Over the past few years, we asked site visitors to submit questions they had about the science behind hydraulic fracturing (or "fracking") and the Marcellus Shale. We’ve selected the most popular and relevant questions and answered them below.
Several factors influence where drilling and hydraulic fracturing operations take place in Pennsylvania.
The first factor is that drilling can only occur in areas where the Marcellus Shale unit is present. This eliminates most of the southeastern part of the state and some of the central valleys, where the Marcellus has either been eroded away or was never deposited in the first place.
The second factor comes down to economics—companies would like to maximize the return on their investment in drilling and fracturing each well—an investment which can exceed 7 million dollars. By studying the geology of the Marcellus in a particular region using geophysics and data from previously drilled wells, companies select parts of the formation that look the most promising.
Once a company has decided which areas to target, it must lease the mineral rights for the land. The exact location of a well pad is often determined by where the company is able to obtain a lease with favorable terms.
Sources:
Tour of a Marcellus Well
General Information on the Marcellus from PA DCNR
Oil and gas companies employ geologists and petroleum engineers whose job it is to predict the nature of the fractures that will form during a fracking operation.
The Marcellus shale, located thousands of feet underground, experiences enormous compressive forces. In order to push the rocks apart and make them fracture, operators need to overcome these forces.
They do this by pumping water into the rock formations. Once the pressure exerted by the water exceeds the pressure acting on the rock, the rock splits. The orientation of the fractures can be predicted by measuring the forces acting on the rocks.
It is in the interest of operators to carefully measure these forces so that they can orient horizontal wells perpendicular to the fractures, maximizing gas production.
Source: Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production, by Norman J. Hyne (2012)
This type of contamination is unlikely for three reasons.
Together, these factors mean that the fractures themselves are not a likely path of contaminant migration. Instead, poorly cemented casing and spillage on the surface are the most likely causes of contamination.
Source: Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production, by Norman J. Hyne (2012)
The number of small (magnitude 3) earthquakes in the eastern and central United States has increased from an average of 21 per year between 1967 and 2000 to around 300 per year between 2010 and 2012.
Most scientists agree that this increase is related to human activity. However, the fracking process itself isn’t thought to be responsible—although fracking does cause seismic activity, the resulting earthquakes are generally too small to be felt or cause damage.
Instead, “induced seismicity” is usually related to deep injection wells. These wells are used to dispose of used fracking fluids by injecting them into rock layers 2,000-8,000 feet below the surface. The injection of fluids deep underground can activate inactive faults, as happened 2011 and 2012, when a series of earthquakes occurred in the Youngstown, Ohio area. The earthquakes resulted in little to no damage, but raised public awareness of induced seismicity and forced the shut-down of the injection well thought to be responsible.
It’s important to note that no injection-related earthquake has occurred more than six miles from an injection well. As of 2014, there are only nine licensed oil and gas injection wells in Pennsylvania.
Sources:
USGS
Ohio Department of Natural Resources
One of the risks associated with gas drilling is that methane will leak from a poorly cemented casing, contaminating groundwater or escaping at the surface. In addition to providing a path for contamination, a poor cement job can threaten the production potential of a well.
Fortunately, there are regulations and tools in place to ensure that the casing is properly cemented. In most states, including Pennsylvania, regulations require that parts of a well passing through a groundwater aquifer be cemented.
Once cement has been placed around the outside of the casing, a special tool called a “cement bond logger” is used to inspect the seal. This tool, which is slowly lowered through the casing, emits sonic pulses. Like an ultrasound, these pulses allow operators to see through the well casing and inspect the cement. If the cement job is lacking, operators are required to conduct a “squeeze” operation to fill in any flaws.
Source: The Geological Interpretation of Well Logs, 2nd edition, by Rider and Kennedy (2011).
It’s important to remember that fracking is only one stage of preparing a well for production. Before the well is ready to be fracked, many other things need to happen.
The process of waiting for favorable economic conditions, choosing a drilling site, and obtaining the needed leases and permits can take months to years.
Once a site has been chosen, the next step is the construction of a well pad. The well pad provides a stable platform for the drilling equipment and is designed to prevent surface spills from causing groundwater contamination. Building a well pad normally takes a week or two.
