What's new in production

Source rock reservoirs (SRRs) have a normal productive life of 20 to 30 years. Compared to conventional oil and gas reservoirs, SSR wells experience more rapid decline, because they typically have low permeability and limited reservoir contact. When the so-called “shale revolution” (a tired phrase) began around 2008, there wasn’t a lot of experience at work—it was a trial-and-error, perf-and-pray situation. 

A lot of the early wells paid off, and the numbers speak for themselves. Since the beginning, over a million oil and gas wells have been hydraulically fractured, and the pace continued through 2014, with something like 35,000 new wells fractured every year, according to FracFocus. 

The lack of knowledge meant that some of the early wells were disappointments. Many underproduced from the start, or produced dramatically, only to decline faster than an untalented pop star’s career. Sometimes two wells of similar potential from the same SRR, alike in all known aspects, would behave very differently, and the success of the frac job was considered the most likely culprit. 

Do it again. Refracturing is not all that new. It is said that George Mitchell’s original “discovery” well in the Barnett shale was restimulated by refracturing four times before 2006. But for the most part, the answer to a poorly performing well was simply to drill and frac a new one.

There were many reasons that older fracture jobs were not as successful. There could be too large a distance between access points, 150–400 ft being typical. Combine  that with a less-than-optimal fracture network, and the result was a lot of stranded oil and gas. During fracturing, the perfs were generally placed in the middle of the designed fracture, but sometimes this meant missing the most productive zones. Sometimes the fault lay with incorrect use of proppant—particles that were too large or too small for the fracture, or had
insufficient strength. 

The object of a frac job is to maximize the exposure of the SRR surface area, and refracturing has exactly the same goal. A refrac also has the wisdom of years behind it—a lot has been learned since 2008.

Sometimes it is practical to reuse the original access points. When a formation has low “stress anisotropy,” or a tendency to split in particular directions (the way wood wants to split along the grain), then the source rock can be fractured in new directions, expanding the fractures and keeping them open with additional proppant. This is often the way to go, if frac sleeves (mechanical valves) were used in the original frac, but this is the case in only about 15% of fractured wells. When, as is more often true, access was attained with perfs, then new perfs can be added between the old ones. In that case, the old perfs must be sealed off. 

There are many ways to isolate the original perfs. One method is to seal them off with cement. This has the advantage of returning the well to its original state, but it is also very expensive and time-consuming. Ball sealers can be used to temporarily plug the old perfs, though they can’t seal non-circular openings. Particle diverters can be added to block the original access points. The idea is that fluids take the path of least resistance, so the diverters enter the cracks with the least amount of pressure (low production) and bypass those with higher pressure. 

Meet the candidates. BP ran a refracturing pilot program last year in Oklahoma’s Woodford shale. The plan was to restimulate dry gas wells that were disappointing producers in the first place. The company refractured five wells initially, and followed that with an additional nine, gaining experience as they went. The wells were all five or six years old. A refrac was considered, if the well met the following criteria: 1) fracture stage spacing was at least 500 ft; 2) of the original stages, a third or more of them were not propped effectively, with only a fraction of the target amount of proppant placed; 3) the unperforated heel section was in the pay zone—an improved understanding of the geology led to a redefinition of the top of the formation, adding 700 ft; 4) there was a large amount of gas-in-place with high pressure; and 5) the wells were producing less than 700 Mcfd. New fracs were added between the old ones, and the result was a doubling of production.

Other companies in other shale regions have had similar success. CONSOL  Energy in Pennsylvania’s Marcellus shale recently hired Halliburton to refrac several wells drilled in 2009, significantly improving production in all of them. The company is considering refracturing another 200 wells. Devon Energy refraced some 13 wells in the Barnett shale of Texas, while Canadian company Encana has begun a refracing pilot program in the Haynesville shale of Louisiana. Gas wells in the Haynesville decline rapidly, and Encana refractured seven wells that had declined an average 27% since 2012. The company is reportedly considering a similar program in Colorado’s Denver-Julesburg basin and the Eagle Ford shale of Texas. Exco Resources also told its investors that it plans a refracturing campaign in the Haynesville.

Hope for service contractors. With the recent turn of events in the U.S. oil patch, and the sharp downsizing of drilling plans, thoughts have turned to the economics of refracing. The cost of a refracture is approximately the same as the original frac job, but without the added cost of pad, permits and drilling. In the past few years, so much more has been learned about stage placement, proppants, fluids and perf clusters, that the number of old wells that might benefit from refracs must be huge and growing. The hope is that refracing will become a common form of well servicing. I mean, all that talent and horsepower needs to do something.