Wettability

By Erle C. Donaldson and Waqi Alam

The wettability of oil reservoirs is the most important factor controlling the rate of oil recovery, providing a profound effect on petroleum production. The petroleum industry has increased the research effort on wettability, but, so far, there has been limited coverage on the topic. Wettability reviews the major research and applications on wettability, capillary pressure and improved recovery. Critical topics including core preservation, the effect of wettability on relative permeability, surface forces such as van der Waals equation of state, petroleum traps and pore size effects are all included in this musthave handbook. Deciphering the techniques and examples will increase the efficiency and production of oil recovery, translating to stronger reservoir simulations and improved well production.

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 25, 2013
• ISBN: 9780127999906

Erle C. Donaldson

Erle C. Donaldson is an independent consultant and managing editor of the Journal of Petroleum Science and Engineering. He was elected to the International Hall of Fame for Science in 1993. He has received a distinguished service award from the Republic of Honduras and other honors from the U.S. Department of Energy, the National Petroleum Engineering Honor Society, and the University of Oklahoma. Dr. Donaldson has written, co-authored, and served as editor on numerous articles and books.

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engineering.

CHAPTER 1

Wettability

1.1 The Four States of Wettability

1.1.1 Water-wet System

1.1.2 Fractionally-wet

1.1.3 Mixed-wet

1.1.4 Oil-wet

1.2 Interfacial Tension

1.3 Contact Angle

1.4 Advancing and Receding Contact Angles

1.5 Core Preservation

1.6 Capillary Pressure

1.7 Amott Wettability Index

1.8 Combined Amott-USBM Wettability Test

1.9 Spontaneous Imbibition

1.10 Oil Recovery by Imbibition

1.11 Spreading

1.12 Effect of Wettability on Relative Permeability

1.12.1 Effect of Wettability on Waterfloods

1.12.2 Procedures

1.12.3 Production Curvess

1.12.4 Effects of Viscosity

1.13 Effects of Wettability on Electrical Properties

1.14 Dispersion and Wettability

1.14.1 Introduction

1.14.2 Applications

1.14.3 Theory

1.15 Influence of Wettability

1.16 Example Problems

1.1 The Four States of Wettability

Wettability is the relative adhesion of two fluids to a solid surface. With respect to two immiscible fluids in a porous media, wettability is the measure of the preferential tendency of one of the fluids to wet (spread or adhere to) the interstitial surfaces of the porous medium in the presence of the other fluid. The surfaces of the pores in rocks contain a wide variety of exposed minerals that have preferential affinities for water, hydrocarbons, or constituents suspended and dissolved in the fluids. Thus for a water/oil/rock system, the term wettability refers to the average, overall, relative wetting preference of the interstitial surfaces of the rock.

Four general states of wettability have been recognized: (1) water-wet, (2) fractional-wettability, (3) mixed-wettability, and (4) oil-wet.

1.1.1 Water-wet System

A water/oil/rock system is considered to be water-wet when more than 50% of its surface is wet by water. Water occupies the smaller pores and dead-end cul-de-sac pores, and exists as a film covering the surfaces of the preferentially water-wet larger pores of the rock. Oil is lodged in the larger pores as droplets resting on a film of water; oil globules may extend through two or more of the larger pores and coat random areas of the pore surfaces containing minerals that are preferentially oil-wet. Therefore, water exists as a continuous phase throughout the porous system and the nonwetting phase (oil) is a discontinuous phase consisting of globules in the larger spores surrounded by water. If the water saturation is reduced to its irreducible saturation (Swi), water remains as a continuous phase in the small pores and crevices through the porous medium. At Swi the oil saturation is high enough for it to also exist as a continuous phase through the larger pores of the rock. As the water saturation increases, the nonwetting phase quickly becomes discontinuous, with globules and fingers of oil in larger pores completely surrounded by water. The wetting phase saturation exists as a continuous phase at all saturations equal to or greater than Swi. If a preferentially water-wet core is saturated with oil then contacted with water, water will spontaneously imbibe into the rock displacing the oil until a state of static equilibrium is attained between the capillary and surface energy forces of the fluids and rock surfaces. If another sample of the same rock is saturated first with water and then contacted with oil, oil will not imbibe into the rock.

1.1.2 Fractionally-wet

The term fractional wettability was proposed by Brown and Fatt (1956) to characterize heterogeneous wetting of the pore surfaces where the preferential wetting is randomly distributed throughout the rock. In some cases, the random distribution of minerals (with a variety of chemical properties) exposed to the surfaces in the pores is such that areas which are either preferentially water-wet or oil-wet are scattered through the rock and there are no continuous oil networks through the rock.

1.1.3 Mixed-wet

The term mixed wettability, as defined by Salathiel (1973), is a condition where the small pores in the rock are water-wet and saturated with water, but the larger pores are oil-wet and filled with oil in contact with the pore walls that form a continuous path through the length of the rock. Salathiel reasoned that this condition could occur during the original accumulation of oil in a reservoir if oil-containing surface active compounds displaced connate water from the larger pores; the surface active compounds would gradually displace the remaining films of water on the pore surfaces. Oil would not enter the smaller pores where the threshold capillary pressure for displacement of water is too large.

The terms fractional-wet and mixed-wet are included in the frequently used general term neutral-wettability. This term only implies that half of the rock surface is water-wet and the other half is oil-wet; it does not distinguish the type of wettability condition.

1.1.4 Oil-wet

When the system is preferentially oil-wet, the positions of water and oil in the rock are reversed. Oil occupies the smaller pores to the exclusion of water, and oil is in contact with the rock surfaces of the larger pores. Where water is present in the larger pores, it is generally in the center of the pores resting on a film of oil. Water (the nonwetting phase in an oil-wet system) also exists as a continuous phase distributed through the larger pores when it is present as a high saturation (near the residual oil saturation, Sor). If the water saturation is decreased (by injection of oil), it rapidly loses continuity and becomes isolated in the larger pores as pockets and fingers of water that are surrounded by oil. Thus in an oil-wet system, oil is a continuous phase for all saturations equal to and greater than Sor.

