Fracture growth stopped more than 5,000 feet below drinking water aquifers "and there was no detectable upward migration of gas or fluids from the hydraulically-fractured [fracked] Marcellus Shale," according to a National Energy Technology Laboratory (NETL) technical report released Monday on the results of a limited field study in Greene County, PA.
"Conclusions of this study are: 1) the impact of hydraulic fracturing on the rock mass did not extend to the Upper Devonian/Lower Mississippian gas field; and 2) there has been no detectable migration of gas of aqueous fluids to the Upper Devonian/Lower Mississippian gas field during the monitored period after hydraulic fracturing," according to the 80-page report.
The researchers' conclusions fly in the face of industry critics who have argued that fracking has introduced natural gas and fluids from the Marcellus Shale into drinking water supplies. In Greene County, at least, that does not appear to be the case.
NETL researchers monitored for inter-formational fluid and gas migration at depth (within the 2,100-foot to 8,200-foot depth interval) prior to, during and after fracking of six horizontal Marcellus Shale gas wells. Additionally, seven vertical gas wells were monitored for tracer (carbon and hydrogen isotopes in the gas, strontium isotopes in the fluids), pressure and production evidence that would indicate possible migration of fluid or gas upward from the fracked shale formation below.
"Gas production and pressure histories from three Upper Devonian/Lower Mississippian gas wells that directly overlie stimulated, horizontal Marcellus Shale gas wells recorded no production or pressure increase in the 12-month period after hydraulic fracturing," they found. "An increase would imply communication with the over-pressured Marcellus Formation below."
While results of the microseismic monitoring "indicate that stress imposed on rock formations by hydraulic fracturing did not extend to the Upper Devonian/Lower Mississippian gas field...numerous microseismic events were observed above the Tully Limestone, which is thought to be an upper barrier to fracture growth from hydraulic fracturing in the Marcellus Shale when intact." Data collected during the research "suggests that energy from hydraulic fracturing was focused along pre-existing joints, low-offset faults and bedding planes"
The NETL research was funded from oil and gas royalties legislatively directed to the Department of Energy under provisions of the Energy Policy Act of 2005.
Greene was the fifth-highest natural gas producing county in Pennsylvania in 1H2014 at 172 Bcf, according to the state's Department of Environmental Protection (see Shale Daily, Aug. 18).