*3.2.3 Wettability of calcium carbonate for crude oil A in limestone-water mixture with inhibitors*

The wettability experiment was carried out on the aqueous limestone suspension in presence of inhibitors, and the data was compared to the wettability profile of crude oil A in absence of inhibitors, as seen in **Figure 8**.

The initial zeta potential value for the limestone-water inhibitor system for both inhibitors, KT and SD, was 16 mV even though KT has a particle charge of −1145 mV, whereas SD has a particle charge of −2097 mV. The zeta potential decreases with the addition of crude oil to the system. Moreover, both inhibitors decrease the zeta potential of the system to a value of about 4 mV, after 80 mL of crude oil was added. Yet, inhibitor SD experiences an earlier decrease in zeta potential value relative to inhibitor KT.

 As compared to the wettability profile of calcium carbonate without inhibitor, it is observed that inhibitor SD decreases the system tolerance to the addition of crude oil, whereas inhibitor KT slightly increases it. In other words, the wettability alteration process, from water-wet to oil-wet, is accelerated by inhibitor SD, while inhibitor KT decreases it. This analysis shows that inhibitor SD is ineffective for crude oil A.

**Figure 9** shows an illustration, showing the possible mechanism that the inhibitor particles follow, in the limestone-water inhibitor system, as crude oil is being added.

**Figure 7.**  *Limestone-oil suspension upon addition of 53 mL of water [26].* 

**Figure 8.** 

*Zeta potential measurements of aqueous limestone suspension without inhibitor and with inhibitors, KT and SD [26].* 

In the limestone-water inhibitor system, the inhibitor gets attracted and adsorbed onto the limestone surface due to its high negative particle charge. Since the inhibitor possesses a higher negative charge than the crude oil, upon addition of crude oil, it remains adsorbed on the limestone surface and tends to resist the wettability alteration, keeping the oil particles in the bulk.

 The transition is resisted due to the negative charges present on both, crude oil and the inhibitor. These negative charges exert forces of repulsion, preventing the crude oil particles from coming in contact with the limestone surface and keeping them afloat in the oil. By extension, this prevents the deposition of heavy fractions of oil, maintaining the composition of the crude oil. However, it can be seen that in **Figure 7**, the zeta potential value of the suspension decreases to an even lower value upon addition of crude oil. Eventually, the inhibitor particles are forced to desorb due to the increasing concentration of crude oil. Hence, the wettability changes.

**Figure 9.**  *Mechanism of inhibitor adsorbing on the limestone surface in the presence of crude oil [26].* 

### *3.2.4 Wettability of asphaltenic solutions on calcium carbonate in limestone-water mixture*

 Asphaltene was extracted from crude oil A, and different asphaltene-toluene solutions of varying concentrations were prepared. **Figure 10** shows the wettability curve for each solution.

 Initially, the limestone surface in the limestone-water system is completely water-wet. Upon addition of asphaltenic solution, the zeta potential value decreases for all concentrations of asphaltene. This signifies the negative character of the asphaltene molecules in toluene as well as in oil, as supported by Bassioni and Taqvi [26]. As the concentration of the asphaltenic solution increases, the zeta potential tends to fall to a more negative value. All concentrations except 5 wt% follow a similar trend in the decrease of the zeta potential value, upon adding the asphaltenic solution. The reason behind the change in trend in case of 5 wt% asphaltenic solution will be explained in Section 3.3, discussing the wettability alteration due to asphaltene.

 Initially, the limestone surface in the limestone-water system is completely water-wet. Upon addition of the asphaltenic solution, asphaltene particles tend to adsorb onto the limestone surface and wet it. The wettability alteration is complete once the wettability curve flattens out, and the plateau, at the end of the wettability curve, marks a completely asphaltene-wet limestone surface.

#### *3.2.5 Wettability of maltenes on calcium carbonate in limestone-water mixture*

In addition to the asphaltenes, a wettability study was carried out on the maltenes extracted from crude oil. **Figure 11** shows the wettability curve for maltenes, asphaltenes, and the crude oil.

