MEASUREMENT OF CRUDE OIL INTERFACIAL TENSION TO DETERMINE MINIMUM MISCIBILITY IN CARBON DIOXIDE AND NITROGEN

Transcription

1 MEASUREMENT OF CRUDE OIL INTERFACIAL TENSION TO DETERMINE MINIMUM MISCIBILITY IN CARBON DIOXIDE AND NITROGEN by Ibrahim O. Awari-Yusuf Submitted in partial fulfilment of the requirements for the degree of Master of Engineering at Dalhousie University Halifax, Nova Scotia August 2013 Copyright by Ibrahim O. Awari-Yusuf, 2013

2 DALHOUSIE UNIVERSITY PETROLEUM ENGINEERING The undersigned hereby certify that they have read and recommend to the Faculty of Graduate Studies for acceptance a thesis entitled MEASUREMENT OF CRUDE OIL INTERFACIAL TENSION TO DETERMINE MINIMUM MISCIBILITY IN CARBON DIOXIDE AND NITROGEN by Ibrahim O. Awari-Yusuf in partial fulfilment of the requirements for the degree of Master of Engineering. Dated: August 19, 2013 Supervisor: Reader: ii

3 DALHOUSIE UNIVERSITY DATE: August 19, 2013 AUTHOR: TITLE: Ibrahim O. Awari-Yusuf MEASUREMENT OF CRUDE OIL INTERFACIAL TENSION TO DETERMINE MINIMUM MISCIBILITY IN CARBON DIOXIDE AND NITROGEN DEPARTMENT OR SCHOOL: Petroleum Engineering DEGREE: MEng. CONVOCATION: October YEAR: 2013 Permission is herewith granted to Dalhousie University to circulate and to have copied for non-commercial purposes, at its discretion, the above title upon the request of individuals or institutions. Signature of Author The author reserves other publication rights, and neither the thesis nor extensive extracts from it may be printed or otherwise reproduced without the author s written permission. The author attests that permission has been obtained for the use of any copyrighted material appearing in the thesis (other than the brief excerpts requiring only proper acknowledgement in scholarly writing), and that all such use is clearly acknowledged. iii

4 DEDICATION I dedicate this work to the two father figures in my life, Mr Ismaila Awari-Yusuf and Mr Yusuf Yusuf Awari, to Mrs Taibat Awari-Yusuf and to all my family and friends who have supported me through the course of my studies. iv

5 TABLE OF CONTENTS LIST OF TABLES... vii LIST OF FIGURES... viii ABSTRACT... x LIST OF ABREVIATIONS AND SYMBOLS... xi ACKNOWLEDGEMENTS...xiii CHAPTER 1 INTRODUCTION Background Objective... 3 CHAPTER 2 LITERATURE REVIEW Miscibility Miscible flooding Carbon dioxide flooding Properties of carbon dioxide Nitrogen flooding Nitrogen properties Miscible flooding mechanism Interfacial tension Minimum miscibility pressure Experimental methods for determining minimum miscibility pressure Crude oil density CHAPTER 3 EXPERIMENTAL The theory behind the pendant drop technique Apparatus Accuracy and reproducibility Materials Requirement of the drop shape analysis Crude oil density measurement Carbon dioxide Density Nitrogen Density v

6 3.6 DSA Measurement CHAPTER 4 RESULTS AND DISCUSSION Crude oil and carbon dioxide systems at 22 0 C Crude oil and nitrogen systems at 22 0 C Gullfaks C using carbon dioxide at 60 0 C CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS Conclusions Recommendations REFERENCES vi

7 LIST OF TABLES Table 3.1: Calibration data...16 Table 3.2: Crude oil density data Table 3.3: Carbon dioxide density data...19 Table 3.4: Variation of nitrogen density with pressure Table 4.1: DSA measurement for Arab AH-50 using carbon dioxide Table 4.2: DSA measurement of Gullfaks C using carbon dioxide Table 4.3: DSA measurement of West Texas Intermediate using carbon dioxide Table 4.4: DSA measurement of Arab AH-50 using nitrogen Table 4.5: DSA measurement of Gullfaks C using nitrogen Table 4.6: DSA measurement of West Texas intermediate using nitrogen Table 4.7: DSA measurement of Gullfaks C using carbon dioxide at reservoir temperature vii

