Wettability measurement

A wide variety of methods in wettability measurement have been discussed both in qualitative and quantitative ways. In this section, wettability measurements which are universally used will be described briefly including contact angle, Amott water index, United States Bureau of Mining (USBM) and Chromatographic wettability test.

Contact angle measurements

When measuring the surface wetting condition, the measurement of contact angles is usually involved as the primary data, which indicates the degree of wetting at the intersection of solid and fluids (Yuan and Lee, 2013). For a rock/brine/oil system, Young’s equation demonstrates the contact angle in equilibrium with the interfacial/surface tensions between the solid and fluids:

σ_os = 𝜎_𝑤𝑠+ 𝜎_𝑜𝑤𝑐𝑜𝑠𝜃 (3.7) Where,

σ_os Interfacial tension between oil and solid σ_ws Interfacial tension between water and solid σ_ow Interfacial tension between oil and water 𝜃 Contact angle of the water/oil/solid interface

Figure 3 Schematic measurement of contact angle at the water/oil/rock system.

Redrawn after Anderson (1986).

The contact angle indicates the wettability of the rock surface. If the rock is preferentially more water wet, a droplet of water is spreading, and the contact angle

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is less than 90º. The rock is considered as preferentially oil-wet when the contact angle is greater than 90º and contact between the water droplet and rock surface is minimized. When the contact angle is 90º or close to this value, the surface is known as intermediate or neutral wettability, and both oil and water do not have a strong affinity to the surface (Dullien, 1992).

Especially for pure fluids and artificial cores, the contact angle measurement is the best wettability measurement method, since neither surfactants nor other compounds change the wettability (Anderson, 1986). However, because of adsorption or desorption of minerals in crude oil which could alter the wettability, there are some difficulties in using contact angle method in reservoir cores.

Amott wettability index

The Amott wettability test is used to measure the average wettability of a core which involves spontaneous imbibition (SI) and forced displacement. The principle of Amott method is that the wetting fluid spontaneously imbibes into the core and displace the nonwetting phase (Anderson, 1986). The core is first prepared by centrifuging under brine to achieve residual oil saturation, and the test is conducted by following four steps (Anderson, 1986; Morrow, 1990).

1) The core, at residual oil saturation, is immersed in oil and the volume of water displaced during the spontaneous imbibition is measured.

2) The core is centrifuged in oil to reach irreducible water saturation, and the total amount of water displaced, including the volume from step 1 is measured.

3) Immerse the core, at irreducible water saturation, in brine and record the volume of oil displaced by the spontaneous imbibition.

4) The core is then centrifuged in water to achieve residual oil saturation.

The total volume of oil produced between both step 3 and step 4 is measured.

The Amott index to water and oil is then defined as follows, respectively:

𝐼_𝑤^∗ = 𝑉_𝑜𝑠𝑝/𝑉_𝑜𝑡 (3.8) 𝐼_𝑜^∗ = 𝑉_𝑤𝑠𝑝/𝑉_𝑤𝑡 (3.9) Where,

𝐼_𝑤^∗ Amott index to water (displacement-by-water ratio) 𝐼_𝑜^∗ Amott index to oil (displacement-by-oil ratio)

15 𝑉_𝑜𝑠𝑝 Volume of oil displayed by SI

𝑉_𝑜𝑡 Total volume of oil displayed by SI and Forced displacement 𝑉_𝑤𝑠𝑝 Volume of water displayed by SI

𝑉_𝑤𝑡 Total volume of water displayed by SI and Forced displacement In strong water-wet cores, most of the oil is displaced by spontaneous imbibition, thus, 𝐼_𝑤^∗ is close to 1, while 𝐼_𝑜^∗ approaches 0. Similarly, in strong oil-wet cores, 𝐼_𝑜^∗ is close to 1 and 𝐼_𝑤^∗ will be 0.

