Hydraulic Fracturing and UDSW protection
ALL Consulting
Lead author: J. Daniel Arthur
Contributing authors: Bill Hochheiser, and Brian Bohm
Introduction
Natural gas production from shale formations is a fast-growing segment of the United States’ natural gas supply, and is considered one of the leading growth sectors to push the United States closer to utilizing stable domestic sources of energy. Natural gas development from shales has also resulted in the movement of the oil and gas industry into areas that have not had previous experience in oil and gas development activities. Production of natural gas from shale formations has become feasible because of advances in hydraulic fracturing and horizontal drilling technologies.
Hydraulic fracturing originally came under scrutiny as a result of concerns that fracturing fluids from coal bed natural gas (CBNG) wells in Alabama were migrating to drinking water aquifers and affecting water wells. These concerns resulted in a 1995 lawsuit seeking regulation of hydraulic fracturing activities in Alabama under the Safe Drinking Water Act’s (SDWA) underground injection control provisions. The court found that hydraulic fracturing for oil and gas development fits the definition of underground injection under the SDWA’s definition and, as a result, Alabama revised its regulations to include hydraulic fracturing in coal beds under its Underground Injection Control (UIC) program. In addition, due to the Environmental Protection Agency’s (EPA’s) concern regarding the use of diesel fuel in fracturing of coal beds, EPA signed a Memorandum of Understanding (MOU) with three major oil and gas service companies to eliminate the use of diesel fuel as part of hydraulic fracturing fluids for CBNG wells across the United States. The actions taken in Alabama prompted an EPA investigation into the threat to drinking water resources from hydraulic fracturing. The results of this investigation, published in 2004, determined the threat to drinking water sources from the hydraulic fracturing of coal bed formations was "minimal"[1]. Shortly after these results were published, Congress exempted hydraulic fracturing from the UIC program as part of the Energy Policy Act of 2005.[2]
The recent increase of shale gas development in a number of areas around the country has heightened public awareness and concern regarding hydraulic fracturing. Shale gas development entails fracturing operations using large volumes of water (generally 3 to 5 million gallons per well), and small amounts of chemical additives to fracture the horizontal wells. While general descriptions of the constituents of these additives and their purposes are publicly available, the precise additive formulas are kept proprietary by the service companies that manufacture and sell them. Much of the public residing in the shale gas basins is unfamiliar with oil and gas drilling and production activities, creating a perception of industry “secrecy” surrounding additive formulas and leading the public to distrust oil and gas producing and service companies. As a result, various non-governmental organizations and public interest groups have been working to raise the alarm about potential groundwater contamination and possible chemical exposure. Concerns have been expressed that the exemption for hydraulic fracturing as part of the Energy Policy Act of 2005 removed prior oversight of hydraulic fracturing by the Environmental Protection Agency.[3] This has led to a movement to repeal the hydraulic fracturing exemption and to regulate hydraulic fracturing under the UIC program, although industry experts and regulatory agencies have found little to no direct evidence that hydraulic fracturing is a threat to human health or the environment. It’s important to note that hydraulic fracturing has never been regulated under UIC program as established by the Safe Drinking Water Act during the 35-year history of the SDWA.[4]
On June 9, 2009, legislation was introduced in both houses of the United States Congress to amend section 1421(d) of the Safe Drinking Water Act (42 USC300h(d)). This legislation was introduced by Diana DeGette, D-Colo., Maurice Hinchey, D-N.Y., and Jared Polis, D-Colo, in the U.S. House of Representatives and by Sen. Bob Casey, D-Pa., and Sen. Chuck Schumer, D-N.Y, in the Senate. The House and Senate bills are both referred to as the “Fracturing Responsibility and Awareness of Chemicals (FRAC) Act”, and propose repealing the 2005 Energy Policy Act exemption of hydraulic fracturing from UIC regulation under the SDWA. Additionally, the bills would require disclosure to regulatory and emergency medical staff of the chemical constituents utilized in the hydraulic fracturing process. This information is currently required for Material Safety Data Sheets (MSDS), which provide contact information for emergency personnel (doctors, nurses and emergency responders) requiring specific chemical identification and concentrations of proprietary formulas from the chemicals manufacturer in the event of a medical emergency.[5]
Overview of Hydraulic Fracturing
The existence of significant quantities of natural gas from deep shale formations has long been known by oil and gas producing companies; however, this type of natural gas was not perceived as being economically accessible until the late 1990s. Shale formations have naturally low permeability, which makes it difficult to access the natural gas resources held within the rock. Engineers were able to determine that by pumping water and sand into the wellbore under high pressure, it was possible to create small fractures or cracks in the shale, thus creating the necessary pathways for natural gas to flow to a well. This process, hydraulic fracturing, has matured and changed since it was first used in the 1940s. It is currently a sophisticated, closely controlled and monitored engineering process, overcoming technical and economic barriers that have traditionally limited the development of shale gas.
