GEO113: ENVIRONMENTAL GEOLOGY
Fossil Fuels: oil, coal, and natural gas. Supply 90% of U.S. energy
requirements. All form from the
remains of once living organisms. Different types of fossil fuels are due to
differences in source and/or geological process producing the fuels.
Demand for fossil fuels spurred by the industrial
evolution of the 19th century where machines replaced animals and
manufactured products replaced homemade items.
Oil shale, tar sand, and methane hydrate are other fossil
fuels that have historically not been utilized to any great extent, but that
potentially may provide a huge resource of energy.
1)
Petroleum
and Natural Gas
Petroleum or crude oil provides approximately 50% of U.S.
energy y needs. It is not a single
composition, but rather a range of substances, all of which are hydrocarbons.
These are usually long, complex chains (polymers) of C, H, and N.
Natural gas is made of shout hydrocarbon chains,
including methane (CH4), ethane (C2H6), and
propane (C3H8). It provides approximately 25% of U.S.
energy needs. Combined, petroleum and natural gas meet almost 3/4 of our
country's energy needs.
Petroleum and natural gas are made in the same way. Need
a number of things to happen. If one doesnıt, then an economic deposit will not
occur.
1)
Microscopic
marine organisms die and their remains fall to the sea floor.
2)
Organic
material must be buried quickly by more sediment.
3)
Buried
organic material must decay under oxygen-poor (reducing) conditions. Otherwise, under
oxygen-rich (oxidizing) conditions, you wind up with H2O and CO2
(seltzer!).
4)
Under
increasing pressure and temperature you breakdown (crack) complex organic molecules into
simpler ones. Need just the right temperature and the right amount of time.
This is known as the oil window and it is between 50 and 100 degrees Celsius
for 10-20 million years. To low a temperature or too short a time and the
organics do not break down. Wind up with tar. Too high or too short a time and
organics are broken down completely to methane that may escape to the surface.
5)
Need
a reservoir rock
of high porosity and permeability to store the oil and gas that has been made
by natural processes.
6)
Need
a low permeability trap rock to keep oil and gas in place and to stop them from
escaping to the surface. There are a variety of types of traps, including fold,
fault, lithologic, and salt dome. Oil and gas are less dense than water and
will migrate upward until stopped by an impermeable rock trap. Oil companies
spend millions of dollars and employ thousands of geologists looking for traps.
A pure petroleum product suitable for human use is
produced only by refining the crude oil in a cracking tower. Get various
products at various levels of the tower.
II. Crude Oil and Natural Gas Supplies and Demand
Oil: U.S. has 30 billion barrels of proven reserves. One
barrel equals 42 gallons. World has approximately 1,000 billion barrels. U.S.
has less than 5% of world's total.
Gas: U.S. has approximately 200 trillion feet of proven
reserves. World has approximately 3,200 trillion cubic feet. U.S. has
approximately 6% of world's total.
Consumption: U.S. consumes approximately 5 billion
barrels of crude oil per year and approximately 20 trillion cubic feet of natural
gas per year. World consumption rates are approximately 20 billion barrels of
oil per year and 60 trillion cubic feet of gas per year. U.S. consumes
approximately 25% of the world's petroleum and 35% of the world's natural gas.
This requires us to import both.
Estimated total resources are 2-3 times proven
reservoirs. Still will begin to run out by the end of the middle to the end of
this century. The exact time depends on market demands. Will rising oil and gas
prices increase reserves (some of the expensive resource moves into the
economic reserve category) or will new technologies replace fossil fuels.
Regardless, the days of cheap oil and gas may be over soon.
If the U.S. had to rely solely on domestic sources, we
have about 6 years of oil and 10 years of gas. New discoveries may only extend
that for a few years.
III.
Extending
Oil and Gas Supplies.
Only 20-40% of the initial oil reservoir gets pumped out
(primary recovery).
Rest is left in the ground unless other ways are utilized to remove more of the
oil. This costs money.
Water may be pumped in to push residual oil towards the
production well. This is known as secondary recovery. May increase percentage
recovered to 50-60%. Still leaves a lot of oil behind. To increase percentage
recovered still further, tertiary recovery methods can be used. These include:
1)
Hydrofracturing: Increase rock permeability by
opening up fractures with high-pressure water.
2)
Explosive
fracturing:
Increase rock permeability by shattering rock with explosives.
3)
Gas
pressurization:
Force oil towards the production well by injecting high-pressure gas.
4)
Hydrothermal
techniques: Use
hot water or steam to reduce viscosity of the oil, making it easier to flow
towards the well and to pump.
5)
Chemical
additives: Can
decrease oil viscosity and surface tension, making it easier for the oil to
flow and be pumped.
May increase total recovery to 75+% of initial reservoir.
All of this enhanced recovery (secondary and tertiary combined) is very expensive and
is done only if the price of oil is high enough, usually above $25-30/barrel.
However, can dramatically increase world reserves by about 2 times, giving
another 50 years of reserves, depending upon demand. Still only puts off the
inevitable.
It is very expensive to drill for oil and gas. On land,
individual wells may cost more than one million dollars each. Off shore they
may cost tens of millions of dollars each. Usually only 1 in 10 wells drilled
produces any appreciable oil or gas. There may be little easy oil left
regardless of price.