The modified Amott index, which is the Amott-Harvey index, 𝐼_𝐴𝐻, is more commonly used by researchers. It is defined as the difference between the displacement-by-water ratio and the displacement-by-oil ratio (Morrow, 1990):

𝐼_𝐴𝐻 = 𝐼_𝑤 − 𝐼_𝑜 (3.10)

This index 𝐼_𝐴𝐻 has a value varies from -1 (strongly oil-wet) to +1 (strongly water-wet). The system is considered as water-wet when +0.3<𝐼_𝐴𝐻<1, intermediate-wet when -0.3<𝐼_𝐴𝐻 < +0.3, and oil-wet when -1<𝐼_𝐴𝐻<-0.3 (Cuiec, 1984).

United States Bureau of Mining (USBM)

Another quantitative method to measure the wettability of the core is the USBM method introduced by Donaldson et al. (1969). This method is relatively fast, and one of its advantages over the Amott method is its sensitivity when it is close to neutral wettability. A minor disadvantage is that the sample for the USBM wettability index should be in plug sized to place it in a centrifuge (Anderson, 1986).

The wettability using USBM is determined by comparing the work required to displace wetting or non-wetting phase with each other. The forced drainage and forced imbibition capillary pressures are measured by centrifuging and plotted versus water saturation (Anderson, 1986), and the work necessary is proportional to the area under those capillary pressure curves. If the core is strongly water-wet, most of the water would be displaced by spontaneous imbibition and the area under the forced drainage curve becomes larger than the area under the forced imbibition curve as shown in Figure 4.

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The wettability index (𝑊𝐼_{𝑈𝑆𝐵𝑀}) is defined by a logarithmic ratio of the areas under the two capillary pressure curves:

𝑊𝐼_{𝑈𝑆𝐵𝑀} = log^𝐴1

𝐴2 (3.11)

where 𝐴₁ is the area under the forced drainage curve and 𝐴₂ is the area under the forced imbibition curve. The USBM wettability index 𝑊𝐼_{𝑈𝑆𝐵𝑀} has a positive value when the sample is relatively water-wet and if 𝑊𝐼_{𝑈𝑆𝐵𝑀} has negative value, the sample is relatively oil-wet. As the absolute value of the index is larger, the greater the wetting preference is.

Chromatographic wettability test

Chromatographic wettability test is recently introduced by Strand et al. (2004) to determine the water-wet fraction of carbonate rock surface. This new method is based on the chromatographic separation between thiocyanate (SCN^-), a

non-Figure 4 USBM Method to determine wettability.

a) Water wet, b) Oil wet (León-Pabón et al., 2014).

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adsorbing tracer, and sulfate (SO42-), the potential determining ion, at the water-wet areas on the carbonate surface (Strand et al., 2006). The schematic illustration of the chromatographic separation between SO42- and SCN^- is shown in Figure 5.

Figure 5 Schematic illustration of the chromatographic separation between SO4

2-and SCN^- (Strand et al., 2006)

SO42- can adsorb onto the water-wet sites of the carbonate surface and there will be a delay of sulfate concentration in the effluent. However, SCN^- does not adsorb onto the rock surface and passes through the surface without any retardations. Ionic concentrations of the effluent are measured by Ion Chromatography (IC) and concentration curves of SO42- and SCN^- are obtained. The area between these effluent curves of two ions is proportional to the water-wet sites of chalk core. A chromatography wettability index (Icw) is calculated using Awet, the area between the tracer and sulfate curves, and Aheptane, the areas of separation between the same ions in a completely water-wet system which uses heptane as reference oil:

𝐼_𝑐𝑤 = ^{𝐴𝑤𝑒𝑡}

𝐴ℎ𝑎𝑝𝑡𝑒𝑛𝑒 (3.12)

The chromatography wettability index (Icw) has a range between 0 and 1, where 0 indicates a completely oil-wet surface, and 1 represents a completely water-wet surface. It is very useful close to a neutral wetting condition which has Icw value near 0.5, however, the limitation is that this method is only applicable to carbonate core material (Strand et al., 2006). Also, since Aheptane refers to a completely water-wet condition, there is a difficulty in application this index in reservoir condition.

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