White paper: Hydraulic fracturing
Hydraulic fracturing, a technique that has helped unlock vast new oil and natural gas supplies in the United States – and which has potential to do the same worldwide – has in this century become a contentious issue in which both proponents and opponents express strong views.
Proponents have said the use of hydraulic fracturing has resulted in historically high levels of recovered oil and gas, which has helped lower energy costs. Opposing arguments maintained hydraulic fracturing, most widely known by the colloquial term “fracking,” was dangerous to U.S. drinking water reservoirs.
Some resolution of the topic was achieved in June 2015, when the U.S. Environmental Protection Agency issued a long-awaited report on the practice, concluding that’s its four-year study found no signs of “widespread, systemic” pollution that were linked to “fracking.”
Still, for many people not in the industry, details about the technology, concepts, intent and utilization of the technique remain a mystery.
This paper, produced by the AAPG Division of Environmental Geosciences (DEG), has been offered to help answer those questions. Part of our mission is to communicate to the public the science behind issues that affect the exploration for and production of petroleum and other energy mineral resources.
As a scientific and professional community of earth scientists, AAPG and its DEG are committed to responsibly developing the energy resources that are the foundation of modern society. We must do so in a manner that protects the environment.
As such, this paper will:
- Provide an overview of the hydraulic fracturing process.
- Illustrate and discuss the impact of hydraulic fracturing to oil and gas production.
- Examine how the technique can be used safely and without threatening the environment.
We believe hydraulic fracturing has indeed helped boost U.S. petroleum reserves to historically high levels, and has enhanced U.S. energy security and lowered fuel costs to consumers across the nation.
We also believe hydraulic fracturing practices, like any industrial process, must be done using procedures and approaches that reduce the risks to humans and the environment. Consumers clearly appreciate the many economic benefits of low oil and natural gas prices, from cheaper gasoline to lower heating and cooling bills, but also can be fearful of potential negative consequences that hydraulic fracturing could bring to their communities.
Industry, governments, regulatory bodies and many advocacy groups understand that hydraulic fracturing operations can be done safely and effectively. All of society benefits when they are.
- 1 The evolution of hydraulic fracturing in the United States
- 2 Hydraulic fracturing – How it works
- 3 A matter of control
- 4 Lessons learned
- 5 Safeguards and mitigating risks
- 6 The future of hydraulic fracturing and energy production
- 7 Annotated bibliography
- 7.1 Basic Information about the Shale Resource and the Hydraulic Fracturing Processes:
- 7.2 Economic Impact of shale development:
- 7.3 Best Practices and Standards:
- 7.4 State and Federal Regulation
- 7.5 Hydraulic Fracturing Fluids:
- 7.6 Induced Seismicity:
- 7.7 Water Use for Hydraulic Fracturing
- 7.8 Methane in Aquifers:
- 7.9 Methane Air Emissions: there is significant variation in measured, estimated or modeled values:
- 7.10 Traffic, Trucking Best Practices:
The evolution of hydraulic fracturing in the United States
Although public discussion and awareness of hydraulic fracturing processes have become widespread in the past several years, the technique has been used for decades. Improvements in technology and implementation practices have helped it to become an essential process for the development of unconventional resources, such as shale gas and shale oil, which continue to transform U.S. energy production. Hydraulic fracturing has been cited as a key technology that has now made the U.S. the world’s largest global oil producer.
“Hydraulic fracturing” is a general term for the technique used to “fracture” rock in an oil or gas reservoir by pumping water (or other fluid) and sand (or other granular material) into a sedimentary rock layer at a pressure that is greater than the earth’s natural pressure at that depth. The oil or natural gas that producers are looking to extract consists of hydrocarbon molecules trapped in the tiny spaces between the grains of sand or tiny clay particles that comprise the rock. By creating artificial cracks in the rock through hydraulic fracturing, these molecules are able to flow out along these cracks to the well, where they are brought to the surface. In nature, there is much more hydrocarbon in the rock than historically has been produced. Hydraulic fracturing as a technology provides a more efficient method of producing the additional oil and gas.
Strategies similar to what we today would call hydraulic fracturing were first used to increase fluid flow in oil and water wells as early as the 1860s, although the first experiment with modern hydraulic fracturing techniques occurred in 1947, by Stanolind Oil and Gas Corp., at the Hugoton gas field in Grant County, Kansas.
