Abstract
Enhanced oil recovery (EOR) potential in unconventional oil reservoirs looks promising but remains largely elusive because the industry is just beginning to realize that primary depletion may recover no more than 7-9% oil-in-place and there is no collective consensus on best EOR practices for tight oil. Moreover, vital scientific understanding of rock-fluid interactions in these formations is presently limited. The objective of this work was to experimentally demonstrate how rock-fluid interactions in nanofluid enhanced oil recovery (EOR) process can be probed at the nanoscale in a rapid and efficient manner.
In this study, atomic force microscopy, a high-resolution scanning tool, was used to characterize adhesion changes which directly relates to the surface energy released in process of wetting. Dispersions of 8-nm sized silicon dioxide nanoparticles were used as nanofluids. Freshly cleaved mica was to mimic shale-like mineral substrates while chips from Tuscaloosa marine shale were used as representative of composite rock surfaces. Methyl-terminated and carboxylic acid-terminated tips were used to represent non-polar and polar oil functional groups. Contact mode chemical force mapping was used for measurements of adhesion at the nanoscale. Each experimental run was completed for a 5 x 5 micron sample area for a duration of 30 min.
Force mapping results showed that adding nanoparticles to the liquid media reduced the adhesion force on the order of pN (10-12 N) to nN (10-9 N) due to reduction in non-electrostatic and electrostatic forces as well as adsorption of nanoparticles onto rock substrates. This finding is consistent with zeta potential measurements and SEM images which reveal that nanoparticles irreversibly adsorb onto phyllosilicate mineral surfaces and improve the wettability by rendering surfaces negatively charged and repelling oil molecules. These studies demonstrate that adhesion measurement can be used as an indicator of nanoscale wettability for rapid screening of different EOR processes in tight oil reservoirs. In addition, it encompasses surface chemistry, material science and core petroleum reservoir engineering for understanding fundamental rock-fluid interactions.