8 LIST OF FIGURES Figure 2.1: Pure carbon dioxide phase diagram... 5 Figure 2.2: Carbon dioxide density as a function of pressure at 60 0 C... 6 Figure 3.1: Schematic of a pendant drop Figure 3.2: shape analysis (DSA 100 V 1.9) and high pressure pendant drop (PD-E 1700) Figure 3.3: Schematic of the axisymmetric drop shape analysis (ADSA) Figure 3.4: Variation of carbon dioxide density with Temperature Figure 3.5: Variation of Gullfaks C density with pressure Figure 3.6: Flow sheet of the PD-E Figure 4.1: Variation of Arab AH-50 drop volume with pressure using carbon dioxide at 22 0 C Figure 4.2: Variation of Gullfaks C drop volume with pressure using carbon dioxide at 22 0 C Figure 4.3: Variation of West Texas intermediate drop volume with pressure using carbon dioxide at 22 0 C Figure 4.4: Variation of Arab AH-50 drop surface area with pressure using carbon dioxide at 22 0 C Figure 4.5: Variation of Gullfaks C drop surface area with pressure using carbon dioxide at 22 0 C Figure 4.6: Variation of West Texas intermediate drop surface area with pressure using carbon dioxide at 22 0 C Figure 4.7: Variation of Arab AH-50 interfacial tension with pressure using carbon dioxide at 22 0 C Figure 4.8: Variation of Gullfaks C interfacial tension with pressure using carbon dioxide at 22 0 C Figure 4.9: Variation of West Texas intermediate interfacial tension with pressure using carbon dioxide at 22 0 C Figure 4.10: Variation of Arab AH-50 drop volume with pressure using nitrogen at 22 0 C Figure 4.11: Variation of Gullfaks C drop volume with pressure using nitrogen at 22 0 C Figure 4.12: Variation of West Texas intermediate drop volume with Pressure using nitrogen at 22 0 C Figure 4.13: Variation of Arab AH-50 drop surface area with pressure using nitrogen at 22 0 C Figure 4.14: Variation of Gullfaks C drop surface area with pressure using nitrogen at 22 0 C Figure 4.15: Variation of West Texas intermediate drop surface area with pressure using nitrogen at 22 0 C Figure 4.16: Variation of drop interfacial tension With pressure Using nitrogen at 22 0 C Figure 4.17: Variation of Gullfaks C With Pressure Using nitrogen at 22 0 C.. 44 Figure 4.18: Variation of West Texas intermediate drop interfacial tension with pressure using nitrogen at 22 0 C viii

9 Figure 4.19: Variation of Gullfaks C s interfacial tension with pressure using carbon dioxide At 60 0 C Figure 4.20: Variation Of Gullfaks C s interfacial tension With temperature ix

10 ABSTRACT Gas injection has been used to enhance oil recovery due to its ability to maintain reservoir pressure, reduce oil viscosity, reduce oil interfacial tension, displace residual oil and induce oil swelling effect. However the type of gas used would have a considerable impact on oil recovery and the cost incurred during injection thereby determining the economics of the process. In this study, the axisymmetric drop shape analysis (ADSA) technique is used to assess the impact of pressure change, injection gas type and crude oil type on the pendant drop volume, surface area and interfacial tension. The ADSA technique is used to measure the pendant drop parameters of the Arab AH-50, the Gullfaks C and the West Texas intermediate crude oil pendant drops in carbon dioxide and nitrogen. It is found that in each test, the pendant drop volume, surface area and reduce linearly with pressure increase. Reduction in the three parameters is more pronounced in the crude oil-carbon dioxide system. The vanishing interfacial tension (VIT) technique is used to estimate first contact minimum miscibility pressure of the crude oil-gas systems from the measured interfacial tension and it was seen that the systems with carbon dioxide required less pressure to achieve miscibility thereby making carbon dioxide a more favourable gas for miscible flooding in comparison to nitrogen. x

11 LIST OF ABREVIATIONS AND SYMBOLS = Oil density at reservoir conditions(lb/ft 3 ) = Fake density used in oil density calculation (lb/ft 3 ) = Pseudo liquid density (lb/ft 3 ) = Oil density at reservoir bubblepoint (lb/ft 3 ) = The difference in density of the two phases (Kg/m 3 ) = Adjustment of oil density due to pressure (lb/ft 3 ) = Adjustment of oil density due to temperature (lb/ft 3 ) Interfacial tension = Weighted average surface gas specific gravity = Stock tank oil specific gravity = Separator gas specific gravity = Interfacial tension (mn/m) a = Capillary length = Constants B 0 = Bond number which represents the ratio of buoyancy force to surface force (dimensionless) = Weighted average oil compressibility from bubble point pressure to a higher pressure of interest, 1/psi g = Gravitational acceleration (M/s 3 ) = Pressure (psia) = Bubblepoint pressure (psia) = Pressure difference at a reference plane (psia) r = Characteristic radius (m) R 0 = Radius at the apex of the drop (m) = Principal radii of curvature xi

