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Wettability significantly affects a variety of measurements critical to describing oil reservoirs, from residual saturations to resistivity indices. In spite of the importance attributed to wettability, no standardized technique exists that completely and adequately characterizes the phenomenon. The most complete review of the subject is given by Andersona<ref name=pt05r13>Anderson, W. G., 1986a, Wettability literature survey—Part 1, Rock/oil/brine interactions and the effects of [[core handling]] on wettability: Journal of Petroleum Technology, v. 38, p. 1125–1144., 10., 2118/13932-PA</ref>–<ref name=pt05r18>Anderson, W. G., 1986f, Wettability literature survey—Part 6, Effects of wettability on [[waterflooding]]: Journal of Petroleum Technology, v. 38, p. 1605–1622.</ref>.

Wettability significantly affects a variety of measurements critical to describing oil reservoirs, from residual saturations to resistivity indices. In spite of the importance attributed to wettability, no standardized technique exists that completely and adequately characterizes the phenomenon. The most complete review of the subject is given by Anderson.<ref name=pt05r13>Anderson, W. G., 1986a, Wettability literature survey—Part 1, Rock/oil/brine interactions and the effects of [[core handling]] on wettability: Journal of Petroleum Technology, v. 38, p. 1125–1144., 10., 2118/13932-PA</ref> <ref name=pt05r18>Anderson, W. G., 1986f, Wettability literature survey—Part 6, Effects of wettability on [[waterflooding]]: Journal of Petroleum Technology, v. 38, p. 1605–1622.</ref>

Physically, wettability represents a balance of forces that occur at the interface between three phases, one of which is a solid. The equation describing this balance was first developed by Young in 1805<ref name=pt05r2>Adamson, A. W., 1982, Physical Chemistry of Surfaces, 4th ed.: New York, John Wiley and Sons.</ref>. For an oil, water, and solid system, the equation would be

Physically, wettability represents a balance of forces that occur at the interface between three phases, one of which is a solid. The equation describing this balance was first developed by Young in 1805.<ref name=pt05r2>Adamson, A. W., 1982, Physical Chemistry of Surfaces, 4th ed.: New York, John Wiley and Sons.</ref> For an oil, water, and solid system, the equation would be

:<math>\sigma_{\rm os} - \sigma_{\rm ws} + \sigma_{\rm ow} \cos \theta_{\rm c} = 0</math>

:<math>\sigma_{\rm os} - \sigma_{\rm ws} + \sigma_{\rm ow} \cos \theta_{\rm c} = 0</math>

==Techniques==

==Techniques==

[[file:wettability_fig1.png|thumb|{{figure number|1}}Wettability of oil, water, and rock system. (After <ref name=pt05r134>Raza, S. H., Treiber, L. E., Archer, D. L., 1968, Wettability of reservoir rocks and its evolution: Producers Monthly, v. 32, p. 2–7.</ref>.)]]

[[file:wettability_fig1.png|thumb|{{figure number|1}}Wettability of oil, water, and rock system. (After Raza et al.<ref name=pt05r134>Raza, S. H., Treiber, L. E., Archer, D. L., 1968, Wettability of reservoir rocks and its evolution: Producers Monthly, v. 32, p. 2–7.</ref>)]]

===Contact angle measurements===

===Contact angle measurements===

If a liquid wets a surface, it tends to spread and cover that surface. Observed on a microscopic scale, the edge of the liquid has a characteristic shape. A knife edge shape indicates wetting, while a beaded edge shape indicates nonwetting. This is shown quantitatively in [[:file:wettability_fig1.png|Figure 1]], which shows a drop of water surrounded by oil and contacting a solid surface. If the edge of the drop forms an acute angle (θ_e 5.)

If a liquid wets a surface, it tends to spread and cover that surface. Observed on a microscopic scale, the edge of the liquid has a characteristic shape. A knife edge shape indicates wetting, while a beaded edge shape indicates nonwetting. This is shown quantitatively in [[:file:wettability_fig1.png|Figure 1]], which shows a drop of water surrounded by oil and contacting a solid surface. If the edge of the drop forms an acute angle (θ_e 5.)

Several techniques used to measure this angle are described by Adamson<ref name=pt05r2 />. The measurements are generally made on a polished surface that simulates the reservoir material. For sandstones, glass slides or polished quartz are often used. Polished marble is usually chosen to simulate a carbonate reservoir. An adaptation of the technique uses a drop of liquid confined between two surfaces<ref name=pt05r39>Craig, F. F., 1971, The reservoir engineering aspects of waterflooding: Dallas, TX, Society of Petroleum Engineers Monograph No. 3, 120 p.</ref>.

