Analysis of Thermodynamic Variables on Fluid and Solid Surfaces

This report provides an analysis of the thermodynamic variables like temperature and pressure on fluid and solid surfaces within a porous medium, considering the impact of wettability and contact angles. The study acknowledges the importance of wettability in multiphase flow and its influence on CO2 storage, a key aspect of carbon emission solutions. The report reviews the work of other scholars, highlighting the challenges in simulating underground conditions and the relationships between wettability and thermodynamic variables. The report explores the influence of wettability on the trapping potential of CO2 in geological formations, covering structural, residual, and other trapping mechanisms. The analysis emphasizes how wettability affects permeability, effective pressure, and CO2 modeling in reservoirs, offering insights into how these variables impact the storage of greenhouse gases. The report also discusses the importance of contact angles, gas densities, and their impact on the solid surfaces of reservoir rocks.

Running head: WETTABILITY AND FLUID AND SOLID SURFACES
The Analysis of Thermodynamic Variables on Fluid and Solid Surfaces
Name
Academic Institution

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Executive Summary
In the modern day when the glob is greatly affected by high rates of carbon emissions,
the capturing and storage of geological carbon and its storage in geological formations seems to
be the future of carbon emissions solutions. However, the security of this approach is dependent
on characteristics of the reservoir mainly its wettability and its impact on the solid and fluid
flows in the porous mediums. As such, it affects the thermodynamic variables of the medium,
which also reduce the capacity of the injected CO2 into the ground. This paper analyzes the
thermodynamic variables like temperature and pressure on fluid and solid surfaces of the porous
medium considering the contact angles within the solid and fluid systems. This is achieved
through the acknowledgement of the role of wettability in multi-phase flow through porous
systems and an analysis of experimental predictions conducted by other scholars who failed to
achieve reliable results due to the complexity and difficulty of simulating underground
conditions. The relationships between wettability and the thermodynamic variables of
temperature and pressure are clearly discussed and analyzed using the research conducted in this
paper through the guidance of the findings of other scholars in this field.

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Table of Contents
Table of Contents.............................................................................................................................2
Introduction......................................................................................................................................3
Justification and Objectives.........................................................................................................3
Literature Review.............................................................................................................................3
Methodology....................................................................................................................................3
Analysis and Discussion..................................................................................................................3
Conclusions and Recommendations................................................................................................3
References........................................................................................................................................4

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Introduction
In the petroleum reservoirs in the earth’s surface, both the natural gas and oil components
coexists with water deposits immiscibly, meaning that the water component is not able to mix up
with the hydrocarbons that make up the oil and gas components. This leaves a tension of
immense strength between this water and oil, as a result of the separation from their immiscible
state. In addition, the natural gas component is also significantly immiscible with the oil
component, making the fluids existing within the reservoirs to contain energy that is surface free,
as a result of the electrical forces resulting from this immiscible fluid matrix (Adamson & Gast,
2007). The immiscible fluid matrix is kept together within the reservoir by these electrical forces
that occur naturally as a result of the resultant cohesive and adhesive forces. The cohesive forces
between the particles of the same component medium are attracted to each other cohesively for
all the fluids, while adhesion works to separate the different individual particles and thus making
them immiscible through adhesion. This phenomenon contributes to the tension within the
reservoir caused by the separation and immiscibility of the fluids and because the surface of the
fluid matrix is held together by both cohesion and tension, to occupy the smallest possible
surface area (Al-Yaseri et al., 2016). The fluid matrix thus in most cases acts a membrane
undergoing surface tension.
Wettability refers to the affinity of any fluid phase in the fluid matrix of natural gas and
oil as well water, to wet a solid surface compared to the rest of the fluids in the immiscible fluid
matrix phase. When looking into wettability, oil and gas are treated as one component of the
fluid matrix, since both of them bare the capability of wetting the solid surface identically, in a
manner that differs from the water component type of wettability (Adamson & Gast, 2007). For

