Clays are the main wetting mineral in sandstone reservoirs and play an important role in the initial wettability and the LS EOR effect. Due to its large surface area and permanently negative charge, cations and organic components can adsorb/desorb on the clay surface as a function of pH in the solution. Adsorption of polar components onto the clay surface creates initial mixed wet condition which can be altered toward more water-wet conditions as LS brine is injected into HS FW reservoir desorbing the organic components from the surface, and thus inducing a tertiary EOR effect. The adsorption of the organic components onto clay minerals are dependent on pH, temperature, ion composition and salinity of the FW (Aghaeifar et al., 2015).
4 Experimental
The objective in this thesis is to evaluate feldspar and their influence on initial wetting and wetta-bility alterations. To achieve this, an optimal mineral preparation is essential. The main objective of this thesis is therefore to perform optimal mineral preparation by using new milling equipment which will help in getting representative adsorption and pH screening results.
Several steps have been done to produce mineral samples that will give reliable and repeatable results. Different apparatus and advanced analysis instruments have been used to analyse the mineral samples after different procedures. To verify stability and reproducibility of the prepared mineral samples adsorption and pH screening tests have been done and compared with results that have been obtained at the University of Stavanger by other students. (Andersen, 2015; Frafjord, 2015; Abdullah, 2016; Harestad, 2017; Tat, 2017; Algazban, 2017). The equipment, brines, minerals and procedures that are used will be described in detail in this section. Some of the procedures are performed identical to the procedures done earlier, and some have been improved.
4.1 Equipment
4.1.1 XRD - Mill McCrone
The XRD Mill McCrone is a small, compact bench top ball mill used for size reduction and homog-enization of rocks, figure 10. This mill has a very specific application area and is primarily used for preparation of samples of materials that are going to be analysed by XRD. A key point is the preservation of the crystal lattice structure. Other ball mills have an aggressive modus operandi which will destroy the crystal lattice and therefore they will not be detected in a XRD analysis.
The McCrone Mill however, employ very gentle modus operandi which will preserve the crystal lattice and is therefore available to analysis. Inside the grinding vessel there are 48 cylindrical grinding elements made of zirconium oxide. Zirconium oxide (ZrO2) is a stabilized oxide which inhibit disruptive transformation to other crystalline forms. (Greenwood, 1984) A unique grind-ing motion grinds the samples gently via friction. Minerals can be grinded both dry or combined with a fluid. Wet grinding in airtight containers reduces crystal lattice deformation and oxidation.
Grinding time can be adjusted to obtain optimum particle size distribution. Samples are grinded from<0.5mmto a lowµmrange. According to the manufacture, this process preserves the crystal lattice, gives almost no sample loss and produce a narrow particle size distribution. The preserva-tion and gentle handling aim to ensure perfectly homogenized and contaminapreserva-tion-free samples for reliable and accurate analysis to give reproducible sample preparation.
A sample preparation kit came as accessory to the ball mill consisting of a percussion mortar, sieve and sieve brush, figure 11. It was designed to rapidly and easily reduce large particles to suitable sizes for the ball mill. (Retsch, 2018)
Figure 10: XRD Mill McCrone (Retsch, 2018)
4.1.2 Micromeritics TriStar II
Micromeritics TriStar II was used to determine the surface area of the minerals. BET method is the basis for the calculations. The apparatus uses nitrogen and helium gases to determine the surface area, by measuring the adsorption of gas molecules on to the surface of the sample at a given pres-sure. VacPrep 061 was used to prepare the samples for surface area measurement. Contaminants, like water vapor and adsorbed gas at surface and pores of the sample, are removed by vacuum and heat. Vacuum was turned on slowly to prevent fluidization of samples. (micromeritics, 2018)
4.1.3 Scanning Electron Microscope
To analyse the feldspars particles a scanning electron microscope (SEM) Zeiss Gemini Supra 35VP was used. SEM is an electron microscope employing a beam of electrons directed to a specimen to obtain an electronic image of the specimen’s surface structure. The beam position is controlled digitally onto the sample, and the resultant image is displayed on a computer screen. The image formation in the SEM is built up sequentially during the scan, giving high pixel resolution and therefore good spatial resolution with a large depth of field. (Goodhew et al., 2000) All of which makes it possible to study the feldspars surfaces, sizes and shapes, and visualize how the particles were affected by the milling.
Emitech K550 was used to prepare the samples prior to the SEM analysis.
4.1.4 Mettler Toledo AB104-S analytical balance Analytical balancer used to weight experimental materials.
4.1.5 Anton Paar DMA-4500 Density meter Density meter used for measuring density of the brines.
4.1.6 Mettler Toledo pH meter
pH meter used to measure pH when analysing and adjusting the samples.
4.1.7 Gilson GX-271 Liquid handler
A Gilson GX-271 Liquid handler was used to dilute the NaCl-brines that had been in contact with anorthite for the static pH screening test. All salinity brines must be diluted before ion compo-sition analysis. Gilson GX-271 Liquid handler provide precise and automate sample preparation, minimizing error in final results (Gilson, 2015).
4.1.8 Dionex ICS-5000+ DP
The Dionex ICS-5000 was used to determine ion composition for the brines that has been in contact with anorthite used in the pH screening tests. The samples are transported with the help of an elute through an ion exchange column where the ions are separated in contact with a resin, the stationary phase. Then the ions together with the eluent passes through a suppressor where the conductivity of the eluent is reduced and increased for the ions, before entering the conductivity detector (Fanali et al., 2013). In the conductivity detector each ion is found based on its conductivity measured in µS.
4.1.9 Stuart SB3 rotater
Rotator used to rotate samples at ambient temperature.
4.1.10 Hettich Universal 1200 centrifuge Centrifuge used to separate mineral from water
4.1.11 Thermo Scientific Genesys 10S UV-VIS spectrophotometer
The Thermo Scientific Genesys spectrophotometer was used to determine the adsorption of quino-line onto the minerals.
4.2 Materials