Wettability in Petroleum Reservoirs

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 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).

Justification and Objectives

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 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.

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 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.

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)

Literature Review

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 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 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 50^o 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.

  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 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 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 action on the surface of the porous medium. This is because the pressure increase causes a corresponding upsurge in the contact angle of CO2 due to the strong increase in the density of the gas as was influenced by the pressure increase. The effect of this was an increase in the relationships between the  mineral compounds and the gas trapped in the porous medium at a molecular level as well as a corresponding increase in the tension between the fluids in the matrix as well as the mineral particles of the medium prompts a controlling and management of the dependency that wettability has on the stress of the solid-fluid system.

The temperature of the reservoirs and the porous medium on the other hand is based in the geothermal gradient of the region where the CO2 gas is injected to and the depth of that reservoir. (Al-Yaseri, 2016) reports the impact of wettability on a quartz reservoir and established that as the temperature within the porous medium increases, there is also a corresponding increase in the contact angle of that medium, although other researchers noted a reduction in the contact angle as temperature continued to increase. Considering the application of the principles of molecular dynamics in a situation where the temperature as a thermodynamic variable is being altered shows that the increase in temperature yields in a corresponding reduction of the contact angle and thus an increase in wettability pore pressure and relative permeability within the porous medium. This means that the solid-fluid system in the porous medium demonstrated an affinity to water-wetness in comparison to any other wettability.

This research was conducted by following a review of secondary sources that have information regarding the thermodynamic variables on fluid and solid surfaces in the porous medium. A secondary source review was mainly conducted as experimental data in this specific topic is difficult to be collected as simulations of the appropriate conditions to replicate the reals situation of a reservoir are not only extremely expensive, but they are also very complicated. Since specialists in the field have been able to conduct the experimental tests and reported their findings and inferences, a review of these inferences can be used to study the impact of changes in the thermodynamic variables on the solid surface of the porous medium in the reservoir. A literature review would result in an inexpensive way of establishing the impact of thermodynamic variable changes in the solid-fluid system (Al-Yaseri, 2016). However, this means that the review would have to rely on the findings of different researchers, which may vary in results, inferences or precision of the findings. The erroneous assumptions or inferences were however eliminated by comparing a variety of literature material from different scholars to be assured that the final results arrived at will be agreeable with what most of the scholars,, researchers and experts in the field arrived at. 

The contact angle is an important parameter in the prediction of the multiphase flows within the porous medium of a reservoir. A theoretical analysis of existing literature was conducted because  there lies a great difficulty in obtaining accurate data in this subject as simulating the same conditions is extremely complicated and expensive. Thus the study relied on existing literature in the field, to establish inferences regarding the effects of thermodynamic variability on the contact angle of CO2 in a variety of materials. Arif, 2016 implies that the tension within the fluid matrix can be computed by the following formula:

According to Al-Yaseri et al., 2016, the contact angle is compared to the density of the gas phase of CO2 according to the following formula, so as to reduce the chances of inaccuracies and lack of precision since acquiring data on contact angles is difficult. The contact angle is first related to the tensions within the fluid matrix using the formula:

This formula is known as the Young formula and it can further be transformed into a a macroscopic equation through the approximation methodology of sharp- kink to form the following equation.

which explains the variance between the density function of the fluid matrix and the density of the CO2 gas. The formulas were combined with the values obtained in the research to find both the receding and advancing water contact angles.

The CO2 wettability in the porous medium of the reservoir within the earth’s surface was found to be of great significance in the determination of the contact angle, which in turn determines the residual and structural capabilities of the medium. (Dake, 2011) also clearly explained how the differences in pressure and temperature within the porous medium of the reservoir will vary due to the differences in the depths of carbon injections and the formations that occur as a result of these injections. The contact angle, according to (Chibowski & Terpilowski, 2008), can be characterized as a function of temperature and pressure , since these aspects vary during carbon storage with depth.

In addition, the relationship between the contact angle and the receding and advancing angles is given by the following formula:

(Chibowski & Terpilowski, 2008) points out that both the receding and the advancing contact angles denoted by  and respectively have a tendency to increase when the pressure in the porous medium is increase. It is reported in this literature that these angles increase to different extents depending on the gas that is being studied. For instance, (Arif, 2006) reports that at 0.1 MPa the calcite background exhibits receding and advanced contact angles of 10^o and 18^o respectively, which coincides with the data from other reports like (Al- Yaseri et al., 2016) which utilized a quartz medium and obtained a  of 0 ^o and of 10^o. These results both yield the same inferences that an increase in pressure yield a strong affinity for water wetness as is signified by the results obtained, where the angles obtained are small angles of less than 50^o. Increases in pressure show a reduction in the water wettability but a great increase in the CO2 wettability because the pressure increase yields an increase in the strength of the kinetic interactions between the gas and the medium as is seen in (Al- Yaseri et al., 2016). This is also characterized by a reduction in the capillary forces and thus an upward movement.

With regard to temperature changes, the values of  and  are seen to continue to decrease with a corresponding increase in temperature. The relationship between temperature and the receding and advancing contact angles is properly document in the report by (Arif, 2016) where the values of  was 108 degrees and 122 degrees at 308 K and they reduced to 44 degrees at 343K. This decrease was also consistent in other reports including (Chiquet

et al., 2007) and (Sarmadivaleh et al., 2015). (Gupta & Mohanty, 2006)  also established a corresponding increase in relative permeability with increasing temperatures meaning that the water wettability of the medium increased with the increase in in temperatures. This behavior of the medium as the temperatures increase is attributed to the increase in the value of the interfacial tension (as a result of the differences in the cohesive energies of the fluid matrix and solid  surface, within the fluid-solid system. In addition, the kinetic interactions between the different phases within the system become hostile as the temperature increases, which promote the porous medium to have a high affinity to water wettability (Burnside & Naylor, 2011). This is because the density of the CO2 gas as the temperature increases will tend to decrease, leaving the medium to be wetted by waste as the gas will tend to rise at a high temperature.

Conclusions and Recommendations

The wettability of the porous medium is important when studying to establish what happens when CO2 is injected to the ground and stored there in a bid to clear the environment (Farokhpoor, Bjørkvik, Lindeberg, & Torsæter, 2013). Wettability is determined using the size of both the receding and the advancing contact angles within the solid-fluid system, although the size of the angle is impacted by the thermodynamic variables of temperature and pressure. Both the receding and the advancing contact angles denoted by   and  respectively are impacted by increases in both the thermodynamic variables of temperature and pressure. An increase in pressure has an impact of increasing these  angles significantly to make the medium weakly water wet at high pressure, as was demonstrated in a number of the reports that were reviewed in this project. The  and values also decreased significantly with an increase in temperature to make the porous medium strongly water wet, and this also corresponded to the results obtained in other reports that were being reviewed in the process. The report thus concluded by saying that lower pressures and high temperatures could yield better residual and structural storage abilities of the medium to CO2 as it promotes a better strength of water wettability and thus higher capillary forces and a high relative permeability of the surface porous medium. 

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