Understanding Penetration Behavior of Microemulsions Into Shale Nanopores

Abstract

Abstract Resource shales have low permeability matrix with nanoporous features. At nano scale in situ fluids experience strong fluid-wall interactions and confinement effects. The hydrocarbons recovery from the nanopore network is therefore limited. Microemulsions are known to be effective in stimulating oil and gas production from non-conventional reservoirs, however their mechanisms of action, as well as their ability to penetrate into nanosize pores of shale matrix during an operation such as hydraulic fracturing needs to be investigated. In this paper, we investigate the conditions for the microemulsion droplet penetration into model nanopores, identify the penetration mechanisms and, following their penetration, analyze their interactions with model organic and inorganic walls, and study their behavior in confinement. Molecular dynamics simulation is employed to simulate the behavior of a nanodroplet dispersion facing a solid surface. The model nanodroplets comprise swollen micelles of C12E7 nonionic surfactant with the d-limonene solvent solubilized in their cores. An oil-wet solid surface is modeled using graphite to represent hydrophobic kerogen in shale, and a water-wet solid surface is modeled using brucite to represent hydrophilic inorganic materials in shale. These surfaces are considered to have nanocapillaries with varying sizes, available for the microemulsion penetration experiment. Our results indicate that penetration into capillaries with sizes less than 10 nm is strongly influenced by the wettability of the solid surface. In the case of an oil-wet solid surface the droplets adsorb on the surfaces and hence impact the penetration ability. In the case of a water-wet surface, however, microemulsion droplets effectively penetrate into the nanocapillaries. The droplets are capable of penetrating into the capillaries that are smaller than their own size. In both of these two cases, the solubilized solvent and the surfactant are delivered into a tight nanocapillary network and come into contact with the in situ hydrocarbons. This research can be extended to include ionic surfactants, varying salinity, and more complex solid surfaces to develop the next generation microemulsions with superior performance in enhancing oil production from the unconventional reservoirs.