If the preferentially oil-wet rock is saturated with water and contacted with oil, the oil will imbibe into the rock displacing water until a state of equilibrium is attained. Water will not spontaneously imbibe into an oil-wet rock.

Most of the world’s oil reserves are found in either sandstone or carbonate rocks. The wettability of sandstones generally ranges from neutral to strongly water-wet; however, carbonates exhibit oil-wet tendencies ranging from neutral to strongly oil-wet. Some sandstone oil reservoirs are oil-wet because the oils contain surface-active polar compounds that are expelled from the bulk oil to the interfacial surface where they are physically and chemically attached to the surfaces of the sandstone: the well-known oil-wet sandstone reservoirs in the U.S. are the Bradford, Pa., sand (Marsden and Khan, 1962); Wilcox, Okla., sand (Katz, 1942); and Tensleep, Wyo., sand (Nutting, 1934).

Wettability is the most important parameter affecting: (1) the microscopic distribution of water and oil in rock pores, (2) the characteristics of capillary pressure curves, (3) the fluid flow properties of oil displacement by water, and (4) the amount and distribution of residual oil saturation. Donaldson and Crocker (1977) present color photographs of residual oil saturation that are visual verification of the four states of wettability.

1.2 Interfacial Tension

When two immiscible fluids (gas-liquid or liquid-liquid) are in contact, the two fluids are separated by a definite interface that is only a few molecular diameters thick. Within the liquid (away from the interface) the molecules exert an attractive force on each other in all directions, Fig. 1–1a. At the interface, however, there is an inward directed force attempting to minimize the surface by pulling it into the shape of a sphere, Fig. 1–1b. The molecular activity at the surface creates a film-like layer (or membrane) of molecules that are in tension; the tension is the surface free energy per unit area that develop at the interfaces: gas-liquid, gas-solid, liquid-liquid, and liquid-solid.

Figure 1-1 Interaction of attractive molecular forces in the bulk liquid (a) and at the surface (b).

Liquids that wet the walls of a capillary tube inserted into the bulk liquid (by adsorption of a thin film of molecules on the capillary wall) will rise in the capillary tube in order to reduce the total surface area, Fig. 1–2a. At equilibrium between the attractive forces and gravity, the free energy of the surface is reduced to a minimum value. The decrease of an element of surface area is 2πr·dh which corresponds to a decrease in surface energy equal to σ·dA, thus

Figure 1-2 (a) Capillary rise of a liquid wets the walls of a tube. (b) Depression of a liquid that does not wet the walls: d = diameter, h = height, σ = interfacial tension, and θ = contact angle.

(1.1)

The free energy required to raise the elemental volume of liquid (with density ρ) to the height h against the downward force of gravity is

(1.2)

At equilibrium the two energies are equal (Ex. 1-1), therefore

(1.3)

If the liquid does not wet (adsorb to) the wall of the capillary, the liquid will be depressed in the capillary because the mutual attraction of the liquid’s molecules is greater than the glass-liquid molecular attraction, Fig. 1–2b. The pressure of the nonwetting fluid in the capillary, P2, is greater than the pressure of the fluid just above the interface, P1; and the nonwetting liquid is depressed a distance, h, below the level of the liquid outside the capillary.

The du Noüy (1919) method for measuring interfacial tension (IFT) uses an analytic balance beam to measure the force required to pull a platinum ring through the interface. The surface tension exerts a force on the inner and outer circumferences of the ring. The difference between the inner and outer diameters of the ring is negligible; therefore, the length of the ring in contact with the fluid is 2*·2πr (where r is the average radius of the ring) and the IFT is σ =g f/4πr (Ex. 1-2).

The Wilhelmy plate method for measuring interfacial tension uses a plane rectangular plate that is pulled upward or pushed downward through an interface. The force acting on the perimeter of the plate and the advancing or receding contact angle of the fluid against the plate are used to calculate the interfacial tension (Ex. 1-3). As the plate is moved into or out of the liquid (Fig. 1-3), the change of the force, F, due to the adhesion of the fluid to the surface of the plate is equal to the length of the interface in contact with the plate (2× width + thickness) times the interfacial tension and cosine of the contact angle (θ).

Figure 1-3 (a) Wilhelmy plate method for measurement of advancing contact angle. (b) Receding constant angle. (after Tiab and Donaldson, 2004)

(1.4)

Other methods for measuring IFT that take the effect of gravity into account are discussed by Adamson and Gast (1997): drop weight, sessile drop or bubble, maximum bubble pressure, and pendant drop (for a detailed description of the pendant drop apparatus refer to Donaldson and Pavelka [1967]).

The air/oil interfacial tensions and other properties at 25°C of several crude oils are listed in Table 1-1. The values are one third, or less, than the IFT of air/water at 25°C (72.8 mN/m). Chu (1982) found that the air-water IFT decreases almost linearly with temperature from 72.5 mN/m at 21.1°C to 60.1 mN/m at 93.3°. Organic solutes, surface active compounds, and caustics decrease the air-water IFT while sodium chloride and calcium chloride produce a small increase of IFT at any specific temperature. The air-liquid interfacial tension of various liquids at 20°C is listed in Table 1-2.

Table 1-1

Air-oil interfacial tensions, viscosity and density of crude oils measured at 25°C (McGhee et al., 1979).

Table 1-2

Air-liquid interfacial tensions and densities of various liquids at 20°C (Weast, 1970: pages F30–F33).

1.3 Contact