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instance within a reservoir, wettability describes the state of the rock and fluid matrix of that
rock of the reservoir, with regard to whether the rock is wetted by the water component or the oil
component of the fluid matrix. Wettability in a reservoir can occur in three different states which
represent the categories of wettability.
Fig 1: The different possibility of occurrence for wettability
The arrows in the above figure represent the bearing of the tangent to the contact angle (θ) that is
formed between the surface of the rock and the water particle, given that the particle is also
surrounded by an oil phase as is the case in the reservoir. In the water wet scenario, water is the
fluid that wets the solid surface of the rock preferentially, because the contact angle between this
rock and the water is an acute angle. Neutral wettability occurs when the contact angle between
droplet of water and the surface of the rock in the reservoir is a right angle while the wettability
situation is considered oil wet if the contact angle is an obtuse angle (Arif et al., 2016).
Wettability also plays a big role in the multiphase flow within the porous rocks of the
earth’s surface as well as interactions between fluids and rocks, as this is what characterizes the
reservoirs where the fluid matrix of oil and gas is found. In addition, the storage of carbon as
well as its capture is yet another play and contributor to the natural process of reducing carbon
emissions in the environment, an approach that has been recently embraced in the reduction of

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these emissions for a cleaner environment (Amott, 2009). Since the capture of carbon and its
storage is a physical process requiring the injection of CO2, wettability plays a great role in
distributing the fluid phases and thus the final destinations of the CO2 that is injected into the
ground through this process. This in turn affects the permeability and the effective pressure
within the voids and the capillary in the site, which in turn significantly affect the CO2 modeling
of the reservoir for storing the gas. The storage of these gases thus mainly depends on
wettability, and thus the thermodynamic variables affecting how the rock and the fluid matrix
behave in conditions of pressure and temperature should be assessed and evaluated. Since these
thermodynamic variables especially within the earth’s surface vary greatly, an analysis of the
thermodynamic variables affecting the fluids and the solid surface of the rocks can be
investigated by following the behavior of the contact angle of brine or even CO2. This is because
the impact of CO2 has been greatly investigated as interest on CO2 storage in this earth’s surface
continues to be investigated in the academic world through research, as this avenue provides a
feasible solution in the reduction of carbon from the atmosphere through its injection in the
earth’s surface (Anderson, 2006). In addition, the densities of gas can also be used in the
exploration of the factors of wettability, allowing for reliable conclusions regarding the concept
of wettability. This clearly also sheds light on the impact of the variability of these
thermodynamic variables on the solid surfaces of the rocks of the reservoirs.
Justification and Objectives
The analysis extends the works of other researchers like Arif and Al-Yasseri, who
investigate the thermodynamic properties of different materials with regard to wettability of solid
and fluid surfaces in a medium. The main aim of the study is to identify and analyze the changes
in wettability of both solid and fluid phases of materials during the extraction of petroleum from

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the ground. Both the microscopic wettability of both brine and CO2 are also analyzed, for
varying temperature and pressure and the implications of this conditions in the trapping potential
of the medium. In so doing, the study is able to make clear the responsible thermodynamic
variables that affect the wettability.
Literature Review
Wettability is known to be the most important factor determining the multiphase flow
within a medium that is porous as it involves one fluid within the fluid matrix and solid rock
surface system to remain in contact. For this reason, it determines the rate and direction of flow
of both the gaseous and liquid phases within the spaces and pores of this porous medium. Thus,
wettability can be said to contribute to the distribution of these fluids in the earth’s surface
especially while they are being formed. It also determines the saturation of the fluid matrix on
the solid surface as the wettability is an aspect that is relative to the rock surface within the
reservoir. (Chaudhary et al, 2013) expresses the concept of wettability and its relationship to the
pore pressure within the porous medium as a major determinant of the capillary pressure within
the reservoir as well as the permeability, because the space within the pores are wetted by this
fluid phase responsible for wettability in a given context. This then implies that the fluid that is
responsible for wettability, either the water component or the oil component is able to take up the
pores of the porous medium, making the pores smaller and thus increasing the pore pressure,
while the other fluid component or phase takes up the space surrounding the pores (Donaldson &
Tiab, 2004)
The contact angle is effective in giving an account what wettability entails and how it
impacts the pressure within the spaces of the porous rock surface. The conspt of the contact

