**2.2 Electroacoustic**

In electroacoustics, the motion is due to the different densities of particle and medium [16]. This motion disturbs the double layer around the particles and eventually forms a dipole moment. The summation of the dipoles results in an electric field, known as colloidal vibration current (CVI). A two-electrode sensor detects this current and uses it to compute the zeta potential.

The measurement depends on several factors such as zeta potential, density difference between the medium and suspending particles, viscosity, dielectric permittivity of the liquid, and particles' weight fraction. This technique has several advantages over the electrophoresis which are [16]:


#### **2.3 Significance to the wettability study**

Zeta potential measurements are used in nearly all industries that involve colloidal systems. In addition, measuring zeta potential is a reflection of characterizing the surface charge of a suspended particle [17]. Recently, zeta potential measurements have been used extensively to study various biomaterials such as proteins, leukocytes, and DNA [18–20]. Moreover, they have been used to understand the behavior of nanoparticles with human cells, beneficial for applications in the field of medicine [21]. Adhesive properties of various polymers and biomaterial have been investigated using the zeta potential approach [20, 22]. In a study, conducted by Bassioni [23] on environment-friendly construction, zeta potential measurements were used to identify the presence of anionic superplasticizers on cement particles. In addition, Bassioni and Ali [24] performed zeta potential measurements to identify effective dosage of additives to oil-well cement.

In another study, conducted by Bassioni [25], CaCO3 showed potential to adsorb significant quantities of anions due to its positive surface charge. Moreover, zeta potential measurements were made to determine the surface charge of the calcium carbonate particles in the presence of varying dosages of the adsorbates [25]. Since wettability refers to the adsorption of a fluid, in presence of immiscible fluids, onto a rock surface, zeta potential measurements are a promising way to measure wettability. As compared to all other wettability measurement techniques discussed, the zeta potential approach is significant as it can study the wettability alteration in situ [26]. Wettability studies can be carried out on the reservoir rock in order to determine its tolerance and its interaction with inhibitors, prior to any injections to the reservoir. Predictions can be made to study the effect of reservoir conditions onto the rock wettability.

Several studies have been conducted where researchers have employed the concept of zeta potential for different applications. Elimelech et al. used zeta potential measurements to study reverse osmosis membranes [27]. In another study, zeta potential was used to determine the optimum dosage of additives to cement for hydration [28]. Bousse et al. conducted zeta potential measurements on metallic oxide thin films [29]. In addition, another study carried out zeta potential measurements on some transition metal oxides at high temperature [30].

## **3. Case study**

#### **3.1 Methodology**

Several experiments were conducted in order to study wettability on reservoir rocks using zeta potential measurements. These include the wettability study of:


 The experimental setup, as shown in **Figure 3**, mainly comprises of a zeta probe, an electronic stirrer and a mixing cell. Prior to the experiment, the particle size of the powdered reservoir rock (i.e., limestone) was determined using a WJL laser granulometer, as it was suspended in water. For each of the experiment, a colloidal mixture was prepared. For the first study, 200 g of limestone was poured into 200 g of water over a period of 1 minute. The mixture was allowed to sit for 1 minute and then stirred vigorously for 2 minutes before transferring to the mixing cell. In the mixing cells, the mixture is continuously stirred using an electronic stirrer at room conditions. All experiments conducted in this study were carried out using distilled water since wettability effects become very important when brine saturation is lowered [31].The zeta probe was placed and the zeta potential was measured. After the initial zeta potential value was recorded, the fluid of interest is added mL-wise to the mixture, and successive zeta potential measurements were made.

 Model DT-1200 Electroacoustic Spectrometer (Dispersion Technology Inc., New York, USA) was used to measure the zeta potential values of the system. Moreover, the device has a zeta potential measurement accuracy of ± 0.1 mV + 0.5, as stated by the manufacturer. Densities of the liquid medium and the suspended solid medium and the suspended solid particle size were the parameter input to the system. The probe was placed in the external cell, and the zeta potential was measured, as illustrated in **Figure 3**. For the proposed zeta potential setup, the zeta probe takes measurements when immersed in a colloidal system. Moreover, the system of solid fluid needs to be smooth so a steady colloidal system exists throughout the analysis. The system works with small particles of sizes less than 10 μm. If aggregation in the system or deposition on the probe occurs, it may result in inaccurate readings.

