Understanding Clay-Oil-Brine Interactions at the Nano-scale:
Implications for Low-Salinity Enhanced Oil Recovery

Abstract

The global crude oil consumption has sharply increased for half century; consequently, there has been a depletion of reserves and currently producing oil fields. Increasing the oil supply for the world markets can be achievable by either developing new oil reservoirs or by improving the recovery of current fields by means of enhanced oil recovery (EOR) technologies. The former choice requires new installations and infrastructure; resulting in high capital costs, environmental impact and the achievement of a productivity of only 30 to 50% of the original oil in place (OOIP). EOR techniques on the contrary do not require large capital costs and can greatly enhance the amount of recoverable oil, therefore extending the life of mature oil reservoirs. Low-salinity enhanced oil recovery (LSEOR) is one of the most interesting EOR methods because it is environmentally friendly, low cost, and has been proven to be able to achieve great effectiveness. It is a well-known fact that low salinity enhanced oil recovery (LSEOR) results from altering the wettability of the oil reservoir towards a more water-wet state. However, many theories exist regarding the determining fundamental mechanism controlling its effectiveness, with up to 17 having been invoked in the literature. Nevertheless, gaining this fundamental knowledge is crucial to increase the application of LSEOR in real-world conditions. In this work, we aim to understand how different chemical parameters (pH, concentration, cation type, ionic strength, Na+:Ca2+ ratio) of bine, clay, and model oil affect the wettability of kaolinite (and pyrophyllite) at the nano and microscopic scale. Kaolinite having been selected because of it is ubiquitous presence in the pore space within sandstones reservoirs, and because clay minerals have been singled out as crucial to LSEOR due to their well-known chemical reactivity. To this end, various techniques were utilized to study wettability and proxy of wettability of clay surfaces, including chemical force microscopy, thermogravimetric analysis and mass spectrometry, contact angle measurement, and, to a lesser extent, environmental scanning electron microscopy and Fourier-transform infrared spectrometry. The main parameters that influence wettability are: a) surface charge of kaolinite and pyrophyllite, b) cation type, significantly Ca2+ which plays a crucial role in increasing oil adhesion through cation bridging, rather than monovalent cation (Na+), c) concentration, increases of Ca2+ concentration directly relates to more oil adsorption, whereas changes of Na+ concentration presents only a small effect on the amount oil adsorption, d) pH which controls the protonation state of the polar molecules (decanoic acid) as well as the charge of clay surface and can lead to increases (or decreases) in the amount of oil sorbed, e) type of functional group in oil components. The main conclusion of the theses is that surface complexation and cation bridging are the most important mechanisms in controlling oil sorption (adhesion) but at some conditions the electrical double layer effect can also play a crucial role.