Hydraulic fracturing, or fracking, is loudly and frequently blamed for contaminating water supplies. However, a new study by the DOE casts doubt on these assertions while another study highlights the need for increased quality control in well casing and sealing.
The National Energy Technology Laboratory conducted the yearlong research for the DOE in Green County, Pennsylvania. The study monitored the fracking of six horizontal gas wells in the Marcellus Shale. In the area, the Marcellus Shale is located at about 8,100 feet and is about 280 feet below the Tully Limestone formation, 3,800 feet below the Upper Devonian/Lower Mississippian (UD) gas field and about 5,000 feet below the nearest drinking water aquifer. The operator of the wells allowed unlimited access to the DOE to conduct the study.
First, the DOE sought to determine the maximum height of fractures created during the fracking process. This determination is an important one because fracking uses pressure along the length of the horizontal well to fracture the rock, allowing natural gas to freely flow into the horizontal well. During the fracturing process, microseismic activity was monitored in six wells by use of a geophone array deployed in two vertical wells in the midst of the six horizontal wells. During fracturing, over 10,000 microseismic events were observed. Most of the events were located below the Tully Limestone formation. However, all events were well below (by about 1,800 feet) the lowermost gas producing zone of the conventionally developed UD gas field. The events above the Tully Limestone were clustered along pre-existing joints, faults and bedding planes.
Second, the DOE sought to determine if natural gas or fluids had migrated upward to the UD gas field. The possibility exists the pressure created by fracking will create pathways between the area fractured and upper zones. Pressure differences between the gas wells in the UD gas field and the six wells in the Marcellus Shale showed no increases in pressure in the upper zones, which would be expected if there was communication between the over-pressured Marcellus Shale wells and the wells in the UD gas field. Additionally, the carbon isotope signature of UD gas is distinctly different from Marcellus Shale gas. A comparison showed no sign Marcellus Shale gas had migrated upward. Finally, tracers were injected along with fracturing fluid; no indication of the tracers was found in the upper wells.
In summary, the study concluded the fracking process did not create microseismic events in the overlying rock formation in such a fashion as to cause any problems to the UD gas field. Additionally, there was no communication found between any of the fracking fluids or gas from the lower zones to the upper zones.
Another study confirmed poor well casing and cementing can create a pathway for methane to seep upward into a drinking water supply or even a basement where an explosive situation could develop. The study, conducted by professors from Duke University and other universities, examined the composition of natural gas in about 141 drinking water wells located in six counties in Pennsylvania. It found drinking water wells within 1 kilometer of a natural gas well were more likely to have methane concentrations. However, the study concluded the most likely cause were faulty or inadequate well casing or imperfections in the cement sealing. In short, the fracking process did not cause methane to impact the drinking water.
These two studies suggest the process of hydraulic fracturing does not create an upward pathway for methane. However, poor quality control in well completion, which could occur on a conventional well, needs to improve to ensure stray gas methane does not contaminate drinking water supplies.