12 =Solution gas oil ratio at bubblepoint pressure (scf/stb) mn/m = Mill-Newton per meter = Temperature ( 0 F) ADSA = Axisymetric drop shape analysis EOR = Enhanced oil recovery = Interfacial tension MMP = Minimum miscibility pressure PV = Pore volume RBA = Rising bubble apparatus VIT = Vanishing interfacial tension xii

13 ACKNOWLEDGEMENTS I am immensely grateful to all faculty members and staff especially Mr Mumuni Amadu and Mr. Matt Kujath for their support and motivation during this project. I will also like to thank my supervisor Dr. Michael Pegg, for the continued assistance, guidance and support given me through the course of this project. I am also thankful to Dr. Jan Healssig for his acceptance to serve on the examining committee. Finally, to my family which has been a constant source of encouragement and support, I say thank you. xiii

14 CHAPTER 1 INTRODUCTION 1.1. Background As crude oil recovery from conventional reservoirs continues to decrease, enhanced oil recovery (EOR) is increasingly becoming significant in the petroleum industry. Gas injection has been used as an EOR process in the petroleum industry for a very long time due to its pressure maintenance capability, its ability to reduce the viscosity of reservoir fluids, and its efficiency in displacing reservoir fluids as well as inducing oil swelling effect which is the expansion of the reservoir oil due to the dissolution of the injected solvent into the reservoir fluid (Sclumberger Limited, 2013). Interfacial mass transfer occurs between the injected gas phase and the reservoir fluid during gas injection until an equilibrium state is achieved. As a result of this phenomenon, the physical and chemical properties of the reservoir fluid are modified leading to a more efficient displacement process (Danesh, 1998). Gas flooding can be classified into miscible, semi miscible and immiscible flooding processes depending on the temperature, pressure, type of injected gas and reservoir conditions (Ali et al., 2013). Lake (1989) stated that fluids that mix in all proportions while still existing in a single homogenous phase are considered to be miscible. The minimum miscibility pressure (MMP) is defined by Johnson and Pollin as the lowest pressure at which an apparent point of maximum curvature can be seen as recovery of oil at 1.2 pore volumes (PV) gas injected is plotted against pressure (Johnson & Pollin, 1981) this can also be said to be the pressure at which the interfacial tension () between two phases is zero (Green & Willhite, 1998). 1

15 2 The classification of gas flooding is based on the MMP (Wang & Gu, 2011). Hence If the pressure of the reservoir is not maintained above the MMP of the injected gas then the injection process would become semi miscible or immiscible (Fanchi, 2006). Determining MMP accurately is a crucial step in the design of an economical miscible injection program. Numerous analytical and experimental methods have been developed to predict or estimate MMP. Traditionally, the slim tube method is considered as the standard technique for MMP measurement in an oil/solvent system (Huang & Dyer, 1993). However, it is very expensive and time consuming (Gu et al., 2013). High pressure carbon dioxide core flood tests can also be used to measure MMP in a similar fashion as the slim tube method (Huang, 1992). The rising-bubble apparatus (RBA) (Christiansen & Haines, 1987) and the vanishing interfacial tension (VIT) technique (Rao, 1997; Rao & Lee, 2002; Rao & Lee, 2003 ; Gu et al., 2013) are faster and less expensive methods of which MMP can be experimentally estimated. It has been shown that reduces in a linear fashion in an isotherrmal system as the pressure reduces (Adamson & Gast, 1997) and that MMP can be estimated through the extrapolation of the linear equation till is zero (Rao & Lee, 2003). The choice of injection gas would depend on the availability of the gas, reservoir conditions and the economic viability of the gas injection process. Carbon dioxide injection is said to be one of the most effective methods to improve the efficiency of the oil recovery process (Alvarado & Manrique, 2010; Farouq & Thomas, 1996). To test the above stated statements, the axisymmetric drop shape analysis (ADSA) technique is used to assess the impact of pressure change, injection gas type and crude oil type on the pendant drop volume, surface area and. In more detail, the ADSA technique is used in this study to measure the DSA pendant drop parameters of the Arab AH-50, the Gullfaks C and the West

16 3 Texas intermediate pendant drops in carbon dioxide and nitrogen systems. The VIT technique is then used to estimate first contact minimum miscibility pressure of the crude oil and gas systems from the measured at 22 0 C and over a pressure of 100 to 600 psi Objective This work is being carried out for the following reasons: 1) To understand the principles behind miscible gas injection as an EOR method. 2) To test the accuracy of statements from literature that state that reduces linearly with pressure increase under isothermal conditions and that carbon dioxide is a more favorable gas for miscible flooding compared to nitrogen. 3) To generate data that can form the basis of correlations to determine MMP of the used crude oil samples in carbon dioxide and nitrogen systems. 4) To determine the variation in pendant drop volume and surface area with pressure and density. 5) To determine if varies linearly with pressure at non reservoir conditions.

