GSA Critical Issue: Hydraulic Fracturing


Schematic of rock grains, pore space and permeability Figure 5:
Schematic of rock grains, pore space, and permeability. The interconnection of spaces between grains allows flow of gas or other fluid. Modified from Bureau of Economic Geology, The University of Texas.

Oil and natural gas, which are hydrocarbons, reside in the pore spaces between grains of rock (called reservoir rock) in the subsurface. If geologic conditions are favorable, hydrocarbons flow freely from reservoir rocks to oil and gas wells. Production from these rocks is traditionally referred to as “conventional” hydrocarbon reservoirs. However, in some rocks, hydrocarbons are trapped within microscopic pore space in the rock. This is especially true in fine-grained rocks, such as shales, that have very small and poorly connected pore spaces not conducive to the free flow of liquid or gas (called low- permeability rocks)(fig. 5). Natural gas that occurs in the pore spaces of shale is called shale gas. Some sandstones and carbonate rocks (such as limestone) with similarly low permeability are often referred to as “tight” formations. Geologists have long known that large quantities of oil and natural gas occur in formations like these (often referred to as tight oil or gas). Hydraulic fracturing can enhance the permeability of these rocks to a point where oil and gas can economically be extracted.

Schematic geology of natural gas resources Figure 6:
Schematic geology of natural gas resources. Modified from U.S. Energy Information Administration and modified from U.S. Geological Survey Fact Sheet 0113-01.

Hydraulic fracturing (also known colloquially as “fracing,” or “fracking,”) is a technique used to stimulate production of oil and gas after a well has been drilled [15]. It consists of injecting a mixture of water, sand, and chemical additives through a well drilled into an oil- or gas-bearing rock formation, under high but controlled pressure. The process is designed to create small cracks within (and thus fracture) the formation, and propagate those fractures to a desired distance from the well bore by controlling the rate, pressure, and timing of fluid injection. Engineers use pressure and fluid characteristics to restrict those fractures to the target reservoir rock, typically limited to a distance of a few hundred feet from the well. Proppant (sand or sometimes other inert material such as ceramic beads) is carried into the newly formed fractures to keep them open after the pressure is released and allow fluids (generally hydrocarbons) that were trapped in the rock to flow through the fractures more efficiently. Some of the water/chemical/proppant fracturing fluids remain in the subsurface. Some of this fluid mixture (called “flowback water”) returns to the surface, often along with oil, natural gas, and water that was already naturally present in the producing formation. This natural formation water is known as “produced water” and much of it is highly saline [16]. The hydrocarbons are separated from the returned fluid at the surface, and the flowback and produced water is collected in tanks or lined pits. Handling and disposal of returned fluids has historically been part of all oil and gas drilling operations, and is not exclusive to wells that have been hydraulically fractured. Similarly, proper well construction is an essential component of all well-completion operations, not only wells that involve hydraulic fracturing. Well completion and construction, along with fluid disposal, are inherent to oil and gas development, and are specifically addressed in this paper because of concern about them and their relationship to hydraulic fracturing.

Hydraulic fracturing of shales and other tight rocks typically is through horizontal or directional (non-vertical) drilled wells (fig. 6) which typically involve longer boreholes and much greater volumes of water than conventional oil and gas wells..

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