Phase Behavior of UAE Crude-Oil/Carbon Dioxide System at Reservoir Temperature

Transcription

1 Phase Behavior of UAE Crude-Oil/Carbon Dioxide System at Reservoir Temperature Papa.M. Ndiaye 1 Faculty/Department Universidade Federal do Paraná / Departamento de Engenharia Química, Curitiba-PR, Brazil papamatar00@gmail.com Abdulrazag Y. Zekri 2 Engineering/Chemical and Petroleum Engineering UAE University/Engineering Al-Ain, UAE a.zekri@uaeu.ac.ae Mohamed Nakoua 3 Faculty/Department Zakum Development Company / Training Department, UAE, ABU DHABI MAlNakoua@zadco.ae Marcelo Castier 4 Engineering/Chemical Engineering Texas A & M University of Qatar Al- Doha, Qatar marcelo.castier@qatar.tamu.edu Reyadh Almehaideb 5 Engineering/Chemical and Petroleum Engineering UAE University/Engineering Al-Ain, UAE reyadh@uaeu.ac.ae Shahin Negahban 6 ADCO/ EOR Strategic Advisor Dhabi Company for Onshore Oil Operations, Abu Dhabi, UAE snegahban@adco.ae Abstract A significant number of Enhanced Oil Recovery (EOR) techniques focus on increasing the displacement efficiency as their main recovery mechanism by injecting carbon dioxide.. Carbon dioxide flooding is among the most promise EOR methods for the light and medium oil reservoirs and has been successfully used in a number of worldwide basins. Carbon Dioxide becomes supercritical when injected under field conditions, and as a natural hydrocarbon solvent, it not only can recover more oil but also considerably reduces greenhouse gas emissions. Several factors affect the mobilization efficiency. These factors include rock geometry, pore structure, flooding rate and fluid properties. Phase behavior of the system is controlled by the properties of the reservoir oil and injected fluid mixture which depend on pressure, temperature, oil composition, and injected fluid composition. In this work, phase behavior data and swelling test at well temperature of UAE crude oil is presented. The static synthetic method is used to perform saturation pressure measurements at well temperature. Results show there is a critical carbon dioxide concentration above which a liquid-liquid-gas equilibrium is observed. Keywords: live oil, high pressure, carbon dioxide, phase behavior. 1. INTRODUCTION: The demand for oil is increasing at a fast rate for the last few years. Demand growth this year is running at its fastest level in 24 years [1] Today, petroleum is produced worldwide at a rate of gallons per second. Efficient application of secondary and tertiary recovery techniques in super giant carbonate reservoirs will help supply the world demand for oil. To estimate the extra oil production due to the application of any new and existing Enhanced Oil Recovery (EOR) techniques require the estimation of recovery efficiency of the selected technique [1]. Given the fact that discoveries of major new oil fields are declining and the demand for oil is increasing at a rate of around 1.4% per year from the current levels of 86 million barrel of oil per day in 2010, Improved or Enhanced Oil Recovery (IOR or EOR) techniques are expected to play a larger role in providing future oil from current oil fields. IOR or EOR processes contribute significantly to 96