17 CHAPTER 2 LITERATURE REVIEW 2.1. Miscibility Miscibility between two or more fluids has been defined in numerous ways by different authors (Rao D. N., 1997; Benham et al., 1965; Lake, 1989). However all definitions acknowledge one of the following (Mohamed, 2009): (1) Inexistence on an interface between the mixing fluids; (2) Occurrence of zero between the mixing fluids; (3) All fluid mixtures mix in all proportions while existing in a single indistinguishable phase Miscible flooding This is a branch of enhanced oil recovery (EOR) where miscible gases are injected into the reservoir. These gases maintain the reservoir pressure as well as improve the recovery of the reservoir fluid due to the reduction in between the injected fluid and the reservoir fluid (Schlumberger Limited, 2013). Many miscible gasses could be injected into the reservoir to achieve the same outcome. The choice of the gas would depend on the cost and its availability. A few gases that are currently being used are liquefied petroleum gas (LPG) such as methane and propane, light hydrocarbon, nitrogen and carbon dioxide. Carbon dioxide is the most widely used gas for miscible flooding because it reduces oil viscosity and is less expensive when compared to LPG (Schlumberger, 2013). 4

18 Carbon dioxide flooding Carbon dioxide has been used in oil recovery since 1952 (Stalkup, 1978). Carbon dioxide can be used for miscible displacement (Rathmel et al., 1971), immiscible displacement (Kumar & Von Gonten, 1973), reservoir pressure maintenance (Holm & Josendal, 1974), well stimulation (Stright Jr., Aziz, & Settari, 1977), etc Properties of carbon dioxide Carbon dioxide is a relatively non-toxic, non-flammable fluid (Mohamed, 2009). It has a critical temperature and pressure of C and MPa respectively; its triple point is C and MPa (Span & Wagner, 1994). Figure 2.1 illustrates the pressure-temperature property of carbon dioxide. It can be seen that carbon dioxide would exist in different phases depending on its temperature and pressure. Figure 2.1: Pure carbon dioxide phase diagram (Zhang et al., 2012)

19 density (g/cm 3 ) 6 Figure 2.2 show the variation in density with pressure at 60 0 C. It can also be seen from Figure 2.2 that pressure has a large impact on density. Figure 2.2 was plotted using data from the peace software thermodynamic data (Peace Software) and the correlation proposed by Ouyang (Ouyang, 2011). 1.2 Density (kg/m3) at 60 degrees Pressure (psia) Figure 2.2: Carbon dioxide density as a function of pressure at 60 0 C (Peace software; Ouyang, 2011) 2.4. Nitrogen flooding Nitrogen flooding has been used in EOR successfully for a long time in the oil industry (Koch & Hutchinson, 1958; Hudgins et al., 1990). It has also been used for other purposes such as gas lift, pressure maintenance and gas cycling (Clancy et al., 1985). Nitrogen is used as an alternative to natural gas and carbon dioxide due to its low cost and non corrosive nature and since it can be

20 7 extracted from atmospheric air via cryogenic processes, its source is largely unlimited (Lindley, 2011). Nitrogen miscibility is favored in reservoirs that are rich in light and intermediate hydrocarbon components at a high reservoir pressure (Hudgins et al., 1990). Due to the high injection pressure that nitrogen flooding requires, the reservoir should be able to withstand this high pressure without fracturing the formation (Lindley, 2011) Nitrogen properties Nitrogen is a nontoxic, non-flammable gas at atmospheric conditions. It has a critical temperature of -147 C and a critical pressure of MPa and critical density of kg/m 3. Its triple point is at C and MPa. Nitrogen makes up at least 78% of atmospheric air by volume (Air Liquide, 2013) Miscible flooding mechanism Miscible gases displace reservoir fluid in very similar ways. Lindley (2011) explained that the injected gas forms a miscible front by vaporizing light components of reservoir oil. The enriched gas then moves away from the injection well into the reservoir, where it further enriches itself by contacting with reservoir oil and vaporizing more light components. This enrichment process continues until the gas becomes miscible with the reservoir fluid and a homogenous phase with new physicochemical properties is formed. Continuous injection of the gas would push oil via the miscible front towards the production well. The produced reservoir fluid can be separated for oil, natural gas and the injected gas (Lindley, 2011).

