Characterization of Mixed-Wettability States in Oil Reservoirs by Atomic Force Microscopy
- Subhayu Basu | Mukul M. Sharma
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 1997
- Document Type
- Journal Paper
- 427 - 435
- 1997. Society of Petroleum Engineers
- 1.6.9 Coring, Fishing, 1.2.3 Rock properties, 5.8.5 Oil Sand, Oil Shale, Bitumen, 4.3.1 Hydrates, 1.8 Formation Damage, 2.7.1 Completion Fluids, 5.1.1 Exploration, Development, Structural Geology, 4.3.4 Scale, 5.2.1 Phase Behavior and PVT Measurements, 4.3.3 Aspaltenes, 4.1.2 Separation and Treating, 4.1.5 Processing Equipment, 5.1 Reservoir Characterisation
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A new technique is described for directly measuring the capillary pressure required to rupture brine films on mineral surfaces. These measurements provide direct evidence for the creation of mixed-wettability states in oil reservoirs. In addition, the measurement of the critical disjoining pressure allows us to observe systematic variations in wettability as the capillary pressure is increased. The effect of brine salinity, pH, surface charge, and surface curvature have been quantified. Results for two reservoirs, one from Texas (Moutray) and another from Alaska (Prudhoe Bay), are presented.
Wettability, or the tendency of surfaces to be preferentially wet by one fluid phase, has a strong influence on the distribution and flow of immiscible fluids in oil reservoirs. The efficiency of oil recovery processes and the displacement and production of oil by fluids injected into the reservoir depend on the wetting properties of the rock surfaces. In strongly water-wet rocks, the oil resides in the larger pores and flows with relative ease. However, large quantities of oil are left trapped in the pore space because it no longer forms a continuous pathway for flow (a sample spanning cluster, in percolation terminology). In oil-wet rock, on the other hand, oil is present in the small pores and its relative permeability is small. However, it can form continuous pathways for oil flow even at small oil saturations, usually resulting in lower trapped oil saturations.
A considerable amount of work in the past1-4 has been directed towards understanding the mechanisms responsible for oil reservoirs being oil-wet, water-wet or mixed-wet. It is generally believed that all reservoirs, being initially water-saturated, were water-wet to begin with. Over geologic time, oil migrated into the pores of the host rock, draining the water from the pore spaces. If thin water films persist on the surfaces of reservoir rocks, they will prevent the surfaces from becoming oil-wet. Under some conditions these water films are broken, as evidenced by the fact that many reservoir rocks show mixed-wet behavior. Several theories have emerged to identify factors that satisfactorily explain this phenomenon1-6. Factors that may affect the reservoir wettability include oil composition, brine chemistry, rock surface characteristics, capillary pressure, and temperature. In this paper, we have measured the surface force curves of a crude-oil/water/mineral system and quantify the conditions under which mixed wettability conditions arise.
In recent years many researchers have emphasized the importance of surface forces in the stability of aqueous films. It is theorized that the wetting properties of a reservoir rock depend on the ability of the oil phase to destabilize and rupture thin aqueous films on solid surfaces5-17. In this case pore size and structure may also play a role in the wettability of oil reservoirs2,18-21.
Past Methods for Measuring Reservoir Rock Wettability
Many laboratory methods are available for measuring the wettability state of reservoir core samples. Among quantitative measures, the Amott method22, the U.S. Bureau of Mines (USBM) method23,24, and the combined Amott/USBM method25 provide good global information regarding the wetting behavior of rock samples at a core scale. Classical contact-angle measurements are, by far, the most useful tool for determining the wettability at a pore scale. However, standard contact-angle measurement techniques (e.g., the sessile drop method26) fail to account for changes in wettability as a function of capillary pressure. Additional issues such as surface roughness, contact-angle hysteresis, and the relationship between macroscopic contact angles and disjoining pressure have been reviewed in the past18-21,27,28,51.
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