- Explain how the process of hydraulic fracturing differs from conventional oil and gas extraction.
- Describe the processes that allow the recovery of oil from tar sands.
- List advantages and disadvantages of unconventional fossil fuel extraction.
Conventional pumping of oil and gas a permeable reservoir is not the only way to obtain them. We now can extract oil and gas from deeply buried shale (tight rock) or sand formations close to the surface. These new or unconventional methods of extracting oil and gas have become important as, over the last 150 years, we have extracted much of the easily-obtained fossil fuel resources that formed ~300,000,000 years ago.
We have picked the “low-hanging fruit.” We are now extracting petroleum and methane from sands that are mixed with tar – tar sands. This method occurs only near Earth’s surface and requires vast amounts of water and energy to heat the tar in order to separate, treat and pump the viscous product hundreds of miles through pipelines that often leak. The surface where this extraction occurs is radically changed and emissions from this process are found in the air, water and soil.
You learned in the last section that sandstone is the most common reservoir rock. This is because sandstone can be permeable. Oil finds its way into these pores. However, shale also can be a reservoir. It has many small pores that may contain oil, but, until recently, we did not know how to get to that oil. With the development of hydraulic fracturing (fracking), vast new stores of oil and gas have been opened.
Let’s look at tar sands and fracking.
Tar Sands / Oil Sands
Video 10.4.1 – Oil Sands 101 (2:56)
Tar sands, or oil sands, are sand deposits that contain viscous petroleum products (like tar) and thus can not be drilled and pumped out of the ground like conventional oil. Tar sands contain bitumen or asphalt which is so thick or resistant to flowing that it must be steam heated while in the ground to extract. This process uses a lot of water and energy, making the tar sands process very polluting and expensive. In some cases, the oil sands can be dredged and heated, though, again, this is a dirty process. Alberta, Canada has the largest known reserves of tar sands in the world. (A resource becomes uneconomic once the cost of extracting it exceeds the revenue which is obtained from the sale of extracted material.)
To create one barrel of oil from this material requires approximately 25 m3 of gas. The other environmental cost of oil sands production is the devastation of vast areas of land where strip-mining (an open pit) is taking place, tailings ponds are constructed, and the unavoidable release of contaminants into the air, groundwater, and rivers of the region.
Oil Shale and Hydraulic Fracturing (Fracking)
Oil shale (or tight oil) is a fine-grained sedimentary rock that has a significant quantity of petroleum or natural gas. Shale is a common source of fossil fuels with high porosity but it has very low permeability. In order to get the oil out, the material can be mined and heated, which, like with tar sands, is expensive and typically has wide spread negative environmental impacts. 
Another process to extract oil and gas from shale and other unconventional tight resources is called hydraulic fracturing or fracking. In this method, high-pressure injections of water, sand grains, and added chemicals are pumped underground, creating and holding open fractures in the rocks, which aids in the release of the hard-to-access fluids, mostly natural gas. This is more useful in tighter sediments, especially shale, which has a high porosity to store the hydrocarbons but low permeability to transmit the hydrocarbons. Fracking has become controversial due to the potential for groundwater contamination and resulting earthquakes (induced seismicity), and represents an area of confrontation between public concerns and energy value .
This kind of energy development requires substantial amounts of water not only for the traditional drilling process, but also for the actual fracturing. Water is first mixed with chemicals, fine sands, and soaps then pumped at extremely high pressure into the shale rock to fracture it, forming pathways for the oil and gas to reach the well borehole. Some locations where fracking is occurring are dealing with water shortages already, setting up conflict between fossil fuel companies and citizens such as occurred in Denton Texas where the city voted to ban fracking in city limits only to have this reversed by the Texas state legislature. This view of the area near (but not in) Denton shows how closely packed the drill sites are. The drill sites are the white pads.
Air quality near fracking sites is diminished. Research has linked pollution from fracking to unhealthy levels of smog and of toxic air contaminants. Exposure to this pollution can cause eye, nose, and throat irritation, respiratory illnesses, central nervous system damage, birth defects, cancer, or premature death, yet the oil and gas industry is exempted from many regulations that limit air pollution from industrial activity. The EPA has issued standards to limit harmful air pollution from the oil and gas industry, but these still contain major gaps. LINK
The water injected into the ground along with produced water (water that was trapped in the rock with oil and recovered) are then brought to the surface and separated from the oil and gas. The water waste must be disposed of, often by re-injecting into the earth. This can result in earthquakes such as those observed in Oklahoma.
Fracking can contaminate groundwater, either from the petroleum resource being produced or from the chemicals used in the fracking process. Fracking fluids or produced water – the fluid pumped out of the well and separated from oil and gas – not only contain the chemical additives used in the drilling process but also contains heavy metals, radioactive materials, volatile organic compounds (VOCs) and hazardous air pollutants such as 12 benzene, toluene, ethylbenzene and xylene. In some cases, this contaminated water is sent to water treatment plants that are not equipped to deal with some of these classes of contamination .
Video10.4.2 – Hydraulic Fracturing 101 (2:08)
Sands or sandstones that contain high-viscosity petroleum