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angle is able to address issues of the thermodynamic variables affecting the behavior of the solid
surface and the fluid matrix such as the thermodynamic variables affecting the porous medium.
According to Bryant & Blunt (2014), the contact angle can be established by considering the
impact of the different phases within the fluid matrix and solid surface due to the force fields
acting within the fluid matrix. The balance between these forces can thus be theoretically applied
to determine the contact angle.
Further, (Al-Yaseri, et al., 2016) establishes a great relationship between the contact
angle and multiphase flow within the porous medium, especially when the gas densities of CO2
are considered. CO2 gets in contact with the fluid matrix from the injection of the gas into the
surface of the earth, following a new technology to reduce the amount of carbon and greenhouse
gases from the environment, to make it safer and cleaner. Arif (2016) also explains that the
wettability of the system also impacts the ability of the porous medium to store the CO2with
regard to different forms of the multiphase flow in the medium causing different forms of
trapping of the gas within the medium. With regard to trapping, structural trapping prohibits the
rising of the CO2 gas spiral as a result of a caprock overlaying the plume. The capillary forces
influence by wettability and contact angle thus limit the movement of the gas plume upwards as
it causes a balance between buoyancy and capillary forces thus assuring a commendable
structural trapping for small contact angles and water wettability. This high level of water
wettability increases the trapping potential of the caprock as the sealing efficiency is improved
by the high levels of capillary forces at the entry the caprock (Armitage, Faulkner, Worden,
2013). Increasing the contact angle will reduce the entry pressures of the capillary and thus
reduce the immobility of the gas and in turn reduce the structural trapping of the rock surface. On
the other hand, residual trapping requires the capillary forces to trap the phase compounds that

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do not participate as the wetting phase in the fluid matrix within the porous medium. (Pini &
Benson, 2013), inferred that residual trapping is determined by both the porosity level and the
gas saturation of the medium, meaning that a contact angle that is acute but greater than 50o
would lead to a residual trapping of commendable quality. This is because weak water
wettability demonstrates a good residual trapping as they are able to trap large amounts of
residual fluid phases that do not wet the pores. Other forms of trapping include the resolution and
mineral as well as the adsorption forms of trapping, which have an indirect relationship with
wettability and contact angles. Mineral and desorption wettability rely on the volumetric amount
of the aqueous fluid phase substances that are absorbed by the brine that has settled in the pore
spaces. They also rely on the configuration of the pores with regard to the number of phases as is
claimed in (Linderberg & Wessel-Berg, 2007). The distribution of the pores within the medium
as well as the volumetric amount of fluid phases in aqueous for are all contributed to by the
wettability levels of the porous material or the rock surface. The former factor has a relationship
with the interface are within all the fluids in the matrix. (Iglauer, 2011) implies that the interface
area of the porous material depends on the dissolution rate within the medium. Even the
reactions related to mining are also related to this configuration and arrangement of the fluid
phase particles within the fluid phase. The importance of mineral trapping is depicted in through
the configuration of fluid particles in the medium which translated to the location and scale of
the pores within the medium, and the volume of a given fluid phase within the medium. These
measures are important in the identification of reaction kinetics within the porous medium. The
relationship emanating from this situation is vital in determining what mining methodologies and
precautions to be selected.

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While mitigating climate change, the CO2 has to be injected using the concepts of
trapping that greatly depend on the trapping potential of the porous medium. According to (Orr,
2009), the wettability of the rock surface as well as the storage and CO2 trapping potential of the
porous medium greatly influence the multiphase flow within it because they all factors of the
capillary forces within the medium. The relationship between the wettability and gas densities of
CO2 are thus made possible by the fact that the capillary forces within the medium provide a
countering effect of buoyancy exerted by the CO2. The contact angle within the fluid matrix is
able to determine the capillary forces to imply that the contact angle is comparable to the
densities of the gases within the porous medium to establish a more stable relationship to
determine the movement of the gases within the medium or vice versa.
(Iglauer et al., 2015) highlights that the contact angle between the gas, rock medium and
the water component and the piecing together of the geometry of the pore network also
contribute to a difference in the capillary forces within the medium. This in turn impacts the
behavior of both the fluid and solid surfaces of the medium, specifically with regard to their
behavior in different temperature and pressures. A contact angle that is acute implies water
wettability of the medium and thus which further leads to commendable structural and residual
trapping abilities. The reverse of this is also true, as high contact angles that are mainly obtuse
angle imply that the medium is oil-wet, and thus a very poor capability of both structural and
residual trapping because the capillary forces in this case are usually very low.
Wettability measurement by contact angle is an important aspect in while establishing the
impact of the thermodynamic variables on the performance of the fluid and solid surface
systems. This is due to the fact that wettability and contact angles do not differ much and the
effect they have on the behavior of the fluid and solid systems is determined by the capillary

