanent entry points are created per target fracturing stage, with multiple perforations per entry point over dozens of target stages along the length of the wellbore. Since multiple perforation clusters are stimulated simultaneously, there is no control over the size of each fracture. Some clusters are not stimulated at all, while others receive varying amounts of treatment. The result is gaping areas of the formation that remain unstimulated.
This lack of control during frac placement opens the door for disruptive technologies that address these concerns in very tangible ways. Which leads us to “pinpoint” fracing …
Pinpoint Entry Fracturing
With limited entry (discussed above), drillers frac multiple clusters (group of perforations intervals, or zones) at the same time. Most drilling operations in the United States include five clusters per stage. Some oil and gas operators are experimenting with an increased number of clusters (up to 16 per stage).
There’s a significant group of important operators (including ExxonMobil, Shell, Apache, among others) that seem to be convinced that it is extremely difficult to accurately and equally frac each zone, all at the same time, with limited entry. Therefore, they prefer to treat each zone individually. That’s where pinpoint, or single-point entry, fracturing comes in.
Pinpoint fracturing can improve near-term and long-term well performance compared to plug-and-perf. It provides consistent, repeatable fracture placement along the entire lateral of each well and from well to well. Zones are isolated with sand plugs and operators can frac each cluster individually with coiled tubing in the well. This allows operators to optimize the stimulation design for a given formation. Locations of fracture initiation are verifiable and consistent, because pinpoint fracturing treats a single cluster at a time. The amount of proppant and fluid pumped into each fracture is consistent.
Pinpoint fracing has been growing in popularity and many service companies have designed completions systems around it. Indeed, there are several completions companies cropping up lately that sell specialized tools for unconventional multistage fracturing. That said, one of the big selling points of pinpoint stimulation is that it requires less equipment and a smaller footprint.
Operators who end up going back to plug and perf say the maximum number of stages they can frac with pinpoint stimulation is a limitation, as are the pump rate they can achieve. They also cite cost and operational inefficiencies. For example, one of the most successful pinpoint frac systems these days comes from NCS Multistage. The systems are limited by the maximum depth and flow rate they can achieve; both issues are due to the fact that they require coiled tubing to activate their sleeves.
What Do You Think?
While many single-point entry fracturing adopters have found it increases oil production, many of the big players stick with the more traditional plug-and-perf method. What’s your opinion? What do you think is next in stimulation technology? What emergent methods will be the next “big thing” in hydraulic fracturing?
Get in touch with us and let us know what you think. And if you need support and stability from a manufacturer with over fifty years of experience in designing, manufacturing, and delivering high-quality oilfield tools, Keystone Energy Tools will be happy to assist you.