March 2016 /// Vol 237 No. 3

Columns

What's new in production

Forget refracs

Don Francis, Contributing Editor

Dr. William Maurer thinks there’s a better way to re-stimulate multi-stage shale wells, and he’s got a pretty strong opinion about it. “This new technology overcomes the major problems with refracs, which have earned the name ‘pump and pray,’ due to their poor reliability in many wells. I think it will replace a lot of refracturing, since it will work on all depleted wells, whereas refracturing only works on part of them. So, a lot of time must be spent selecting the best candidates. It also will stimulate all the fracs, whereas microseismic has shown that fewer than 50% of the fracs are stimulated by refracturing.”

With a Ph.D. in mining engineering from the Colorado School of Mines, and more than 40 patents related to advanced drilling tools competing for wall space in his office, Dr. Maurer is not without expertise in this area. Still, his assertion is bold, and it bears a closer look. The major problem with shale wells, as Maurer (and some other experts) see it is “constriction zones” that form at the intersections of the fracs and the horizontal well, and reduce flowrates.

The constriction zone. “Although the fracs are typically 50–100 ft tall, away from the horizontal wells, they initiate in a constriction zone only about 8 in. tall at the horizontal well, resulting in severe plugging of this zone, by crushed proppant particles and other solids, as the wells are produced. The fracs initiate parallel to the well, and then rotate 90 degrees perpendicular to it, which further increases the plugging problem. Frac plugging is, therefore, the major cause of the rapid decline in shale wells and greatly reduces the economics of these wells,” Maurer says.

“Only about a half-cubic-foot of crushed fines will fill and plug a 10-ft diameter, damaged zone around a horizontal well. These fine particles migrate along the fracs, as the wells are produced, until they enter the near-wellbore constriction zones and progressively increase plugging of these zones.” Maurer mentions parenthetically that plugging is also caused by embedment of proppants into the formation, which also creates fine crushed particles, as well as by silica-sand proppant solubility, blowback closures, and frac-fluid viscosifiers. Maurer thinks eliminating the effects of the constriction zone will increase increase flowrates and recovery rates by 20–50%.

Eliminate them how, I hear you thinking. This is where it gets interesting. After considerable head-scratching over the problem, Maurer concluded that the solution is to bypass the constriction zone, rather than trying to eliminate it. Doing so, he thinks, will open up 100% of the fracs and eliminate the need to refrac them every two to three years. And so it was that in 2015, Maurer, of Maurer Engineering, filed a patent on an alternative stimulation system that consists of drilling drain holes that bypass the constriction zones, intersect fracs, and provide large flow paths from the fracs to the wellbore. These drain holes are intentionally drilled across hydraulic fractures to increase production rates.

New shale stimulation system. This new system targets older shale wells that were originally drilled and fractured conventionally, but have declined by 50–70% after two to three years. The idea seems elegantly simple: drill small-diameter drain holes (2–5 in. in diameter) that, according to Maurer, accomplish several useful things. They eliminate expensive and unreliable refracturing; they bypass near-wellbore damaged zones; and they provide the aforementioned large flow paths from fracs to horizontal wells, or to surface. In theory, these flow paths remain open for the life of a well (eliminating the need to refrac every 2–3 years); allow oil and gas flow between plugged and open fracs; and allow low-cost completions with slotted liners or open hole.

There are several basic drain hole configurations. For example, horizontal drain holes can be drilled as new wells from the surface, in shallower formations; or drilled from the vertical leg of a horizontal well; or branched drain holes can be used to intersect fracs at multiple points. Drain holes can be sidetracked from a horizontal well, using existing sidetracking tools. Drain holes also can be drilled underbalanced, to maximize production from these wells.

Needless to say, nothing in this business is infallible, so Maurer points out that if problems occur with drain holes, they can be plugged with cement, allowing conventional refracturing to resume. “Drain holes completed openhole have the advantage that they can be fraced using straddle packers to create new fracs or to stimulate existing fracs. This will increase inflow into the drain holes by intersecting oil-and-gas-filled natural fractures.”

System potential. According to Maurer, the new system serves up quite a list of benefits:

  • Reduces stimulation costs 50%, or more;
  • Stimulates 100% of fracs instead of 50% with refracturing;
  • Eliminates the need to re-stimulate every two to three years;
  • Reduces new well costs 25–50% (wider frac spacing and better communication);
  • Eliminates environmental and disposal problems associated with fracing;
  • Can be used in fields where fracing is banned.

Sometimes, the best solution to a knotty problem can be to apply familiar tools in different ways. Maurer’s solution may be the case here. As this is being written, field trials are being arranged. In the meantime, you be the judge. 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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