Adhesion tests

As discussed in chapter 6, adhesion tests can be used to characterize crude oil/brine/solid interactions. We thus studied the adhesion behavior of two different crude oils, crude oil A

Chapter 7. Experimental study of oil-water-solid interfaces

and B, as a function of pH and salt concentration and how this was affected by adding different enzymes. Solutions with pH ranging from 3 to 12 and concentrations of NaCl from 0 to 1 M were used in the experiments. A drop of crude oil is formed under brine solution by using a micro syringe with an inverted needle. After some period of contact between oil and solid surface (about 2 min), the oil droplet is drawn back into the needle. At this stage three different behavior, adhesion, non-adhesion and temporary adhesion may observe. The standard procedure for the adhesion experiments is provided in more detail Appendix B.

It has been shown by adhesion map for clean glass slides that the pH and salt concentration of the brine, as well as the crude oil composition, affect the wetting behavior of the original water-wet glass surface (Skauge et al., 1996; Buckley et al., 1998). Tables 7.1 and 7.2 show adhesion maps for crude oil A and B, respectively. According to the adhesion maps, adhesion occurs at low pH and low NaCl concentration, in agreement with other experimental studies (e.g. Buckley and Morrow, 1990; Buckley et al., 1995; Skauge et al., 1996). Figure 7.1 shows an example of oil droplet which adheres to a clean glass surface.

Table 7.1: Adhesion map for crude oil A with clean glass.

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Chapter 7. Experimental study of oil-water-solid interfaces

Table 7.2: Adhesion map for crude oil B with clean glass.

12

9

6

5

4

3

0 0.1 0.5 1.0 M M M M [NaCl]

pH

12

9

6

5

4

3

0 0.1 0.5 1.0 M M M M [NaCl]

pH

Figure 7.1: Photo of an oil droplet that adherer to the clean glass surface.

At low salinity, where electrostatic forces are important, pH is the dominant variable for adhesion. Buckely and Morrow (1990) found a characteristic pH above which adhesion does not occur. This can be explained by considering double layer forces. At low pH, the basic functional groups at the crude oil-water interface are positively charged. The glass-water interface, on the other hand, is negatively charged above pH 2 (Iler, 1979; Anderson 1986-a).

This likely result in crude oil adhesion by thinning of the water film separating the two oppositely charged interfaces.

Chapter 7. Experimental study of oil-water-solid interfaces

Non-adhesion was observed at high pH ( 9) for both crude oils at all salinities. This is likely due to both the glass-brine and crude-oil brine interfaces being negatively charged. At pH 12 it was not possible to test adhesion for crude oil B as the physical properties of the oil was altered so that it would not form droplets at the syringe tip. This behavior may be attributed to a high fraction of acidic functional groups for crude oil B which would be completely ionized at this high pH, this giving rise to a very low oil-brine interfacial tension.

At pH 6, however, adhesion or temporary adhesion was observed at low salinities for crude oil A, while non-adhesion was found at all salinities for crude oil B. Both crude oils showed adhesion or temporary adhesion at low pH (pH 3) and salinities. At higher salinities, however, crude oil B gave adhesion while crude oil A showed temporary adhesion. This is likely due to differences in oil composition between crude oil A and B which, according to Buckley and Morrow (1990), can have a major effect on adhesion behavior, especially at high salinities. At high salinities, electrostatic forces are less important and the influence of DLVO forces in stabilizing the thin film of water between solid and crude oil is not significant. In this condition the adhesion behavior depends on the net charge at the interfaces of oil/water and solid/water (Buckley and Morrow, 1990). For a given crude oil or solid surface, the surface charge depends on the extent of acid/base dissociation reactions. If both oil/water and solid/water interfaces have like charge, it results in repulsion of interfaces and consequently the stabilization of intervening water film (non-adhesion behavior).

Adhesion map for crude oil B was also provided using aged glass as the solid surface. This was done to provide a qualitative measure of whether or not the surface properties of the glass slide were changed upon aging. Table 7.3 shows the adhesion map of crude oil B when aged glass was used as the solid surface. As the map shows oil droplet was adhered to the glass slide in all conditions and effect of different pH and salt concentration were overshadowed by wetting behavior of the solid surface. It should be pointed out that glass slides were washed first with toluene and then with decane prior to the experiments. Attempt was done to treat all the glass slides identical respect to washing procedure.

Chapter 7. Experimental study of oil-water-solid interfaces

Table 7.3: Adhesion map for crude oil B with aged glass.

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7.1.1 Effect of enzymes on adhesion

The effect of a selected enzyme (NZ2) on the adhesion behavior of crude oil B was examined.

A condition in which crude oil B showed adhesion behavior, pH 3 and 0.5M NaCl concentration was chosen as the initial condition. NZ2 enzyme was added to the brine in 0.1 wt% concentration. Crude oil droplets were introduced at the surface in 2 minutes time interval sequence i.e. droplet number 1 is the first drop in the measurement series, two minutes later, drop number two is tested etc. Table 7.4 shows the results from the experiment.

The oil droplet starts to destabilize and move toward a non-adhesion condition after about 6 minutes (droplet 5). Thereafter the oil droplets did not adhere to the glass. As will be discussed in more detail in section 7.4, reasons why adding NZ2 may shift the crude oil B/0.5 M NaCl, pH 3/glass from adhesion to non-adhesion include adsorption of protein and cleaving of ester bonds which may change the crude oil/brine/solid interactions.

Table 7.4: Adhesion behavior of crude oil B at pH 3 and 0.5M NaCl concentration by NZ2 enzyme introduction.

Adhesion

Chapter 7. Experimental study of oil-water-solid interfaces