Today’s post looks at ways injected hydraulic fracturing fluid might move through the subsurface and contaminate groundwater. The information in this post is based on the U.S. Environmental Protection Agency’s (EPA) December 16, 2016 report: “Hydraulic Fracturing for Oil and Gas: Impacts from the Hydraulic Fracturing Water Cycle on Drinking Water Resources in the U.S.”

 

Wells are constructed to withstand the pressures applied during injection of hydraulic fracturing fluid. The fluid is mostly water and sand with on average about 1 % chemical additives. Wells are constructed by inserting steel casing down a borehole and cementing it next to the rock formation. The cement prevents fluid and gas from coming back up along the sides of the casing. State regulators generally require protection of groundwater resources and so casings are cemented (at a minimum) from the surface to below the groundwater zones. The goal of hydraulic fracturing is to inject solution down the hole under enough pressure to produce long horizontal and vertical fractures in the oil and gas formation. The sand in the solution then props the fractures open, so the gas and oil can flow out of the rock and up the interior of the casing to the surface. Once the pressure on the well is released, most of the hydraulic fracturing fluid comes back up the well and is captured in tanks for disposal. It is the fate of the fraction of fluid lost in the fractures which is a concern in EPA’s study. There are fluids and minerals already existing in the oil and gas formation. Some of these fluids and minerals come back up the well with the hydraulic fracturing fluid. EPA’s report examines whether these naturally occurring fluids and minerals can be activated by the hydraulic fracturing process and move through the subsurface. EPA is also concerned with whether the hydraulic fracturing process might cause gas in the targeted oil and gas formation or in the overlying rocks to move through the subsurface into groundwater.

 

Unfortunately the EPA has only been able to identify a limited amount of research on fluid and gas movement once an oil or gas well has been hydraulically fractured.

As a result there are a lot of limitations on EPA’s ability to provide any definitive conclusions on fate and transport of gas, hydraulic fracturing fluid, or naturally occurring groundwater and minerals through the subsurface. EPA examines two pathways fluids and gas might move into groundwater during and after hydraulic fracturing. The first pathway is a result of the age old method of human frailty, in other words, a bad well design or poor construction of the well. Also included in this pathway is the lack of permanence of human construction or your basic entropy – the eventual degradation of the steel casing or cement. The second pathway EPA examined is the movement of gas and fluid through the fractures created by hydraulic fracturing into naturally existing fractures, faults, and nearby operating or abandoned oil and gas wells, or in other words, unintended consequences.

 

What kind of scientific or technical evidence did EPA find to support the existence of these two pathways? First there’s the technical data EPA collected from the oil and gas industry, although it is incomplete. There is the research by non-EPA scientists who have examined data collected from the oil industry or who have produced independent data themselves, and then there is groundwater modeling. As someone who has conducted and used groundwater models, I always caution on the usefulness of this methodology. There are so many variables in the subsurface; it is often hard to use models as more than just a tool to assess the many probabilities which might occur. But in this case, groundwater modeling has provided some interesting and pretty definitive insights into what can happen to fluids and stray gas during and after hydraulic fracturing.

 

So did research show there are subsurface pathways through which groundwater contamination can occur during and after hydraulic fracturing takes place? EPA reviewed data collected from the oil and gas industry and found that in roughly 3 % of wells hydraulically fractured, there was a mechanical failure of the well. This means some fluid or gas may have escaped along the outside of the casing during the hydraulic fracturing process. Where it went is a guess. Just because the casing or cement failed, it doesn’t mean fluid or gas moved into groundwater. Only in about 0.5 % of these cases did the well not have cement or some other type of barrier protecting the groundwater. Of course there are some notable well failures which have contaminated groundwater, like the ruptured casing resulting in a large surface spill in Killdeer, North Dakota during 2010. Based on groundwater chemistry, scientists think there are also some hydraulically fractured gas wells in the Marcellus Shale in Pennsylvania which have contaminated drinking water resources with natural gas as a result of poorly cemented casing. And there’s another incident in Ohio where gas moving up the exterior of casing into groundwater was thought to have been caused by poor cementing. A number of oil and gas wells being hydraulically fractured are not new wells. Of the oil and gas wells EPA examined about 19 % were older wells. They ranged in age from 8 days to 51 years. EPA noted there were more casing and cement failures in these older wells. So yes there is a limited amount of data showing oil and gas wells being hydraulically fractured can fail and be a pathway for groundwater contamination.

