Fracking (Hydraulic Fracturing)
As the reserves of recoverable liquid petroleum diminish, oil companies are increasingly turning to unconventional resources. Unconventional resources include such things as extra heavy oil and shale. It is estimated that as much as 30% of total world oil reserves are in the form of shale. Because shale is a fine-grained, sedimentary rock, the oil it contains does not easily flow and therefore must be “released” before it can be pumped from the ground. The process employed is called hydraulic fracturing, and generally consists of four stages.
Hydraulic fracturing is most often performed in horizontally drilled wells. After a period of vertical drilling in order to reach shale deposits, a lateral extension of up to 5000 feet is drilled parallel to the rock layer containing the shale. Lateral drilling has many advantages, including reduce ground surface disruption.
In the next step, water is injected into the recently bored hole. Occasionally, other substances such as gels, foams, compressed gases and even air are injected. Chemical mixtures are usually included in the injection. Chemicals are intended to increase the permeability of the rock by dissolving various components. The exact composition of the chemical injection is based on the geological composition of the area.
Added chemicals include acids, biocides to kill bacteria, corrosion inhibitors, and surfactant. Specifics include hydrochloric acid, methanol, ammonium chloride, ethylene glycol (antifreeze), isopropanol (rubbing alcohol). Approximately 750 chemicals are listed as possible additives for hydraulic fracturing.
The fluid is injected under high pressure with the intent of fracturing the soft shale. Pressures can reach as high as 15,000 pounds per square inch (100 Mpa) and injection rates can be as high as 265 liters per minute. Injected fluid is recovered, to some degree, and stored in surface containers. Do to the chemical additives, this material is often toxic.
Seismic monitoring is used to estimate how large the induced fractures are as well as their orientation, resulting in a rough geometric map of the fracture. This mapping helps to ensure that injected fluids are not leaked during progressive lengthening of the fracture zone. Fracturing monitoring also allows for the identification of underground “reservoir zones,” where a shale deposit may expand beyond the lateral bore hole.
This refers to the cessation of fracturing and the active recovery of liquid petroleum. This process usually involves preparing the “bottom” of the well, running piping, and cementing a casing. It is no different than the process that would be used to pump liquid oil from “traditional” deposits. Additional fracturing may occur at later dates in an effort to “stimulate” or enhance production in a slowing well.
Environmental risks include air quality damage, migration of gases to the surface, and groundwater contamination. The Energy Policy Act of 2005 amended the Safe Drinking Water Act to exclude the chemical and fluids used in fracking from EPA jurisdiction. There have been claims of water contamination due to fracturing in Texas, North Dakota, Pennsylvania, Colorado, and Louisiana.
The EPA has jurisdiction over waste disposal of any fluids recovered from fracking and brought to the surface. They have limited authority over those fluids that remain in the ground. A leak from a surface containment facility is within EPA jurisdiction, but a leak in the underground borehole may not be.