21 Interfacial tension Interfacial tension, commonly expressed in mn/m or dyne/cm, is a property of the interface that exists between two immiscible phases. It is referred to as interfacial tension when both phases are liquid and it is called surface tension when one of the phases is atmospheric air. However, they both refer to the Gibbs free energy present per unit interface area at a particular temperature and pressure (Schlumberger, 2013) Minimum miscibility pressure The minimum miscibility pressure (MMP) is defined by Johnson and Pollin as the lowest pressure at which an apparent point of maximum curvature can be seen as recovery of oil at 1.2PV gas injected is plotted against pressure (Johnson & Pollin, 1981). It can also be defined as the minimum pressure whereby the injected gas phase becomes miscible with the residual oil in place (ROIP) after a multi-contact process at the existing reservoir temperature. (Stalkup, 1987). MMP is dependent on factors such as the composition of the injected gas, the ROIP and the reservoir temperature and independent of the velocity of displacement and the condition of the porous media (Alomair et al, 2011)

22 Experimental methods for determining minimum miscibility pressure A number of experimental methods have been developed to measure MMP. Traditionally, the slim tube method is considered as the standard technique for MMP measurement in an oil/solvent system (Huang & Dyer, 1993). This method reproduces the multiphase fluid flow through a porous medium under reservoir conditions. It is however very expensive and time consuming (Gu et al., 2013). High pressure carbon dioxide core flood tests can also be used to measure MMP in a similar fashion as the slim tube method (Huang, 1992). The expensive and time consuming nature of the above methods led to the development of more favorable methods that are faster and more cost effective such as the rising-bubble apparatus (RBA) (Christiansen & Haines, 1987). The use of the RBA to determine MMP is faster and requires less crude compared to the slim tube method and the core flooding approach; however, this method could overestimate MMP for some systems (Gu et al., 2013). The VIT, which is the technique used in this study, has recently been used to measure MMP (Rao D. N., 1997; Rao & Lee, 2002; Rao & Lee, 2003;Gu et al., 2013). The between crude oil and carbon dioxde can be accurately measured using ADSA and the MMP can be extrapolated from data at reservoir conditions.

23 Crude oil density There are a number of correlations that can be used to estimate crude oil density. However, in this work, the use of correlations was not feasible due to the large amount of data needed by the correlations. Hence the measurement of crude oil density was carried out in the laboratory using a pycnometer (purchased from VWR International) according to ASTM standard D

24 11 CHAPTER 3 EXPERIMENTAL 3.1. The theory behind the pendant drop technique The pendant drop technique uses axisymmetric drop shape analysis (ADSA) to determine interfacial properties through ascertaining the profile of the liquid droplets formed. This experimental profile is then fitted with the theoretical Laplace equation reported by Cheng, (1990). Hydrodynamic equilibrium is a requirement for this technique, i.e. the only forces acting on the drop should be gravity and surface tension (Neumann & Rio, 1997). Figure 3.1 below shows the schematic of a pendant drop. Figure 3.1: Schematic of a pendant drop (Mohammed, 2009)

25 Chiquet et al, (2007) reported that Cheng s equation can be represented by the following ordinary differential equations with an arc length of s: 12 (17) (18) (19) R 0 2R 0 + (20) Where: θ = angle between the horizontal and the tangent to the drop contour B 0 = Bond number which represents the ratio of buoyancy force to surface force (dimensionless) r = characteristic radius (m) = The difference in density of the two phases (kg/m 3 ) g = Gravitational acceleration R 0 = radius at the apex of the drop (m)

26 13 The ADSA determines B 0 and values that minimizes the difference between the solution to equation 20 and the digital profile of the drop. Hence, the capillary length a is determined. Capillary length can be expressed as: (21) The can therefore be determined by solving equation 21 after the difference in density between the two phases has been obtained. Hence: ( ) = (22) 3.2. Apparatus The high pressure pendant drop apparatus (PD-E 1700) and the drop shape analysis (DSA 100 V ) are used to measure the equilibrium and dynamic s of crude oil/carbon dioxide systems at different temperatures and pressures. The PD-E 1700 was made by EUROTECHNICA and the DSA 100V was made by K SS. The equipment is shown in Figure 3.2.