Initially, the limestone surface in the limestone-water system is completely water-wet. When maltene is added to the system, the zeta potential value decreases until the curve flattens out, as seen in **Figure 11**. Resins, in the maltenes, adsorb onto the surface, and the wettability curve flattens out when the limestone surface is completely maltene-wet.

When compared to the wettability of limestone for crude oil A, the zeta potential values of the maltene wettability curve seem to coincide with it. Moreover, the maltene wettability curve attains a plateau at a zeta potential +20 mV,

**Figure 10.**  *Wettability of aqueous limestone mixture for asphaltenic solutions of varying concentrations of crude oil A [26].*  *Understanding Wettability through Zeta Potential Measurements DOI: http://dx.doi.org/10.5772/intechopen.84185* 

**Figure 11.**  *Wettability profile of limestone for maltene, asphaltene, and crude oil [33].* 

corresponding to the wettability alteration in the wettability curve of limestone for crude oil. On the other hand, the plateau of the wettability curve for asphaltenic solution coincides with the plateau of the wettability curve for crude oil A. In addition, both wettability curves seem to coincide at a similar value of about +12 mV.

The behavior can be explained such that when crude oil is added to limestonewater mixture, the maltenes, in the crude oil, readily adsorb onto the surface of the limestone particle. After the surface is completely maltene-wet, asphaltenes start adsorbing onto the surface of calcium carbonate. The wettability curve approaches a plateau when the limestone surface is completely asphaltene-wet.

#### *3.2.6 Summary*

 In contrast to the quantitative techniques discussed in **Table 1**, the zeta potential technique presents an entire wettability profile for the reservoir rock. Wettability measurements are unique for each system. Cases have been presented where different wetting states can be understood from the proposed zeta potential technique. As stated earlier, the quantitative measurement methodologies stated here cannot determine wettability in situ*.* Thus, if samples of the mixture are drawn at different wetting states, respective values would be observed, as shown in **Table 2**. For the study presented in Section 3.2.1, if a sample from the mixture was obtained anywhere from the beginning of the experiment until where the plateau's first steep drop was observed in **Figure 4**  (i.e., ~44 mL of crude oil for oil A), the contact angle method would yield θ values of less than 90°. However, if a sample is drawn from beyond the plateau, the contact angle value would result in greater than 90°.

 In all, from the results analyzed above, it was observed that calcium carbonate, in water, is positively charged and has the potential to adsorb particles onto its surface. Oil, on the other hand, consists of negatively charged components which adsorb onto the calcium carbonate surface due to electrostatic interaction.

 In a strongly water-wetted calcium carbonate system, upon the addition of crude oil, the zeta potential is observed to decrease until oil particles adsorb onto the rock surface. Effective inhibitors in such systems have been able to increase the surface resistance to adsorb negatively charged oil particles. In a strongly oil-wetted system, the zeta potential fluctuates upon the addition of water and eventually leads to the formation of large chunks of calcium carbonate with asphaltene deposition. Inhibitors have been found to be ineffective in such systems.

As evident from the different wettability studies carried out, crude oil adsorbs onto the limestone surface, initially, with a steady decrease, followed by a steep decrease in the zeta potential value and a plateau. On the other hand, maltenes have been observed to adsorb onto the surface with a steady decrease in the zeta potential value, while asphaltene molecules have been observed to readily adsorb onto the limestone surface, resulting in a steep decrease in the zeta potential value, followed by a plateau. The superimposition of zeta potential values for the wettability profiles shows a strong relationship that will be explained, in detail, in the following section.

#### **3.3 Wettability alteration due to asphaltene**

 All wettability studies, carried out, indicate the adsorption of oil particles when crude oil or its individual components are added to a limestone-water mixture. However, a detailed explanation is required for such a behavior and the role of asphaltene in the wettability alteration process. **Figure 12** provides a mechanism that the oil components undergo through the wettability alteration process, at a microscopic level.

