AADE ’17: Constructing better wells can increase integrity during fracturing, reduce production problems

HOUSTON -- As the North American shale revolution has evolved, spawning new technologies and strategies for drilling and completing horizontal wells with longer laterals and increased frac staging, the upstream oil and gas industry is still learning how to construct wells for maximum, long-term production.

Companies are now fracing wells with increasingly high pressure rates, multiple times, to produce as much as possible, but at the same time, challenging the limits of today’s wells’ tubing, casing connections and cementing. Common problems include leaky tubing, cracked casing couplings and compromised cement, which can reveal themselves during the production phase of the well, long after it has been drilled and completed.

The best solutions to these problems are possible during well construction, including the drilling and hydraulic fracturing phases, according to George King, Apache Corporation’s distinguished engineering advisor. King gave a presentation entitled, “Preserving Well Integrity During Fracturing” at the 2017 American Association of Drilling Engineers’ (AADE) National Technical conference last week in Houston.

“As a drilling engineer, you deal with a well for about four to six weeks,” King said, noting that completion engineers will be in charge of the well for one to two weeks, before finally handing it over to production engineers, who will operate the well for five to 50 years. “Build the best well you can, to hand off.”

What some drilling engineers may not realize, is that a large share of horizontal shale wells will be fractured many more times than conventional wells, he said. “Last November, Apache (had a well) in the Permian with 147 single-entry frac stages down the length of a mile-and-a-half-long lateral,” King said. “Consider this—I’m blowing up that same balloon of steel and casing 147 times. There’s got to be some give in there somewhere.” Older conventional wells haven’t experienced the same downhole problems that horizontal shale wells are experiencing today. “We’re using conventional technology, what we learned on conventional reservoirs, to try to frac shales. They flow completely different and they react completely different, so we’re having to come in and do it again and again, and that leads to problems that will continue cropping up,” King said.

One problem that can ultimately lead to well failures during fracing is the significant wellbore undulations that happen while drilling horizontally. Most often, drilling horizontally doesn’t result in a perfect curve and lateral “J” or “L” shape, but peaks and valleys in the curve and lateral sections that occur as the bit makes corrections to stay on track. These undulations hurt the integrity of the well, and make them more susceptible to failure during fracturing. “We’re seeing some examples, where we’ve drilled 29 wells and we have 18 or 19 casing failures during fracturing, and the failure isn’t in the vertical section… the failure is downhole, and sometimes we lose the entire wellbore.” Causes of this problem include too much dogleg severity and drilling too rapidly, he said.

Another problem he discussed is the strength of casing connections in horizontal wellbores. In the onshore U.S., 50% of wells are still completed with API 8-round threads. “It’s an easy thread to find, but when you start putting in bending forces, we are starting to see failures within a foot or two of the connections,” King said. “We need to look at a better thread, something where the junction or the connection is at least as strong as the pipe bond, and also watch your quality,” he said, noting that irregularities in the threads can cause weak connections.

He also mentioned cementing as one part of well construction that could be improved for unconventional shale wells. While 50 ft of cement should be enough to separate and isolate zones holding 10,000 psi, he suggests using 200 to 600 ft of cement, because pressure fluctuations during fracturing operations are hard to predict, and cement can break down and crack when refractured many times. He also recommended using a second intermediate casing string for added stability in wells that would be refractured.