Medad T. Tweheyo

Surface Characterization of Model, Outcrop, and Reservoir Samples in Low Salinity Aqueous Solutions

The purpose of this study was to characterize model, outcrop, and reservoir samples under low salinity, different cationic valency, and pH ranges. Potentiometric titrations and zeta potential measurements were performed in order to determine the surface properties at different pH. The extent of ion adsorption on the mineral surfaces was found by the difference between the two approaches. X-ray diffraction and cation exchange capacity measurements were also performed. All samples except calcite showed high, negative zeta potentials in fresh water at pH > 6, followed by NaCl, low salinity solution (LSS), and solutions with divalent cations. The effect of ion valency on the zeta potential at low pH values was not prominent. Above pH 8, H+ and OH− were not potential determining ions for samples that contained calcite and dolomite more than 1.5%. Above pH 8, the presence of carbonates in outcrop and reservoir samples significantly affected the zeta potential by reversing the charge in divalent solutions. The cation exchange capacity of predominant quartz, mixed quartz-clay-carbonate, and quartz-carbonate sandstones was 0.0, 0.2, and 2.2 meqv/100 g, respectively. The compositional differences between the samples were also reflected in the point of zero charge (pzc) values. Samples containing more than 1.5% carbonates had pzc in the range of pH 8–9, while samples with small fractions of carbonates or without carbonates had pzc values in the range of pH 2.9–3.3.

Interfacial Tension Measurements Between Oil Fractions of a Crude Oil and Aqueous Solutions with Different Ionic Composition and pH

Dynamic and equilibrium interfacial tensions between crude oil fractions and aqueous solutions of various compositions and pH were measured. The basic oil components seemed to determine the interfacial tensions at pH 2, while the non-dissociated and dissociated acidic components governed the interfacial tension at the natural pH and pH 9, respectively. The ionic composition of the aqueous phase influenced the degree of dissociation of the acidic components at pH 9: Na+ ions in the aqueous phase promoted dissociation of the interfacial acidic components (compared to pure water), while Ca2+ ions resulted in complexation with the dissociated acids and most likely formation of stable interfacial films. The amount of Ca2+ determined which of these phenomena that dominated when both ions were present in sea water solutions. Generally, the interfacial tensions of the oil fractions were lower when measured against the high salinity aqueous solutions than against the corresponding low salinity solutions.

Electrophoretic Measurements of Crude Oil Fractions Dispersed in Aqueous Solutions of Different Ionic Compositions—Evaluation of the Interfacial Charging Mechanisms

Electrophoretic measurements were carried out as a function of pH for various fractions of an acidic crude oil dispersed in pure water and low salinity solutions of NaCl and CaCl2. The acidic and basic components were selectively extracted from the crude oil in order to elucidate their importance for the interfacial charges and implicitly the zeta potentials. Ionization of basic and acidic components at low and high pH, respectively, could not fully account for the obtained zeta potentials. The results could, however, be explained by additional consideration of interfacial hydroxide ions, coming from the water phase.

Sample Mechanistic Modeling of Alkaline Surfactant Polymer Flooding for Snorre Field from Core-scale to Larger Scale of One-Spot Pilot

A considerable amount of oil resides in the Snorre reservoir in the North Sea. The impact of low-salinity-water flooding was investigated from core-scale using coreflood tests to larger scale of one-spot pilot using Single-Well-Chemical-Tracer (SWCT) tests before. Since the results showed a negligible amount of oil recovery, the alkaline/surfactant/polymer (ASP) flooding has been selected after the evaluation of the feasibility of all possible chemical enhanced oil recovery (EOR) methods based on the reservoir conditions. The potential of ASP has been evaluated through mechanistic modeling from core scale to large scale of one-spot pilot using SWCT test. First, the mechanistic modeling of ASP coreflood has been performed to make sure about the proper propagation of alkali, in-situ surfactant (soap), and surfactant. Second, the ASP injection has been also evaluated at larger scale of one-spot pilot using SWCT method. Mechanistic modeling of ASP flooding is highly sophisticated because of the complicated ASP phase behavior and the reactions that affect the process. Almost all effective reactions have been taken into account. Although low-salinitywater flooding as a standalone method could not improve oil recovery, the effect of low-salinity-water on the ASP efficiency has been studied to design and optimize the ASP method. Keywords— ASP flooding, Single-Well-ChemicalTracer (SWCT) method, low-salinity-water flooding, oil saturation.