27 14 Figure 3.2: shape analysis (DSA 100 v 1.9) and high pressure pendant drop (PD-E 1700) The major components of the PD-E 1700 are a high temperature and pressure cell with two windows C and 69 MPa are the maximum operating temperature and pressure. The DSA 100 (V ) consists of a light source and a high resolution CCD camera. The high pressure cell is placed between the camera and the light source to enable illumination through the two windows. The is determined by analyzing the shape of a pendant drop and this is considered as the most powerful method of measuring interfacial properties because of its versatility, accuracy and simplicity (Cheng & Neumann, 1992; Jennings & Pallas, 1988). The pendant crude oil drop is formed within the carbon dioxide phase using a needle installed at the top of the high pressure cell. A digital image of this drop is acquired using a digital image acquisition system.

28 15 This image is then fitted with the Laplace equation of capillarity and the is automatically calculated by generating an interfacial profile that best fits the actual drop. Figure 3.3 shows a schematic of the process. Image Image Analysis Numerical Optimization Physical Properties (ρ,g) ( ) Interfacial tension Figure 3.3: Schematic of the axisymmetric drop shape analysis (ADSA) adapted from (Hoorfar & Neumann, 2006)

29 Accuracy and reproducibility The accuracy of the pendant drop equipment was tested by calibrating with the measurement of the of deionised pure water /atmospheric air system until the typical value of mn/m was obtained.the calibration data are shown in Table 3.1 below. The standard deviation of the data was calculated as [mn/ m] Table 3.1 : Calibration data Theta(L) [deg] Theta(R) [deg] Theta(M) [deg] Fit-Er [um] Density L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y L-Y

30 Materials Three crude oil samples were used in this study: Arab AH-50, Gullfaks C and west Texas intermediate. Carbon dioxide (99.5% purity) and nitrogen (99.995% purity) were purchased from Praxair and were used without further purification. Carbon dioxide s critical temperature and pressure are o C and 7.38MPa. Nitrogen s critical temperature and pressure are -240 o C and 1.30MPa Requirement of the drop shape analysis The drop shape analysis software used (DSA ) requires the capillary needle diameter, the local gravitational acceleration, and the density difference between the liquid (crude oil) and gas phase (CO 2 /N 2 ) Crude oil density measurement In this work, the use of correlations was not feasible due to the large amount of data needed by the correlation. Hence the measurement of crude oil density was carried out in the laboratory using a Pycnometer (purchased from VWR International) according to ASTM standard D

31 18 Table 3.2 shows the calculated density data. Table 3.2. : Crude oil density data Crude Oil Temperature Density (g/cm 3 ) Gullfaks C 22 o C Arab AH o C West texas intermediate 22 o C Gullfaks C 60 o C Carbon dioxide Density There are numerous correlations that have been generated to estimate carbon dioxide density. However the most popular data for carbon dioxide density are does published by Span and Wagner in Hence carbon dioxide density data at the pressure and temperature conditions of interest i.e., psia and 22 0 C and 60 0 C were calculated by using peace software. These data were validated and compared with data presented by Span & Wagner, (1994) obtained from the National Institute of Standards and Technology (NIST) website. The average density of carbon dioxide decreases with increasing temperature and increases with pressure.

32 19 Table 3.3: Carbon dioxide density data Density (g/cm 3 ) Temperature ( 0 C) Pressure (psia) 22 0 C 60 0 C Figure 3.4 shows that the change in density of the Gullfaks C crude with temperature is more obvious as pressure increases.

33 Density (g/cm 3 ) Degrees 60 degrees Pressure (psia) Figure 3.4: Variation of carbon dioxide density with temperature Nitrogen Density Nitrogen density at the pressure and temperature conditions of interest i.e., psia and 22 0 C were calculated by using peace software. This data was compared and validated with data presented by Span et al., (2000) obtained from the National Institute of Standards and Technology (NIST) website. The average density of nitrogen increases with increasing pressure. The nitrogen densities were calculated using the ideal gas law as the as the error due to compressibility at the highest pressure of 600 psia is 2.5 percent.

34 Density (g/cm 3 ) 21 Table 3.4: Variation of nitrogen density with pressure Density (g/cm 3 ) Temperature ( 0 C) Pressure (psia) 22 0 C Pressure (psia) Figure 3.5: Variation of nitrogen density with pressure at 22 0 C

35 DSA Measurement Figure 3.6: Flow sheet of the PD-E1700 (EUROTECHNICA, 2008) The schematic of the pendant drop apparatus is shown in Figure 3.6. The equipment was first calibrated using deionized water and atmospheric air until a satisfactory value of mn/m was obtained. The view cell was then filled with gas (CO 2 or N 2 ) until the predetermined pressure was reached using the screw pump on the gas cylinder. About ten minutes was allowed for the pressure in the chamber to stabilize. Finally, crude oil was then added to the liquid supply tank (TL1) with valve A shut. Valve B was then shut and valve A opened. The screw piston pump (PG1) was then operated anti-clockwise to suck the crude oil into the cylinder. Once the cylinder was full, PG1 was operated clockwise with valve A open to allow some crude into TL1 and allow trapped air bubbles to be released. Valves A and B were then closed and valve C opened. PG1 was operated clockwise again till a small amount of crude oil was released into a glass beaker. Valve C was then closed and valve B slowly opened while PG1 was operated clockwise till crude oil emerged at the capillary. After the crude oil pendant drop was formed in the gas phase, a digital image of each drop was taken and stored on the computer hard drive. The

