An Unconventional Challenge Can Casing Failures During Hydraulic Fracturing Be Stopped?

Shale producers around the world have learned in recent years that they cannot take wellbore integrity for granted. During pressure-pumping treatments, the interplay of hydraulic fracturing and formation geomechanics is deforming and even shearing steel casing. In the worst of cases, this means that some operators have lost access to long sections of a wellbore before it produces a single barrel of oil. This is largely not viewed as an inter-well challenge similar to frac hits. Rather, it is considered to be intrawell phenomena. The problem can take several forms and has no universal driver. George King, an independent consultant and leading technical voice on hydraulic fracturing, points out that not every unconventional asset is affected—indicating that geology and understanding of the rock fabric matter. However, information he has collected from client operators shows that in certain US shale and tight oil fields, between 20%–30% of horizontal wells are impacted to some degree. “Sometimes it is with the cement support, and sometimes it is with the casing itself,” said King, listing two of the problem’s general root causes. He added that the casing deformation is “an artifact of some damage that we are seeing right now in the realm of fracturing, and [during] the time period of fracturing.” A commonly reported version of the problem emerges after just a few stages at the toe-end of a horizontal well have been fractured. As another plug is pumped downhole to continue the completion, it suddenly stops 1,000 ft to 5,000 ft short of its target. This could happen in the bend, or it could be somewhere deep in the lateral. Often the deformation is described as ovalling. If a restriction consumes much more than a tenth of an inch of the pipe’s original internal diameter, there is the potential that unstimulated stages beyond the stopping point are lost to conventional plug-and-perf methods. This fate can sometimes be avoided by running slim-profile perforating guns, but then other isolation technologies are still needed to replace conventional plugs. Unconventional operators in China, Argentina, Canada, and the US have all faced the issue. In response, the technical community has recently stepped up collaboration efforts at industry conferences to find answers. In China, the issue appears to be more widespread on a per-well basis. National oil companies working the Sichuan Basin, the country’s most active unconventional area, have reported that more than 40% of gas wells have experienced casing deformation or failures. Like their US counterparts, Chinese operators typically realize the problem during a post-fracturing tool run. Argentina’s Vaca Muerta is not immune either. A 2015 technical paper coauthored by researchers at Chevron and YPF (URTeC 178620) casts casing shear and restrictions as common and “extremely detrimental to the performance of the well.” When explaining why the issue is cropping up today, more than a decade into the shale revolution, experts are quick to point out that a lot has changed since the first horizontally drilled and hydraulically fractured wells.

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