Surfactant Flooding in Heterogeneous Formations
- Arild Lohne (Intl. Research Inst. of Stavanger) | Ingebret Fjelde (IRIS LLC)
- Document ID
- Society of Petroleum Engineers
- SPE Improved Oil Recovery Symposium, 14-18 April, Tulsa, Oklahoma, USA
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 5.5.3 Scaling Methods, 1.8 Formation Damage, 4.1.2 Separation and Treating, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 1.6.9 Coring, Fishing, 4.1.5 Processing Equipment, 5.2.1 Phase Behavior and PVT Measurements, 1.2.3 Rock properties, 5.7.2 Recovery Factors, 5.3.4 Reduction of Residual Oil Saturation, 5.3.2 Multiphase Flow, 5.4.1 Waterflooding, 4.3.4 Scale, 5.1.5 Geologic Modeling
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The potential for surfactant flooding of oil fields can be obtained using field-scale surfactant simulation model with capillary number (Nc) dependent relative permeability, kr. Typically, kr(Nc) is solely based on core flood experiments. In this paper, surfactant recovery mechanisms in presence of heterogeneities smaller than the field simulation grid block are investigated. Effects of reducing the interfacial tension, IFT, are investigated in various models by numerical simulation of the displacement process and by steady-state upscaling. It is demonstrated that measured core-scale results should go through appropriate averaging before they are used in a field-scale surfactant model.
Macro-scale effects of IFT in presence of small-scale heterogeneities are examined separately. Capillary trapping of oil in heterogeneous formations depends on the geometric distribution of permeability and wettability and on IFT. In general, reduced IFT has little or possibly negative effect on production from water-wet, low-permeability inclusions embedded in a mixed-wet background. Production from mixed-wet, low-permeability inclusions will in general be increased at reduced IFT.
The IFT level needed to mobilize capillary trapped oil is proportional to the heterogeneity scale. At normal field flow rates, it was found that oil recovery was sensitive to variation in IFT at scales less than approximately 10 m. IFT reduction down to 1 mN/m was found sufficient for heterogeneities of length scale 10 cm. It is shown that the effect of capillary trapping in heterogeneous formations can be represented as Nc-dependent relative permeability function at the larger scale and that the larger-scale functions can be obtained from rate dependent steady-state upscaling. The macro-scale effects described here comes in addition to the measured core-scale effects, and may increase the potential for surfactant flooding.
Mechanisms that might be important in surfactant flooding of an oil field are:
1. Micro-scale mechanisms
a. Reduced residual oil saturation, Sor.
b. Altered relative permeability.
2. Macro-scale mechanisms
a. Capillary trapping due to the presence of heterogeneities.
b. Segregated flow due to gravity.
3. Wettability alteration (affects flow on both scales).
This list is not complete, e.g., large scale volumetric sweep efficiency will normally be reduced when the micro-scale mobility is increased by the surfactant. Typically, polymer would be used to reduce the mobility of the surfactant slug, and thus the effect of polymer should also be evaluated before implementation of the method in a field.
The micro-scale mechanisms (1) as well as the wettability alteration (3) would be natural targets for experimental investigations. The reduction in Sor is known to be a main mechanism in surfactant flooding of water wet formations [1, 2]. The potential for surfactant flooding at water-wet conditions is typically represented by a measured capillary desaturation curve (CDC), which is a plot of Sor versus the capillary number, Nc. In mixed-wet formations, remaining oil saturation (ROS) after water flooding is typically much lower  than in water-wet medium. The final Sor can be very hard to reach experimentally due to capillary end effects and the low permeable tail of the oil relative permeability curve [4, 5]. Practical ROS in a water flooded field will normally be much higher than Sor and it might be more important that an IOR method like surfactant flooding can accelerate the oil production and reach an already low Sor than an actual reduction in Sor itself. Therefore the total relative permeability and not only the end points should be measured at high Nc  in mixed-wet formations.
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