A Pore-Level Investigation of Relative Permeability Hysteresis in Water-Wet Systems
- A.B. Dixit (Edinburgh Petroleum Services Ltd.) | S.R. McDougall (Heriot-Watt U.) | K.S. Sorbie (Heriot-Watt U.)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- June 1998
- Document Type
- Journal Paper
- 115 - 123
- 1998. Society of Petroleum Engineers
- 5.1 Reservoir Characterisation, 5.3.4 Reduction of Residual Oil Saturation, 5.2.1 Phase Behavior and PVT Measurements, 5.3.2 Multiphase Flow, 1.8.5 Phase Trapping, 5.3.1 Flow in Porous Media, 4.1.4 Gas Processing, 5.7.2 Recovery Factors, 5.5 Reservoir Simulation, 5.4.1 Waterflooding, 5.1.5 Geologic Modeling, 4.3.4 Scale
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Oil-recovery efficiency is known to depend crucially upon the surface chemistry of the particular crude-oil/brine/rock system under investigation. Mineralogy, brine chemistry/pH, crude-oil composition and ageing conditions all serve to modify the effective contact angles operating within a mixed-wet porous medium. Recent network modelling studies have helped to clarify the important role played by wettability during multiphase flow in non-uniformly wet systems and the consequences of modified contact angles during waterflooding have been described in terms of various "Regimes". These have subsequently been used to classify a wide range of apparently contradictory wettability experiments.
The purpose of the present paper is to apply these ideas to the study of hysteresis phenomena in both strongly and weakly water-wet drainage/imbibition relative permeability curves. Such hysteresis effects have long been recognised by the industry but the precise direction of hysteresis appears to vary from study to study. Indeed, in water-wet media, even the degree of sample consolidation appears to play an important role.
A 3-D network model is reported which takes into account a great deal of the underlying pore-scale physics. Initial water saturation, wettability alterations, film flow, phase trapping and realistic variations in advancing and receding contact angles are all incorporated into the simulations.
More specifically, this study examines relative permeability hysteresis in systems from "Regime IA" which corresponds to strongly and weakly water-wet porous media i.e. media where some degree of modification in the distribution of effective contact angles has occurred. The consequences of different combinations of pore scale displacement mechanisms (viz. snap-off and piston-like displacement) for relative permeability hysteresis has been calculated using the pore-scale simulator. The effects of combining these mixed mechanisms with other network properties such as co-ordination number (z) and certain volume and conductivity exponents (v and ) are also reported. The nature of hysteresis is found to change quite dramatically as these parameters are varied and all experimentally-observed trends in relative permeability hysteresis are reproduced under suitable conditions. This appears to be the first time that a pore-scale model has been used to provide a clear interpretation of all experimentally observed hysteresis trends during primary drainage, secondary imbibition and secondary drainage displacements in both strongly and weakly water-wet systems.
Relative permeability curves provide the reservoir engineer with a macroscopic description of the way in which two (or three) phases flow together locally in a porous medium. However, these functions tend to be history-dependent and often exhibit hysteresis when phase saturation reversals occur. The theoretical and experimental aspects of the hysteresis between primary drainage (PD) and secondary imbibition (SI) relative permeability curves have been discussed by many authors, although most of these studies have focused upon strongly water-wet porous media. It is now widely accepted, however, that an appreciable number of reservoirs are either mixed-wet or oil-wet in nature and it is the secondary drainage (SD) process (i.e. the displacement of water by oil from the residual oil saturation, Sor) which actually reveals a great amount of information about the wettability of the sample. Indeed, a number of wettability measurement techniques are based on the data obtained from the secondary drainage capillary pressure curves.
The main difference between the primary and secondary drainage processes is the presence of trapped residual oil during secondary drainage and this is found to play a pivotal role during the subsequent displacement. The distribution of residual oil after waterflooding greatly depends upon the pore geometry, the pore network topology and the wettability of the system.
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