Research on Mechanism of Oil Displacement with Natural Gas Based on Nuclear Magnetic Resonance (NMR) Technology

Aiming at the problem of poor effect of waterflooding in ultra-low permeability reservoirs in Ordos Basin, a study on enhanced oil recovery (EOR) technology by natural gas drive is conducted with the Zhenbei C8 reservoirs as the research object. In view of the problem of unclear understanding of the microscopic dynamic process and oil displacement mechanism of natural gas drive in ultra-low permeability reservoirs, oil displacement tests with water-drive, gas-drive, and gas-foam-drive are carried out respectively by using the microscopic visualized oil displacement device based on simulation model of glass etching, so as to study the remaining oil type and the law of producing remaining oil after starting water-drive with different media (natural gas, natural gas foam). At the same time, the diameter limits of pores-throats with water-drive and gas-drive producing reserves in ultra-low permeability reservoirs are clarified on the basis of the NMR technology. The research shows that the swept area of natural gas drive is smaller than that of water drive, but the oil displacement efficiency can be improved by dilating, reducing the viscosity and extracting the crude oil in the area where it flows (or percolates). Natural gas-foam flooding increases the injection pressure by means of foam, and can divert the gas, thus expanding the microscopic swept area and slowing down the gas channeling at the same time. The radius of minimum pore-throats with water-drive producing reserves in ultra-low permeability reservoirs is 0.1 μm. The radius of available pores-throats with gas-drive reserves is two orders of magnitude smaller than that with water drive, and the radius of those with gas-foam drive is one order of magnitude smaller than that with water drive. The research results provide a new way for further enhancing oil recovery of related reservoirs.