What's New in Production: Unconventional well refracturing: Something old, something new

When it comes to oilfield technology innovation, there is often a preference for developments that are viewed as “something new,” as opposed to applying something “old,” which already exists and has been used in one form or another in the past. It’s just more intriguing and even considered of greater value to technology proponents, to focus on transformative innovations, including those that can only be imagined but don’t yet exist. Moon shots! However, it is those technologies which are old, even forgotten from past generations, which can have the greatest impact, especially more immediately. 

Striking a balance between the old and the new, and the near and the far, is the key to a successful innovation program. But there is that inclination to dismiss the old, which can result in missing opportunities for well and field value enhancement in the here and now. 

With that in mind, I want to talk about hydraulic fracturing in unconventional reservoirs and, specifically, refracturing—and how old technologies can be applied as new to provide potentially significant well production uplift and increased recovery. 

Let’s recall a few old well completion and stimulation technologies that became new again and a few that are old now, but which can be applied as new in refracturing long, horizontal unconventional wells. 

Proppant. One of the earliest fracturing proppants, along with river sand, was walnut hulls. Sand, being stronger and more abundant, soon took over as the proppant of choice. But decades later, walnut hulls reappeared—now resin-coated and marketed as lightweight, near neutrally buoyant proppant! The walnut hulls were also now a proprietary material, revealed only in the patent literature. 

Lightweight proppant (LWP) offerings today are no longer walnut hull “technology,” but synthetics such as from Sun Specialty Products, which combine strength, sphericity and buoyancy. LWP, as an additive to standard proppant stages, may find a preferred application in refracturing, in conjunction with smaller job sizes with less overall proppant volumes. They can be combined with microproppants, another “new” fracturing technology, but old in the paint and building construction industries, among others. 

Microproppant can be up to 10 times smaller than standard sand proppant. Both specialty proppants have demonstrated successes in enhancing well productivity, when added in a few percent to regular proppant stages. They travel and settle deeper in the main fracture than the larger and less-buoyant sand and, in the case of microproppants, conceivably prop open small, secondary microfractures. But field trials of both have been limited. Further trialing, including together, is warranted. If usage increases, costs, currently a hindrance, can come down. 

Diverting agent. One of the first diverting agents used in acid stimulation was chicken feed—cheap, abundant, and disposable. Chicken feed was quickly phased out, though, in favor of various solid diverters over time. Chicken feed never made a comeback as walnut hulls did. 

But regarding the evolution of solid diverters, a true breakthrough was that of ball sealers, invented by Exxon in the 1950s. Various embodiments followed, including biodegradable ball sealers, or bioballs, made of collagen or other materials soluble in produced fluids at elevated temperature. Bioballs are now an old technology too, largely forgotten.  

However, as an alternative to cementing liner, plug & perf refracturing, lower-cost bullhead refracturing, with bioballs to divert from one fracture to the next, has shown promise. In multi-zone, horizontal well refracturing, it is necessary to use bioballs in significant excess (e.g., 300% of perforation count) and pump at high frac rates, e.g., 90+ barrels per minute. Also enabling bioball application is the fact that they are lighter than conventional ball sealers. Effective diversion with bioballs beyond 3,000 to 5,000 ft of horizontal length is not achievable now. However, restimulation to that extent can still result in significant production uplift at much lower cost than cementing a liner, reperforating and refracturing. And it is repeatable. 

Expandable liners are not so old, but they are no longer new technology, either. Common uses have been casing repair patches and for sand control in offshore wells, as an alternative to gravel pack screen completions. But what about for refracturing unconventional wells? 

Mohawk Energy, for one, provides multiple sizes of expandable liners, which have found early application as an alternative to a cemented liner for refracturing. With an expandable liner, cementing is, of course, eliminated, and a larger-diameter wellbore can be retained, through which reperforation and refracturing can then be conducted. Presently, expandable liners are limited to ~5,000 to 7,000 ft of lateral length, but if a lateral is longer, it need not be entirely restimulated to provide a substantial uplift. 

Other than in the most mature plays, such as the Barnett shale, refracturing is not yet a priority in unconventional field development. For the most part, refracturing has been limited to applying standard, new well, cemented liner plug & perf designs. But expandable liners for plug & perf refracturing, minimal proppant designs, including lightweight and/or microproppant and bullhead refracturing with biodegradable ball sealers, are among the potential opportunities for developing lower-cost procedures specific to refracturing. 

As older technologies, these aren’t viewed as transformative. But they don’t need to be! What is more important than achieving well productivity uplift and enhanced hydrocarbon recovery? Refracturing, which may be the necessary way of the future in sustaining unconventional resource development, calls for such technology consideration, old or not.  

About the Authors

Leonard Kalfayan

Contributing Editor

Leonard Kalfayan has 42 years of oil, gas and geothermal experience. He has worked for Hess, BJ Services, Unocal, and as a consultant. He is an SPE Distinguished Lecturer and Distinguished Member. He has authored numerous publications, and also holds 13 U.S. patents.