1. H84MPS Multiphase System
Department of Chemical and environmental Engineering
University of Nottingham
Impacts of Multiphase Flow on Hydraulic Fracturing
Youhong Yao
4177603
26 March 2014

2. 1. Introduction
Hydraulic fracturing (“fracking”) is a stimulation technique used to
exploiting and producing unconventional oil and gas, such as shale gas. In
spite of the utilization of hydraulic fracturing could be traced back to the
early 2000s, energy exploration and production companies commenced
actively to integrate horizontal drilling with hydraulic fracturing to exploit
shale gas until 2003 (Boudet et al 2014). In the last decade, due to the
depletion of conventional energy resources and the constant increasing of
energy demand of the world, driven by the population growth, there is a
significant breakthrough has been made on the technique of hydraulic
fracturing in order to exploit unconventional oil and gas to meet the energy
demand of the world (Fink 2013).
Due to the gas is trapped in pore spaces of the rock with limited permeability,
so gas cannot continuously flow into a well, therefore, the gas flow need to
be stimulated by widening fractures with the hydraulic fracturing stimulation
method. The hydraulic fracturing process includes drilling horizontally
through a rock layer and injecting a huge volume of high pressure fracturing
fluid involves water, sand, and other chemicals that fractures the rock and
thus increases the permeability so that facilitates the flow of oil and gas
(Kotsakis 2012).
Boudet et al. (2014) points out that the developments of hydraulic fracturing
have made significant contributions to the exploitation and production of
unconventional oil and gas and made wells more profitable because of
increasing the productivity of wells. It is clear, therefore, that an advantage
of hydraulic fracturing is economic benefit. Nevertheless, environmental and
safety issues associated with hydraulic fracturing have become increasingly
controversial, such as water contamination and earthquake.

3. 2. Discussion and Justification
This section presents multiphase issues with hydraulic fracturing and
solutions for the issues, and then demonstrates economic effects, safety
aspects and potentials of hydraulic fracturing in the future.
2.1 Multiphase issues with hydraulic fracturing
Multiphase flow is simultaneous fluids flow of different phases such as gas,
water and oil. Boudet et al. (2014) indicates that when multiphase flow of
different phases occurs in hydraulic fracture, aspects such as relative
permeability, capillary pressure, and wettability in the fracture and the rock
could have effects on hydraulic fracturing cleanup process and the fracture
fluid performance, and in the long term unconventional oil and gas
exploitation and production will be dramatically affected by these factors.
Figure 1 demonstrates the rate of gas production for single phase flow and
multiphase flow. Obviously, gas production rates considerably decreased
when multiphase flow occurs.
Fig.1 Impact of single phase flow and multiphase flow on gas production rate
Source: Boudet et al. (2014)
With regard to wettability, in the area of unconventional oil and gas
exploitation and production, it represents the relative affinity of a certain

4. phase to contact reservoirs rock and the ability of a certain phase to
maintain contact with reservoirs rock. If rock prefers water than for oil or
gas, it could be defined to be water-wet rock; in this case, water layer will
cover a majority part of rock surface in pores. Generally, wettability is
affected by the minerals in the pores. According to Fahes and Firoozabadi
(2007), wettability alteration has significant effects on increasing liquid
mobility in reservoir rock. Due to the wettability characteristics of reservoir
rock, a long time will be taken in the hydraulic fracturing cleanup process
and the accumulation of gas or oil in the fracture will be reduced when
pressure reduces below dew-point pressure, and lead to decrease of
productivity.
Multiphase flow through porous is an interactional result of capillary, viscous,
and gravitational forces, and capillary force generally dominates in reservoir
rock flow. Referring to capillary pressure, Boudet et al (2014) reveal that the
pressure difference between the wetting phase and the non-wetting phase is
capillary pressure (Pc). Capillary pressure equation for a gas-water system is
defined as:
Pc = Pg − Pw ……………………………………………… (1)
Fig.2 Impact of wetting phase saturation on capillary pressure
Source: Kotsakis (2012)
Figure 2 demonstrates the impact of wetting phase saturation on capillary