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forces within the system. These forces are greatly affected by these thermodynamic properties of
the medium as a result of the temperature and pressure range within the earth’s surface where the
porous medium is located. This phenomenon is explained clearly by Bikkina (2012), where the
impact of temperature is seen to increase the volume of the porous medium and therefore
ameliorating the relative permeability of the porous medium. This scenario also increases the
capillary pressure of the volumes of the fluid phases within the pore network system and the non-
wetting components which exist around the pore network thus increasing the capillary pressure
of the porous system. The effect of a higher capillary pressure and a higher relative permeability
within the medium is an increase in the structural trapping and the residual trapping capabilities
of the medium to the CO2 gas, implying that the contact angle tends to reduce due to the
volumetric changes of all the fluid phases within the system (Bachu et al., 2007). These aspects
also affect the porous medium reversely in the scenario where the temperature thermodynamic
variable reduces. With regard to pressure, an upsurge in pressure within the earth’s core leads to
a resultant upsurge of the pore pressure and a further increase in the penetrability of the network
of pores, thus increasing the residual and structural trapping of the CO2 gas in the medium.
With regard to wettability, of the fluid and solid system, the thermodynamic properties
also greatly impact the wettability and the contact angle of the fluid matrix within the system. In
(Arif, 2016), any alterations in these variables lead to a corresponding change in the contact
angle and the wettability of the medium. For instance, at the operating pressure where the CO2
gas is injected into the medium, the temperature within the reservoir as well as the compositions
or volumes of the amount of fluid in the brine have a great impact on the wettability of the
system. This is because the thermodynamic variable changes also have an impact on the
hydrophobic nature of the fluid matrix within the medium and also the roughness of the rock

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surface. (Chiquet et al. 2007) implies that when the above mentioned factors are slightly altered,
the ions in the brine of the fluid matrix and the strength of the forces that hold the individual
components of the fluid matrix are impacted leading to a change in the contact angle and thus the
wettability of the porous material. The minerology of the fluid and solid system would also be
greatly impacted by changes in any of the thermodynamic properties of the porous medium, as
the differenced in pressure and temperature will impact the solid surface and change it (Dake,
2011). For example, during the injection of the CO2 gas into the earth’s surface, the pressure that
is used is determined by the depth of the storage site and the difference between that injection
pressure and the pressure of the porous medium in the injection site to prevent the occurrence of
explosions. (Budisa & Schilze-Makuch, 2014) explains that there are standard pressures and
temperature values set for the injection exercise, since at these levels, the CO2 gas has a behavior
of supercritical phase. Under these conditions, the CO2 gas demonstrates properties of both the
gaseous and the liquid phases which imply that the gas is then stored in a thermodynamic
supercritical state in the porous mediums or reservoirs within the earth’s surface.
The impact of pressure changes on the contact angle within the porous systems cannot be
underplayed, as an increase in pressure causes a corresponding decrease in the contact angle as
has earlier been explained. While this causes an increase in the water wettability of the medium,
it also increases the CO2 wettability (Broseta et al. 2012 ) has also conducted experimental tests
that prove that there might not be much of a significant change of the contact angle with the
increase in pressure, although the CO2 wettability is reported to have a positive increase. This
result however greatly lack the assumptions that would make the precision in the results
collected and the inferences made precise. Thus, Espinoza and Santamarina, (2010) conclude
that the increase in the pressure within the porous material has an impact of reversing the wetting