2 overall oil production by increasing the recovery factor, the fraction of oil that can commercially be produced from the total oil deposited in geologic formations, from less than 10% for primary recovery to up to 20-40% for secondary recovery and 50-60% for EOR/IOR processes [2]. Carbon dioxide flooding is currently the 2 nd most important commercial technique for EOR after thermal techniques. The physical model for estimating overall recovery consists of displacement efficiency, volumetric sweep efficiency, mobilization efficiency, and capturing efficiency. A significant number of EOR techniques focus on increasing the displacement efficiency as their main recovery mechanism by injecting carbon dioxide. Several factors affect the mobilization efficiency. These factors include rock geometry, pore structure, flooding rate and fluid properties. Phase behavior of the system is controlled by the properties of the reservoir oil and injected fluid mixture which depend on pressure, temperature, oil composition, and injected fluid composition. Previous experimental [3] work results revealed that lower viscosity of displacing fluid, and lower interfacial tension between displacing and displaced fluid lead to reduction of residual oil saturation and improvement in the mobilization efficiency. In this work, a experimental investigation of the phase behavior of UAE live oil sample and CO 2 at reservoir temperature is carried out. Experiments include recombination of live oil using the two stage process and measurements of the saturation pressure and also the swelling factor of the system live oil/ CO MATERIALS AND METHODS: The experimental scheme used in this work is based on the static synthetic method and is showed in Figure 1. Stock tank oil and gas from first stage separator with known composition were recombined to obtain the live oil. The experimental procedure begins with loading the gas from the first stage separator into the syringe pump chamber. Since the gas from the first stage separator is a mixture of hydrocarbons up to C-7, its average vapor pressure is relatively low. Thus, the mere opening of valve cylinder containing this gas is not sufficient to move a sufficient amount of solvent into the syringe pump chamber. Usually, with the cylinder and V1 opened for about thirty minutes and V2 closed, the temperature the syringe pump chamber is kept at 283 K. This arrangement allows a natural flow from ambient temperature zone (gas cylinder) to reduced temperature zone (syringe pump). While the chamber is being filled, the cell assembly can be started. A typical recombination process starts by insertion of precise amounts of stock tank oil into the cell together with the magnetic stirrer. The cell is then closed and connected to the process line. Keeping V5, V6, and V7 closed, V2, V3 and V4 are opened. With V1 closed, the entire line is pressurized using the syringe pump and stabilized at 100 bar, 283 K. The stabilization of the system (zero pump flow) requires about ten to fifteen minutes, and should be done carefully because any trace of flow may lead to systematic errors of the volume of gas injected. Once the system is stabilized, the volume of gas inside the syringe pump chamber is recorded and a given volume of gas is injected into the cell through the micrometric valve V7. For mass of stock tank oil equal to 9 g, 10 ml of gas are injected at 283 K and 100 bar. With the first gas injection process concluded, the pressure of the system is lowered up to 50 bar and the valve V6 is opened (keeping V1, V7, V5 closed and V2, V3, V4 opened). The recombination process is carried out at ambient temperature (296 K,) so that during this step no heating is required. A sequence of procedures aiming to obtain a monophasic system is then started. By means of the magnetic stirrer the system is continuously stirred and the pressure inside the cell gradually increased until the condition of single-phase system. The pressure is then lowered to 74 bar (available experimental saturation pressure) and the resulting vapor phase is removed by opening V7, V5 and V8. The second stage for recombination is done injecting fresh gas and repeating the procedure described above. Once completed the recombination process, the saturation pressure of the recombined oil is measured at two temperatures (385 K and 398 K) and the result compared to available experimental data from bottom hole sample of the same field. For this purpose, the heating device is turned on and monitored by a PID controller device. For saturation pressure measurements, the light source from the lateral window of equilibrium cell is turned on and an infrared device which allows a precise phase transition detection even at low or without eye visibility is used to record phase transitions. Once the recombination process performed, a given amount of at 283 and 22.4 MPa is injected and the saturation pressure of the system is measured using the same procedure employed for the Typical results of the synthetic static method are bubble point and dew point curves. 97

3 Figure 1: Experimental Scheme used in this work. 98

4 2.1 Swelling Test Measurement The swelling factor is defined as the ratio between the volume of saturated mixture live oil/carbon dioxide and the volume of saturated live oil at reservoir temperature. For volume measurements of both live oil and carbon dioxide/live oil mixture, the syringe pump along with carbon dioxide material balance is used. First the saturation pressure of live oil is measured. Then the pressure is lowered up to 60 bar in order to leave the piston at its maximum position at the backside of the PVT cell. The valve V6 is then closed and the pressure of the syringe pump is increased up to the saturation pressure previously measured. The temperature of the syringe pump chamber is kept at 10 o C and one has to wait until the complete stabilization of the system (zero flow in the syringe pump controller). The initial volume (V i ) of the syringe pump is registered and while keeping the pressure constant valve V6 is opened, allowing the piston motion up to the position that keeps the live oil at saturated condition, keeping the pressure and temperature constant. After stabilization (zero flow), the final volume is registered (V f ). The displaced volume inside the syringe pump (V CO ) is then computed (V i V f ). The density of carbon dioxide at this given condition of temperature and pressure is known (Angus) and the displaced carbon dioxide mass (m 0 ) inside the syringe pump is determined. This mass is equal to the mass displaced in the cell. Since the cell temperature is known (125 o C), the volume at the backside of the piston (V CO ) can be computed. The volume of saturated live oil (V O ) can be obtained by the difference between the overall available cell volume (26.22ml) and V CO. A given amount of carbon dioxide is then injected in the cell and the procedure described above is used to compute the displaced volume (V S01 ), the displaced carbon dioxide mass (m 01 ) inside the syringe pump, the volume at the backside of the piston (V C01 ), and the volume of the mixture (V 01 ). 3. RESULTS AND DISCUSSION: 3.1 Phase Transition Figures 2, 3 and 4 show pictures of phase transition at several carbon dioxide concentrations. Phase transition are detected by means of an infrared device. The transition from the monophasic fluid (Figure 3a) to a two phase system is concomitant with a light explosion (Figure 3b). (a) (b) Figure 2. Monophasic live oil (a) and incipient phase formation in live oil at 125 o C 99