**Figure 3.**  *Experimental setup [26].* 

## **3.2 Results and discussion**

This section states the results obtained from each analysis and experiment. In addition, it discusses each observation in detail, with possible explanations for each of the finding.

 The *D*50 size of the limestone particle was found to be 7.06 ± 0.211 μm with a surface area of 8.628 m<sup>2</sup> /g and a density of 2.66 g/cm3 . On the other hand, the PCD analysis showed that both inhibitors, SD and KT, carry a surface charge of −2097 and −1145 mV, respectively. Additionally, the moisture content in each inhibitor was found to be 96.2 and 71.3%, respectively. The ICP-MS analysis, carried out on the SD inhibitor samples, showed significant concentrations of Rb, Ca, and Mg, in descending order. On the other hand, Ni, Al, and Cr were found in significant concentrations in the KT inhibitor (from highest to lowest, respectively).

Various wettability studies were carried out for different crude oil samples on limestone, in presence and absence of inhibitors, as well as of oil derivatives, such as maltene and asphaltenes. The following sections state the findings of each and discuss the results in detail.

#### *3.2.1 Wettability of calcium carbonate for crude oil in limestone-water mixture*

**Figure 4** shows the results of the zeta potential measurements of the limestonewater suspension as different crude oils are added to the system.

 Initially, the surface charge for aqueous limestone suspension was found to be about +30 mV. At this stage, the CaCO3 is completely wetted by water since no crude oil is added yet. Upon addition of crude oil, the zeta potential of the aqueous limestone suspension starts to decrease steadily until a certain limit beyond which a steep fall is observed. This limit differs for each type of oil, as seen from **Figure 4**.

 As oil comprises of negatively charged particles [3, 32], oil droplets tend to adsorb to the positively charged CaCO3 surface as a result of electrostatic interaction, displacing the water particles to the bulk. **Figure 5** depicts this competitive adsorption behavior. Once the entire surface is completely wet with the oil droplets, the wettability curve experiences a steep decrease in the zeta potential value, signifying a complete oil-wet surface. The curve flattens out to indicate the end of the adsorption process.

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

**Figure 5.** 

*Negatively charged crude oil particles adsorbing onto positively charged calcium carbonate surface [33].* 

 It can be seen from **Figure 4** that all crude oil samples experience a similar trend upon the addition of crude oil. However, crude oil C tends to have a slightly different trend. This behavior will be explained in the section discussing the wettability alteration due to asphaltene.

As seen from **Figure 4**, it is observed that there exists a general trend in the wettability curve for all crude oil samples. However, the curve appears to be shifted vertically. In accord with our own investigation [26], a previously reported study showed C7 asphaltene particles, in presence of resins, are negatively charged [34]. Therefore, the more the asphaltene content present, the lower the zeta potential decrease observed. Hence, the vertical shift is observed.

#### *3.2.2 Wettability of calcium carbonate for water in limestone-oil A mixture*

**Figure 6** shows the zeta potential measurements of the limestone-crude oil mixture as water is added.

The initial zeta potential of the limestone-oil suspension was found to be −15 mV. This signifies that oil carries components with high negative charge, as compared to limestone, lowering the zeta potential value of the limestone suspension to a negative value. Upon addition of water, the zeta potential fluctuates around the average value, as exhibited in **Figure 6**.

 Initially, the limestone surface is completely coated by oil (i.e., oil-wet), and the mixture is being stirred smoothly. When water is added to the system, it washes

away the negatively charged adsorbed oil droplets from the surface into the bulk. However, due to electrostatic interactions, these oil droplets tend to return to the limestone surface and adsorb onto it, with time. This continuous behavior results in fluctuations in the zeta potential values. **Figure 7** shows the remainder of the limestone-oil water system when the experiment was halted.

It can be seen in **Figure 7** that deposition of heavy fractions of oil on the limestone surface occurred, forming large chunks of solid matter. The formation of solid chunks became an obstacle for the stirrer in mixing, and the experiment was halted. This situation arose due the addition of water. Water tends to alter the composition of the system, destabilizing the limestone-oil suspension. Due to such disturbance, the heavy fractions aggregate and deposit onto the surface of limestone, as seen from **Figure 7**.
