Wettability and Relative Permeability of Prudhoe Bay: A Case Study in Mixed-Wet Reservoirs
- G.R. Jerauld (Arco Exploration and Production Technology) | J.J. Rathmell (Arco Exploration and Production Technology)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- February 1997
- Document Type
- Journal Paper
- 58 - 65
- 1997. Society of Petroleum Engineers
- 5.7.2 Recovery Factors, 5.1.1 Exploration, Development, Structural Geology, 1.2.3 Rock properties, 4.3.3 Aspaltenes, 6.5.2 Water use, produced water discharge and disposal, 1.11 Drilling Fluids and Materials, 5.2.1 Phase Behavior and PVT Measurements, 5.6.1 Open hole/cased hole log analysis, 1.6.9 Coring, Fishing, 5.4.1 Waterflooding, 1.8 Formation Damage, 5.6.5 Tracers, 5.5.2 Core Analysis, 5.6.2 Core Analysis, 5.3.4 Reduction of Residual Oil Saturation, 5.4.2 Gas Injection Methods, 5.2 Reservoir Fluid Dynamics, 4.3.4 Scale
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This paper describes the variation in wettability and relative permeability at Prudhoe Bay. We show that measurements are consistent with recent theories of the relationship between water-saturation, pH, wettability and relative permeability. In particular, the wettability of the reservoir changes from water-wet low on structure near the water-oil contact to mixed-wet behavior higher on structure. Increasing oil-wetting character is correlated to decreasing water saturation. Changes in wettability are also accompanied by changes in waterflood recovery efficiency and are optimum at Amott indices approaching zero. Lithology also impacts recovery efficiency with more clay-rich rocks having higher residual oil saturations. Wettability data include Amott indices, Cryo-SEM measurements made on frozen pressure cores, contact angle measurements and micromodel studies.
Prudhoe Bay is a large and prolific mixed-wet reservoir. Because of the economic importance of Prudhoe Bay and because of the variety of oil recovery mechanisms operative or possible within the reservoir, the waterflood recovery behavior has been extensively studied. Waterflooding has been studied to understand the amount of the reservoir which should be waterflooded or gravity drained through gas-cap expansion. Expansion and optimization of multi-contact miscible gas injection has also relied on a accurate understanding of waterflood behavior. It has long been known that wettability is a primary determinant of waterflood recovery efficiency, but reports of variation of wettability within a reservoir have been uncommon and largely speculative. This paper provides a case study in which a variety of data have been measured to characterize wettability, its influence on waterflood behavior and the data demonstrate a variation in wettability within a reservoir.
Wettability is defined as the tendency of one fluid of a fluid pair to spontaneously coat the surface of a solid. Thus, water-wet rocks have surfaces in which the molecular forces between water and the surface are greater than the molecular forces between oil and that same surface. Most sandstones are thought to first contain water and later oil migrates into the rock. Thus, oil must displace water away from the rock surface to wet it. Because the force between the oil-water interface and the solid surface depends upon distance from the surface in a complicated way, in some instances surfaces become oil-wet only after the capillary pressure becomes very large or the water saturation becomes small. Once oil contacts a surface it may alter it by adsorbing constituents of the oil onto the surface changing its properties. Because of the complicated composition of crude-oils, rock surfaces and the colloid interactions precise understanding of the most important variables controlling wettability has been difficult to achieve. Instead, a syndrome of reservoir wettability has developed.
Over the last decade an understanding of the general Characteristics of native state wettability has emerged. The wettability in many sandstone reservoirs is characterized as mixed-wet as in Prudhoe Bay. The distinguishing feature of mixed-wet systems is the existence of a large region of two-phase flow with a finite but very small oil flowrate continuing down to low oil saturations. This behavior is associated with the existence of thin films of oil coating a connected portion of the pore-walls of the rock. Mixed-wet systems spontaneously imbibe some water starting from connate water saturation and some oil from residual oil saturation. Native-state wettability as we have come to know it has a broader set of characteristics which we whimsically call the WIMP-wet syndrome, features of which are displayed to varying degrees in Prudhoe Bay data. Native-state samples have weak capillary forces with capillary pressure in imbibition and secondary drainage being much less than that in primary drainage. This is an indication of intermediate contact angles (30 < <150 ) or mixed patches of oil-wet ( >150 ) and water-wet ( <30 ) regions on a scale less than a pore. Rocks are composed of different minerals which are preferentially wet by oil or water, for example Kaolinite clay is often oil-wet in Prudhoe rock. The wetting preference also depends on the oil-type, with asphaltene rich oil displaying more oil-wet behavior, and the brine pH and salinity, with acidic solutions showing more of a tendency for oil wetting.
We begin our discussion of Prudhoe Bay wettability with basic contact angle and wettability data and move on to more macroscopic and native state data. Micromodel and Cryo-SEM work gives pore-level insight into wettability.
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