Can also extend supplies by conservation measures,
including insulating homes and driving high-mileage cars. This has caused us
rate of increase of consumption to level off. However, beginning to increase
again as third world develops. Conservation may be difficult for many
underdeveloped countries because of lack of funds for expensive technologies.
Do we have the right to deny other countries because of our previous wasteful
habits?
Environmental impacts of oil and gas drilling,
exploitation, transport, and use are often very high. New oil often now found
in very sensitive areas. Should we destroy these areas for the oil and gas they
contain. Can reduce environmental impacts, but at a price. Once introduced into
the environment, it may take tens or hundreds of years for habitats to recover
from large oil spills. Combustion of oil and gas produces CO2
potentially leading to greater global warming.
Do we want to rely on energy sources that are in short
supply, are getting ever more expensive, and that produce a number of negative
environmental impacts? Is there a better way?
IV. Coal
Presently meets about 20% of U.S. energy needs. Prior to
the utilization of petroleum and natural gas, coal was a very important source
of energy. Fell into disfavor during the 20th century. Now may
become again our most important fossil fuel.
Original organics are land plants, typically found in
swamps and tropical environments. Need to bury their decaying remains under
reducing conditions. Initially peat forms at the surface. This can be used as a very
low-grade source of energy. As
pressure, temperature, and time increase, peat converts to lignite or brown coal, then bituminous or soft coal, and then finally anthracite or hard coal. At each grade, the amount of
carbon and the energy content in the material increases while the amounts of H,
O, N, and P decrease. If cooked too much you get pure carbon in the form of graphite. It has no energy value at all.
Determining the abundance of coal is easier to estimate
than for oil or gas since coal occurs in well-defined layers.
Worldwide proven reserves are approximately 700 billion
tons out of a total resource of nearly 10 trillion tons. U.S. proven reserves
are approximately 200 billion tons out of a total resource of 2 trillion tons.
U.S. has the world's single biggest supply of coal and we have consumed less
than 5% of our reserves. We have hundreds of years of domestic supplies. So why
don't we use it more than we do?
IV.
Problems
With Coal
1) Coal is a solid and this limits its use as an energy
source. Can't be used in cars, planes and other transportation applications.
However, coal can be converted to a natural gas-like
product through a process called gasification. This process has been known
since the 1830s. Unfortunately the process produces a gas with only 25-30% of
the original energy content of the coal. Also the process is expensive.
Coal can also be converted to a petroleum-like liquid
through a process called liquefaction. Germans did it in WWII. South Africa does it today.
However, requires large volumes of water and is more expensive than simply
buying and refining natural crude oil.
2)
There
are many environmental problems associated with the mining and utilization of
coal.
a)
Coal
typically contains high sulfur. Results in acid mine drainage and acid rain.
b)
Coal
burning leaves and ash residue that contains 5-20% by volume of toxic
materials. Where do we store this toxic residue?
c)
Coal
mining, particular strip mining (the cheapest and preferred way) destroys
habitats and vegetation. Land reclamation is possible, but expensive.
d)
Underground
mining is hazardous to miner's health. Explosions of methane gas can occur,
radon concentrations are high, and black lung disease can be fatal. It is the
most hazardous job around.
e)
The
combustion of coal produces large amounts of CO2.
V.
Non-traditional
Fossil Fuels
1)
Oil
shale. This is a
misnomer. Name actually refers to kerogen (a waxy hydrocarbon) in any
sedimentary rock. Need to crush rock and cook rock, extract kerogen, and then
refine it into a petroleum-like liquid. It is estimated that the world has
reserves equivalent to 2-5 trillion barrels of crude oil. This is much more
than actual worldwide oil reserves. Since approximately two-thirds of the
world's reserves are in the U.S., why don't we use oil shale and become energy
independent?
Problems with oil shale:
a)
Dispersed
in rock. Therefore, need to mine very large volumes.
b) Large environmental impacts of
surface mining. Do we want to dig up large portions of the U.S.?
c)
Refining
process requires large volumes of water, approximately 3 barrels of water/
barrel of oil produced. Much of western U.S. already running short of water.
d)
Refining
process increases the volume of rock. Where do we put excess (20-30%)?
2)
Tar
sands: Sediments
containing a semi-solid tar deposit. The origin of these deposits is uncertain.
Perhaps they are immature oil deposits or perhaps they are the residues left
over after lighter oils and gases have migrated elsewhere. Refining and
problems are similar to those for oil shale.
Canada presently utilizing
Athabasca tar sands in Alberta (near Calgary). These may allow Canada to become
largely energy self-sufficient. U.S. has limited reserves.
3)
Gas
hydrates:
Crystalline solids of methane and water ice. Found in deep marine sediments and
arctic areas. May be the worldıs largest source of methane and may contain more
energy than all other fossil fuels combined! However, there are technical and
legal problems with exploiting these deposits. Many of them are located in
international waters. Who owns them and who regulates their recovery. There
also is the potential for catastrophic ocean-floor collapse if the deposits are
removed improperly. Methane also is a very strong greenhouse gas. Global
warming would increase dramatically if the trapped methane escaped into the
atmosphere
Regardless of which of these alternative fuels are used
in the future (if any) they all produce CO2 when burned. The world
is presently trying to reduce the emissions of CO2. The expanded use
of any fossil fuel will only tend to increase emissions.