5. pressure; capillary pressure difference between the fracturing face and the
reservoir will result in fracturing fluids to move to reservoir. Increasing the
water phase saturation could be achieved by forcing the water flow into the
fracture of rock, thus, the water pressure need to increase above the gas
pressure. During the process of drainage, the water pressure will slightly
decrease, while the gas will subtly imbibe. Obviously, a negative capillary
pressure is needed for a higher water injection pressure than the gas
pressure in order to displace gas (Kotsakis 2012).
In terms of relative permeability, Boudet (2014) points out that permeability
is the single phase flow conductivity of pores. Base on Darcy's law, the
permeability K is defined as:
K =
qμL
AΔP
…….…………………………………………… (2)
Where: μ= the viscosity of the fluid
q= flow rate of the fluid
L= length of porous medium
A= cross section area porous medium
ΔP= pressure difference across the length of the porous medium
If multiphase fluids flow simultaneously through a porous medium, the
relative permeability of individual could be defined.
qi = (
Kkri
μi
)A
Δpi
Δx
…………………………………………… (3)
Where: qi= the flow rate of phase i
kri= the relative permeability of phase i
μi =the viscosity of phase i
ΔPi= the pressure drop within the phase i
Kkri
μi
= the mobility of the phase i
Kkri=the total permeability of phase i
Because the relative permeability of oil or gas is lower in reservoir rock, the
accumulation of oil or gas in the fracture will be reduced when pressure
reduces below dew-point pressure, and lead to the decrease of productivity.

6. According to Ricardo (2011), when multiphase fluids flow is mobile within
the fracture, pressure loss in a fracture may increase by an order of
magnitude. In detail, relative permeability changes, alteration of fluid
saturation, and phase interaction between fluids will make contributions to
multiphase pressure losses.
If the saturation of one phase increases, it results in flow area decrease of
the other phase. In the hydraulic fracture process, initially, gas saturation in
the reservoir rock is 100% before hydraulic fracturing process, due to the
reservoir rock are being opened by hydraulic fracturing, the saturation of
liquid increases to 30% from 0%, at the same time, the saturation of gas
phase decrease to 70% from 100%. Eventually, the flow area of the gas
phase has been reduced by 30% and result in decrease of gas production
Ricardo (2011).
Fig.3 Impact of fractional flow of liquid on the multiplier of pressure drop
Source: Ricardo (2011)
Relative permeability changes suggest that because of the saturation of one
phase increases, the permeability decreases related to the other phase. For
instance, in shale gas wells, if the water saturation increases in the fracture,
the gas permeability in the fracture will decrease. The phase interaction
between multiphase fluids shows differences of mobility. As two phase move
through a porous media with significantly different velocities, one phase

7. could interfere with the flow of the other. This phenomenon is particularly
apparent in gas and water flow. Figure 3 illustrates the impact of multiphase
flow on the multiplier of pressure drop, which demonstrates that a massive
pressure drop resulted from a really small increase of water fraction.
2.2 Solutions to multiphase issues
Due to the wettability characteristics and low permeability of reservoir rock,
as well as pressure lose in fracture, the hydraulic fracturing cleanup process
will take a long time and the accumulation of gas or oil in the fracture will be
reduced when pressure reduces below dew-point pressure, and lead to
decrease of productivity (Fahes and Firoozabadi 2007). Therefore, it is
indispensable to seek solutions for multiphase issues.
According to Li and Firoozabadi (2000), with method of chemical treatment
of the rock to change the wettability to intermediate gas-wetting is
approachable means for multiphase issue. An essential element in liquid
accumulation resulted from lower liquid mobility due to extreme liquid
wetting. Increasing liquid mobility is accomplished by means of the altering
wettability of the rock from extreme liquid-wetting to intermediate
gas-wetting by chemical solutions at higher temperature. As a result, the
high saturation of the liquid could be prevented, and the high gas
productivity could be achieved.
In the study of Li and Firoozabadi (2000), steady relative permeability of
untreated and treated Berea cores is measured at 24 ℃ to indicate the
impact of chemical solution treatment on water-gas relative permeability.
Figure 4 presents gas-water system relative permeability of treated Berea
core with chemical solution is dramatically higher. Altering wettability to
intermediate gas-wetting lead to increase of water mobility in certain water
saturation, so gas mobility is increased by this means.

8. Fig. 4 Treated and untreated Berea cores water-gas relative permeability
Source: Li and Firoozabadi (2000)
For multiphase flow issues of hydraulic fracturing, Ricardo (2011) reveals
that using a viscous disproportionate permeability modifier (VDPM) is a
reasonable approach to increase oil production. This polymer, VDPM is
injected with fracturing fluid to reservoir rock could improve the irreducible
water in pores, and then it is absorbed to pores surface and decrease the
relative permeability of water without impacting the relative permeability of
oil. Therefore, the rate of oil production is improved dramatically by this
approach, as Figure 5 shows the productivity is substantial higher when
VDPM is applied, compared with conventional technology of hydraulic
fracturing.
Fig.5 Impact of VDPM on oil production
Source: Ricardo (2011)

