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| Fig. 1: Hydraulic fracturing illustration: well depth, aquifer depth, and water usage. [5] (Courtesy of the EPA). |
The recent dramatic increase in shale gas production has benefitted the United States by reducing energy costs, lowering carbon emissions, and increasing energy security. [1] Hydraulic fracturing, combined with horizontal drilling, enabled the shale gas revolution. In simple terms, hydraulic fracturing is the process of breaking rocks in tight (low permeability) shale rocks by injecting high pressure fluid into the reservoir. The reservoir rock is broken by creating new fractures and opening preexisting fractures. Moreover, a small amount of sand is injected with the fluid to keep the fractures from closing after the injection stops. Once the fractures are in place, the previously trapped oil and gas can freely flow out. In addition to hydraulic fracturing, horizontal drilling is done to maximize the flow by increasing the contact area with the reservoir. Fig. 1 shows an illustration of a typical hydraulic fracturing set-up. The total horizontal drilling length can be as high as 10,000 feet with fracture stages that are several hundred feet apart. [1] Furthermore, the wellbore consists of several alternating layers of steel pipe and cement to stabilize the well and prevent oil and gas leakage. [1]
Contrary to popular opinion, both hydraulic fracturing and horizontal drilling technologies have existed for a long time. The first commercial hydraulic fracturing job was done in the 1950s while the first horizontal well was drilled in the 1930s. [2] In addition to the number of years of experience, the oil and gas industry has drilled more than 100,000 horizontal and multi-stage fractured wells in the United States and Canada. [3] Thus, the industry is well beyond the experimental phase with an extensive amount of knowledge and data from past experiences.
One main concern for hydraulic fracturing is the creation of earthquakes. The injected fluid can leak into small natural fractures found in the surrounding rock. This increases the pressure within these small fractures and can cause small earthquakes to occur. [1] Generally, these small earthquakes have magnitudes of less than -1.5 on the Richter scale. [1] According to Mark Zoback, a Stanford geophysics professor, this amount of energy is equivalent to "a gallon of milk falling off a kitchen counter". [4] Larger earthquakes can occur when injecting into potentially active faults. [3] However, with proper planning and site characterization, injection into preexisting faults can be avoided and injection rates can be controlled to reduce the risk of triggered earthquakes. [4]
Another common misconception is on the distance of shale gas reservoirs from aquifers. There is a legitimate concern about hydraulic fractures reaching the aquifers and contaminating drinking water sources. Typically, however, there are several thousands of feet separating the aquifer base from the shale gas formation as shown in Fig. 1. [1,5] In addition, hydraulic fracture lengths are designed to be limited to the height of the formation (around 300 ft to 500 ft). [1] Moreover, it is highly unlikely that fractures can cross multiple impermeable formations which serve as barriers. [1] Another related concern is on injection fluid composition and water usage. In hydraulic fracturing, chemicals are added to prevent scale formation and bacterial growth. [1] These chemical additives account for 0.5 to 2 percent by volume of the injected fluid. [5] In terms of water usage, 2 to 4 million gallons of water are needed for a typical shale gas horizontal well. [5] Water can be sourced at the site or brought in using trucks. [5] For the initial stages of hydraulic fracturing, 25 to 75 percent of the injected fluid can be recovered and recycled. [5] Furthermore, aside from fresh water, other options for hydraulic fracturing fluid include saline or brackish water. [3,5]
Effective policy making can be achieved with industry players, community stakeholders, and government agencies working together. Discussions on hydraulic fracturing should be based on facts and sound engineering principles. Everyone agrees that hydraulic fracturing should be conducted in a safe and environmentally responsible manner. There are several measures that can be taken to achieve this goal. Close monitoring of seismic activity can track the extent of the fractures and the development of small earthquakes. [1] In addition, proper well design and construction are necessary to prevent the contamination of groundwater sources. [1] Frequent monitoring of emissions and proper maintenance of facilities can ensure that air pollution is minimized. [6] Moreover, transparency is needed in terms of disclosing the chemicals used in hydraulic fracturing. [6] Water usage can be minimized and controlled with water recycling and nonpotable water usage. [3] Moreover, water quality can be protected by the regular monitoring of nearby groundwater sources to detect contamination. [1]
© Carla Co. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. Zoback, S. Kitasei, and B. Copithorne, "Addressing the Environmental Risks from Shale Gas Development," Worldwatch Institute, July 2010.
[2] G. E. King, "Hydraulic Fracturing 101," Society of Petroleum Engineers, One Petro 1525960-MS, 6 Feb 12.
[3] P. Morrison, "Fracking Expert Mark Zoback: We Need Good Science, Good Engineering, Good Regulations and Good Enforcement," Los Angeles Times, 22 Apr 14.
[4] M. Zoback, "Written Testimony to the Committee on Energy and Natural Resources, United States Senate," 19 Jun 12.
[5] "Plan to Study the Potential Impacts of Hydraulic Fracturing on Drinking Water Resources," U.S. Environmental Protection Agency, EPA/600/R-11/122, November 2011.
[6] "Ninety-Day Report"," Shale Gas Subcommittee, Secretary of Energy Advisory Board, U.S. Department of Energy, 11 Aug 11.