Student: Yujun Chen
Writ 340
Instructor: Dr. Ramsey
Title: Hydraulic Fracturing --- Engineering for Rocks
Biography: Yujun Chen is a senior undergraduate student in University of Southern California
majoring Chemical Engineering. He chose Petroleum Engineering as his emphasis in his major
and has taken 5 courses related to oil and gas extraction and production. He has been doing
relative research for Hydraulic Fracturing and gained many insights of this technology. He aims
to purse a Master Degree of Petroleum Engineering and is interested in career in oil industry.
Abstract: This paper introduces the technology of Hydraulic Fracturing, which is now the most
popular-used method in oil industry to produce oil from unconventional formations. The paper
is written for general audiences who don’t necessarily possess knowledge of Petroleum
Engineering. It covers the background knowledge of oil extraction, the principle of hydraulic
fracturing and its applications.
Keywords: engineering, petroleum, rock, oil, Darcy’s law, energy
Hydraulic Fracturing --- Engineering for Rocks
Introduction
Gasoline may be most familiar to Californians because our daily commute depends highly on it.
However, not too many people understand how oil is produced out of the ground. One misconception is
that somewhere deep in the ground there is a big “pool” that is full of oil, and production of oil is simply
sticking a “straw” to suck it up. However, the reality is that there is no such a “pool” that allows you to
extract oil easily. Oil exploration and production is a complex engineering process that involves logging,
drilling, cementing and perforation. Although these technologies are fairly complicated, they are wellestablished sciences that have been put into practice for years. However, challenges are still remain. As
the demand for oil is increasing rapidly and the oil price is skyrocketing in recent years, the traditional
extraction methods can no longer meet our needs. According to the research of Leonardo Maugeri, who
is currently an associate with the Geopolitics of Energy Project and the Environment and Natural
Resources Program at the Harvard Kennedy School, the production rate of many U.S. oil wells are facing
dramatic decline. In order to maintain high production rate, drilling intensity must increase. For example,
“by December 2012 it took about 90 new producing wells per month just to maintain North Dakota’s
Bakken-Three Forks oil production of 770,000 barrels per day” [B1]. Hydraulic Fracturing technology is
now playing an important role of alleviating this situation. Hydraulic Fracturing is aimed to improve the
production rate in each individual well to maintain the overall production rate without dramatically
increasing the drilling intensity. The definition of Hydraulic Fracturing is given by Boyun Guo in his book
Petroleum Production Engineering: “Hydraulic Fracturing is a well-stimulation technique that is most
suitable to wells in low-and-moderate-permeability reservoirs that do not provide commercial
production rates even though formation damages are removed by acidizing treatment” [B2]. As human
demand for energy is increasing rapidly, voices for promoting alternative energy have been put on the
table. But so far, oil and gas are still playing the core role of supporting the global energy infrastructure.
So it is important to understand the current energy situation we are facing and the choice of technology
we have in our hands. The purpose of this essay is to explain the principle and applications of hydraulic
fracturing by introducing simply mathematical models and basic terminologies. In the following, we will
start with the background knowledge of oil extraction.
Background Knowledge
Detail explanation of Hydraulic Fracturing requires a basic understanding of how oil is extracted
from subsurface. As mentioned above, oil (formally called hydrocarbons) is not concentrated in one area
like a “pool”. A sponge is a good model to illustrate how oil is widely spread out in the pores of the rock
formations. Treating a sponge as the rock formation and the water in the sponge as oil, the status of the
rock before extraction is like a sponge soaked with water. One cannot see any water by the sponge’s
appearance because water is stored in the porous materials (fibers) inside. When the sponge is
squeezed, the pressure will force the water come out of the porous materials. Same principle is
applicable to oil extraction from rock formation. Although rock is difficult to squeeze, specialized
equipments can generate force to remove oil from rock pore by creating pressure gradient at two ends.
This is a well-studied engineering process for which many equations are formulated to calculate the oil
flow rate and pressure gradient, which will be shown next. Its academic name is recognized as
“Transport Process in Porous Medium” [B3].