36 DSA software determined the and other output parameters which were also stored on the computer hard drive. 23 The of each crude oil drop was measured and the drops were replaced five times as old drops were withdrawn from the capillary and new drops were created to ensure repeatability and accuracy of the data. The measurements were done within five to ten seconds of contact with the gas phase. At the end and the beginning of each test, the entire system was cleaned by flushing it with methanol three times and drying with compressed air. measurements were taken for the three crude samples at 22 o C degrees and six pressures ranging from psi. The first contact miscibility pressure of the Gullfak C sample was measured by measuring at an assumed average temperature of 60 0 C and three pressures ranging from psi. The summary of data collected is shown in Tables 4.1 to 4.7.

37 CHAPTER 4 RESULTS AND DISCUSSION Interfacial tension was measured for the Gulffaks C, Arab AH-50 and West Texas intermediate dead crude oil samples using carbon dioxide and nitrogen gas over a pressure of 100 to 600 psia and 22 0 C and was also taken for the Gullfaks C dead crude at an assumed reservoir temperature of 60 0 C and over a pressure of 100 to 300 psia. The DSA results are presented in Tables 4.1 to 4.7 below. These tables contain analyzed parameters which include interfacial tension [MN/m], pendant drop volume and pendant drop surface area. Other parameters also present in the tables that were not considered includes the pendant drop left, right and middle contact angles (Theta (L,R,M ) [deg]), the drop base diameter, the fit error, the method L-Y and the magnification factor. Interfacial tension, pendant drop volume and drop surface area measurements were plotted against pressure ranging from 100 to 600 Psi and at 22 o C. The Gullfaks C crude sample s interfacial tension, pendant drop volume and pendant drop surface area measurements using carbon dioxide gas was also plotted against pressures of 100, 200 and 300 Psi at 60 o C. The DSA measurement for this is shown in Table 4.7. Tables 4.1 to 4.3 shows the measurement data for the three crude oil samples using carbon dioxide at 22 0 C while Table 4.4 to 4.6 shows the measurement data using nitrogen also at 22 0 C. Figures 4.1 to 4.3 shows the variation of the three crude sample s pendant drop volume with pressure using carbon dioxide, Figures 4.4 to 4.6 shows the variation in the pendant drop surface area with pressure using carbon dioxide and Figures 4.7, 4.8 and 4.9 shows the variation of interfacial tension with pressure using carbon dioxide. All of these tests were carried out at an experimental temperature of 22 0 C. 24

38 25 Figures 4.10 to 4.12 shows the variation of the three crude sample s pendant drop volume with pressure using nitrogen, Figures 4.13 to 4.15 shows the variation in the pendant drop surface area with pressure using nitrogen and Figures 4.16, 4.17 and 4.18 shows the variation of interfacial tension with pressure using nitrogen. All of these tests were carried out at an experimental temperature of 22 0 C. DSA measurement was also carried out at an assumed reservoir temperature of 60 0 C for the Gullfaks C crude sample using carbon dioxide over a pressure ranging from 100 to 300 psia. Table 4.7 shows these DSA measurements and Figure 4.19 shows the variation of interfacial tension with pressure while Figure 4.20 shows the variation of interfacial tension with temperature.

39 Crude oil and carbon dioxide systems at 22 0 C Table 4.1: DSA measurement for Arab AH-50 using carbon dioxide 100 Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average

40 27 Table 4.2: DSA measurement of Gullfaks C using carbon dioxide 100 Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y Average Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average

41 28 Table 4.3: DSA measurement of West Texas Intermediate using carbon dioxide 100 Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y Average Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 22 0 C Fit- Er L-Y L-Y L-Y L-Y L-Y Average

42 Arab AH-50 Linear (Arab AH-50) y = x R² = Pressure (psia) Figure 4.1: Variation Arab AH-50 drop volume with pressure using carbon dioxide at 22 0 C Gullfaks C Linear (Gullfaks C) y = x R² = Pressure (psia) Figure 4.2: Variation of Gullfaks C drop volume with pressure using carbon dioxide at 22 0 C

