Researchers from two major universities and a subsidiary of Royal Dutch Shell plc said water used in hydraulic fracturing (fracking) operations remains locked in the targeted shale formation and poses no threat to underground water supplies.
Pennsylvania State University geosciences professor Terry Engelder, Cornell University earth and atmospheric sciences professor Lawrence Cathles, and Shell International Exploration and Production Co. geologist Taras Bryndzia performed experiments on drill cuttings from the Union Springs member of the Marcellus Shale, and on core plugs from the Haynesville Shale.
Their findings were published in the September issue of the Journal of Unconventional Oil and Gas Resources.
"In terms of public concern, the water that gets driven into the shale stays there," Engelder told NGI's Shale Daily on Wednesday. "There was concern that flowback was only a small fraction of the water put in the ground, and that water that's left in the ground would contaminate groundwater. The answer is no, because it gets sequestered in the shale, very much like a sponge would soak up water."
According to the report's abstract, "more than [5 million gallons] of water containing additives is commonly injected into a typical horizontal well in gas shale to open fractures and allow gas recovery. Less than half of this treatment water is recovered as flowback or later production brine, and in many cases recovery is [less than] 30%."
The researchers said while flowback and brine is managed at the surface, the water that remains underground, also known as residual treatment water (RTW), is beyond the control of engineers.
"Some have suggested that this RTW poses a long term and serious risk to shallow aquifers by virtue of being free water that can flow upward along natural pathways, mainly fractures and faults," the researchers said. "These concerns are based on single phase Darcy Law physics, which is not appropriate when gas and water are both present. In addition, the combined volume of the RTW and the initial brine in gas shale is too small to impact near surface aquifers even if it could escape.
"When capillary and osmotic forces are considered, there are no forces propelling the RTW upward from gas shale along natural pathways. The physics dominating these processes ensure that capillary and osmotic forces both propel the RTW into the matrix of the shale, thus permanently sequestering it."
Engelder, well noted for his early cheerleading for the Marcellus Shale’s potential, said the dueling forces created, in essence, a "permeability jail."
"When more than one phase is present, then the presence of two phases more or less interferes with flow, and generally the result is to slow the flow down a great deal, if not stop it altogether. The concepts associated with capillary pressure have the ability of one phase to keep another phase in place," he said.
The researchers also refuted the idea that fracking could accelerate brine escaping to aquifers near the surface and lead to groundwater contamination, insisting that the exact opposite was true. After a series of standard temperature and pressure (STP) counter-current imbibition experiments on the aforementioned cuttings and core plugs, the researchers asserted that fracking and gas recovery would actually reduce the risk.
"If one wants to dispose of fracking waters, one could probably not choose a safer way to do so than to inject them into a gas shale," Engelder said Wednesday.
The report was supported by the Research Partnership to Secure Energy for America and Penn State’s Appalachian Basin Black Shale Group, a program of the geosciences department with industrial affiliates that studies the Marcellus and Utica shales in Pennsylvania, West Virginia and New York.