Through years of practice by engineers, many drilling and production technologies are
developed to extract oil. However, many conventional methods have met their bottlenecks because the
production rate is declining as more and more oil is extracted. According to an academic article
Production Rate Decline in Pressure-Sensitive Reservoir published by Fernando Samaniego, the
production rate would decline as the pressure gradient between wellbore and reservoir becomes
smaller [B4]. Samaniego’s paper indicates that reservoir’s productivity has weak relation with the total
oil volumetric storage in the reservoir itself. The main factor influencing the productivity is the pressure
difference between the wellbore and the source rocks (where hydrocarbons are generated in the first
place). In other words, though there is significant amount of oil left in the reservoir, due to the low
pressure gradient, there is not enough force to move the oil from the reservoir to the wellbore. This
relation is well-explained by a widely-used mathematical model called Darcy’s Law, which is presented
by Dandekar in his book called Petroleum Reservoir Rock and Fluid Properties:
𝐾𝐴
𝜇
𝑄=
∗
∆𝑃
∆𝑥
[B3]
Where Q is volumetric flow rate, K is permeability, A is flow area, 𝜇 is viscosity, P is pressure and x is
distance [B3]. For a regular reservoir, all the parameters could be considered fixed except ∆𝑃 the
pressure gradient. Since the flow rate is directly proportional to pressure gradient, the low pressure
gradient of the reservoir will result in low oil flow rate, which is the situation faced by most oil wells
produced with conventional methods. A new method has to be invented to solve this issue, and that’s
why we introduce Hydraulic Fracturing.
How does Hydraulic Fracturing work?
Generally speaking, hydraulic means the pressure created by the liquid column on the vertical
axis. The principle of hydraulic pressure can be explained by a simple mechanical equation from Physics
For Scientists and Engineers written by Serway,
𝑃 = 𝑃𝑜 + 𝜌𝑔𝐻
[B4]
Where P is hydraulic pressure, Po is surface pressure that is generated by high-power pumps, 𝜌 is liquid
density; g is gravity constant (9.81 m/s^2) and H is the height of the liquid column [B4]. In practice,
water is common medium for this process because it has a relatively high density and it’s cheap and
widely accessible. Mathetically, if we fix the liquid density 𝜌, the only variable is the height of the water
level 𝐻. We can control the pressure by varying H. Figure 1 demonstrates the scheme of how to apply
this principle to create a large hydraulic pressure on the well wall. Basically, the well consists of two
portions: vertical portion and horizontal portion. All the hydraulic force of water is created by the
vertical portion of the well that extends all the way up down to the bottom. The pumper truck generates
the surface pressure Po . The bottom of the well is about 4000m. This height is able to create a high
pressure for the horizontal portion of the well.
Figure 1: http://en.skifergas.dk/technical-guide/what-is-hydraulic-fracturing.aspx
For fracturing, what it means is to create a “crack” in the rock matrix. As mentioned above, rock
matrix is like a sponge, however, extracting oil from rock is not as easy as squeezing a sponge because
the volume of rocks are very difficult to compress. There are different pore sizes inside a rock, and oil
can only flow between big pores. The flow rate of oil is related to both porosity and permeability. High
rock porosity indicates potentially more oil can be stored inside the pore, and high rock permeability
indicates the easiness of extracting the oil. However, high porosity doesn’t necessary mean high
permeability. These two variables are not proportionally related and they are highly dependent on
individual rock type. There are many examples that a rock has high porosity but with zero permeability,
which means no matter how high the pressure gradient is, the oil will not flow out from the pore space.
