Source: Pittsburgh Post-Gazette, July 31, 2013
Posted on: http://envfpn.advisen.com
The question of fracking the shale layers above and below the Marcellus has transitioned from an “if” to a “when” for many oil and gas operators in Pennsylvania. On that, they agree; how to do it is another story.
In recent discussions with analysts, executives at three of southwestern Pennsylvania’s largest oil and gas firms shared contrasting views about what they believe happens when two wells are fracked on top of each other.
They were talking about their companies’ experiments with the Upper Devonian formation, which is a group of shales that lies only a few hundred feet above the Marcellus.
Downtown-based EQT Corp. has changed its strategy after monitoring four Upper Devonian wells for a few years. Rather than drill Marcellus wells now and come back for the Upper Devonian bounty later, the company decided to drill more of these shallower shale wells at the same time as the Marcellus ones.
Their Upper Devonian wells aren’t the most stellar in terms of gas production, executives said, but it makes sense in the context of an already constructed well pad and paved access road, with all the necessary equipment already on site, to toss another horizontal spoke into the ground.
Philip Conti, EQT’s CFO, told investors that waiting too long after the Marcellus is fracked before tapping the Upper Devonian could deplete the shallower formation or cause interference between the two.
Oil and gas companies have said for years that the layers of limestone book-ending the Marcellus Shale from what lies above and below it serve as natural barriers so that fracking fluids and gas stay within that zone.
The Tully Limestone separates the Marcellus from the Upper Devonian and it’s not absolutely impermeable, according to Kris Carter, mineral resources division chief for the Pennsylvania Geological Survey.
While companies are not required to submit their microseismic tests to the state, Ms. Carter said she’s been told by some firms that their fractures reached from the Marcellus into the Upper Devonian, penetrating the Tully. That doesn’t necessarily ruin a well, she said.
Albert Yost, senior adviser with the National Energy Technology Laboratory in Morgantown, noted that even without a direct crack linking the two, there could still be microseismic reverberations from blasting just a few hundred feet away.
If fracking one shale rattles the rocks that make up the shale above or below it, that could affect the rate at which gas flows out of that formation.
That’s something that Consol Energy is studying now, following the completion of its first Upper Devonian well.
Consol decided to drill in the Burkett, the closest Upper Devonian shale to the Marcellus, partly to test how the two would interact.
“One of the biggest impacts, maybe the most important impact we found is not the specific well profile and production numbers from the Upper Devonian well, (but) more importantly, what impact it had on the Marcellus horizontals that we had below it,” Nick DeIuliis, Consol’s president told analysts last week.
The company warned the results are preliminary and may not apply everywhere in its operation area, but it appears that fracking the Upper Devonian well first created a “pressure ceiling” for the Marcellus, so that when the lower shale was fractured, those fracs were more contained, more concentrated and more effective, yielding a higher gas flow rate and less of a decline in production over time.
Range Resources Corp., on the other hand, is in no rush to try out these theories.
It has drilled several Upper Devonian wells, but says its near-term focus will remain on the Marcellus.
Range plans to first review the production results from other companies’ Upper Devonian and Utica wells and then chart its course through these zones.
“To the extent those other operators have success, we’ll be able to capture that,” said Range CEO and President Jeff Ventura. “To the extent they don’t, we’ll have saved some risk investment.”