July 2015 /// Vol 236 No. 7

Columns

What's new in production

A trick up their sleeves

Don Francis, Contributing Editor

Unsurprisingly, some thought leaders think mainstream, multi-stage completion methods are suboptimal. It’s true that “factory” wells in U.S. shale plays have yielded remarkable results, but problems remain.

Among other service companies, the boffins (Editor’s note: a term used in Britain during World War II, to refer to technical experts) at NCS Multistage have done some head-scratching about solving these problems, which they believe is best done by multiple-cycle sleeves. According to NCS, this relatively new frac sleeve development is a tidy solution in the completions and production phases of unconventional well development, which they describe in a recent conversation:

During completion operations, proppant flowback into the wellbore can be a problem. It’s even worse in unconsolidated formations, where there’s a lot of sand flowback when the well is put in production through the perforations. With a closable sleeve, operators can frac each stage individually and close the sleeve immediately following the frac; that allows the frac to heal while finishing stimulation of the rest of the well. At the end of that process, each frac sleeve is reopened individually. The formation pressures will have stabilized, greatly reducing or eliminating proppant flowback into the wellbore.

Fracture sequencing. In addition to frac healing, there’s the option of fracturing stages out of sequence, to deal with shadow stress, in which a fracture in a single zone creates pressures that affect the frac in the subsequent zone. Operating companies have developed techniques for dealing with shadow stress, e.g., zipper fracs, and the latest entry in the oilfield lexicon of colorful terms, the “Texas Two Step,” which calls for fracturing out of sequence.

For example, they’ll fracture zone one, then three, and then come back and fracture between them after the stress, to take advantage of the stress shadowing to maybe create a more complex frac structure between those two stages, and then alternate that same way throughout the well. Companies have never had this option before. They’ve been limited to fracturing from the toe of the well to the heel, in sequence.

Zipper frac, a technique involving the simultaneous fracturing of parallel wells at the same time, adds tremendous logistical challenges and operational risks, due to added equipment and personnel, and concurrent operations. Using multiple-cycle sleeves to perform out-of-sequence frac operations from a single wellbore has the potential to take advantage of stress alterations, but without the costs and risks of simultaneous completion operations on multiple wellbores at the same time.

During production, a multiple-cycle sleeve allows management of the whole well, stage by stage. For example, with unwanted gas or water production, or a thief zone, a coiled tubing unit could be used to close any combination of sleeves to isolate and shut off zones of the well. This capability is used, now, in waterflood projects, where sleeves are opened and closed, as needed, to enhance flood efficiency.

Recompletions. Large potential can be seen in recompletions. The biggest problem facing operators, who want to recomplete a multi-stage well, is that they can’t control wellbore pressure, especially in plug-and-perf completions. With closable sleeves, you could go down and close as many sleeves as needed, to re-establish wellbore integrity, so that you could focus the refrac energy in a single stage at a time.

Considering that thousands of multi-stage wells have been drilled and completed, how serious does the industry believe these problems to be? Just a nuisance? Something that in an ideal world would be nice—but nonessential—to solve?

Proppant flowback. According to NCS, in some areas, proppant flowback is worse than in others, but generally more of a nuisance than a crisis. Operators always can go back in and clean up the well, but they’re also losing some proppant from the fractures. Resin-coated sand is used sometimes, but proppant placement dynamics cannot guarantee that the resin-coated proppant will prevent flowback.

As usual, there’s a spectrum between the extremes. But even at the nuisance level, costs are involved. One of the ways by which proppant flowback is addressed is by over-flushing each frac stage, which means pumping a lot more fluid into it than is needed to fill the frac, to place proppant further from the wellbore. This adds to fluid costs and pumping time.

So why has the industry resisted addressing these problems? NCS says it’s because operators haven’t had any options until now. Operators have completed wells, in the way that they’ve completed them, because that was the only technology available. They weren’t thinking about refracturing, because it wouldn’t have made any difference. They were limited to plug-and-perf and ball-drop sleeves with open-hole packers, as open-hole completions.

Of course, there is always risk in anything that you run downhole, including multiple-cycle sleeves. However, NCS says the risk of catastrophic failure is small. Operators always have an option to go back in and plug-and-perf to complete the well. Although the well wouldn’t be lost, the ability to manage the wellbore would be. But that’s no worse than the current state of affairs in multi-stage completions.

Net of the costs and risks, it appears that operators stand to gain considerably by solving these problems with multiple-cycle sleeves. Among these gains are extended well life, greater ultimate recovery, and the big one—wellbore management. Refracturing, a big subject and grist for another mill, also can be enabled under the correct set of conditions by multiple-cycle sleeves.

Well, there you have it. The industry may be on to something here. Several flavors of the multiple-cycle sleeve concept have popped up, and this portends a wholesale change in the way that multi-stage shale wells are completed. Stay tuned. wo-box_blue.gif  

The Authors ///

Don Francis DON@TECHNICOMM.COM / For more than 30 years, Don Francis has observed the global oil and gas industry as a writer, editor and consultant to companies marketing upstream technologies.

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