Hydraulic fracturing of oil and gas reservoirs (often abbreviated as “fracking”) has received a great deal of public attention and some controversy. While media outlets often imply that it’s a relatively new and poorly understood technology, the first experimental job was performed in 1947 and it has been done over 2.5 million times since then (Montgomery and Smith, 2010). More recent applications to shale gas production in the Marcellus and Utica deposits have led to a significant energy boom for the region and the country. Increased public awareness of this technology has also brought about concern over potential environmental impacts. Decisions made by land owners, business leaders, and government officials will have significant economic and environmental consequences over the next several decades, and it is vitally important that science play a significant role in these decisions.
Hydraulic Fracturing Basics
Oil and natural gas resources are stored within the pore space of rocks, far beneath the surface of the earth. Production of these resources requires installation of a deep well with tubing to carry these fluids to the surface. The effectiveness of a well depends greatly on the flow of fluid from the surrounding area to the well bottom, usually through the pore spaces in the rock. This flow depends mainly on the viscosity of the fluid (ability to flow) and the permeability of the rock (pathway for flow). Hydraulic fracturing increases flow by creating a network of new flow paths (fractures), particularly in very low permeability rock where the rock pores are not well connected. It is done by rapidly pumping high pressure fluid into the rock formation until cracks form. A proppant (usually sand) is included with the fluid as a means of holding the new fractures open enough to allow oil or gas to flow. It has been estimated that hydraulic fracturing has increased US oil reserves by 30% and natural gas reserves by 90% (Montgomery and Smith, 2010).
Concern #1 – Uncontrolled Fractures
One of the most significant fears concerning the hydraulic fracturing process comes from perceived uncertainty in the length of fractures. There is often a public misperception that fractures can penetrate all the way to the surface or to shallow, fresh-water aquifers. This scenario is not feasible for a few reasons (Fisher and Warpinski, 2012).
- The leading tip of a fracture propagates due to a force (fluid pressure) applied at the base to spread the sides of the fracture apart. A fracture can only propagate as long as there is pressure at the tip. There are physical limitations to how far the force applied at the well base can propagate away from the well, particularly considering that the fracture is confined in all directions by surrounding rock.
- There are limits to fracture height from the natural layering of rock in the earth’s crust. Every interface between rock types crossed by a propagating fracture dissipates force and thus reduces or stops growth.
Real world field data has demonstrated that fracture propagation is limited. Microseismic monitoring is a technology that can map induced fractures by locating the source of vibrations induced by fracturing rocks. This technique has been applied to thousands of hydraulic fracturing events to determine the height of fracture propagation in four different shale gas reservoirs (Fisher and Warpinski, 2012). In every case, the highest fracture remained thousands of feet below the deepest freshwater aquifer or water well.
Concern #2 – Stray gas migration
The uncontrolled migration of natural gas into shallow freshwater aquifers, commonly referred to as “stray gas”, presents safety concerns, particularly for homeowners that rely on well water. A demonstration of this phenomenon that has gotten attention on TV/film is lighting of a flowing kitchen faucet on fire. While an oft-cited study by Osborn et al (2011) showed a correlation between methane concentration in water wells and distance to the nearest gas well, critics of this study (eg. Davies, 2011) point out that the sample size was very small (60 wells) and not random since many of the available data points were in the vicinity of a single known contamination incident. The lack of baseline (pre-drilling) data also makes their results difficult to interpret, particularly since more recent studies (Baldassare et al., 2014; Wilson, 2014) have determined that natural background methane from both biogenic (created by microbes) and thermogenic (from high temperature cracking of deep organic material) sources is common throughout the state of Pennsylvania, and can be detected prior to any shale gas well drilling activity in the area.
However, most agree (Osborn et al., 2012; Davies, 2012; Darrah et al., 2014, Baldassare et al., 2014) that the most likely mechanism for gas migration is wellbore leakage rather than hydraulic fracturing. In fact, all investigated incidents of stray gas(above background level) in Pennsylvania have been attributed to failure in production casing or in cement allowing flow from shallow, gas bearing zones that are penetrated by the well (Darrah et al., 2014, Baldassare et al., 2014 ). Such failures can be diagnosed and repaired as needed and are not an indictment of the fracturing process occurring thousands of feet below the surface.
Concern #3 – Groundwater quality
Groundwater quality is an important concern for the shale gas production industry particularly with the processing of hydraulic fracturing fluid or high salinity water from the deep subsurface. It is important to note that there has not been a single case of groundwater contamination attributed to hydraulic fracturing fluid or formation water leaking from a shale gas reservoir (Brantley et al., 2014).There are multiple reasons why such fluids will not leak from the reservoir to the near surface:
- There are multiple layers of rock separating the reservoir from any potential source of drinking water, many of them impermeable. This impermeability has trapped natural gas in the subsurface for millions of years.
