Abstract: The L-9 Reservoir in Xinjiang is a thin-layer sandstone ordinary heavy oil reservoir with edge and bottom water. Due to the influence of edge/bottom water and CaCl₂ water type, conventional chemical flooding systems are not applicable. To address this, a field test of air-assisted microbial flooding was carried out for enhanced oil recovery. During the test, to curb bottom-water coning and water channeling in high-permeability channels, and to expand the sweep volume of microbial flooding, reservoir fine geological modeling, and tracer monitoring numerical simulation techniques were used to identify preferential migration pathways. It was found that the remaining oil saturation was high on both sides of these pathways. Then, the compatibility of the thickening nutrient activation system (mainly bio-polysaccharides) with the reinjected wastewater on site and formation water was assessed. Also, the activation effect of the system on major oil-producing bacteria and the rule of changes in microbial community structure during activation were studied. The results show that The thickening nutrient activation system without additives shows high viscosity and stability, with a core plugging rate of 93.1% and an enhanced oil recovery of 15.5 percentage points. After adding the activator, the concentration of oil-producing function genes and total bacteria increased by 1-2 orders of magnitude, promoting the degradation of the thickening system. After injecting the thickening nutrient activation system, high-permeability channels were effectively blocked, achieving good oil-increase and water-reduction effects. This research offers useful guidance for the regulation of microbial flooding in heavy-oil reservoirs during the high-water-cut period.

Key words: microbial flooding, heavy oil, thickening nutrient system, edge water, bottom water, CaCl₂ water type