Managing Environmental Risks During Fracturing

Although fracturing has been in use for more than half a century, the recent proliferation of the use of the technology to access unconventional resources has led to increasing awareness and concern about potential environmental impacts.

Opportunities come with risks, and it has long been a priority of industry and activists alike to balance the risks and benefits associated with oil and gas development. The good news is that, given educated regulators and effective regulations, the risks of hydraulic fracturing can be reduced to a manageable level.

In this section, we’ll cover the science behind the environmental concerns associated with hydraulic fracturing and what can be done to make sure these risks remain at a reasonable and socially responsible level.

Groundwater Protection

The possibility of groundwater contamination has long been one of the most controversial aspects of hydraulic fracturing. In this section, we’ll take a look at the science.

First, remember that most horizontal wells are thousands of feet deeper than the deepest groundwater. Underground Sources of Drinking Water (USDW’s in the EPA’s terminology) are usually a few hundred feet deep, rarely deeper than 1000 feet.

In contrast, most of the oil and gas rich shale formations that we are drilling in the U.S. now are on the order of 6,000-12,000 feet deep. The depth to the Barnett Shale, for instance, is about 7,000 feet. Marcellus wells are averaging around 8,000 feet deep. The Bakken is over 10,000 feet deep. So there are a lot of layers of rock separating near-surface, fresh water aquifers and these target oil and gas formations being fractured. This rock serves as a barrier between the fracturing fluids and groundwater resources.

Fracture Height Containment

Many studies have been done in an attempt to better understand exactly what is happening in the subsurface during fracturing operations. We can look to these studies to provide evidence about whether a fracture could extend into a groundwater zone, providing a conduit for fracturing fluid and hydrocarbon migration to cause groundwater contamination.

One thing the studies indicate is that we have a rather fortuitous set of geologic conditions in the subsurface. First, if a fracture reaches the margin of a producing shale, it’s likely to terminate rather than extend into another formation. That’s because oil and gas shales are relatively more prone to propagating fractures than the layers above and below them. The rocks above and below our target reservoirs are often more ductile so they won’t fracture, or they have higher stresses and strengths so we can’t create enough pressure to open fractures in them, or they are too permeable and porous and suck the fluid out of the fracture, stopping its growth. In addition, since a fracture will leak off faster and faster as it grows bigger, and its total volume is limited to the amount of fluid pumped, it is unlikely that a fracture could extend thousands of feet through rock.

From an economic standpoint, when an oil and gas company fractures a well, they want to contain the fractures to the reservoir zone. There is no reason to fracture anything else and they are actually wasting money if the sand and fluid go somewhere other than into the reservoir rock. Therefore there is a financial disincentive to fracturing out of zone.

The size of a hydraulic fracture (its length, width, and height) can be calculated based on the volume of fluid we pump.  For any given fracturing job, we know how much water we’re putting into the ground – and we know where that water leaves the wellbore.

The distance a hydraulic fracture can go away from the wellbore (whether up toward the surface or laterally within the formation) is limited by the volume of fluid pumped into it. It’s a straight-up volumetric limitation. Thick fractures have less length and height, while thin fractures can reach further from the well bore, but the dimensions are finite and predictable.

In the unlikely situation where a fracture might escape the immediate vicinity of the reservoir, other factors come into play, the most influential of which is that as a fracture approaches the earth’s surface, the earth stresses change to promote horizontal fractures instead of vertical ones.  So if a fracture rises to a relatively shallow depth, the fracture will turn and go horizontally because that requires less effort than trying to go straight up to the surface. The natural state of subsurface stresses is actually a lucky break for energy companies engaged in fracturing operations.

Finally, from a research perspective, there is a good study on this topic of height growth that uses microseismic data from thousands of fracture treatments. These researchers used microseismic monitoring, and they collected data from fracturing activity in numerous states.

The summary of this work, as it relates to this topic, is that there is no evidence in any of these thousands of treatments that any fractures have covered even half the distance to the freshwater aquifers.1Fisher, K., and Warpinski, N., 2012, “Hydraulic fracture height growth: Real data”, SPE Production and Operations (February), 8-19.

Given all of this, fracture propagation from the target formation into the near-surface aquifers does not appear to be something that we need to be concerned about.

Potential Contamination Paths

That sounds encouraging, so are we safe from the risk of groundwater contamination?

Not quite yet.

The most likely path for fracture fluid leakage into groundwater systems is not through fractures, but rather through a corroded or poorly cemented well casing. Of course this isn’t a problem inherent only to hydraulic fracturing – any oil or gas drilling activity has the potential to leak fluids into a groundwater zone through the casing. The only difference with hydraulic fracturing is that some consider the chemicals used in a fracturing process to be more harmful than those used during normal drilling and production operations.

Fortunately, we have access to a wide range of technologies capable of assessing cement jobs and casing integrity. In addition, a legal and procedural framework for minimizing the risks of leaks is already in place. The best thing we can do to prevent groundwater contamination is to ensure that well casing is installed carefully and according to specifications.

Hydraulic Fracturing Drinking Water Assessment

From our assessment, we conclude there are above and below ground mechanisms by which hydraulic fracturing activities have the potential to impact drinking water resources. These mechanisms include water withdrawals in times of, or in areas with, low water availability; spills of hydraulic fracturing fluids and produced water; fracturing directly into underground drinking water resources; below ground migration of liquids and gases; and inadequate treatment and discharge of wastewater.

We did not find evidence that these mechanisms have led to widespread, systemic impacts on drinking water resources in the United States. Of the potential mechanisms identified in this report, we found specific instances where one or more mechanisms led to impacts on drinking water resources, including contamination of drinking water wells. The number of identified cases, however, was small compared to the number of hydraulically fractured wells.

This finding could reflect a rarity of effects on drinking water resources, but may also be due to other limiting factors. These factors include: insufficient pre- and post-fracturing data on the quality of drinking water resources; the paucity of long-term systematic studies; the presence of other sources of contamination precluding a definitive link between hydraulic fracturing activities and an impact; and the inaccessibility of some information on hydraulic fracturing activities and potential impacts.

Excerpt from EPA Hydraulic Fracturing Drinking Water Assessment Executive Summary[JL1]2United States Environmental Protection Agency, 2015, Assessment of the Potential Impacts of Hydraulic Fracturing for Oil and Gas on Drinking Water Resources, http://ofmpub.epa.gov/ (accessed July 1, 2015).

Images: “Environmental Risks” by Jim Blecha