Wettability modification of oil reservoir rocks studied by atomic force microscopy (AFM)
Creators: name: Yesufu-Rufai, Sherifat Omokhuwa
Publisher: name: Imperial College London
Year: 2020

description: In oil/brine/rock systems, oil recovery is influenced by the wetting preference of the rock surface, which is dependent on numerous factors including rock mineralogy, brine chemistry and reservoir temperature and pressure. These factors make the assessment of rock wettability challenging and subjective and extensive analysis is required to obtain reliable data. In this work, the alteration of surface wetting behaviour when the chemical environment is changed was studied in detail. It presents the fundamental characterisation of wettability and its dependence on surfactants and oxidation/reduction agents for minerals in contact with brines of various compositions. The impact of these potential wettability modifiers was ascertained via sub-pore investigations of molecular surface interactions using Atomic Force Microscopy (AFM). The contribution of clay mineral orientation to the low salinity effect, that is the incremental oil recovery obtained during waterflooding with brine of low salinity, was investigated by studying adhesion of an "oil-modified'' probe to the edge surfaces and basal planes of clay minerals in brine of high and low salinity. Novel workflows were developed whereby rock samples were assessed in-situ with minerals in their native orientations, thereby assuring that potential results are transferable and representative of oil reservoirs. The results suggest that the low salinity effect observed in clay-bearing sandstones is enhanced by the charge anisotropy at clay edges. This is owing to the discovery that there is about a five-fold decrease in oil adhesion at clay edges than at the flat basal planes in low salinity brine driven by a change in polarity at the edges as a result of the lower ionic strength. This study enables, for the first time, a clear interpretation of the contribution of clay mineral orientation to wettability alteration during low salinity flooding. The impact of oxidation/reduction conditions on wettability was also investigated by comparing adhesion forces between "oil-modified'' probes and the rock surface before and after the inclusion of a reducing agent; denoting aerobic, oxidising laboratory environments and anaerobic, reducing reservoir conditions, respectively. Findings indicate that the wetting behaviour of air-exposed cores differs from in-situ reservoir rocks considerably as active iron oxides on the surface of air-exposed cores bind with polar oil components, altering the wettability significantly. There was about a 70% decrease in adhesion of polar amine molecules to the rock surface under reducing conditions. This illuminates the detriment of performing laboratory experiments that ascertain the success of potential low salinity flooding protocols or chemical enhanced oil recovery (cEOR) formulations in conditions that are not representative of the reservoir. In addition to salinity and redox conditions, brine chemistry was also established to affect surface interactions as divalent cations present in brine were observed to have an affinity for the sulfonate headgroups of anionic surfactants. At higher surfactant concentrations, calcium ions in brine mitigate the adhesion of the headgroups to the sandstone surface, as these ions preferentially form salt bridges between neighbouring headgroups, effectively enhancing micellization. The consequences of this in cEOR points to the role of divalent ion presence in surfactant loss and the associated economic impact. The outcome of this work establishes AFM as a viable screening tool for assessing the response of rock surfaces to wettability altering conditions, essentially judging the effectiveness of oil recovery procedures before deployment in the field.