Analyses

The various analyses utilized in this experimental work is explained in this section.

6.2.1 pH measurements

The pH in brines and produced water was measured using the pH meter seven compact^ä from Mettler Toledo, with the electrode semi-micro pH. There were taken several measurements to quantify the results and the repeatability was ±0.01 pH units at room temperature.

6.2.2 Density measurements

The densities of the brines and oils were measured using an Anton Paar DMA-4500 density meter at room temperature. Initially, the density meter was cleaned with white spirit, acetone and DI-water. Then a small amount of fluid was injected into the glass tube and the density was determined. There were taken several measurements to quantify the results, and the repeatability was ±0.001 g/cm³.

6.2.3 Viscosity measurements

The viscosities of the brines and oils were measured using an Anton Paar rotational rheometer Physica MCR 302. Approximately 0.650 ml fluid was placed on a metallic surface, and a metal plate was lowered towards the fluid and the position between the two surfaces were measured.

The viscosities of the oil and brines were determined through shear stress/shear rate relation.

The repeatability was 0.01 mPa.s. Figure 6.1 presents a viscosity measurement of RES 40-0.

Three equal and stable measurements are done before the viscosity is confirmed.

Figure 6.1 Example of viscosity measurements by Anton Paar rotational rheometer

6.2.4 Interfacial tension measurements

The IFT between a liquid-liquid interface was measured using a Krüss tensiometer with the Du Noüy ring method. Initially, liquid one was placed in a glass container, and a platinum-iridium ring was lowered into the glass container, then liquid two was introduced. The ring was moved from liquid phase one to liquid phase two. A lamella was produced when the ring moved through the phase boundary, and the force acting on the optimally wettability ring was measured in mN/m (du Noüy, 1925). Figure 6.2 illustrates measurement of IFT between DI-water and oil with the Du Noüy ring method.

Figure 6.2 Illustration of IFT measurement between oil and DI-water with the Du Noüy ring method. A lamella is produced between the two immiscible fluids.

6.2.5 Determination of AN and BN

The crude oils were analyzed for the amount of acidic and basic polar components, in mgKOH/g. The AN (Acid number) and BN (base number) for the oils were measured using a Mettler Toledo T55 auto-titrator with an international standard developed by (Fan & Buckley, 2007). The standard is a modified version of ASTM D664 for acid number titration and ASTM 2895 for basic number titration. The instrument is using a blank test as a reference during potentiometric titration of oil samples, where measurements of electronic potential is converted to equivalent acid numbers. Each measurement of the oil samples requires a titration solvent and a spiking solution, the composition of these two solvents are listed in appendix A. To quantify the measurements, the weight of the samples was taken with a Mettler Toledo weight instrument with an accuracy of ±0.0001 g. Calibrations and blank samples were done regularly to compensate for changes in the electrode properties when it was exposed to air. The reproducibility of the analyses was better than 0.02 mgKOH/g oil added.

6.2.6 Ion chromatography

The effluent samples from the chromatographic wettability test were diluted 1000 times with DI-water using the trilutionä LH system from a Gilson GX-271 liquid handler. The diluted samples were then placed in a Dionex ICS-5000⁺ Ion Chromatograph, and chemical analyses of cations and anions was determined. The software controlling the chromatograph used retention time, which is travelling time through the columns, and plotted conductivity versus the retention time. The area below each peak corresponds to the ion’s relative concentrations, and the concentrations of each ion was measured using external standard methods.

6.2.7 Scanning Electron Microscopy (SEM), EDAX

Small samples from the chalk outcrop material was analyzed with a Scanning Electron Microscopy (SEM). Images was taken by scanning a focused electron beam over the rock surface. The electrons in the beam interact with the sample, and various signals is produced which can be used to obtain information about the surface topography and composition. The material was prepared with assistance of an Emitech K550. The rock samples were exposed to vacuum and coated with palladium in an argon atmosphere. The coating will reduce the thermal damage, enhance secondary electron emission and increase the electrical conductivity of the sample which is important for Scanning Electron Microscopy (Emitech, 1999; Instruments).

Elementary analyses were also taken of the rock samples with Energy Dispersive X-ray Spectroscopy (EDAX). The analyses are used to obtain quantitative results of chemical composition of a specific location within the rock sample. The technique can detect elements from carbon and uranium with a capacity as low as 1.0 wt%. When SEM and EDAX is combined, elemental analyses of the specific area for a given sample can be adjusted based on the magnification the sample is being observed (Marickar et al., 2009)

6.2.8 Simulating with SENDRA

The two-phase core flooding simulator SENDRA was used to history match experimental data.

Relative permeability curves at different initial wettings were created based on the output data from the history match. SENDRA utilize a fully implicit black-oil formulation which is based on Darcy’s law and the continuity equation with a fully automated history matching routine and a forward simulation of an experimental performance (Chukwudeme et al., 2014). Experimental pressure-drop data and oil production profiles was implemented in the program, and the

simulated curves was determined from an automated history matching approach (Prores).

Initially, a water-oil experiment and imbibition displacement were chosen, then experimental core data and recovery test data was implemented into the program, and finally a SENDRA analyses were done.

The initial water saturation was Swi=20% and the residual oil saturation, Sor was calculated by equation (5.4) in section 5.2, and the endpoint relative permeabilities was calculated by equation (3.8) and (3.9) in section 3.3.1. Relative permeability of water (krw) was calculated at Sor, and relative permeability of oil (kro) was calculated at Swi. The initial saturation and endpoint relative permeabilities were used as input data in SENDRA. Capillary pressure curves and fractional flow curves were also constructed based on the output data and relative permeability curves from the history match in SENDRA.