Modeling of EOR PVT data using PC-SAFT equation
- Karen Schou Pedersen (Calsep A/S) | Sukit Leekumjorn (Calsep Inc.) | Kristian Krejbjerg (Calsep Inc.) | Jawad Azeem (Calsep FZ-LLC)
- Document ID
- Society of Petroleum Engineers
- Abu Dhabi International Petroleum Conference and Exhibition, 11-14 November , Abu Dhabi, UAE
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 1.8 Formation Damage, 4.1.2 Separation and Treating, 4.1.5 Processing Equipment, 5.2 Reservoir Fluid Dynamics, 4.3.3 Aspaltenes, 4.6 Natural Gas, 5.2.2 Fluid Modeling, Equations of State, 5.4.2 Gas Injection Methods, 5.2.1 Phase Behavior and PVT Measurements, 5.5 Reservoir Simulation
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The paper presents a C7+ characterization procedure for the PC-SAFT equation of state. The characterization procedure was applied to model both routine and EOR PVT for a Middle East reservoir fluid. The injection gas contained 60 mole% of CO2. No other parameter adjustment was needed than to determine the optimum binary interaction parameters for CO2. Among the data matched was a liquid-liquid critical point on a swelling curve for a CO2 mol% of 43. The PC-SAFT simulation results suggest that the fluid for this CO2 concentration has two critical points. The one at the lower temperature agrees with the critical point found in the swelling test. The study shows that the potential of the PC-SAFT equation of state in the oil industry is not limited to modeling of asphaltene precipitation and other specialized applications. Extensive routine and EOR PVT data including a minimum miscibility pressure has been modeled using the PC-SAFT equation. Unlike cubic equations, a volume correction does not have to be applied to match liquid densities.
For decades cubic equations of state (EoS) have been the industrial standard for simulation of the phase behavior of petroleum reservoir fluids. These models capture the phase behavior without being overly complex. The Soave-Redlich-Kwong (Soave, 1972) and the Peng-Robinson (Peng and Robinson, 1976) equations were developed in the 1970s and a few years later extended with a volume correction (Peneloux et al., 1982). With the development of C7+ characterization methods (e.g. Pedersen et al., 1983), the cubic equations became a valuable and widespread tool in the oil and gas industry.
Though it has been proven that cubic equations work well for many reservoir fluids, it is also well known that the models have limitations. It would be desirable with a better match of oil compressibilities and gas Z-factors. Some fields developed today are at a very high reservoir pressure and highly undersaturated. Such fluids are difficult to model with a cubic EoS because accurate liquid compressibilities are needed for a pressure range that could span over more than 1,000 bar. Another limitation of the cubic equations is for simulation of liquid-liquid equilibria (LLE). Examples are CO2 induced LLE and asphaltenes splitting out in a separate phase either because the pressure drops or as a result of gas injection.
The PC-SAFT EoS (Chapman et al. (1988 and 1990)) and Gross and Sadowski (2001)) has been shown to give better results for oil compressibilites (Larsen et al., 2011). This is a requirement in asphaltene modeling and PC-SAFT has shown good abilities to capture how the asphaltene onset pressure develops with temperature and composition (e.g. Ting et al., 2007). While much focus has been on the PC-SAFT equation for that type of specialized applications, the versatility of the PC-SAFT equation has been given less attention. If the equation is to gain widespread use in the oil industry, a reliable PC-SAFT C7+ characterization must exist to enable simulation of routine PVT data as well as EOR data for reservoir fluids ranging from light condensates to heavy oils.
Though the cubic equation of state parameters, Tc, Pc and acentric factor, are well defined for light components, it is not straight forward to determine those parameters for the heavy end of a reservoir fluid. Because the critical state is unattainable for a heavy hydrocarbon, it has always been a matter of discussion how to assign critical properties to heavy hydrocarbons for use with cubic equations. Cumbersome parameter tuning is often needed to get a good match of an extensive PVT data material. With segment length, segment diameter and segment interaction energy as component model parameters, the PC-SAFT model appears to be well suited for heavy hydrocarbons.
While a cubic equation of state is bound to match the critical point of a pure component, this is not the case with the PC-SAFT equation. This could be a potential problem when simulating gas injection EOR processes for which miscibility develops through a critical point.
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