Fracture Fluid Additives
Slickwater fracturing, the predominant hydraulic fracturing method used for development of natural gas wells in shale formations, is a technique that was refined in the Barnett Shale play of Texas during the late 1990s. Slickwater fracturing is partially responsible for improving the economic viability of the play.[6] Previous fracture techniques for extremely tight rock formations like shales used thicker gel-based fracturing fluids to transport proppants into the formation.[7] These thicker gel-based fluids were too expensive to use economically at the volumes necessary for the fracturing of the horizontal wells in shales. Slickwater fracture fluids are less expensive because these fluids are predominantly water. Gelled fracturing fluids use a polymer base, typically an organic guar, that when hydrated with water forms a viscous gel that is used to carry the sand during the fracture treatment.[8]
Slickwater fracturing requires the addition of chemicals to the water-sand mixture for specific purposes such as reducing viscosity, preventing bacterial growth, or preventing the corrosion of the piping used in the well. The make-up of fracturing fluid will vary from one basin to another, from one contractor to another, and even from one well to another. Well characteristics, formation properties, and fracturing fluid compositions are a few of the criteria used to select the most appropriate additives for the stimulations. Unique challenges such as scale buildup, bacteria growth, etc., require specific additives to prevent degradation of the well performance; however, not all wells require all categories of the additives for a treatment. Furthermore, while there are many different formulas for each additive, only one of each category of additive is added per well. For example, only one biocide is used at a time, even though there are many different biocides to choose from.
A typical fracture fluid will include four to six additives, but some may require a dozen or so. However, even when a large number of chemical additives are used to address multiple potential issues like scale development and bacteria growth among other things, the fluid is still overwhelmingly water. For instance, a Fayetteville Shale slickwater fracture fluid which included 12 additives still contained 99.5 percent water with less than 0.5 percent other compounds. Notably, most of these compounds are similar to ones used in everyday consumer products like food additives, cosmetics, and household cleaners. Even those components that have toxicity associated with them in their pure forms, such as acids used to clean the near-wellbore area, are highly diluted in the pumped fluid. These chemicals are further altered and diluted by reactions in the subsurface. One example would be the acids that are pumped in fracturing fluids reacting with the rock in the subsurface such that the acid ends up being converted into inert salts.[9]
Protection of Ground Water
The initial public concerns related to groundwater impacts associated with hydraulic fracturing came from the hydraulic fracturing of CBNG wells in Alabama. Those wells were hydraulically fractured with fluids that, instead of having a water base, were primarily composed of diesel fuel with additives added to help carry proppant and address other formation issues such as bacteria growth, scale development and corrosion of metal equipment. The concern with CBNG wells is that these wells are typically shallow (in some basins less than 1,500 feet deep[10]) and in some cases may actually be completed in drinking water aquifers.[11] In contrast, shale gas formations are often located a few thousand to over ten thousand feet below ground. The very nature of shale gas geology is one of the key pieces of evidence supporting the limited risk to groundwater from hydraulic fracturing activities. With average depths ranging from 5,000 feet to 11,000 feet below the land surface (depths which are 3,000 or more feet deeper than most of the coal beds investigated by the EPA in their 2004 study) gas shales such as the Barnett Shale, Fayetteville Shale, Marcellus Shale, and Haynesville Shale are located a great distances below fresh groundwater formations.[12] The rocks present between the shale formations and groundwater aquifers create a substantial barrier to fluid migration from hydraulic fracturing activities. In fact, it is the impermeability of these rock layers that has kept the natural gas in the shale allowing operators to produce the gas that is present today.
