Synergistic Effects Between Supercritical CO2 and Diluted Microemulsion on Enhanced Oil Recovery in Shale Oil Reservoirs

Abstract

Summary The development of unconventional liquid-rich reservoirs, supercritical carbon dioxide (scCO2) considers a promising fluid to further improve oil recovery of shale oil reservoirs in and after hydraulic fracturing. However, the scCO2 has some disadvantages to limit its application in hydraulic fracturing, such as ultra-low viscosity, asphaltene deposition and high miscible pressure. Diluted microemulsion (DME) shows great potential as the additive of fracturing fluid to improve the well productivity through strengthening the spontaneous imbibition during the shut-in period after hydraulic fracturing. Therefore, it is essential to further understand the synergic effects between scCO2 and DME at the pore scale. In this study, three soaking sequences are designed and compared, which include only scCO2 soaking, water-scCO2-DME soaking sequence, and DME-scCO2-DME soaking sequence using shale cores from the Lucaogou Formation. Low-field nuclear-magnetic-resonance (NMR) technique are utilized to quantify the oil distribution among different pores in each soaking stage. Furthermore, component change of the produced oil is characterized by the gas chromatography (GC). Notably, T1-T2 spectra are introduced to verify the results of T2 spectra and GC. Results show that DME can replace the oil from small pores into large pores and thus improve the extraction effects of scCO2. The solid-liquid and oil-water/scCO2 interactions determine the adhesion work of heavy components. DME can enhance the heavy component (C17+) mobilization through interfacial tension (IFT) reduction and wettability alteration. Combing scCO2 and DME can effectively improve the mobilization of both light and heavy components of crude oil, and thus achieve a better ultimate oil recovery rate.