9. 2.3 Economic effects of hydraulic fracturing
With regard to the economic impacts of hydraulic fracturing, generally, it
could be more profitable because of the improvement of the productivity by
hydraulic fracturing. As sdudy of Boudet et al (2014) indicates that the
potential economic effects is a significant advantage of hydraulic fracturing
technique development, such as creating job opportunities, increasing local
revenues and individual incomes, as well as improving property values and
public services. Nevertheless, a research about economic effects of the
Marcellus shale in Pennsylvania showed that economic aspects were not as
previous prediction. Shale gas exploration will cost more because the
consumption of the hydraulic fracturing treatments is substantially high.
2.4 Safety aspects of hydraulic fracturing
It is accepted that water in injection fracturing fluid is significant during
hydraulic fracturing, 2–4 million gallons of water is needed per well, which
contributes the depletion of ground water. Moreover, chemicals occupy
0.5-2.0% of fracturing fluid, for example, 4 million gallons fracturing system
needs from 80 to 330 ton of chemicals, which could result in ground water
contamination and public health issue. The Environment Protection Agency
(2011) has been prompted to study the relation between ground water
quality and hydraulic fracturing (Boudet et al 2014).
Furthermore, Wang et al. (2014) indicates that the hydraulic fracturing
technique might cause earthquake, due to a large volumes of fracturing fluid
are injected to reservoir rock. In April and May 2011, two small earthquakes
occurred near Blackpool because hydraulic fracturing was applied in
Lancashire’s Bowland Shale reservoir. According to the study of Wang et al.
(2014), the fracturing fluid injection alters the strains and stresses on the
earth’s crust and relieves the effective stress, which could trigger
earthquakes.

10. 2.5 Potentials of hydraulic fracturing in the future
Although potential economic effects of hydraulic fracturing are noticeable
because of the improvement of the productivity of gas and oil, it is a
controversial aspect for different study. In order to create potentials of
hydraulic fracturing, further research should pay more attentions to
optimization of economics and physics of hydraulic fracturing technique
(Marongiu 2013). Additionally, Miller (2011) shows that future hydraulic
fracturing relies on effectiveness of water treatment due to the injection of
fracturing fluids in reservoir rock.
3. Summary and Conclusions
In this paper, I explored multiphase flow issues associated with hydraulic
fracturing from relative permeability, capillary pressure, wettability in the
fracture and the reservoir rock 3 aspects, due to the multiphase flow occurs,
which have substantial effects on hydraulic fracturing cleanup process and
the fracture fluid performance, and in the long term unconventional oil and
gas exploitation and production will be dramatically affected.
Furthermore, I sought to find approachable solutions for multiphase issues,
in the study of Li and Firoozabadi (2000), with method of chemical
treatment of the rock to increase liquid mobility and alter the wettability to
intermediate gas-wetting is approachable means for multiphase issue.
Besides, Ricardo (2011) reveals that using VDPM to improve the irreducible
water in pores of the rock and decrease the relative permeability of water
without impacting the relative permeability of oil in order to reduce the
impacts of multiphase flow. In terms of economic effects, it is a controversial
aspect for different researchers. Additionally, considering safety aspects,
water contamination and earthquake are two serious parts about hydraulic
fracturing. With regard to potentials of hydraulic fracturing in future,
optimization of economics and physics should be paid much attention.
Finally, further study should take the public health and water contamination
into consideration, this view is in line with Fink (2013).

11. References
Boudet, H., Clarke, C., Bugden, D., et al. (2014) ‘“Fracking” Controversy
and Communication: Using National Survey Data to Understand Public
Perceptions of Hydraulic Fracturing’ Journal of Energy Policy 65, 57-67
Fahes, M. and Firoozabadi, A. (2007) ‘Wettability Alteration to Intermediate
Gas-Wetting in Gas-Condensate Reservoirs at High Temperatures’ SPE
3, 397-407
Fink, J.K. (2013) Hydraulic Fracturing Chemicals and Fluids Technology
Oxford: Gulf Professional Publishing
Kotsakis, A. (2012) ‘Regulation of the Technical, Environmental and Health
Aspects of Current Exploratory Shale Gas Extraction’ RECIEL 21, 28-36
Li, K. and Firoozabadi, A. (2000) ‘Experimental Study of Wettability
Alteration to Preferential Gas-Wetting in Porous Media and Its Effects’.
SPEREE 3 (2), 139-149
Marongiu, M., Economides, M. and Holditch, S. (2013) ‘Economic and
physical optimization of hydraulic fracturing’ Journal of Natural Gas
Science and Engineering 14, 91-107
Miller, P. (2011) ‘Future of hydraulic fracturing depends on effective water
treatment’ Journal of Hydrocarbon Processing 4, 18-22
Ricardo, P. (2011) ‘A New Approach to Hydraulic Fracturing Using Relative
Permeability Modifiers Increases Oil Production While Holding Water in
Check’ SPE 5, 16-17
Wang, Q., Chen, X., Rogers, H., et al. (2014) ‘Natural gas from shale
formation – The evolution, evidences and Challenges of shale gas
revolution in United States’ Journal of Renewable and Sustainable
Energy Reviews 30, 1-28