Darcy’s Presence in Hydraulic Fracturing
The development of the process known as hydraulic fracturing or ‘fracking’ has been a work in
progress for well over a century. Although variations of this technology have been present for a
long time, it has only become a heated issue of debate due to the recent discovery of the US’s
shale gas reserve. The technology that drives this exploration is derived from fundamental laws
and should therefore be first understood when approaching the topic of ‘fracking’. This article
offers a brief history of the technology along with a basic explanation of how hydraulic
fracturing works.
Introduction
Hydraulic fracturing, also know as fracking, has received lots of attention in the media in
recent years due to the US’s current shale boom. The concept, however, is not new at all and in
fact can be traced back to the late 1800’s. To gain a better understanding, let's try to first
establish why the technology is where it is today and how engineers over the past few centuries
utilized a general law of nature.
In the 1860’s pioneers in the oil industry were experimenting with ways to maximize the
production of wells that were on the verge of depletion [1]. What this means is that the
unrecovered oil remaining in the well was trapped within rock formations due to the rock’s low
permeability, or the high viscosity of the oil. To recover this oil, these cowboys of engineering
reasoned that they could unlock this black gold by dropping steel tubes packed with liquid (and
later solidified) nitroglycerin down the wells to rupture the rock to obtain a quick surge in
production [2]. This process was indirectly the first frac job, which at the time was known as
“shooting” [1]. Unfortunately, this process would damage the well and render it unusable after
the quick surge in production. As a result, engineers had to resort to other devices to extract oil
and for this they turned to an empirical law deduced by the French engineer Henry Darcy.
Darcy’s Contribution
During the years of 1855-1856 Henry Philibert Gaspard Darcy performed column
experiments to investigate the flow of water through sand filters [3]. In search of innovation in
water purification, these experiments eventually led to the famous Darcy’s law (famous to those
familiar with fluid flow through porous media). This law, as depicted in equation 1, draws on the
relation of fluid flow through porous media and it was later established that his proportionality
constant, K, could be extended to other fluids by taking it as the ratio of permeability of the rock
to the fluid’s viscosity (k/ ) [3].
Eq 1: Darcy’s Law. Q= volumetric flow, A= flow area perpendicular to L, K= hydraulic
conductivity (proportionality constant), L= flow path length, h = hydraulic head.
Because engineers wanted to improve the flow of the oil (volumetric flow Q) to increase
production, they turned to this law for guidance. There are three things that can be altered
according to Darcy’s law that can improve upon the flow: (1) the pressure difference of the upper
surface to the low part of the well, (2) the surface area of the wellbore, and (3) the opening of the
pores in the rock formation (increasing the permeability). So the first place engineers started with
was with the pressure. Advances in pumps and condensers allowed for additional production of
numerous wells. The second adjustment that excited engineers was the introduction to horizontal
drilling. This increased the surface area of the wellbore (the vertical hole drilled) to the oil-
bearing rock. Although this technology was not the most economically sound approach at the
time, it paved the way for future advancements in this field, which eventually prevailed in the
21st century [1][2][4]. Finally we reach permeability, which the cowboy engineers of the 1800’s
caught onto — only this time the modern engineers didn’t want to destroy the well. So instead of
packing tubes with nitroglycerin and dropping them down the well, a perforating gun is placed
into the well and guided to the target zone with the objective of inducing fractures in the rock
formation. From here, millions of gallons of water along with sand and other chemicals are
injected into the well at high pressures, forcing open the cracks in the formation. This directly
increases the permeability, which has a direct relation with the flow for fluids that are being
produced. This final step, the injection of fluids to induce the fractures in the rock formations, is
what is commonly known today as fracking and has led to the current technologies, which oil
companies use to exploit the vast shale formations that exist in our country today.
The Process of Well Completion
Well completion today is a somewhat more complex and refined process but still utilizes
that fundamental relationship derived by Darcy. First, a well must be drilled 6 to 8 thousand feet
deep [4][5]. The well is drilled vertically past the deepest underground water supplies or aquifer
(see figure 1).
