Environmental Geology lecture outline
Primary energy sources?
Quick background information:
- definition of
energy: "the property of matter and radiation that is manifest as a capacity to perform work (such as causing motion or the interaction of molecules)." - Google definition.
- units: varies - joules, btu, calories, some details.
- first law of thermodynamics: energy can be transformed, but not destroyed or created.
Chart of U.S. energy use from Lawrence Livermore National Laboratory. What is perhaps most surprising as you look at this chart? Source: https://publicaffairs.llnl.gov/news/energy/energy.html#2008
The changing picture for China of energy production. Image source: http://www.eia.doe.gov/emeu/cabs/china/part2.html
USGS site concentrating on hydrocarbon energy sources.
Geology of hydrocarbon formation (3 requirements)
What follows is some of the science behind 'conventional' oil and gas. Oil and gas in tight reservoirs (often shales) and fracking have caused a significant shift in our energy landscape and are now so widely practiced that it is hard to call it unconventional. These are discussed afterward.
The three components of conventional oil and gas:
- source rock:
- need to preserve organics in the rocks.
- euxinic environments with slower aneorobic decay favorable to preservation found in
restricted marine bodies or lakes. Where found today?
- sulfur compounds also common to euxinic environments.
- basin subsidence (to bury and preserve the sediment and fossil organics).
- black shales or limestones common source rocks - often stinky when break a fresh face open.
- some 50% of source rock formed in the Cretaceous Period.
- thermal window (thermogenic component):
- kerogen: waxy and complex organics in source rock - not mobile.
- kerogen needs to be cooked into oil (low simmer) or gas (hotter).
- cooking increases mobility of the organics.
- temperatures too high and change organics into less useful forms.
- reservoir or trap:
- need reservoir rock and seal that the mobilized oil and/or gas can collect in.
- stratigraphic traps (e.g. originally reef, channel or offshore sand
bodies within finer grained sediment) - they have high porosity.
- structural traps: the classic anticline.
- Depiction of several types of traps with green representing crude oil and red as overlying gas. Image source: http://energy.er.usgs.gov/gg/research/petroleum_origins.html
- time element?
- not such that it is renewable.
- need the right history (e.g. trap formation during or before thermal mobilization).
Tight reservoirs and fracking
Have known for many decades that there was oil and gas trapped in "tight" formations such as the Bakken shale up in North Dakota. Tight means low permeability. However, typical vertical wells could not pump the oil and or gas from these tight rocks. Fracking is part of a 4 part technology that has allowed us to extract this oil and gas. The unconventional deposit became a conventional deposit as technology developed.
Four components of fracking operations:
- directional drilling often horizontal) along the tight formation, often 1 mile out from a central well.
- sealing off of sections of the horizontal bore hall, and pumping in water to increase pressure to a point that a series of new fractures are created (this technically is the fracking part). This is repeated until a network of fractures has made a tight rock into a highly fracture reservoir from which the oil and gas can now flow out.
- sands or other propants are added to the fracking fluids to prop the fractures as the fluid pressure decreases as oil and gas is pumped out.
- often chemicals are added to the well fluids to help the oil flow by reducing the viscosity. In addition, biocides are added to prevent microbial growth which can produce a clogging sludge.
- resource intensive and therefore expensive.
Image from USGS and US EPA showing a schematic of a fracked oil and gas well. Source: http://water.usgs.gov/owq/topics/hydraulic-fracturing/ .
Other 'unconventional' hydrocarbon energy resources?
- oil shales: (not been through the thermal window, and so oil and gas not generated yet, but organics there).
- tar sands: e.g. up in Canada and what the Keystone pipeline was partially about.
- methane hydrates.
- coalbed and black shale methane:
- abiogenic (due to deeper heating).
- biogenic (can be shallow, due to microbes).
- if you buy land find out it you own the mineral rights or not.
Oil and gas from a carbon cycle perspective
Carbon cycle consists of three linked parts:
- Short term organic cycle: describes the linkages between atmosphere, primary producers (by photosynthesis), decomposers, and everything between (like humans).
- terrestrial part.
- marine part.
- Long term organic cycle: describes the linkages between organic carbon stored in geologic environments and the short term organic cycle.
