Shale
is a fine-grained, laminated sedimentary rock composed of clay- and silt-sized
sediments typically deposited in low-energy environments, along with algal-,
plant- and animal-derived organic matter.
Formation of natural gas within shale primarily occurs via thermogenic degradation
(cracking) of organic material (kerogen) through time, although biogenic
production can also occur. As the
organic-rich shale rocks are buried and subjected to increasing pressures and
temperatures (at a typical geothermal gradient of 30oC/km),
breakdown of organic matter to petroleum hydrocarbons begins to occur. The oil generation window occurs at
approximately 60-120oC (2-4km depth), while the gas generation
window occurs at 100-200+oC (3-6km depth). The thermal maturity of the shale rock,
generally measured by vitrinite reflectance (%Ro), is thus used to
determine the type and quantity of hydrocarbons present. Vintrinite, composed of cellulose and lignin,
is a common organic constituent in coal and woody kerogen that has a vitreous
luster and characteristic reflectance when heated. Gas-generating shale rock has typical % Ro
values ranging from 1.5-3.0%, where as oil generation predominantly occurs in
sources rocks with a <1.5% Ro.
In
addition to the thermal maturity of the source rock, the volume of produced gas
is also dependent on the source of the organic matter and overall TOC content. Kerogen is the insoluble portion of organic
matter that is not extractable using organic solvents (versus bitumen). There are four main types of kerogen,
differentiated by their composition.
Type III, composed of woody terrestrial source material, is typically known
to generate gas. Globally averaged
organic contents of shale are estimated to range between 2-10% carbonaceous material,
with higher percentages being indicative of deposition under anoxic, reducing environments. In general, the higher the TOC content within
shale, the higher the probability of finding economically viable quantities of gas.
Once
generated, natural gases are stored in natural fractures and pores between
individual shale grains, and are also adsorbed onto the surfaces of, and also
pores within, kerogen and clay particles.
Due to shale’s extremely low porosity and matrix
permeability (< 1 millidarcy, mD), the organic-rich “black shales” can serve as both source rock and
reservoir, trapping large estimated quantities of natural gas.
Figure 2. Conventional and unconventional petroleum reservoirs.
Numerous
questions and concerns are arising regarding the environmental effects of such drilling
and extraction practices, with particular concern of the fluids used in
hydraulic fracturing. What is the exact
composition of the “hydrofrack” fluids? What are the geochemical reactions of
the fluids interacting with the shale? What
will happen to the integrity of the shale reservoir and surrounding rock
formations over time? While residual
fluids left in the shale reservoir become trapped within fractures and pore
spaces, ultimately inhibiting further natural gas generation or extraction over
time?
My
work ultimately aims to determine the chemical and physical interactions of
different types of fracking fluids (water-based vs. CO2-based) on
shale rock formations of varying TOC and carbonate contents, thermal maturity
and brittleness index, and thus determine their impact on gas recovery
efficiency. A more complete and
quantitative understanding of the impacts of hydraulic fracturing fluids in
shale reservoirs is imperative if we are to continue to depend on natural gas
as a major energy resource.
References:
Arthur,
J.Daniel, Bohm, Brian, and Layne, Mark, 2008.
Hydraulic Fracturing Considerations for Natural Gas Wells of the
Marcellus Shale.Ground Water Protection
Council Annual Forum.
U.S. Department
of Energy (DOE), Office of Fossil Energy and National Energy Technology Laboratory,
2009. Modern Shale Gas Development in
the United States: A Primer.
DE-FG26-04NT15455.3-5.
Your article is very instructive and easy to understand. Right now in China-the world's factory, the government and the big oil companies in China are VERY eager to obtain the technical skills of the shale gas exploration and development! Large amount of money has been out in this research field! As far as I know, recently there are several academic conferences on shale gas prepared to be held in Chinese institute and universities. So I think your research is very important and is expected to bring considerable benefits to the oil companies.
ReplyDeleteThis was very easy to read - you kept it simple enough without 'dumbing down' your science. This sounds very interesting and I'm excited to see how your research progresses!
ReplyDeleteThis was little heavy on the technical facts for the average reader. I would suggest adding a picture of shale for non-geologists. I really like the second to last paragraph.
ReplyDeleteIt might be helpful to focus on your project first, before going into the formation of oil and gas. Start with why understanding fracking is important, then provide the rest of the information as background.
ReplyDeleteDefinitely too technical, although I like how you sometimes offered less technical synonyms (i.e. thermogenic degradation (cracking), in the first paragraph). I would consider replacing the technical terms altogether, rather than simplifying them in parentheses. And if you are trying to grab a reader's attention, starting with the definition of "shale" is not the way to go.
ReplyDeleteThe intro is way to technical, but the science is great. I like the paragraphs with the questions. It outlines the types of questions that need to be answered before hydrofracture can be used as a major source of petroleum. I like the comparison / contrast between conventional oil extraction methods and extraction from gas rich shale.
ReplyDeleteThe title is rather dull but the topic has so much potential and relevance to the average reader. Restructuring the blog to get the readers attention earlier would be a good change.
ReplyDelete