The industry often paints a picture of the geology in the Fayetteville Shale that is not actually what it is like. They often show the fracturing taking place many thousands of feet below groundwater, with layers of "impermeable" rock in between. This is the case in some shale plays, however the Fayetteville Shale is relatively shallow and the layers above and below it are highly faulted and fractured.
Multiple groundwater studies by the USGS, AGS, and various universities show that the Fayetteville Shale area lies within an aquifer system that extends from the surface down to 15,000 feet. An aquifer is an underground region of groundwater. Gas wells that are fractured in the Fayetteville Shale are being done so literally in the middle of an aquifer.
The AOGC is charged with protecting "fresh water zones." Groundwater becomes saline, or too salty at a certain depth and is generally unfit for human use. In 2011 the AOGC increased the required casing depth for wells in the Fayetteville Shale from 500 feet to 1,000 feet deep. The depth of the fresh water zones in the area varies but the groundwater zone that fracturing is occurring in is interconnected with fresh groundwater zones above that are used for drinking water and agriculture.
Also, wells in the Fayetteville Shale area are relatively shallow compared to most other major shale plays. While most wells in places like the Marcellus Shale range from 5,000 feet deep to 12,000 feet deep, the shallowest wells in the Fayetteville Shale are only 1,200 feet deep. Over half are less than 4,000 feet deep and over 80% are less than the one mile (5,280 feet) that is often touted by the industry.
As you read about these aquifers there are two terms that you need to know, porosity and permeability. These are terms to describe the characteristics of rock. These characteristics determine the nature of the aquifer in the area. Porosity is the amount of open space within a rock that can hold water. Permeability is how easily water can pass through the rock.
What you would want are layers of rock above the Fayetteville Shale with low porosity and low permeability. This would be more solid rock than that with high porosity and permeability. Permeability is the more important characteristic when looking at whether water will pass through it. If a layer is low in porosity but high in permeability that means the rock itself is a dense, solid rock, but there are cracks in the rock that allow water to pass through it easily. This is what exists in rock layers above the Fayetteville Shale. There are layers with low porosity but the natural faults and fractures in the area give these layers high permeability.
Normally the groundwater cycle from the surface, to deep in an aquifer, and back to the surface takes thousands of years. After injecting billions of gallons of fluid at high pressure into over 4,000 wells in the area, this process is sped up. New fractures and faults are created, and the chemicals dissolve the rock and change the chemistry of the rock. Fluid migration can also be helped by all the wells in the area themselves. As casings fail over time or from other causes like earthquakes and poor workmanship, these wells provide additional avenues for this fluid to migrate up into shallow groundwater zones.
Aquifer systems in Northern Arkansas
Aquifer systems in Northern Arkansas (side view)
Western Interior Plains Confining System
Stratigraph of rock layers compared to aquifer systems in Fayetteville Shale area