Desorption due to acidic and basic reactions

The desorption of polar organic components is clearly pH dependant and Austad et al. (2010) proposed that the main mechanism for the pH increase could be due to the desorption and adsorption of cations onto mineral surfaces. Clay could in a way act as cation exchanger due to its permanent negatively charged site on the surface. Initially in the reservoir, there will be a chemical equilibrium and the clay minerals will have adsorbed acidic and/or basic organic components in addition to inorganic cations (i.e. Ca²⁺, Mg²⁺) present in the FW. When

introducing an injection brine with low cation concentration, Ca²⁺ could dissolve from the clay surface. Protons from water molecules could compensate the negative charge on the clay surface.

In other word, a local pH increase will occur due to their higher affinity towards to the clay

surface. The water wetness of the system increases as OH^- will interact with the basic or acidic material. The proposed mechanisms are illustrated in Figure 3.4 and in equation (3.1 – 3.3).

The polarity and the reactivity of the polar components towards the negatively charged mineral surface is pH dependant. The acidic material that are present in crude oil is often represented by a carboxylic type, R-COOH. Most of the basic components contain nitrogen as a part of aromatic molecules, R3N, with a reactive par of electrons (Strand et al., 2016). The NSO components increases with increasing molecule weight of the crude oil and are represented in the heavy end fraction. But also crude oils with high API could have considerate amount of both acidic and basic components. After the low salinity water interact with the polar components there will also be a desorption of Ca²⁺ as Figure 3.4 illustrates.

𝐶𝑙𝑎𝑦 − 𝑅𝐶𝑂𝑂𝐻 + 𝐻₂𝑂 ↔ 𝐶𝑙𝑎𝑦 + 𝑅𝐶𝑂𝑂⁻+ 𝐻₂𝑂 (3.1) 𝐶𝑙𝑎𝑦 − 𝑅3𝑁 + 𝐻2𝑂 ↔ 𝐶𝑙𝑎𝑦 + 𝑅3𝑁𝐻⁺+ 𝑂𝐻⁻ (3.2) 𝐶𝑙𝑎𝑦 − 𝐶𝑎²⁺+ 𝐻⁺↔ 𝐶𝑙𝑎𝑦𝐻⁺+ 𝐶𝑎²⁺ (3.3)

Figure 3.4: Illustration of the proposed low salinity mechanism due to pH increase

Adsorption of basic material

There have been several studies performed investigating the effect pH dictates on the adsorption and desorption of organic material. Burgos et al. (2002) studied quinoline as a basic material and its ability to adsorb onto kaolinite and montmorillonite clay in CaCl2-solutions. Quinoline is a basic polar component that are present in crude oils. Figure 3.5 illustrates the results from the experiments and clearly shows that the adsorption of quinoline is a pH dependent process.

Figure 3.5: (a): Quinoline adsorption onto kaolinite. (b): Quinoline adsorption onto montmorillonite (Burgos et al., 2002). The stippled line is the fraction of protonated Quinoline.

The adsorption of quinoline decreases as the pH is increasing, whereas the largest adsorption seems to be observed at approximately pH 4. Relatively, the decrease was more significant when adsorbing onto kaolinite than montmorillonite. For the kaolinite experiment, when passing a pH value of 5, the adsorption was less than 1 mmol/kg. However, for the montmorillonite

experiment, the adsorption was over 100 mmol/kg when passing a pH of 7. In any case, there seem to be decreasing adsorption of basic components with increasing pH for both high and low concentrations of Ca²⁺. Highest adsorption observed for the LS brine with a concentration of 1000 ppm rather than 25000 ppm.

RezaeiDoust et al. (2011) also experimented with quinoline, but studied the adsorption onto kaolinite only. They showed that the quinoline adsorption was a completely reversible process, with regards to pH. It is very interesting to observe that as the experiment progressed the adsorption decreased when pH increased from 5-8, but also decreased adsorption from pH 5 to 2.5. The lower adsorption at low pH can be explained by the fact that the concentration of H⁺ will be very high. H⁺ is the cation with the highest affinity towards the negative clay charge and will compete with the other active species present in the brine (Helmy et al., 1983).

Figure 3.6: Reversible adsorption of Quinoline onto Kaolinite regarding pH at ambient temperature. Sample 1-6 contains salinity of 1000 ppm.

Sample 7-12 contains salinity of 25000 ppm

Further studies with quinoline was performed by Aksulu et al (2012) and the ability quinoline has to adsorb onto illite. As observed in previous experiments (Burgos et al., 2002; RezaeiDoust, 2011) the adsorption was highest for the low salinity brine. Furthermore, the adsorption peaked when the pH was close to the pKa value for quinoline (≈4.9). The active specie is the protonated form of quinoline, (R3N-H)⁺ (Aksulu et al., 2012). So when the system experiences alkaline conditions (8>pH) the adsorption drastically drops due to the lower concentration of positively charged species (Aksulu et al., 2012). Figure 3.7 illustrates quinoline adsorb onto illite using both HS and LS brines at different pH values.

Figure 3.7: Adsorption of quinoline onto Illite using both HS-brine (25000 ppm) and LS-brine (1000) ppm as a function of pH at ambient temperature (Aksulu et al., 2012)

Figure 3.8: Illustration of quinoline. Left is protonated form, right is neutral form

Adsorption of acidic material

As there seem to exist a general trend for the adsorption of basic components, surely a trend can be found for acidic components as well. Madsen and Lind (1998) performed experiments to study exactly that. They used benzoic acid in an NaCl solution and observed its ability to adsorb onto kaolinite. The result from their test shows that acidic adsorption is also highly pH dependant and the result can be seen in Table 3.1 below:

Table 3.1: Adsorption of Benzoic acid in NaCl-brine onto Kaolinite at 32ºC, as a function of pH (Madsen &

Lind, 1998)

pHinitial Gmax at 32°C mmole/m²

5.3 6.0 8.1

3.7 1.2 0.1

Benzoic acid has a pKa value of 4.2. When the pH value equals the pKa value, the concentration of benzoic acid on the protonated form and the deprotonated form will be equal. The neutral protonated carboxylic material could adhere towards the clay surface through hydrogen bonds.

Figure 3.9: Adsorption of carboxylic group onto clay by H-bonding (Austad et al., 2010)

4. Materials and Methods