- Inorganic carbon cycle: describes the linkages between Carbon Dioxide in the atmosphere, weathering processes, tectonics, and geologic release of Carbon Dioxide.
Carbon cycle diagram (excluding the inorganic) from NASA showing estimates of amounts and fluxes. Oil, gas and coal are part of the "Sediments" reservoir. Source: http://earthobservatory.nasa.gov/Features/CarbonCycle/carbon_cycle4.php
Short and long term carbon cycles together in a box and arrow diagram. The weathering of exposed sedimentary rock, with its carbon in it returns C back to the surface environment and to the short term cycle. That 'weathering' also includes a suite of microbes that eat these organics, producing methane.
Image from USGS showing some transfer estimates between the atmosphere and other carbon sinks that are contributing to carbon dioxide build up in the atmosphere. Image source: http://pubs.usgs.gov/fs/fs137-97/fs137-97.html
Cradle to grave approach
There is a Hindu tale of the three blind wiseman, which basically indicates it is wise to be able to see the whole beast and not just parts. The cradle to grave approach is a systems thinking type of approach, to be ore holistic in one's considerations. It is an excellent way to identify unintended consequences, and true cost effectiveness.
This diagram captures a lot, but one missing component is the production of waste water during the extraction phase. A standard approach is to take what are usually very salty waters (saltier than seawater in some cases) and inject them back down into an old field. This is what can induce seismicity. There are other missing components.
What are exploration
techniques for hydrocarbons?
- early on could drill where oil seeped out of ground.
- Schematic diagram and old photograph of oil seeps, 100s of which exist in California alone, the most famous of which are the La Brea tar pits. The smooth, dark material in the flat area is slowly moving, very viscous oil/tar. Images from: http://geomaps.wr.usgs.gov/seeps/index.html
- geologic mapping allowed inference as to geology at depth and location of possible reservoirs.
- now in search of offshore and/or hidden/subtle oil fields -> need to use geophysics.
- seismic reflection profiling.
- latest is 3-D seismic reflection profiles.
Seismic section image off the coast of California showing sedimentary layering, faults, and other geologic features. Some of the features, especially in the deeper and lower parts, are artifacts of the imaging technique, and it is helpful to be trained in the interpretation of seismic sections. This type of data is crucial and common in oil exploration. Note the vertical scale of depth. Image source: http://walrus.wr.usgs.gov/mapping/csmp/data_collection.html
Extraction and secondary recovery techniques.
- drilling depths of several 1000' common, 20,000' done
regularly, and up to 30,000' have been done by industry. 1982 average cost
per foot was $52.56. One 18,000' hole in Gulf of Alaska in 1983 cost $44
- Off shore platforms crucial.
- overpressurized reservoirs a major concern.
- drilling mud density such that weight of column in drill
stem yields bottom pressure greater than reservoir pressure. Increase density
by adding barium chloride.
- Campeche 1979-1980 history:
exploratory well in Gulf of Mexico had a blowout on June 3rd, 1979. Caught
fire. March 23rd 1980 intercepted and under control. More than 3 million
barrels of oil lost, oil slick 120-160 km long that fouled Texas beaches.
- Oil recovery rates and secondary/Tertiary recovery techniques:
- initial pumping may leave more than half the oil in the ground (clinging to grains).
- hydrofracturing: increasing fracture permeability.
- increase reservoir pressure by water injection.
- increase reservoir pressure and decrease oil viscosity by pumping in Carbon Dioxide.
Environmental concerns associated
with transportation, refinement?
- oil spills from tankers:
- Amoco Cadiz, March 1978, offshore France, 1.6 million barrels lost, 160 km of shoreline affected.
- Exxon Valdez, March 24th 1989, Bligh Reef in Prince William Sound, 250,000 barrels lost, steam cleaning of shore sections not such a good idea in the long run.
- Mega Borg had fire on June 8th, 1990, 80 km from Galveston Texas with 4.3 million gallons released. Tried bioremediation. (source Keller)
- double lined tankers.
- emissions, groundwater contamination from refinement.
What are larger concerns
associated with intense fossil fuel use?
- depletion of the nonrenewable sources.
- contribution to global warming, global climate change.
- geopolitical distribution and ramifications.
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