Several techniques used to measure this angle are described by Adamson.<ref name=pt05r2 /> The measurements are generally made on a polished surface that simulates the reservoir material. For sandstones, glass slides or polished quartz are often used. Polished marble is usually chosen to simulate a carbonate reservoir. An adaptation of the technique uses a drop of liquid confined between two surfaces.<ref name=pt05r39>Craig, F. F., 1971, The reservoir engineering aspects of waterflooding: Dallas, TX, Society of Petroleum Engineers Monograph No. 3, 120 p.</ref>

Contact angle measurements can be precise, but even for ideal systems, measurements can show significant variation. Variations are related to surface preparation, equilibration of the solid and liquids, and surface roughness.

Contact angle measurements can be precise, but even for ideal systems, measurements can show significant variation. Variations are related to surface preparation, equilibration of the solid and liquids, and surface roughness.

Utilization of a surface balance provides information on the product of the interfacial tension and the contact angle<ref name=pt05r152>Teeters, D., Anderson, M. A., Thomas, D. C., 1989, Formation wettability studies that incorporate the Wilhelmy plate technique, in Borchardt, J. K., Yen, T. F., eds., Enhanced and Production [[Stimulation]]: Washington, D., C., American Chemical Society Symposium Series No. 396.</ref>. This technique is quantitative and flexible but subject to some of the same limitations as those of contact angle measurements.

Utilization of a surface balance provides information on the product of the interfacial tension and the contact angle.<ref name=pt05r152>Teeters, D., Anderson, M. A., Thomas, D. C., 1989, Formation wettability studies that incorporate the Wilhelmy plate technique, in Borchardt, J. K., Yen, T. F., eds., Enhanced and Production [[Stimulation]]: Washington, D., C., American Chemical Society Symposium Series No. 396.</ref> This technique is quantitative and flexible but subject to some of the same limitations as those of contact angle measurements.

===Displacement techniques===

===Displacement techniques===

Measurements that use larger samples are usually employed for reservoir materials that show varying composition, mineralogy, and structure. Two procedures most commonly used are the Amott<ref name=pt05r11>Amott, E., 1959, Observations relating to the wettability of porous rock: Petroleum Transactions, AIME, v. 216, p. 156–162.</ref> and the U.S. Bureau of Mines (USBM) method<ref name=pt05r47>Donaldson, E. C., Thomas, R. D., Lorenz, P. B., 1969, Wettability determination and its effect on recovery efficiency: Society of Petroleum Engineers Journal, v. 9, p. 13–20., 10., 2118/2338-PA</ref>.

Measurements that use larger samples are usually employed for reservoir materials that show varying composition, mineralogy, and structure. Two procedures most commonly used are the Amott<ref name=pt05r11>Amott, E., 1959, Observations relating to the wettability of porous rock: Petroleum Transactions, AIME, v. 216, p. 156–162.</ref> and the U.S. Bureau of Mines (USBM) method.<ref name=pt05r47>Donaldson, E. C., Thomas, R. D., Lorenz, P. B., 1969, Wettability determination and its effect on recovery efficiency: Society of Petroleum Engineers Journal, v. 9, p. 13–20., 10., 2118/2338-PA</ref>

===Amott method===

===Amott method===

If a sample spontaneously imbibes only brine, it is considered water wet. Similarly, if it imbibes only oil, it is considered oil wet. If the sample imbibes neither, it is described as neutrally wet.

If a sample spontaneously imbibes only brine, it is considered water wet. Similarly, if it imbibes only oil, it is considered oil wet. If the sample imbibes neither, it is described as neutrally wet.

A modification of the test in general use first prepares the sample by centrifuging it in brine. This is followed by centrifuging in oil to irreducible water saturation. The Amott procedure is then followed, but a combined index—the Amott-Harvey wettability index<ref name=pt05r26>Boneau, D. F., Clempett, R. L., 1977, A surfactant system for the oil wet sandstone of the North Burbank unit: Journal of Petroleum Technology, v. 29, p. 501–506., 10., 2118/5820-PA</ref>—is calculated by subtracting the displacement by oil ratio from the displacement by water ratio. Figure 2 shows these volumes, and Equation 2 gives the calculation:

A modification of the test in general use first prepares the sample by centrifuging it in brine. This is followed by centrifuging in oil to irreducible water saturation. The Amott procedure is then followed, but a combined index—the Amott-Harvey wettability index<ref name=pt05r26>Boneau, D. F., Clempett, R. L., 1977, A surfactant system for the oil wet sandstone of the North Burbank unit: Journal of Petroleum Technology, v. 29, p. 501–506., 10., 2118/5820-PA</ref> is calculated by subtracting the displacement by oil ratio from the displacement by water ratio. Figure 2 shows these volumes, and Equation 2 gives the calculation:

:<math>\mbox{Index} = \overline{BC}/\overline{BD} - \overline{DE}/\overline{DB}</math>

:<math>\mbox{Index} = \overline{BC}/\overline{BD} - \overline{DE}/\overline{DB}</math>

