Changes

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.

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 [http://petrowiki.org/Glossary%3AIrreducible_water_saturation 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} = \frac{\overline{BC}}{\overline{BD}} - \frac{\overline{DE}}{\overline{DB}}</math>

The range of values and the corresponding wettability characteristics are listed in Table 1.

The range of values and the corresponding wettability characteristics are listed in Table 1.

! || Water Wet || Neutrally Wet || Oil Wet

! || Water Wet || Neutrally Wet || Oil Wet

|-

|-

| Contact angle || || ||

| colspan=4 align=center | Contact angle

|-

|-

| Minimum || 0° || 60 to 75° || 105 to 120°

| Minimum || 0° || 60 to 75° || 105 to 120°

| USBM wettability index || W near 1 || W near 0 || W near -1

| USBM wettability index || W near 1 || W near 0 || W near -1

|-

|-

| Amott wettability index || || ||

| colspan=4 align=center | Amott wettability index  

|-

|-

| Displacement by water ratio || Positive || Zero || Zero

| Displacement by water ratio || Positive || Zero || Zero

===USBM method===

===USBM method===

The USBM method developed by Donaldson et al.<ref name=pt05r47 /> uses the same types of data, but considers the work required to do the forced displacement. This requires calculating the area under the [[capillary pressure]] curve obtained during the forced displacement. Generally, the capillary pressure displacement is done by centrifuging, but other capillary displacement techniques can be used.

The USBM method developed by Donaldson et al.<ref name=pt05r47 /> uses the same types of data, but considers the work required to do the forced displacement. This requires calculating the area under the [[capillary pressure]] curve obtained during the forced displacement. Generally, the capillary pressure displacement is done by centrifuging, but other capillary displacement techniques can be used.

|+ {{table number|2}}Alternative wettability measurement techniques

|+ {{table number|2}}Alternative wettability measurement techniques

|-

|-

! Method

! Method || Observation

! Observation

|-

|-

| Microscopic examination

| Microscopic examination || Visual examination of the fluid surrounding grains

| Visual examination of the fluid surrounding grains

|-

|-

| Flotation method

| Flotation method || Distribution of grains at water/oil interface or air/water interface

| Distribution of grains at water/oil interface or air/water interface

|-

|-

| Glass slide method

| Glass slide method || Displacement of the nonwetting fluid from a glass slide

| Displacement of the nonwetting fluid from a glass slide

|-

|-

| [[Relative [[permeability]]]] method

| [[Relative permeability]] method || Location and relative magnitudes of ''k''_ro and ''k''_rw curves

| Location and relative magnitudes of ''k''_ro and ''k''_rw curves

|-

|-

| Reservoir logs

| Reservoir logs || [[Basic open hole tools#Resistivity|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 || Changes in the longitudinal relaxation time

| Changes in the longitudinal relaxation time

|-

|-

| Dye adsorption

| Dye adsorption || Adsorption of a water soluble dye

| Adsorption of a water soluble dye

|}

|}

[[Category:Laboratory methods]]

[[Category:Laboratory methods]]