When Fracturing, Heed the Plugs

Abstract

Plugs used in fracturing do more than just plug. In addition to isolating stages during fracturing, they often provide the first warning when casing has been deformed - an all-purpose description for pipe that has been bent, squeezed, or gashed. A vague label for the damage is useful because most often those dealing with the problem will never know more about the damage than the inner diameter of the casing has been narrowed - ovalized is the popular term for it - and that they need to find a smaller-diameter tool to get past the obstacle. Eric Davis, the global completions chief for ConocoPhillips, offered a rare public presentation about what can be learned by digging deeper into the reasons why a plug, frac gun, or bottomhole assembly (BHA) cannot pass through a casing diameter. A major message in a panel discussion during the recent virtual 2020 Unconventional Resources Technology Conference (URTeC) was that investigating casing can yield surprising results. Davis told the online audience he hoped the presentations would “scare you into getting more data.” Davis shared an observation made while watching fracturing in progress on a screen displaying temperature and acoustic data collected by fiber-optic cables attached outside the casing for distributed temperature sensing and distributed acoustic sensing. As the stage progressed, cooler injected water (shown in green) mixed with the warm reservoir fluid (in blue) (Fig. 1). Temperature data from the fiber-optic cable showed the previous stage as all blue, indicating the plug was sealing. Then the colors of the previous stage started changing as cooler water leaked in and green streaks began bleeding into the blue of the previous stage; then “it all went blank.” The fiber had been severed, Davis said, adding that seeing it in real time “was quite shocking.” The fiber optic was still working on the yet-to-be-fractured upper stages, but “as we moved up the hole, we kept parting the fiber.” For the team involved it was a big loss in terms of the cost of the fiber installation and the data they had planned to collect. But for Davis the value of what was learned after the fiber was cut “is far more valuable than the lost fiber-optic installation.” Collecting the information required sending diagnostic tools downhole to inspect the damage. Gashes were found in the casing. They appeared to have been cut by the fast-flowing water-sand mix that may have begun as a small leak, grew in volume, and severed the metal tube holding the fiber-optic cables on the outside of the casing. “You can see it looks like the packing element is where the erosion occurred on a lot of cases,” Davis said. They also learned that the metal in the casing is softer than the plugs. Davis said, “We were preferentially eroding casing over the plug.” They found “total casing breaches” created by the high-pressure stream of fluid and sand on 25% of the plug locations examined on multiple wells.