Use of the technique expanded with little public fanfare over the next several decades, improving the performance of thousands of wells across the United States.
The first large scale U.S. hydraulic fracturing operation was conducted on a well in Stephens County, Oklahoma, in 1968, when water with half a million pounds of sand (as a proppant to keep the artificial cracks open) was injected into the rock formation. (In comparison, today’s large-scale hydraulic fracture programs may inject more than 300,000 pounds of proppant into oil or natural gas bearing rock formation.)
Success brought more confidence in using the technique, and as a result massive hydraulic fracturing programs have been utilized since the 1960s to stimulate wells in oil and natural gas producing areas across the United States.
In a related development, horizontal drilling started in the 1940s and became common in the 1980s. Horizontal wells, by design, have a much larger area of contact with the producing rocks, which in turn allows greater oil or natural gas or oil flow to the well.
In the 1980s operators began applying massive hydraulic fracturing techniques to horizontal wells, most notably in the Austin Chalk Formation of Texas. The technique proved to be highly effective, and since then has been applied successfully for recovery of both oil and gas from shale reservoirs, which would otherwise produce small, uneconomic volumes. This advancement has significantly increased the available global volume of fossil fuel resources.
Hydraulic fracturing – How it works
The concept behind hydraulic fracturing is simple and straightforward: Natural cracks and fractures in brittle rocks that generally are tightly closed by the force of the weight and pressure of overlying rocks – but by pumping water into these closed fractures you can force them to temporarily open (and even generate new cracks), which provides a pathway for oil and gas to flow from the rock to the well.
The fractures stay open only as long as there is a pressure applied. Therefore, to help keep them open longer, a proppant also is pumped into the cracks to “prop” them open even after the injection process ends, releasing the pumping pressure. This allows the oil and natural gas in the rock layer or unit to continue to flow to the well.
(“Proppants” are tiny sand grains, although man-made or specially engineered grains, such as resin-coated sand or high-strength ceramic materials like sintered bauxite, also may be used.)
Hydraulic fracturing is also called a “stimulation technology,” because its purpose is to stimulate additional oil and natural gas to flow into the well. The most visible process of a hydraulic stimulation occurs when the equipment arrives to pump fluids and proppant from the surface into the targeted zone of stimulation, the rock layer containing oil and natural gas. This part of the operation includes large industrial trucks and other equipment.
It is not uncommon on a large stimulation project to have 30-40 trucks and several dozen storage tanks in a scenario that looks like a very large construction site concentrated in a very small space; the drilling operation itself may cover about three acres.
The technology is essentially the same for offshore wells, accomplished with boats and equipment modules that can be placed on the drill ship or offshore platform.
A hydraulic stimulation project must have all of this equipment linked together to be able to push a slurry of water and sand down a narrow pipe to a depth of one to two miles – and then along a lateral pipe to a distance of possibly more than a mile and a half. The pumps must overcome not only the friction of the water in the pipe, but also the natural pressure of the geologic formation at depth. Thus, many pumps working together are necessary to both overcome friction and squeeze the water and sand mixture into the fractures. The horizontal part of the well is stimulated in multiple sections. The stimulation of each section takes a few hours and the entire hydraulic fracturing operation will be spread over two to five days.
A matter of control
Before a well can be stimulated several concentric columns of pipe are cemented in the wellbore. These serve to isolate the oil and natural gas producing formation from any aquifers and often shallower oil and gas zones. All pressurized pumping operations are controlled from a centralized operations station that monitors pressure, flow rates and other variables. The equipment can digitally monitor the overall hydraulic pressure both on the surface and down in the well as the hydraulic stimulation is performed.
Specifically, the control station monitors:
- The pressure on the rocks.
- When the fractures open.
- The pressure necessary to pump into the fractures.
- The volumes of water and sand being injected.
There also are natural physical properties of the rocks and regional tectonic stresses that help to control the hydraulic fracturing process.
Different types of rocks have different physical properties and contain different fluids: oil, natural gas and water. The location, size and design of the rock’s existing fractures, density and variations in subsurface pressures also affect the hydraulic fracturing operation, so all of these factors must be considered for every type of rock that will be hydraulically stimulated.
