Many methods have been used to measure wettability. This paper describes the three quantitative methods in use today: contact angle, Amott method, and the U.S. Bureau of Mines (USBM) method. The advantages and limitations of all the qualitative methods-imbibition, microscope examination, flotation, glass slide, relative permeability curves, capillary pressure curves, capillarimetric method, displacement capillary pressure, permeability/saturation relationships, and reservoir logs-are pressure, permeability/saturation relationships, and reservoir logs-are covered. Nuclear magnetic resonance (NMR) and dye adsorption, two methods for measuring fractional wettability, are also discussed. Finally, a method is proposed to determine whether a core has mixed wettability.
This paper is the second in a series of literature surveys covering the effects of wettability on core analysis. Changes in the wettability of cores have been shown to affect electrical properties, capillary pressure, waterflood behavior, relative permeability, dispersion, and simulated EOR. For core analysis to predict the behavior of the reservoir, the wettability of the core must be the same as the wettability of the undisturbed reservoir rock.
When a drop of water is placed on a surface immersed in oil, a contact angle is formed that ranges from 0 to 180 deg. [0 to 3.14 rad]. A typical oil/water/solid system is shown in Fig. 1, where the surface energies in the system are related by Young's equation,
sigma = interfacial energy [interfacial tension (IFT)] between the oil and water, sigma = interfacial energy between the oil and solid, sigma = interfacial energy between the water and solid, and theta = contact angle, the angle of the water/oil/solid contact line.
By convention, the contact angle, theta, is measured through the water. The interfacial energy sigma is equal to or, the IFT.
As shown in Fig. 1, when the contact angle is less than 90 deg. [1.6 rad], the surface is preferentially water-wet, and when it is greater than 90 deg. [1.6 rad], the surface is preferentially oil-wet. For almost all pure fluids and clean preferentially oil-wet. For almost all pure fluids and clean rock or polished crystal surfaces, sigma, and sigma, have values such that theta=0 deg. [0 rad]. When compounds such as crude-oil components are adsorbed on rock surfaces, these interfacial energies are changed unequally. This changes theta and hence the wettability. The farther theta is from 90 deg. [1.6 rad], the greater the wetting preference for one fluid over another. If theta is exactly 90 deg. [1.6 rad], neither fluid preferentially wets the solid. As shown in Table 1, when preferentially wets the solid. As shown in Table 1, when theta is between 0 and 60 to 75 deg. [0 and 1 to 1.3 rad], the system is defined as water-wet. When theta is between 180 and 105 to 120 deg. [3.1 and 1.8 to 2.1 rad], the system is defined as oil-wet. In the middle range of contact angles, a system is neutrally or intermediately wet. The contact angle that is chosen as the cutoff varies from paper to paper.
The term a sigma - sigma is is sometimes called the adhesion tension, theta :
The adhesion tension is positive when the system is water-wet, negative when the system is oil-wet, and near zero when the system is neutrally wet.
Many different methods have been proposed for measuring the wettability of a system. They include quantitative methods-contact angles, imbibition and forced displacement (Amott), and USBM wettability method-and qualitative methods-imbibition rates, microscope examination, flotation, glass slide method, relative permeability curves, permeability/saturation relationships, permeability curves, permeability/saturation relationships, capillary pressure curves, capillarimetric method, displacement capillary pressure, reservoir logs, nuclear magnetic resonance, and dye adsorption.
Although no single accepted method exists, three quantitative methods generally are used:
the Amott method (imbibition and forced displacement), and
the USBM method. The contact angle measures the wettability of a specific surface, while the Amott and USBM methods measure the average wettability of a core.
A comparison of the wettability criteria for the three methods is shown in Table 1. The remaining tests in the list are qualitative, each with somewhat different criteria to determine the degree of water or oil wetness.