Dependence of Waterflood Remaining Oil Saturation on Relative Permeability, Capillary Pressure, and Reservoir Parameters in Mixed-Wet Turbidite Sands
- G.J. Hirasaki (Rice University)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1996
- Document Type
- Journal Paper
- 87 - 92
- 1996. Society of Petroleum Engineers
- 5.5.8 History Matching, 5.4.1 Waterflooding, 4.1.5 Processing Equipment, 5.3.2 Multiphase Flow, 4.3.4 Scale, 2.4.3 Sand/Solids Control, 5.7.2 Recovery Factors, 1.2.3 Rock properties, 5.1.1 Exploration, Development, Structural Geology, 5.3.4 Reduction of Residual Oil Saturation, 5.2.1 Phase Behavior and PVT Measurements
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The dependence of waterflood oil recovery on relative permeability, capillary pressure, and reservoir parameters was investigated by numerical simulation. The relative permeability and capillary pressure curves were based on laboratory measurements on unconsolidated sands and were evaluated for water-wet and mixed wet states. The reservoir model was a prototype turbidite sand with a range of thickness and permeability values. The economic oil recovery was based on an economic limit water cut of 50%.
The remaining oil saturation in the swept region for the water-wet cases was close to the residual oil saturation. The remaining oil saturation of the mixed wet cases ranged from low values near the residual oil saturation to far above the residual oil saturation. It is dependent on the reservoir parameters that govern: (1) the vertical "film surface drainage" of oil by gravity, (2) accumulation of a high oil saturation and thus a high relative permeability under the cap rock, (3) updip migration of the oil that accumulated under the cap rock. The dependence on the reservoir parameters can be summarized by dimensionless groups. There is a dimensionless time for the vertical displacement of oil by gravity. The accumulation of a high oil saturation under the cap rock is dependent on the ratio of the capillary transition zone and the sand thickness. The updip migration is dependent on a combination of the gravity number and the end point mobility ratio.
The oil remaining after waterflood operations is sometimes divided into mobile, unswept oil and immobile, "residual oil saturation" in the swept region. Here we will focus only on the swept region. We will show that the much of the remaining oil saturation, ROS, in the swept region may be mobile. The residual oil saturation, Sor, is defined here as the saturation at which the oil relative permeability goes to zero. We show that the ROS in the swept region can be very different from Sor in mixed wet systems.
It is recognized that the Sor of mixed wet sandstone systems is sometimes very low, e.g. 10%. Also, the oil relative permeability at these low saturations are very low. It can be measured only if the duration of the experiment is long enough and the driving force for displacement (gravity or pressure gradient) is large compared to the capillary pressure retaining the oil as a wetting phase end effect. The low oil relative permeability near Sor translates into a water fractional flow near unity. Welge's integration of the Buckley-Leverett equations shows that many pore volumes of throughput will be required to reduce the oil saturation close to Sor in mixed wet systems. Thus the ROS in the swept region of mixed wet systems can be very different from Sor.
Welge integration of the Buckley-Leverett equation results in a ROS at breakthrough for the mixed wet system that is very similar to that of the water-wet system. A well in deep water environment may have to be recompleted soon after water breakthrough so the ROS at breakthrough may be close to that which remains when the sand unit is abandoned. One may ask why it was necessary to make measurements on a mixed wet system if the ROS is going to be nearly the same as for the much more easily measured, water-wet system. The ROS at breakthrough for the mixed wet and water-wet, 2-D systems can be either very similar or very dissimilar, depending on the sand thickness and permeability.
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