43 West texas intermediate Linear (West texas intermediate) y = x R² = Pressure (psia) Figure 4.3: Variation of West Texas intermediate drop volume with pressure using carbon dioxide at 22 0 C The DSA measurements for the three crude oil samples using carbon dioxide gas are shown in Tables 4.1 to 4.3. All the pendant drops formed for this study formed from the outer surface of the capillary needle. Figures 4.1 to 4.3 above show that the pendant drop volume reduces in a linear fashion as pressure increases. The volume of the first crude oil sample, the Arab AH-50 reduced from 9.17µl to 5.52µl, that of the Gullfaks C sample reduced from 10.03µl to 6.54µl and the West Texas intermediate sample s pendant drop volume reduced from 10.12µl to 5.92µl.This occurred due to increase in the force exerted by the carbon dioxide gas on the pendant drop. This same phenomena explains the decrease in the pendant drop surface area as volume is directly proportional to surface area. It can be seen that the change in volume of the three different crude samples is approximately the same.

44 Arab AH-50 Linear (Arab AH-50) y = x R² = Pressure (psia) Figure 4.4: Variation Arab AH-50 drop surface area with pressure using carbon dioxide at 22 0 C Gullfaks C Linear (Gullfaks C) y = x R² = Pressure (psia) Figure 4.5: Variation of Gullfaks C drop surface area with pressure using carbon dioxide at 22 0 C

45 y = x R² = West texas intermediate Linear (West texas intermediate) Pressure (psia) Figure 4.6: Variation of West Texas intermediate drop surface area with pressure using carbon dioxide at 22 0 C The DSA measurements for the three crude oil samples using carbon dioxide gas are shown in Tables 4.1 to 4.3. Figures 4.4 to 4.6 above show that the pendant drop surface area reduces in a linear fashion as pressure increases. The pendant drop surface area of the first crude oil sample, the Arab AH-50 reduced from mm 2 to mm 2, that of the Gullfaks C sample reduced from 21.3 mm 2 to mm 2 and the West Texas intermediate sample s pendant drop surface area reduced from mm 2 to mm 2. This change in the pendant drop surface area can be explained as due to the increased force exerted by the carbon dioxide gas on the pendant drop. It can be seen that the change in the pendant drop surface area for the three different crude samples is approximately the same.

46 Arab AH-50 Linear (Arab AH-50) y = x R² = Pressure (psia) Figure 4.7: Variation of Arab AH-50 interfacial tension with pressure using carbon dioxide at 22 0 C Gullfaks C Linear (Gullfaks C) y = x R² = Pressure (psia) Figure 4.8: Variation of Gullfaks C interfacial tension with pressure using carbon dioxide at 22 0 C

47 West Texas Itermediate Linear (West Texas Itermediate) y = x R² = Pressure (psia) Figure 4.9: Variation of West Texas intermediate interfacial tension with pressure using carbon dioxide at 22 0 C The DSA measurements for the three crude oil samples using carbon dioxide gas at 22 0 C are shown in Tables 4.1 to 4.3. Figures 4.7 to 4.9 above show that the pendant drop reduces as pressure increases in a linear fashion. The of the first crude oil sample, the Arab AH-50 reduced from mn/m to mn/m, that of the Gullfaks C sample reduced from mn/m to mn/m and the West Texas intermediate sample s reduced from mn/m to 14.6 mn/m. It can be seen that the change in for the three different crude samples over the same pressure range is approximately the same. Solving the trendline equations of the three crude oil samples for zero yields a first contact miscibility pressure of 1205 psia for the Arab AH-50, 1214 psia for the Gullfaks C sample and 1288 psia for the West Texas intermediate sample.

48 Crude oil and nitrogen systems at 22 0 C Table 4.4: DSA measurement of Arab AH-50 using nitrogen 100 Psi, 220C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 220C Fit- Er L-Y L-Y L-Y L-Y Average Psi, 220C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 220C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 220C Fit- Er L-Y L-Y L-Y L-Y L-Y Average Psi, 220C Fit- Er L-Y L-Y L-Y L-Y L-Y Average

49 36 Table 4.5: DSA measurement of Gullfaks C using nitrogen 100 Psi,220C Fit-Er L-Y L-Y L-Y L-Y Average Psi, 220C Fit-Er L-Y L-Y L-Y L-Y L-Y Average Psi, 220C Fit-Er L-Y L-Y L-Y L-Y L-Y Average Psi, 220C Fit-Er L-Y L-Y L-Y L-Y L-Y Average Psi, 220C Fit-Er L-Y L-Y L-Y L-Y L-Y Average Psi, 220C Fit-Er L-Y L-Y L-Y L-Y L-Y Average