Carbonate rock reservoirs comprise approximately 60% of the world's oil and gas reserves. Complex flow mechanisms and strong adsorption of crude oil on carbonate formation surfaces can reduce hydrocarbon recovery of an oil-wet carbonate reservoir to as low as 10%. Low salinity waterflooding (LSW) has been confirmed as a promising technique to improve the oil recovery factor. However, the principal mechanism underpinning this recovery method is not fully understood, which poses a challenge toward designing the optimal salinity and ionic composition of any injection solution. In general, it is believed that there is more than one mechanism involved in LSW of carbonates; even though wettability alteration toward a more desirable state for oil to be recovered could be the main cause during LSW, how this alteration happens is still the subject of debate. The main aim of this work is to gain fundamental understanding of physicochemical mechanisms involved in wettability modification of carbonate minerals in the presence of diluted brines. This work utilises a combination of experimental and theoretical approaches examined carbonate surface characteristics using different surface analytical techniques to investigate the effect of ionic type and ionic strength by examining the same systems from different perspectives. Thereby, this work investigates the influence of different types of salts (NaCl, CaCl2, MgCl2, NaHCO3, and Na2SO4), and their concentration (0-100 mM) on the surface characteristics, such as surface charge, surface wettability, and surface forces of unconditioned and model oil (stearic acid and/or asphaltene) conditioned carbonate (calcite and/or dolomite) surfaces, by zeta potential, static contact angle, and atomic force microscopy measurements. Additionally, this work utilises theoretical calculations to validate experimental results, including the minerals surface energy using the Van Oss Chaudhury Good (VOCG) theory and and colloidal forces at oil-rock brine interfaces using the Derjaguin, Landau, Verwey and Overbeek (DLVO) theory. Our results indicate that the adsorption of potential determining cations (Ca2+ and Mg2+) driven by coulombic interactions results in increased electrostatic attraction, and charge reversal of carbonate surfaces from negative to positive. While adsorption of potential determining anions (HCO−3 and SO24−) increased electrostatic repulsion, it maintained negative zeta potential of carbonate surfaces and increased its magnitude up to 10 mM, before decreasing at higher salt concentration and, therefore, enhanced water-wet conditions. Additionally, the adsorption of model oil components alters the zeta potential of the carbonate surfaces to more negative values, reduces total the surface energy of carbonates, due to reduction in the polar acid-base contribution and, consequently, results in more oil-wet surfaces. Evidence of ion-bridging forces in the presence of Mg2+ cations, and unbinding and stretching of the adsorbed stearic acid layer on calcite surfaces in the presence of SO24− anions, were observed. The suggested mechanism in this study for LSW is surface charge modification of carbonates, due to adsorption of ions, which alters the diffuse layer structure by expansion of the electric double layer (EDL), which may result in wettability alteration toward a more water-wet condition to improve oil recovery, by having a positive contribution to the disjoining pressure and facilitating the formation of a thicker and more stable water film. Therefore, this work utilises experimental and theoretical processes to extend the current knowledge base for dynamic water injection design by determining the effect of ionic composition and ionic strength on surface characteristics of carbonate formation.
|Date of Award||3 May 2019|
- University Of Strathclyde
|Sponsors||University of Strathclyde|
|Supervisor||Ashleigh Fletcher (Supervisor) & Maxim Fedorov (Supervisor)|