The Marcellus shale is fast becoming one of the nation's premier natural-gas plays, thanks to its vast extent and choice rock properties. Reservoir characteristics such as thickness do vary from one region to the next, however, according to speakers on the “Meet the Marcellus” panel at Hart Energy Publishing’s Developing Unconventional Gas–East Conference, (DUG-East) in Pittsburgh.

Jack Ward, executive vice president, exploration and production, for Houston-based PetroEdge Energy LLC, discussed results derived from regional mapping. Ward looked at the distribution of porosity in the shale, which carries two main pore types. Inorganic porosity exists between fractures and grains, and organic porosity occurs within diagenetically altered kerogen.

An area where both types of porosity occur could be especially prospective, he noted

While it seems intuitive to suspect that rocks with higher total organic carbon (TOC) content have higher porosities, the correlation is not clear. Ward also stressed the need to carefully calibrate log data, to adjust for the presence of kerogen.

The Marcellus is quite thick in northeastern Pennsylvania, but thin on the southwestern side of the state. And yet, both areas are delivering excellent wells.

“The net differential between the plays is less than the gross differential, because northeast Pennsylvania has thick intervals of low organic content,” said Ward.

On a net/gross basis, the difference between the two major play areas is actually small. “We are seeing the impact of the coarse clastic influx on the eastern side of the basin,” he said. “As we add terrigenous material, our ratio of organic to inorganic material is changing.”

His conclusion? The southwestern play has high porosity feet and high TOC together, and is dominated by organic porosity. The northeast play is quite different, displaying a mix of organic and inorganic porosity.

James Coleman, Reston, Virginia-based geologist with the Eastern Energy Resources Team, U.S. Geological Survey, cited four key factors that affect successful exploration in the Marcellus shale.

The starting point for any resource play is an understanding of the areas of gas occurrence and the potential volumes present. Coleman noted that numerous sources validated the immense size of the Marcellus resource, with estimates ranging up to 220- to 867 trillion cubic feet of gas, on a P90-P10 spread, for ultimate yield of gas from 117 counties on 80-acre spacing.

Water availability is a concern across the Marcellus play as well. Coleman noted that on average some 17 trillion gallons of precipitation fall in the Marcellus fairway each year, and 7 trillion gallons recharge the groundwater.

“On an annual basis, adequate supplies are probably available,” he said. “However, seasonally low periods may restrict on-demand calls.” Operators need to plan ahead and secure adequate storage capacity for temporary periods when water availability may be low.

Another question swirling about the Marcellus is the availability of adequate disposal and/or recycle capacity for drilling and frac fluids. Up to 300,000 gallons of water per day per well are needed for drilling operations, and that water is typically recycled at the well site. Hydraulic fracturing has major water requirements: 2- to 8 million gallons of water are required for each frac job.

Treatment options for water are surface storage and evaporation, injection, recycling and wastewater treatment. One persistent issue for operators in the Marcellus is that permitted water-disposal locations are scarce to nonexistent, and the permitted sites that do exist are unable to properly treat used frac water.

At least one operator is building its own water-treatment facilities to reuse frac water in response to this issue.

Finally, Coleman addressed the possible consequences of habitat fragmentation caused by drilling and production activities. Drillsite access roads, drill-pad footprints and pipeline corridors each impact the land.

A particular concern in the Marcellus is the effect of development on threatened and endangered bird species, including bald eagles, osprey, blackpoll warblers and yellow-bellied flycatchers. Horizontal pad drilling is one mitigating solution, since development with horizontal wells would require five wells per square mile, versus 16 wells per square mile for vertical-only development.

Coleman stressed that the U.S.G.S. is currently addressing all these major concerns in the Marcellus area of interest, through improved data collection and new studies.

The Marcellus covers such a vast area that its regional stratigraphy is quite complex, said Ralph Williams, principal and founder, Reservoir Visualization Inc.

RVI, based in Humble, Texas, is digitizing and normalizing well logs, working with a set of 80,000 Appalachian Basin logs, of which 30,000 penetrated the Marcellus. Within the Marcellus fairway proper, it has about 3,000 well logs.

The Marcellus shale, characterized by a hot gamma ray above 300 API units, sits on top of the Onondaga series. The Tully establishes the top of the Marcellus.

The shale itself is internally stratified. “We recognize 11 distinct Marcellus layers,” said Williams.

Although RVI’s work is in its initial stages, it has already revealed some interesting depositional features, including onlap of the Cherry Valley and Onondaga, facies changes within the shale, and indications of paleohighs that affected Marcellus deposition.

“We are building an isopach of the Marcellus layer, with net shale values,” said Williams. The work will lead to a fuller understanding of the Marcellus and other shale intervals in the Appalachian Basin.

Operators and producers face many challenges in the Marcellus, as in other resource plays. Gas shales sit midway in the resource triangle, said the final speaker, Doug Pferdehirt, president, reservoir production group, Schlumberger. The Marcellus is in the class of large-volume resources that are difficult to develop.

“One thing we’ve learned over the past 50 years is that all shales are not created equal,” says Pferdehirt. “Each has unique challenges.”

Specifically, the Marcellus has high clay content, and Schlumberger has observed a clear relationship between high clay content and the ability to establish and maintain conductivity. As clay content goes up, relative stresses rise. This can affect the ability to stimulate an entire stage if sections with disparate clay contents are treated at the same time.
Sometimes, additional proppant may be required.

“The Marcellus also has highly mineralized natural fractures and faults, which affects the complexity of the fracture system,” he said. “It also appears to result in very high levels of dissolved chlorides in produced water.”

Additional complicating factors are variable reservoir pressures and rock properties, which affect wellbore stability and horizontal well landing locations.

A significant challenge is fracture height containment. And, in addition to its regional variations across its broad geographic extent, the Marcellus is also highly variable at the local scale, in tens to hundreds of feet both vertically and laterally.

The solution to these problems lies in an integration of technologies and processes, he concluded. Technologies such as core analysis, image logs and microseismic monitoring can be combined to deliver the best results; reservoir characterization and management strategies are needed in concert.

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