For most oil and gas wells, the force of gravity on the rock generally determines the vertical pressure at any depth. Vertical pressure on the rock increases as you go deeper, just as water pressure increases as you go deeper in the ocean.
An additional complication is that rocks are deposited in basins and are not all horizontal – and they are pushed as the continents shift. The vertical pressure always generates the largest stress on a rock; the horizontal stresses on a rock will control the lateral spreading of the hydraulic fractures. Fractures tend to form in the direction of the least stress, which is generally horizontal.
Because of this, individual hydraulic fractures and the zone of fractured rock always are wider than they are tall. While each is unique, an average fracture volume created by a hydraulic fracturing job in shale is on the order of 800-1,000 feet wide and only 300-400 feet tall.
This is an important point: The volume of overlying rock and the pressure this creates at an average well depth of about 8,000-9,000 feet is so great that the risk of creating a fracture that could extend vertically to the surface of the earth or fresh drinking water aquifers is extremely low.
Also, clay beds or salt layers that are located between the stimulated zone and groundwater make it extremely unlikely for fractures to extend vertically upward across such layers, because they act as strong barriers, effectively absorbing the energy of the hydraulic stimulation.
In some cases – the Marcellus Shale, for example – the distance between the gas/oil-bearing strata and the deepest fresh water aquifer is more than 5,000 feet. In others, as in the San Joaquin Valley, this distance can be more than 25,000 feet.
The location and magnitude of created fractures typically are monitored using microseismic and tiltmeter technology, geophysical methods that measure very small seismic events and motion caused by breaks in the rock as it is fractured. The sensors used to measure these events, described as the equivalent to jumping one foot in the air and landing, allows operators to have a better understanding of the volume of rock that they are fracturing and where the cracks are forming.
These monitoring programs show that the hydraulically formed fractures are located far from aquifers.
Exploration successes tied directly to the use of hydraulic fracturing caused a significant uptick in U.S. exploration, including in areas where hydraulic stimulation was not commonly known or well understood by all operators or regulators.
This, in part, led to suspicions and objections by anti-fracking advocates, who supported their concerns with stories of:
- Oil field operators draining streams dry.
- Contaminated water being placed back into low-flow streams.
- Causing drilling contamination of shallow fresh water producing zones.
Such reports, understandably, generated concerns among the public.
Indeed, some of these damaging practices did occur.
But fortunately, scientific expertise into the geology of producing zones, experienced insight into the use of best practices, rigorous standards in oil and natural gas producing states and improvements in the technology itself have made such results increasingly rare. State regulators, with their regional geological understanding, provide oversight over industry operations and are accountable to their local communities for the safe and responsible of use of hydraulic fracturing and oil and natural gas production.
For example, today – in typical cases where a well consists of an 8,000 feet vertical section and a long horizontal section of another 6,000-10,000 feet length at depth – the fracture stimulation of the horizontal section is conducted in multiple stages, each over a short horizontal interval. This approach uses much less water while giving the operator much greater control over the process, allowing operators to target specific sections of the well with tailored and more efficient stimulations.
Depending on the geology in a particular locale the fracture stimulation of a well may take four-to-five million gallons of water. Until recently, hydraulic fracturing used fresh water – and major concerns arose about the sources of and total volumes of water used, especially in areas with significant oil and gas operations and in arid areas.
Recent innovations in hydraulic fracturing fluids, however, have made it possible to use produced and recycled water from oil and gas operations. Beneficial use of produced waters for hydraulic fracturing or other commercial applications is expanding – particularly in areas with limited water resources.
Typically more than half of the injected water used for fracking routinely flows back up the wellbore to the surface. Produced water is water and minerals from the treated formation, and may have variable quantities of salts, dissolved solids and organic compounds, that also flows back to the surface. Some produced waters are highly saline, sometimes saltier than seawater, while others are potable (U.S. Geological Survey, 2014).
For all oil and gas production – both wells that are hydraulically fractured and those that are not – about 65 percent of produced water is injected back into the producing formations from which it was extracted; 30 percent is injected into deeper rock units full of saline water; and 5 percent is discharged to the surface (PWS, 2014).
Water and sand make up more than 99.5 percent of the fluid used to hydraulically fracture a well (fracfocus.org).
