Pallise, Janna, Science Scope
Hydraulic fracturing, also known as "fracking" or "hydrofrack-ing," seems to be everywhere these days. Reports on major news networks, on Nation-al Public Radio, and online are aplenty. There have even been antifracking protests within the Occupy Wall Street movement. What is hydraulic fracturing and why is it such a hot topic? This month's column will address these questions and the concerns surrounding the issue.
Hydraulic fracturing (HF) is an unconventional technique in gas production that has been around in some form since the 1940s (EPA 2011; NYT 2011). The gas extracted through HF is highly dispersed in rock, instead of in a concentrated underground location, and dispersed gas is produced only by special stimulation techniques. This relatively recent method has opened up new areas of gas development in natural gas reservoirs such as shale, coalbed, and tight sands (EPA 2011). (This article will focus on HF in shale reservoirs.)
Shale gas refers to natural gas that is trapped in shale (fine-grained sedimentary rock) formations. In 2009, about 14% of natural gas production came from shale formations. Shale gas is found in shale "plays"--shale formations with significant amounts of natural gas. Important plays include the Marcellus Shale in the eastern United States and Barnett Shale in Texas (see Figure 1) (U.S. EIA 2011).
How it works
HF creates fractures in the rock formation that stimulate the flow of natural gas. Wells are drilled vertically hundreds to thousands of feet below the land surface and can include horizontal or directional sections extending thousands of feet. Fractures are created by pumping large quantities of fluids at high pressure down a wellbore into the target rock formation. The fluids consist of millions of gallons of water, chemical additives, and proppants (sand, ceramic pellets, or other incompressible particles) that open and en-large fractures within the rock formation. Fractures can extend hundreds of feet away from the wellbore. The proppants hold open the newly created fractures and natural gas flows from the shale to the well (EPA 2011; U.S. EIA 2011) (see Figure 2).
After fluids have been injected, the internal pressure of the rock formation causes the fluid to return to the surface. Known as "flowback" or "produced water," this fluid can contain the injected chemicals as well as naturally occurring materials (hydrocarbons, brines, metals, and radionuclides). The flowback is stored on site in tanks or pits before treatment, disposal, or recycling. Often, it is injected underground for disposal. Flowback can also be treated and reused or processed at a wastewater treatment plant and then discharged to surface water (EPA 2011).
HF is a booming, rapidly growing industry. Advocates cite the generation of domestic jobs and revenue as a benefit of HF (NYT 2011). As compared to other natural resources (e.g., coal and oil), natural gas is cleaner (NYT 2011). The combustion of natural gas emits significantly lower levels of carbon dioxide, nitrogen oxides, and sulfur dioxide than the combustion of coal or oil. The natural gas produced through U.S. HF operations means less reliance on foreign sources of natural gas (U.S. EIA 2011).
Hazards associated with natural gas production and drilling are not as well known as with other fossil fuels, and regulations have not kept pace with production (NYT 2011). Escalating concerns include adverse effects on drinking water, human health, animals, and ecosystems.
Perhaps the greatest concern with HF is the effect on water, including drinking-water supplies. Concerns about potential indirect impacts include surface discharge of wastewaters, depletion of drinking-water supplies, and methane migration (NYT 2011) (see The Fuss Over Fracking video in Resources).
During the fracturing process, injected fluids can flow to other areas of the formation ("fluid leakoff"); if not controlled, fluid leakoff can reach 70% of the injected volume and may result in fluid reaching drinking-water aquifers (NYT 2011). …