Mass and Momentum Transfer Considerations for Oil Displacement in Source Rocks Using Microemulsion Solutions
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Existing strategies for hydrocarbon extraction have been designed primarily based on macroscopic properties of fluids and rocks. However, recent work on tight formations and source rocks (such as shale) revealed that the fluid properties and phase change of the hydrocarbons stored in the lower end of the pore size distribution inside the organic nanopores deviate significantly from their bulk phases in the large pores. The cause for such deviations is primarily the presence of strong fluid-wall molecular interactions in the nanopore. Organic nanopores, in source rock, store more hydrocarbons than those pores in a conventional reservoir for the same pore volume because nanopore confined hydrocarbons are more compacted and denser than the bulk phase. However, the recovery factor from these pores were reported to be considerately lower. Surfactants, introduced in the form of micelle or microemulsion, have the potential to increase the recovery. Whereas the transport behavior of micelles and their adsorption on solid walls are well-established, the role of microemulsion on the recovery of hydrocarbons under confinement remains poorly understood. In this work, molecular dynamics (MD) simulations were employed to investigate the two-phase flow in kerogen nanopores containing oil, water, and a microemulsion droplet. A slit-shaped pore was modeled representing the organic nanopore, and a mixture of hydrocarbon was chosen to represent the oil phase. Initially, the microemulsion droplets containing nonionic surfactant dodecylhepta(oxyethylene)ether (C12E7), swollen with solvent (d-limonene), were introduced to the water phase. We showed that the droplets were dispersed under the strong molecular interactions existing in the nanopore space. Subsequently, both the solvent and the surfactant components played essential roles in displacing the oil phase. The surfactant molecules were deposited at the interface between the aqueous phase and the oil, thereby reducing the interfacial tension. The solvent molecules, originally solubilized in a microemulsion droplet, penetrated the oil film near the pore walls. Those solvent molecules were exchanged with the adsorbed oil molecules and transformed that portion of oil into free oil for enhanced recovery. In addition, we considered the Couette flow of water near the organic wall with a film of oil, and found that the oil phase, which consisted of free and adsorbed molecules, could be mobilized by the viscous force caused by the flowing water. Hence, the chemicals introduced by the water mobilized both the free oil and a portion of adsorbed oil inside the oil-wet pores. However, there existed a slip at the oil/water interface which inhibited the momentum transfer from the water phase to the oil phase. When the surfactants were present at the interface, they acted as a linker that diminished the slip at the interface, hence, allowing the momentum transfer from the water phase to the oil phase more effectively. As a result, the fractional flow of oil increased due to the presence of both the surfactant and the solvent. At the final part, we extended our study from a single channel to three-dimensional (3D) kerogen pore network, where the pore sizes were less than or equal to 7 nm. The MD results showed that the dispersed microemulsion droplets also mobilized and displaced the oil present within the kerogen pore network. The results of this work are important for our understanding of flow and displacement under confinement and its application to oil recovery from source rocks.