 

What about movement of fluid and gas through new and existing fractures? What technical evidence did EPA find to determine if this was a clear pathway for contamination? Studies have shown that hydraulic fracturing results in vertical fractures in rocks at depths greater than 2000 feet and horizontal fractures in rocks shallower than 2000 feet. EPA was interested to know if fractures can extend out of the rock formation being fractured and into overlying rocks containing groundwater. EPA cites studies showing vertical fractures from hydraulic fracturing operations can extend several thousand feet upward, however fracture length is really dependent on the rock formation being fractured. For example in the Marcellus Shale, most vertical fractures only extend for a few hundred feet. EPA argues the depth of most oil and gas rock formations and their great distances from overlying aquifers prevents movement of fluids and gas along induced fractures into drinking water. Based on the data reviewed by EPA, only 20 % of wells had less than 2000 feet between the shallowest point where fractures could extend upward and the base of the drinking water formation.  This doesn’t mean fractures are not a pathway through which groundwater could be contaminated. It just means it is highly unlikely in most cases. EPA has more concern about this potential pathway for contamination when hydraulic fracturing is either in the same formation as a drinking water resource or a drinking water resource is less than 200 vertical feet from the oil and gas formation, as occurs in several places in Wyoming and California.

 

EPA examined several studies assessing the likelihood of what happens to hydraulic fracturing fluid which does not come back up a well once a well is depressurized. These fluids are called “leak off” and most studies show they are absorbed onto clays or are inhibited from moving out of the local area of the well by various physical forces in the subsurface. You’ve got to remember, the rocks being fractured lack permeability (the ability of liquid and gas to move through them.) It’s why the rocks are being fractured in the first place. But then again, some studies have shown the “leak off” fluid can displace gases in the pore space, which means until the oil and gas well starts operating and sucking up all the oil and gas in the area there is a period when you might have some movement of the gas into and along the fractures. Most modeling studies show the gas shouldn’t reach overlying drinking water resources without a direct conduit upward like a fault or an abandoned well. And then there are a few conflicting studies, where natural gas from underlying oil and gas formations has been found in shallow aquifers and EPA has been unable to identify a clear subsurface conduit.

 

So has EPA proved the potential of poorly constructed and damaged wells to transmit gas and fluid to drinking water resources – yes and no. They have certainly shown a few documented cases where contamination has occurred as a result of poor well construction and well failure. EPA has not however provided a good thesis for how frequently contamination along this pathway occurs. Not their fault – there is simply not a lot of available data which EPA could rely on to make such a determination. How about the potential pathway along natural fractures, faults, and abandoned wells? Has EPA proven or disproven the existence of such routes for fluid and gas movement and the degree of impacts groundwater is suffering as a result? Again yes and no – EPA has cited a couple of studies which have shown direct movement of gas up uncemented casing into natural fractures and other subsurface features then into ground and surface water. But then the modeling of such scenarios presented in the EPA report pretty clearly shows that once production starts all gas and liquids should flow into the production well and not be migrating elsewhere.

 

EPA states “The limited amount of available information hinders our ability to evaluate how frequently drinking water impacts are occurring, the probability that these impacts occur, or to what extent they are tied to specific well construction, operation and maintenance practices. This significantly limits our ability to evaluate the aggregate potential for hydraulic fracturing operations to affect drinking water resources or to identify the potential cause of drinking water contamination in areas where hydraulic fracturing occurs.”