The presence of thousands of feet of rock formations between gas shale formations and shallow fresh groundwater represents just one of the protections that prevent fracture fluids from reaching groundwater aquifers. A tremendous force would be required to create a fracture that would extend from thousands of feet of depth upward into a groundwater formation; far more force than is imparted in a hydraulic fracturing operation. Furthermore, it is in the shale gas developers’ best interest to limit the growth of the fractures to the formation that is being developed as fractures which extend out beyond the producing shale formation may encounter other formations which have poor quality water that has been present in the formation since it was originally deposited. If an operator’s fractures extend into a deep source of poor quality water, the economic success of the well can be affected by the cost of producing and disposing of this water, or the hydrostatic pressure created by this water may reduce the volume of gas released from the shale formation. In order to prevent fractures from extending outward into other formations, hydraulic fracturing jobs may be monitored with downhole equipment such as microseismic monitors and tiltmeters.
In addition, there is oversight by state oil and gas agencies, which regulate the drilling and hydraulic fracturing operations to ensure protection of fresh groundwater.[13] These agency programs place a great deal of emphasis on the protection of groundwater through the establishment of well construction standards along with required agency approval of all well completions (or recompletions), including proper placement and installation of multiple layers of protective steel casting and cement that is designed specifically to protect fresh water aquifers by preventing the migration of fracturing fluids upward to overlying groundwater sources and aquifers. [14] The effectiveness of the steel casing and cements was demonstrated by risk analysis sponsored by the U.S. Department of Energy and performed under the guidance of the American Petroleum Institute in the 1980s.[15] The analysis evaluated the effectiveness of well construction practices on protecting groundwater and fresh water aquifers by evaluating the potential for a leak to occur in an injection well. The analysis concluded that the probability of a well in which the lowest source of drinking water has been 100 percent cased corroding to the point of developing a leak that could potentially reach a groundwater source is between a 2 x 10-5 (one in 200,000) and 2 x10-8 (one in 200,000,000) chance per well year.[16] This analysis was based on wells operating as injection wells in which fluids are injected at a moderate pressure into the well every day for extended periods of time. This operation is in contrast to the manner in which gas shale wells are operated where hydraulic fracturing occurs over a short period of time (usually not more than a week) but at higher pressures. Analysis of the details of the research shows that most of the leaks that were documented resulted from long-term corrosion and not short-term damage; thus the protections offered by a modern horizontal shale gas well in which 100 percent of the drinking water aquifers are protected by cement and casing should be even greater than what was shown in the API research.
Summary
Shale gas is a key part of the current United States energy policy, which aims to ensure an adequate, dependable, affordable, domestic energy supply. Hydraulic fracturing is a necessary completion practice for the development of natural gas from shale formations. It is a highly engineered, closely controlled and monitored process, which is overseen by state regulatory agencies. The fluids used in the hydraulic fracturing of shale gas formations are over 98 percent water, with additives consisting of chemicals that are generally found in everyday consumer products and are always very dilute in the fracturing fluid.
The hydraulic fracturing of shale gas formations takes place thousands of feet below fresh water aquifers and thus has considerable additional protections when compared to fracturing of shallow CBNG wells. Additionally, the current state oil and gas regulatory requirements, such as well construction standards which include casing integrity and completion techniques, provide considerable environmental protection. Through these oil and gas regulatory requirements, hydraulic fracturing has always been regulated, regardless of the exemption that was created in the 2005 Energy Policy Act; hydraulic fracturing is regulated today in the same way it was regulated before the 2005 Energy Policy Act exemption.[17] Furthermore, the geological stratification in shale gas production areas provides additional protections to public health and fresh water aquifers which are almost impossible to overcome by the forces generated by hydraulic fracturing of gas shales. Overall, the hydraulic fracturing of shale gas occurs at such distances from fresh groundwater sources that the natural barriers offered by the intervening geologic layers combined with required well construction practices prevent fracturing fluids from migrating to fresh groundwater aquifers.