The following is an example.
figure 2: http://echo2.epfl.ch/VICAIRE/mod_3/chapt_1/main.htm
Figure 2 is an illustration of low permeable rock formation and the process of hydraulic
fracturing. The white areas of the left image represent porous paces in an oil rock. They are relatively
large pores given that some pore sizes are comparable to the grain size (shade areas). However, there
would be no flow between each pore because all spaces are isolated. This is a typical example of high
porosity with low permeability. To mathematically explain this phenomenon, recall Darcy’s Law 𝑄 =
𝐾𝐴
𝜇
∆𝑃
∗ ∆𝑥 in which K represents permeability. The flow rate Q is dependent on the permeability, not
porosity. Assuming the other variables are fixed, the high flow rate would only be achieved by
maintaining high permeability. Therefore in the above case, small K results in small Q. Given the fact
that this figure is an extreme case of high porosity because most oil rocks have very small porous spaces,
such low permeability is even more difficult to extract oil. This is why hydraulic fracturing comes into the
play.
Hydraulic Fracturing uses extreme high pressure to have the water penetrated the rock, creating
“channels” between each pore such that oil movement can be generated. This is shown on the right
hand side of the figure. Suggested by Darcy equation, once K increases, the volumetric flow rate Q will
increase. Therefore even though the pressure gradient is low, as long as we can create a high
permeability, we can maintain a high oil flow rate. This is the principle of hydraulic fracturing.
Applications of Hydraulic Fracturing
Hydraulic Fracturing is mainly applied to tight formations such as shale formation due to its low
permeability but with abundant hydrocarbons storage. According to Schlumberger, shale formation is
defined as “A fine-grained, fissile, detrital sedimentary rock formed by consolidation of clay- and siltsized particles into thin, relatively impermeable layers…… Shale has potential to become a rich
hydrocarbon source rock” [B5]. Shale rocks have two characteristics: rich hydrocarbons storage and
extremely low permeability. It is difficult to produce oil under normal circumstance. Hydraulic fracturing
alleviates this problem by highly increase shale’s permeability and creates a “way” to introduce
chemicals into rock pores to simulate faster flows. The effect of this use of technology is tremendous.
This is because shale formation is widely existed though U.S. According to U.S. Department of Energy,
Natural gas from shales has the potential to significantly increase America’s security of energy
supply, reduce greenhouse gas emissions, and lower prices for consumers. Although shale gas
has been produced in the United State for many decades, it was not considered to be a
significant resource until the last decade when new horizontal drilling and hydraulic fracturing
technology facilitated economic production [B6].
In addition, hydraulic fracturing is also suitable for other types of well development such as water wells.
This technology could be potentially used to obtain clean water from deep underground in the water
scarcity areas such as Sub-Saharan Africa.
Conclusion
The invention of Hydraulic Fracturing could be traced back to early 1930s, and the recorded first
use of this technology was already 60 years ago. However, Hydraulic Fracturing was not given much
attention in the old day because human demand for energy was still relatively low. As society is
becoming more industrialized, many labor-intense jobs are replaced by machines that require supply of
energy. Energy usage and shortage became widely discussed issues that have massive implications of
economical and political aspects. Hydraulic Fracturing is now playing an important role of securing the
domestic energy supply and affecting the international oil trade. As this technology becomes more
sophisticated in the future, U.S. is expecting to switch from being a large oil import country to a large oil
export country. The global influence of such change would be tremendous.
Bibliography
[B1] Maugeri, L., The Shale Oil Boom: A U.S. Phenomenon, Cambridge, Harvard University, 2014
[B2] Guo, B., “Hydraulic Fracturing,” Petroleum Production Engineering: A Computer-Assisted Approach,
Gulf Professional Pub, 2007, ch. 17, pp.251-265.
[B3] Dandekar, A., Petroleum Reservoir Rock and Fluid Properties, Boca Raton, CRC Press, 2006, ch.3 &
ch.4, pp.13-54.
[B4] Serway, R., “Fluid Mechanics,” Physics For Scientists and Engineers, 7th ed. Belmont, Brooks/Cole,
2005, ch.14, pp.389-416.
[B5] Schlumberger. (2014). Shale [online]. Available FTP: http://www.glossary.oilfield.slb.com/en/
Terms/s/shale.aspx
[B6] U.S. Department of Energy. Shale Gas R&D [online]. Available FTP: http://energy.gov/fe/scienceinnovation/oil-gas/shale-gas-rd