5 . Papa Ndiaye et. al. / International Journal of Modern Sciences and Engineering Technology (IJMSET) (a) (b) Figure 3. Monophasic live oil (a) and phase transition in live oil / CO 2 at 125 o C with 25% of CO 2. Figure 4. Gas-liquid-liquid transition at 125 o C using 35% of CO 2 in mass. Figures 5 and 6 show respectively the saturation pressure and the swelling factor at several carbon dioxide concentrations. From Figure 5 one can see that there is a critical CO2 concentration above which a gas-liquid-liquid transition is detected. 100

6 Figure 5. Saturation pressure as function of carbon dioxide mass fraction at T =125 o C. Figure 6. Swelling factor of carbon dioxide live oil/system as function of saturation pressure, 25 o C. 101

7 3.2. Proposed Segregation Mechanism Phase Based on the observation that at carbon dioxide mass fraction higher than 0.3, there is a vapourliquid-liquid-vapor transition, the live oil can be divided in three fractions: a light fraction, an intermediate fraction and a heavy fraction. At lower carbon dioxide concentration, the interaction between hydrocarbon molecules is not affected enough by the presence of carbon dioxide and only the conventional vapor liquid transition is observed (Figure 7). As the carbon dioxide concentration increases, the interaction between the hydrocarbon molecules fraction is affected in such a way a heavy phase segregation is induced when the pressure is lowered, followed by a vapor phase formation essentially formed by the light fraction (Figure 8). This suggest when the amount of carbon dioxide injected is above the critical concentration, the interaction between the intermediate and the heavy fraction of crude oil is weaker than that between carbon dioxide and the intermediate fraction and this fact can affect significantly the reservoir mobility. Figure 7. Proposed mechanism of phase formation a lower carbon dioxide concentration. Figure 8. Proposed mechanism of phase formation a high carbon dioxide concentration. 102

8 4. CONCLUSION: A new and reliable phase behavior apparatus has been designed for the two stages recombination process and phase behavior measurements. For the oils and conditions studied, a critical CO 2 concentration at which a liquid-liquid-vapor transition has been observed. The value of this CO 2 concentration is 0.3 in mass fraction. With the oils and conditions studied, for CO 2 mass fraction varying from 0.0 to 0.6 the corresponding swelling factor was found to range from 1 to ACKNOWLEDGEMENTS: The authors would like to thank ADNOC and ADNOC R&D Petroleum Committee for their financial support in carrying out this research work. Special thanks to UAE University Research Affairs for their support and encouragements 6. REFERENCES: [1] Gardner, J.W.; Orr, F.M.; Patel, P.D. The effect of phase behavior on CO 2 -flood displacement efficiency, 54th SPE Annual Technical Conference and Exhibition, Las Vegas, [2] McCain, W.D.; Alexander, A.A., Sampling gas in condensate wells. Society of Petroleum Engineers Reservoir Engineering, (Aug. 1992), [3] Sehbi, B.S.; Frailey, S.M.; Lawal A.S. Analysis of factors affecting microscopic displacement efficiency in CO 2 floods, SPE Permian Basin Oil and Gas Recovery Conference Midland, Texas, AUTHOR S BRIEF BIOGRAPHY: Prof. Ndiaye Matar Papa concluded a PhD in Technology of Chemical and Biochemical Processes by the Federal University of Rio de Janeiro in He published more than 20 articles in the area of applied thermodynamics with emphasis on phase equilibria in Chemistry at Federal University of Rio de Janeiro (UFRJ) as associate professor and thermodynamics lecturer Prof. Zekri: received his B.Sc., M.S., and Ph.D. degrees from the University of Southern California. He has spent more than two decades in the petroleum industry. Professor Zekri worked as a consultant to the management committees of Waha Oil Co., Agip Oil Company. He has authored or co-authored more than 90 papers on new developments and technical issues in the areas of improved oil recovery, aspects of petroleum production, and petroleum contracts, and Enhanced Oil Recovery. 103

9 Dr. Mohamed A. Al Nakoua: He is Advisor in Manpower Development Department, Zakum Development Company (ZADCO), Abu Dhabi, UAE. He served as a lecturer and assistant professor in various academic institutions, His research focuses on the development of catalytic plate reactors, syngas generation, phase behavior and mass transfer with chemical reaction mathematical modelling. He published several papers in various reputable International Journals. Dr. Al Nakoua enriched the knowledge of hundreds of students, fresh graduates and engineers. He hold several posts as a Deputy Dean, Department Head and Group Head Supervisor. Prof. Marcelo Castier: He is Professor of Chemical Engineering at Texas A&M University at Qatar where he is faculty fellow of the Mary Kay O Connor Process Safety Center Qatar and member of the Gas and Fuel Research Initiative. His research focuses on the development of thermodynamic models; algorithms for thermodynamic equilibrium; applications of thermodynamics to chemical process safety; energy integration; development of educational software. He has supervised (or co-supervised) more than 35 graduate students and published about 95 journal papers. 104