A molecular study of viscosity-causing mechanism and viscosity reduction through re-emulsification for Jimsar shale oil

Abstract

The high viscosity caused by oil–water emulsification and the lack of clarity on the viscosity-causing mechanism have seriously affected the development of Jimsar shale oil. In this paper, the mechanism behind the increase in viscosity due to emulsification for Jimsar shale oil is revealed at the molecular level, using high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with Electrospray ionization (ESI) technique. The high amount of N1 compounds (carbazoles and indoles) in shale oil combine with water through the N–H bond with the pyridine ring adsorbing formation water, which enhances crude oil's emulsification and water-binding ability. A stable W/O emulsion was formed with water and significantly increased the viscosity of the formation fluid. DFT calculations showed that the interaction energy (Eint) of anionic surfactant AS-1 and AS-2 with polar macromolecules was −258.70 and −77.03 kcal/mol, respectively. The interaction energy of these two systems was much lower than that between H2O and polar macromolecules (-50.87 kcal/mol). It indicated that AS-1 and AS-2 could penetrate the oil–water interface and form stronger interactions with polar macromolecules. Thus, the W/O emulsion is in a reversed-phase, and the viscosity is reduced. The laboratory results showed that the viscosity reduction rates (VRR) of AS-1 and AS-2 due to re-emulsification were greater than 94% at 30℃. The O/AS-1/W and O/AS-2/W emulsions formed during re-emulsification have pseudoplastic fluid flow characteristics and are relatively stable. The emulsions also exhibit spontaneous destabilization and water separation properties, lowering the cost of oil–water separation of produced liquid. Through weak dipole–dipole interaction, CH-π type interaction, and some weak interactions (O∙∙∙N, O∙∙∙S, O∙∙∙H), AS-1 and AS-2 make the space between crude oil and emulsion components more open and inhibit the aggregation of polar macromolecules furthering the viscosity reduction effect.