- Saline waters found at depth are of higher density than fresh water, so the tendency is to sink deeper rather than flow upward.
- The pressure from hydraulic fracturing is applied over a period of only a few hours, not enough time for migration of fluids through thousands of feet of rock.
- Gas production throughout the life of the well continuously decreases the pressure of the reservoir. For this reason the long term effect is to draw surrounding fluids down into the reservoir rather than push fluid upward.
There is some risk of groundwater contamination for any deep well, but it is related to well construction and waste management rather than hydraulic fracturing. The most significant risk is related to handling and disposal of waste water (Rozell and Reaven, 2012). When fracturing fluid is pumped back to the surface (“flow back” fluid), it is typically stored on-site in tanks or containment ponds before being transported away and disposed. It is important to design containment and disposal systems for drilling, hydraulic fracturing and flow back fluid to minimize the risk of leakage and to avoid unwanted releases.
Concern #4 – Earthquakes
Public fear also focuses on the potential for hydraulic fracturing to induce seismicity. The potential for earthquakes triggered by pumping fluids in the subsurface, particularly in the vicinity of a critically stressed fault, has been recognized (NRC, 2012). Almost all documented occurrences of induced seismicity were associated with the long duration injection of large fluid volumes rather than the short term (hours) pressurization that characterizes hydraulic fracturing (NRC, 2012). For hydraulic fracturing, induced seismicity is extremely rare – only 1 occurrence for 35,000 hydraulically fractured shale gas wells in the United States – and the impact has been negligible. The one incident tentatively tied to hydraulic fracturing (Holland, 2013) consisted of a series of minor earthquakes with maximum magnitude of 2.9 that occurred in 2011 in the vicinity of a well in southern Oklahoma. Barely detectable by local residents, the magnitude of the tremors was well below the threshold for potential damage to structures.
As with any industrial process, the development of shale gas resources requires consideration of potential environmental impacts. Although much public scrutiny has focused on hydraulic fracturing, the process itself does not introduce new risk beyond that of more conventional oil and gas production, which mainly relate to well integrity and handling waste materials at the surface. The industry has over 50 years of experience with hydraulic fracturing with millions of jobs performed. By following existing regulations and continually improving best practices, the risk to the environment can be minimized.
Baldassare, F.J., McCaffrey, M.A., Harper, J.A. (2014). “A Geochemical Context For Stray Gas Investigations In The Northern Appalachian Basin: Implications Of Analyses Of Natural Gases From Neogene-Through Devonian-Age Strata,” AAPG Bulletin 98(2), 341-372.
Brantley, S.L., Yoxtheimer, D., Arjmand, S., Grieve, P., Vidic, R., Pollak, J., Llewellyn, G.T., Abad, J., Simon, C. (2014). “Water Resource Impacts During Unconventional Shale Gas Development: The Pennsylvania Experience,” Int. J. Coal Geol. 126, 140-156.
Darrah, T.H., Vengosh, A., Jackson, R.B., Warner, N.R., Poreda, R.J. (Sept 15, 2014). “Noble Gases Identify the Mechanisms of Fugitive Gas Contamination In Drinking-Water Wells Overlying the Marcellus and Barnett Shales,” Proc. Natl. Acad. Sci.- Early Edition, Published online before print.
Davies, R.J. (2011). “Methane Contamination of Drinking Water Caused by Hydraulic Fracturing Remains Unproven,” Proc. Natl. Acad. Sci. 108(43), E871.
Fisher, K., Warpinski, N. (2012). “Hydraulic-Fracture-Height Growth: Real Data,” SPE Production and Operations, SPE-145949.
Holland, A.A. (2013). “Earthquakes Triggered by Hydraulic Fracturing in South-Central Oklahoma,” Bull. Seismol. Soc. Am.103(3), 1784-1792.
Montgomery, C.T., Smith, M.B. (2011). “Hydraulic Fracturing: History of an Enduring Technology” Journal of Petroleum Technology 62, 26–32.
National Research Council, (2012). Induced Seismicity Potential in Energy Technologies, National Academies Press, Washington, DC.
Osborn, S.G., Vengosh, A., Warner, N.R., Jackson, R.B. (2011). “Methane Contamination of Drinking Water Accompanying Gas-Well Drilling and Hydraulic Fracturing,” Proc. Natl. Acad. Sci. 108(20), 8172-8176.
Rozell, D.J., Reaven, S.J. (2012). “Water Pollution Risk Associated with Natural Gas Extraction and the Marcellus Shale,” Risk Analysis 32(8), 1382-1393.
Wilson, B. (2014). “Geologic and Baseline Groundwater Evidence for Naturally Occurring, Shallowly Sourced, Thermogenic Gas in Northeastern Pennsylvania,” AAPG Bulletin 98(2), 373-394.