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|+ {{table number|1}}Approximate relationship among wettability, contact angle, and the USBM and amott wettability indexes

|+ {{table number|1}}Approximate relationship among wettability, contact angle, and the USBM and Amott wettability indexes

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[[file:wettability_fig2.png|thumb|{{figure number|2}}Combined Amott and USBM method. (After <ref name=pt05r14>Anderson, W. G., 1986b, Wettability literature survey—Part 2, Wettability measurements: Journal of Petroleum Technology, v. 38, p. 1246–1262., 10., 2118/13933-PA</ref>; from <ref name=pt05r143>Shamra, M. M., Wundlerlich, R. W., 1985, The alteration of rock properties due to interactions with [[drilling fluid]] components: Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, Las Vegas, NV, Sept. 22–25, SPE 14302.</ref>.)]]

[[file:wettability_fig2.png|thumb|{{figure number|2}}Combined Amott and USBM method. (After Anderson;<ref name=pt05r14>Anderson, W. G., 1986b, Wettability literature survey—Part 2, Wettability measurements: Journal of Petroleum Technology, v. 38, p. 1246–1262., 10., 2118/13933-PA</ref> from Shamra and Wunderlich.<ref name=pt05r143>Shamra, M. M., Wundlerlich, R. W., 1985, The alteration of rock properties due to interactions with [[drilling fluid]] components: Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, Las Vegas, NV, Sept. 22–25, SPE 14302.</ref>)]]

===USBM method===

===USBM method===

:<math>W = \log \frac{A_{1}}{A_{2}}</math>

:<math>W = \log \frac{A_{1}}{A_{2}}</math>

The Amott and the USBM methods can be combined into a single test<ref name=pt05r143 />. This combination and the capillary pressure plot used in the USBM calculation, are shown in [[:file:wettability_fig2.png|Figure 2]].

The Amott and the USBM methods can be combined into a single test.<ref name=pt05r143 /> This combination and the capillary pressure plot used in the USBM calculation, are shown in [[:file:wettability_fig2.png|Figure 2]].

===Comparison of the two methods===

===Comparison of the two methods===

Both the Amott and the USBM test are commonly used in the oil industry, but direct comparisons of the two techniques show only minimal correlation<ref name=pt05r40>Crocker, M. E., Marchin, L. M., 1986, Evaluation and determination of cross correlations between wettability methods—Status Report Project BE7: Bartlesville, OK, National Institute for Petroleum Energy and Research.</ref>. The most significant deviations occur near the neutral wettability region. The Amott method is more sensitive in this area and may be a better indicator. The Amott method can also be used to indicate mixed wettability if a sample spontaneously imbibes both oil and water.

Both the Amott and the USBM test are commonly used in the oil industry, but direct comparisons of the two techniques show only minimal correlation.<ref name=pt05r40>Crocker, M. E., Marchin, L. M., 1986, Evaluation and determination of cross correlations between wettability methods—Status Report Project BE7: Bartlesville, OK, National Institute for Petroleum Energy and Research.</ref> The most significant deviations occur near the neutral wettability region. The Amott method is more sensitive in this area and may be a better indicator. The Amott method can also be used to indicate mixed wettability if a sample spontaneously imbibes both oil and water.

===Additional techniques===

===Additional techniques===

Several techniques are described in the literature for determining wettability<ref name=pt05r14 />. Some are modifications of the Amott or the USBM methods, while others represent significant departures from the standard techniques. These vary from microscopic examination of imbibed fluids to measurement of nuclear magnetic resonance (NMR) longitudinal relaxation. Table 2 lists some of these techniques and their observed variables.

Several techniques are described in the literature for determining wettability.<ref name=pt05r14 /> Some are modifications of the Amott or the USBM methods, while others represent significant departures from the standard techniques. These vary from microscopic examination of imbibed fluids to measurement of nuclear magnetic resonance (NMR) longitudinal relaxation. Table 2 lists some of these techniques and their observed variables.

{| class = "wikitable"

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| Reservoir logs

| Reservoir logs

| Resistivity logs before and after injection of a reverse wetting agent

| [[Basic open hole tools#Resistivity|Resistivity logs]] before and after injection of a reverse wetting agent

|-

|-

| Nuclear magnetic resonance

| Nuclear magnetic resonance

* [[Permeability]]

* [[Permeability]]

* [[SEM, XRD, CL, and XF Methods]]

* [[SEM, XRD, CL, and XF Methods]]

* [[Thin section analysis]]

* [[Thin section analysis]]

* [[Rock-water reaction: Formation damage]]

* [[Rock-water reaction: formation damage]]

* [[Overview of routine core analysis]]

* [[Overview of routine core analysis]]

* [[Core-log transformations and porosity-permeability relationships]]

* [[Core-log transformations and porosity-permeability relationships]]