The possibility of hydraulic fracturing (hydrofracking) contaminating groundwater by leaking to the surface through natural fractures remains "very, very unlikely," according to the forefather of Marcellus Shale geology.

"We can study and understand cracks in shales very much the way people who curate masterpieces like the Mona Lisa understand fracture propagation in these very famous pieces of artwork," Terry Engelder, a professor of geosciences at Penn State University, said during GasMart 2011 in Chicago on Wednesday.

That's because the Mona Lisa, like most old oil paintings, reveals intricate patterns of cracks across its surface when viewed up close, and these cracks follow the same principles as geologic fractures, Engelder said.

Engelder called these "natural hydraulic formations" because, just as an operator pumps large amounts of fluid underground to create enough pressure to crack open a rock formation, these natural fractures are the result of a large amount of high-pressure fluid, in this case natural gas, causing the rock formation to crack open from within.

Those cracks will continue to spread outward until the pressure in the rock formation drops.

Because operators don't know the location of these natural fractures, they must fracture unstimulated rock formations until they hit a natural network. Horizontal drilling is "equivalent to drilling across Mona Lisa's forehead." By running perpendicularly to these natural fractures, horizontal drilling creates slips or "micro-seismic events." These reverberations can be used to measure geologic information about deep shale formations.

"People have concerned themselves with the possibility of fluid leaking from these deep stimuations to the surface. We know from micro-seismic surveys that the vertical height of these stimulations is often a few hundred feet, never more than a thousand feet," Engelder said. That leaves 7,000 feet of unstimulated and highly impermeable rock between the shale and groundwater, making migration to the surface "very, very unlikely," Engelder said.

While a recent Duke University study found increased levels of methane contamination in water supplies near hydrofracked gas wells, Engelder noted that the study did not find any evidence that the chemically laced hydraulic fracturing fluids leaked from shale formations into groundwater supplies (see Shale Daily, May 11).

Engelder helped jump start the Marcellus Shale boom in 2008 with an optimistic resource assessment of the massive formation, but his research into shale goes back more than 30 years (see Daily GPI, Nov. 4, 2008).

During his time at Columbia University he helped the Department of Energy (DOE) drill into the Huron Shale of Kentucky in 1978. With Penn State he helped Shell Oil Co. drill into the Antrim Shale of Michigan in 1985.

Engelder's perspective runs counter to the opinion of Antony Ingraffea, a Cornell University expert on rock fractures and one of the leading academic critics of shale development (see Shale Daily,March 23).

Ingraffea believes that natural fractures make it difficult to predict the effects of "induced fracturing."

Like Engelder, Ingraffea's work on shale also goes back decades, in his case to a three-year stint in the unconventional gas program at the DOE-backed Lawrence Livermore National Laboratory in the mid-1980s. In work with Schlumberger Ltd. he hydraulically fractured sandstone slabs in a laboratory -- using ideal settings as a starting point for understanding the real world complexities of fluids moving through rock formations.

"What we found out after doing all these simulations is that we could get any answer we want, and that's still the case, in my perspective," Ingraffea told an audience at the University of Pittsburgh in March.

According to NGI's Shale Daily Unconventional Rig Count, there were 146 drilling rigs working the Marcellus Shale for the week ended May 6, 2011, up from 125 rigs one year ago (see chart). However, drilling activity in the region has slowed recently, with the current rig count down from its all-time high last fall, when more than 160 rigs were operating.