Water-based nanofluid-alternating-CO_2 injection for enhancing heavy oil recovery: Considering oil-nanofluid emulsification

摘要

The efficiency of CO_2 water-alternating-gas (WAG) flooding is highly limited in low-permeability heavy oil reservoirs due to the viscosifying action of W/O emulsification and high mobility contrast between W/O emulsion and CO_2. Here we examine an enhanced oil recovery (EOR) process which involves water-based nanofluid-alternating-CO_2 (NWAG) injection, and investigate its underlying mechanisms governing the flooding efficiency. Firstly, the bulk rheology and interfacial properties of the oil-nanofluid and oil-water emulsions were tested. Then, the interaction of silica nanoparticles (NPs) and heavy oil was analyzed by scanning electron micrograph (SEM) after the carbonization of emulsions. Finally, core flooding tests were conducted to examine the NWAG flooding efficiency and its underlying mechanisms. Results showed that the bulk viscosity of oil-nanofluid emulsion was decreased by 84.39% compared to oil-water emulsion, and part of the emulsion was changed from W/O to O/W. Meanwhile, the addition of silica NPs enhanced the interfacial adsorption of crude oil heavy components with interfacial activity (e.g., resin and asphaltene). However, the self-assembly structure of the oil-nanofluid interfaces was destroyed due to the interaction of silica NPs and heavy components, which was confirmed by the reduction of the interfacial viscosity and the broken layer of heavy components after carbonization. During the core flooding experiments, NWAG injection could reduce the displacement pressure by 57.14% and increase oil recovery by 23.31% compared to WAG injection. Meanwhile, the mass content of middle components (C_(10)～C_(16)) in produced oil by CO_2 flooding process of NWAG injection was 9.55%, which was much lower than that by WAG injection (15.21%) and that in crude oil (22.69%), indicating that more of middle components were extracted by CO_2 and the interaction between crude oil and CO_2 was improved after oil-nanofluid emulsification. This work could provide a new insight into the high-efficiency exploitation of low-permeability heavy oil reservoirs.