Working on the North Slope, AK Pt. III
Checking out the non-existent traffic, I indicate my intent to turn & ease into the empty lot. My shift begins at midnight, and there are few vehicles on the roads. A sign has been posted in the parking lot, declaring the maximum speed as 5mph, not unusual. I glance around the rental equipment: massive land movers, cranes, generators and other machinery necessary for oil production.
Petroleum is essential for our current lifestyle. Although our focus should be shifting to the next energy source, it is essential that we understand the basics of petroleum production in order to be more educated and make informed decisions about our future. Wikipedia has a great explanation of oil production basics. To summarize that information, there are three main steps: discovery, drilling & production.
During the first step, geologists and geophysicists search for regions that have high potential for the presence of petroleum. Geologists use many pieces of information to determine if a region has this potential. First, they use the basic research on a region, understanding the rock layers and the geologic history. The classic conventional petroleum reservoir has 5 factors- a source rock, a reservoir rock, adequate environment for petroleum generation, a path for the petroleum to travel from the source into the reservoir, & a trap/seal. Each of these factors are required; petroleum will not be produced if a single factor is missing.
Petroleum consists of microscopic plants and animals deposited on the ocean floors after dying. They were buried with the fine muds transported great distances from land. These organic-rich muds were buried, and as the Earth’s tectonic cycles changed, new layers were deposited, such as sands if the water levels dropped or salts if the salt-rich (saline) waters were trapped and evaporated. Temperature and pressure increase with depth, and once the buried mud reached an appropriate depth, the organic materials were broken down into simple hydrogen and carbon (hydrocarbon) molecules we know as petroleum. Oil is created at a lower temperature/pressure point than natural gas. Since these molecules are less dense than the surrounding materials, they will rise if pathways (fractures/faults) exist. If not, then they remain trapped in the mudstones/shales. These are now referred as “unconventional reservoirs,” which will be discussed later.
The hydrocarbon molecules will fill the available space in the neighboring rocks. This space is referred to as “pores,” the amount of space within the rocks is called “porosity,” and the ability of molecules to move through the rock called “permeability.” Rocks like sandstones and limestones can have large amounts of space for the hydrocarbon molecules to stay. These are the conventional reservoir rocks. Since the pores tend to be connected within the rock, they are considered highly permeable, which means the molecules continue to rise until there is something that stops them.
Rocks like shale and mudstones tend to have low permeability, and they can form a seal to prevent the hydrocarbons from moving onward. This results in the petroleum building up within the rock. Faults and other geological features can also trap the petroleum if they are not permeable. Natural gas will often form a cap on top of an oil reservoir because the natural gas has a lower density than the oil. If there is no seal or trap for the petroleum to collect, it will seep out of the ground or into the ocean. Oil seeps are an obvious indicator that there is a high potential for petroleum production.
Unconventional reservoirs are different from conventional reservoirs because they lack the pathways for the hydrocarbons to move out of the source rocks. Instead, the source rock acts as the reservoir and the trap/seal. It is difficult to estimate how much petroleum is stored within these rocks. Fracking is a method of creating fractures within these rocks. The fractures increase the permeability of the rocks, and the oil or natural gas can move out. Fracking is a controversial method of extraction, and there will be a later post dedicated to this subject.
To be continued…
Sources: https://en.wikipedia.org/wiki/Extraction_of_petroleum https://en.wikipedia.org/wiki/Petroleum_reservoir https://en.wikipedia.org/wiki/Petroleum