Fig 1: A cross-section view of a well
These depths are usually within the first 1000 ft. of drilling [4]. Once this depth is reached, the
drill is removed and a steel pipe casing is then inserted and lowered in to separate the drilling
processes from the water source (see figure 1). Cement is then forced down the steel casing, out
the shoe (end of the pipe) and up the sides (the annulus), cementing in the steel casing. Once this
is done the drill bit is reinserted back down into the steel pipe and the drilling continues until a
depth called the “kickoff point” where the curving leading towards the horizontal section can be
drilled. The purpose of this is to remove the original drilling tool and insert one with
measurement-while-drilling (MWD) instruments for the intent of horizontal drilling [6]. The
distance from the kickoff point to where the drilling becomes horizontal is roughly 300 meters.
In the case of the Marcellus, a depth of around 7000 ft. is reached when horizontal drilling
begins through the rock formation of interest (shale in the case of shale gas exploration) [4].
Once this is done, production casing is inserted down the full length of the well and cemented in
by the same methods mentioned earlier. The drilling rig is now no longer needed and a
temporary wellhead is installed along with a workstation for the service crew who will now be
responsible for the perforations and hydraulic fracturing of the well. After this then a perforating
gun is sent down to the target zone where charges are fired, which penetrates through the steel
casing and into the rock formation creating perforations. Once this is all done and everything is
removed from the well then the hydraulic fracturing begins.
Hydraulic Fracturing
It is here where millions of gallons of water and sand along with chemicals making up .5
to 2% of the total mixture are then injected into the well at high pressures [4]. These high
pressures exploit the perforations created along with the weak points that naturally exist within
the formation to create cracks in the formation called fissures (micro fissures already exist within
the rock and those are the weak points that are exploited, see figure 2).
Fig 2: A cross-section image of the fissures being held open by proppant
The sand in the mixture, or proppant, is used to prop the cracks open, which effectively allows
the gas to flow more freely (see figure 2). This can be imagined similarly to propping a window
open. After the initial frac a temporary plug is inserted to isolate the gas and the next segment
that is to be fractured. This fracking process occurs 5 to 7 more times per well before all the
plugs and fracking fluid is recovered (multi-stage fracking) [4]. The backflow of fluid from the
fracking process is stored in a pit and is called produced water. The water from that process that
can be retrieved is drawn up for storage and 95 percent of it can be recycled [7]. The rest of the
backflow is to be disposed of in accordance with state and federal regulations. After this is all
done, a permanent wellhead is placed along with its necessary equipment. With the use of
horizontal drilling this procedure can be repeated from the same pad.
Conclusion
In regards to the oil industry, engineers have come a long way from dropping explosives
down wells. These efforts will surely continue, as new methodologies become necessary to meet
the energy demand of the future. It is remarkable how much technological advancement can be
made by simply utilizing universal relations such as Darcy's law. But the engineer shouldn’t stop
there. Environmental and biological laws should also be considered when approaching new
solutions. Implementations of technologies that lack the proper research might compromise
access to resources that are vital to life, such as clean water. As much as the engineer is mindful
of the physical laws that guide their work, they should keep in mind how their actions affect
ecology, biodiversity, and the quality of human life itself. I have presented fracking as it is today,
because crude depictions are not now uncommon, and because if we are to have an informed
debate about the subject we should first get a basic understanding of the science that governs the
technology.
References
[1] C. Montgomery, M. Smith, Hydraulic Fracturing: History of an Enduring Technology: 2010,
p. 26-32.
[2] P. Tertzakian, Mr. Darcy's Earth Shattering Results: SPE, 2012, p. 6-7.
[3] J. Amyx, D. Bass, R. Whiting, Petroleum Reservoir Engineering: Physical Properties:
McGraw-Hill, 1960, p. 610.
[4] G. King, Hydraulic Fracturing 101: What Every Representative, Environmentalist,
Regulator, Reporter, Investor, University Researcher, Neighbor and Engineer Should Know
About Estimating Frac Risk and Improving Frac Performance in Unconventional Gas and
Oil Wells: SPE, 2012, p. 80.
[5] S. Harris, " Before You Criticize, Get the Facts Straight on Hydrofracking - Livingston
County News," Livingston County News, 26 July 2012:http://thelcn.com/2011/07/26/beforecriticize-get-facts-straight-on-hydrofracking. [Accessed 01 July 2012]
[6] L. Helms, "Horizontal Drilling,"DMR Newsletter, Vol. 35, no. 1, pp. 13:https://www.dmr.nd.gov/ndgs/newsletter/NL0308/pdfs/Horizontal.pdf. [Accessed 1 July
2012]
[7] R. Beckwith, Hydraulic Fracturing: The Fuss, The Facts, The Future: 2010, p. 34-41.