The accompanying figure and table lists the additional ingredients in typical hydraulic fracturing fluid. The website FracFocus.org, a project of the Groundwater Protection Council and the Interstate Oil and Gas Compact Commission, lists the chemicals used in over 75,000 U.S. wells.
Safeguards and mitigating risks
As previously discussed, hydraulic fractures follow the weaker horizontal stress direction in the rock, making it physically impossible to generate fractures at depths of 3,000 feet or more that can vertically reach shallow aquifers. However it is true that some type of pre-existing open conduit – such as an old unplugged and abandoned oil well – could convey fracture fluids into these zones.
If this were to occur, the result would be a failed stimulation, creating added costs and potential liability for the oil and gas producers. As a result, operators work diligently to locate and properly plug old wells, before they initiate a hydraulic fracturing operation.
The chemicals used in hydraulic fracturing are also a concern for the public and both a safety and health concern for the oil and gas operators. As evidenced by the recent EPA report on the practice, strong regulatory programs exist to monitor and evaluate the use of chemicals in hydraulic fracturing as well as rigid corporate safety programs. The U.S. Department of Energy, National Energy Technology Laboratory (Hammack and others, 2014) conducted a year-long study in western Pennsylvania and found no evidence that chemicals used during the hydraulic fracturing process had contaminated drinking water aquifers adjacent to the drilling site.
Similarly, the risk of induced seismicity (meaureable earthquakes and tremors that are caused by human activity) by injecting water in the subsurface for the hydraulic fracturing process is real, but extremely rare and very manageable. Regulators in many states and the Environmental Protection Agency recommend monitoring the factors that can lead to earthquakes during hydraulic stimulation, such as high injection volumes or pressures and/or the presence of nearby subsurface faults. Standard mitigation measures are reductions in injection volumes and/or pressures, or stopping all injection. In all cases these type earthquakes are very small.
In the United States there are tens of thousands of wells used to dispose of oilfield wastewater, including excess produced waters from hydraulic fracturing. While not directly associated with the hydraulic fracturing-stimulation process in an oil or gas well, a small number of wastewater injection wells have been implicated in generating earthquakes in the U.S. midcontinent and intermountain west. Studies indicate that the injection of large volumes of wastewater under pressure into the storage formations used by these wells has most likely changed the local stress conditions to cause small magnitude and felt earthquakes.
Research is ongoing by state and federal agencies on how to correct and mitigate these issues, and in some cases injection has been terminated.
The future of hydraulic fracturing and energy production
Hydraulic fracturing has helped to more than double U.S. oil production from 2008 to 2015. Over the same period U.S. natural gas production has increased almost 30 percent.
Moreover, U.S. production is expected to continue growing for many years. The 2011 Massachusetts Institute of Technology’s (MIT) “Future of Natural Gas” states “Global natural gas resources are abundant. The mean remaining resource base is estimated to be 16,200 [trillion cubic feet (Tcf)] … 150 times the annual [natural gas] consumption in 2009.”
Exploration of shale basins outside the United States is only beginning, but the International Energy Agency projects that shales have the potential to produce 650,000 barrels of oil per day in 2019, primarily in Canada, Russia, Argentina, Mexico and Australia.
In other words, hydraulic fracturing – thanks to improved application processes in its use – has produced a staggering result: Vast new oil and natural gas supplies that deliver important new sources of energy to the world’s energy resource base.
These engineering and technological advancements, which make the drilling of horizontal wells and hydraulic stimulation techniques a reality, are revolutionary tools, and like any tool must be used properly.
Global demand for oil and natural gas continues to grow, and will for decades to come. Meeting the needs of the developed and developing world alike will require the petroleum industry to find and produce new oil and natural gas resources. Hydraulic fracturing will help make this possible.
Basic Information about the Shale Resource and the Hydraulic Fracturing Processes:
- AAPG Energy Minerals Division information on shale resources
- U.S. Department of Energy, Shale Gas 101 describes what it is, why it is important, and explains environmental challenges associated with its production.
- Hydraulic Fracturing: How it Works, including the process, the site setup and fracturing fluid management, by FracFocus Chemical Registry.
Economic Impact of shale development:
- Duke University (May 2016) finds that even a year after oil prices collapsed oil and gas development had positive impacts on most local governments that struggled to maintain roads and services during the boom.