Next, drillers need to come onto the well pad and actually drill the well. A typical Marcellus well, which starts out vertically and ends horizontally, takes two to four weeks to drill.
After this, the drill rig is moved off the site and the fracking process itself begins. Fracking operations are normally completed in a short period of time—typically between one and five days.
The oil and gas industry estimates that wells drilled in the Marcellus will produce for 20-40 years.
Source: Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production, by Norman J. Hyne (2012)
The advent of directional drilling technology—which allows drillers to “steer” the drill bit and create a curved well—is partially responsible for the proliferation of unconventional wells in the Marcellus shale. Typically, the drill bit is steered from the surface with the aid of a variety of sensors installed near the drill bit. These sensors allow geologists on the surface to track the progress of the bit and adjust its course if necessary.
In contrast to a vertical well, which would only intersect the horizontal Marcellus shale unit at one point, a directional well can follow the unit for thousands of feet. The ability to access large parts of a shale formation from one well is one of the factors that has made drilling and producing gas from such formations economically feasible. Directional drilling has the added benefit of allowing drillers to drill more than one well from the same surface site. This reduces the cost and surface environmental impact of gas development.
Source: Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production, by Norman J. Hyne (2012)
Abandoned and improperly plugged wells pose environmental risks. They can serve as conduits for the migration of methane from deep layers to shallow groundwater or to the surface, where it creates an explosion hazard. Drilling a new well near an abandoned well can also lead to problems. In June 2012, a horizontal well being drilled in Pennsylvania intersected an abandoned well drilled in 1932, resulting in the eruption of a 30-foot column of methane and brine from the abandoned well.
Fortunately, there are mechanisms in place to ensure that gas companies responsibly plug wells when they are done producing. When a company applies for a permit to drill in Pennsylvania, it must submit a detailed technical plan for plugging the well at the end of production. As an additional safeguard, gas companies must post a bond when drilling a new well. If the company fails to meet its obligation to plug the well, the bond is forfeited.
Unfortunately, these mechanisms don’t always work. A company may go out of business, or records might get lost. Unplugged wells with no solvent responsible parties are termed “abandoned” or “orphan” wells.
Since no company claims responsibility for an abandoned well, it’s up to the state government to plug it, nominally using fees collected during the well permitting process. There are thought to be tens to hundreds of thousands of abandoned wells in Pennsylvania, most dating to before the modern regulatory era. At the current rate it will take decades to plug them all.
Source: Pennsylvania Orphan Well Program
Although there are hundreds of chemicals available for modifying frack fluid, generally only ten to fifteen are used in any one well. A petroleum engineer is tasked with developing a “frack program” for each well. The goal is to develop a blend of water and additives that does all of the following:
Plain water is often not dense enough or viscous enough to transport proppants to the fractures. To get around this problem, frack crews include thickeners in the frack fluids. In some advanced applications, the thick fluid used to deliver the proppant is thinned using a “gel breaking agent” after the proppants are in place, allowing gas to flow more easily from the well.
Pumping water down a narrow well and into an even narrower fracture is like drinking a thick milkshake through a long, skinny straw. Some frack fluid additives are used to create what’s called “slickwater.” By reducing the friction along the borehole, slickwater reduces the amount of energy needed on the surface to fracture the rocks.
Some frack fluid additives are used to protect expensive equipment and prevent the pathway the frack fluids must take from becoming constricted or clogged. These additives include biocides, which prevent bacteria from growing and clogging the wellbore, and stabilizing agents, which protect the well casing and other equipment from corrosion.
There are several measures in place to prevent surface spills.
Pennsylvania Act 13, passed in 2012, requires that unconventional wells be set back 300 feet from streams. In addition, if a well pad is built in a flood plain, any storage tanks or pits must be located outside of that floodplain.
Surface storage tanks are surrounded by containment berms, designed to contain spills related to leaks or tank failure, while storage pits are lined with impermeable membranes. In addition, the entire well pad is isolated from the surrounding environment by a geotextile membrane.
Despite these precautions, leaks and spills can and do occur. In Pennsylvania, companies are required to replace any private or public water sources contaminated by the drilling process. In addition, companies may bear the cost of any cleanup associated with the contamination.
Source: Citizen’s Guide to Act 13
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