Initially, the limestone surface in the limestone-water system is completely waterwet. When crude oil is added, the resins, major oil component in the light fraction of oil (i.e., maltene), competitively adsorb onto the limestone surface. Resins are negatively charged in oil with a dipole moment of 2.4–3.2 D, while water has an average dipole moment of 1.85 D [32, 35]. Due to greater forces of electrostatic attraction between the positive limestone surface and negatively charged resins, the resins adsorb onto the limestone surface. At the end of the steady decrease, as indicated in **Figure 12**, a monolayer of resins is formed, pushing the water molecules to the bulk.

Following the steady decrease of zeta potential, the limestone surface experiences a steep decrease in zeta potential value. This is attributed to the competitive adsorption between the resins and the asphaltene molecules. Asphaltene molecules, as reported earlier, have a dipole moment over the range of 3.3–6.9 D [36]. This signifies the ability of asphaltene molecules to carry a greater charge than resins. Therefore, during the wettability alteration process, asphaltenes carry a greater negative charge than resins and competitively adsorb onto the limestone surface,

**Figure 12.**  *Mechanism oil particles undergo during a wettability alteration process.* 

#### *Understanding Wettability through Zeta Potential Measurements DOI: http://dx.doi.org/10.5772/intechopen.84185*

 displacing the resin molecules to the bulk. The wettability curve flattens out when the asphaltene molecules form a monolayer on the limestone surface. Wettability studies of limestone for crude oils, A and B, tend to follow this behavior. Moreover, wettability of limestone for all asphaltenic solutions except 5 wt% is believed to follow this mechanism. According to a study conducted by Plank and Bassioni [37], CaCO3 was found to correspond to a Type II isotherm [38].

According to Gregg et al. [39], the Type II isotherm, observed in physical adsorption, indicates the formation of a monolayer of particles onto the surface near the inflection point in the isotherm. Moreover, it allows for multiple layers to be formed above that monolayer of particles. The additional phenomenon, indicated in **Figure 12**, is believed to be observed in the wettability curve of limestone for crude oil C and is the reason behind the difference between the wettability of limestone for all asphaltenic solutions and 5 wt%. As the concentration of crude oil increases in the limestone-water crude oil system, another steep decrease in the zeta potential value is observed. The curve flattens out, and fluctuations are observed toward the end of the wettability curve. Discussing the mechanism following the monolayer formation of asphaltene molecules on limestone, it is believed that more asphaltene molecules approach the monolayer and adsorb onto it. This results in the formation of multilayers, leading to asphaltene deposition. This behavior can be attributed to the adsorption isotherm IV [38].

 As described by Gregg et al. [39], the points of inflection in the adsorption isotherm type IV reflect the completion of monolayer as well as the onset of multilayer adsorption. Moreover, capillary condensation is associated with this isotherm where the adsorbate fills the small pores of the solid. In the case of the wettability alteration by crude oil, the pores can be assumed to be filled by asphaltenes which causes fluctuations. The wettability study, of limestone-oil system with water added, shows fluctuations at a significant level where oil is destabilized by water the system. Significant fluctuations, toward the end of the wettability profiles, are also evident in **Figure 4**, the wettability study of limestone for crude oil C, and evident in **Figure 10**, the wettability study of limestone for 5 wt% asphaltenic solution. When a concentrated liquid coats a porous solid, it tends to permeate through the solid rock and fill its pores. As observed from the wettability study of limestone-oil system with water added, water presence in such a system causes deposition of heavy fractions, leading to instability in the system.

Commonly, physical adsorption gives rise to such an isotherm resulting in multilayer adsorption. Components, comprising of aromatic rings, in the crude oil adsorb onto the monolayer of oil particles adsorbed on the CaCO3 surface. As multilayers adsorb, the system experiences a steady decrease in the zeta potential value until a plateau is reached. Due to the multilayer adsorption, the thickness is believed to increase causing the CVI to measure a potential value at a distance beyond the electric double layer.