- National Bureau of Economic Research study (October 2105) by Dartmouth professors focuses on the income and employment consequences of fracking, finding that over a third of fracking revenue stays within the regional economy.
Best Practices and Standards:
- U.S. Secretary of Energy Advisory Board Natural Gas Subcommittee report (2011) gives best practice recommendations and documents their implementation in the final report.
State and Federal Regulation
- EPA requires “Green Completions” by 2015
- Bureau of Land Management (BLM) has proposed hydraulic fracturing rules for federal lands. Federal courts have blocked enforcement of the rule.
- BLM released a proposed rule to reduce venting and flaring of natural gas in February 2016. The final rule is expected in late 2016.
- EPA plans to require data on equipment types and methane emissions from all well and facility operators. The information would be used to apply the May 2016 rule for new sources to existing wells.
Hydraulic Fracturing Fluids:
- FracFocus, sponsored by the Interstate Oil and Gas Compact Commission and the Ground Water Protection Council, lists chemicals used in over 100,000 wells.
- Baker Hughes, world’s third largest hydraulic fracturing service provider, has since 2014 provides a complete, public list of chemicals in its hydraulic fracturing fluids.
- U.S. Geological Survey: “What you do and don’t know about induced seismicity" & U.S. Geological Survey frequently asked questions about earthquakes caused by wastewater injection.
- New U.S. Geological Survey “New Hazard Model for Induced Earthquakes” maps risk of damage from earthquakes (April 2016).
- U.S. National Research Council study on induced seismicity (2013) (the PDF is free to download)
- The StatesFirst Induced Seismicity Work Group released its report (October 2015), “Potential Injection-Induced Seismicity Associated with Oil & Gas Development: A Primer on Technical and Regulatory Considerations Informing Risk Management and Mitigation.” StatesFirst is an initiative of the Interstate Oil and Gas Compact Commission (IOGCC) and the Ground Water Protection Council (GWPC).
- “Oklahoma’s recent earthquakes and saltwater disposal”: F. Rall Walsh III* and Mark D. Zoback (2015, Science Advances). Saltwater disposal is predominantly produced water, not hydraulic fracturing flow-back.
- Kansas Geological Survey monitoring and mitigation of earthquakes related to saltwater disposal, report to the state legislature, January 2016.
Water Use for Hydraulic Fracturing
- Comparison of Water Use for Hydraulic Fracturing for Unconventional Oil and Gas versus Conventional Oil, B.R. Scanlon and others, 2014, Environ. Sci. Technol.
Methane in Aquifers:
- EPA draft assessment of the potential impacts of oil and gas hydraulic fracturing activities on the quality and quantity of drinking water resources in the United States released in June 2015. Read executive summary or full report.
- Science Advisory Board Hydraulic Fracturing Research Advisory Panel documentation of its public meetings and peer review is here.
- Methane and other gases occur in shallow aquifers and formations above the Marcellus, and predate Marcellus drilling: ”A geochemical context for stray gas investigations in the northern Appalachian Basin: Implications of analyses of natural gases from Neogene-through Devonian-age strata”, Fred J. Baldassare, AAPG Bull. V. 98, No. 2 (February 2014), P. 341–372.
- Methane found in water wells in the Denver-Julesburg basin, Colorado, is mostly microbially generated in shallow coal seams. About 0.06 to 0.15 percent of wells leaked thermogenic methane due to inadequate surface casing, casing leaks or wellhead-seal leaks (Owen Sherwood and others, 2016).
Methane Air Emissions: there is significant variation in measured, estimated or modeled values:
- National Energy Technology Lab: Life Cycle Greenhouse Gas Emissions: Natural Gas and Power Production presentation to EIA June 2015 Energy Conference.
- EPA National Inventory shows that 2013 methane emissions from natural as systems declined 12 percent from 2005, while production increased about 28 percent over the same period.
- Barnett Coordinated Measurement Campaign, seven articles in the July 7, 2015, Environ. Sci. Technol. v. 49, issue 13, document airborne measurements over the Barnett shale production region in Texas and the small number of “super emitters”.
Traffic, Trucking Best Practices:
- The American Petroleum Institute, the American Trucking Associations and the National Tank Truck Carriers have collected nearly two dozen recommendations for roadway safety and more considerate driving practices.