The HDNS technology can improve near wellbore fluid flow ability in horizontal well by making use of the combined viscosity reduction effects of oil soluble viscosity reducer, nitrogen and steam, and the laterally and vertically mixed function of mass transfer in reservoirs.
Nitrogen is firstly injected, followed by steam. The compressed nitrogen which is trapped by crude oil would expand and gather, the continuous oil would be divided by small nitrogen slug into slug oil, resulting in the break of continuous oil phase, change in flow pattern, reduction in mutual force, and consequently increase in the fluidity of oil. Although nitrogen has low solubility in water and oil, but it can form microbubbles in the reservoir, pushing steam forward, enhancing heat conduction, increasing steam heat carrying ability, and aiding viscosity reduction. Nitrogen made steam front more even, increasing steam sweep volume. After entering into oil reservoir, nitrogen will take up the oil channels in porous media, making original irreducible oil into movable oil, reducing residual oil saturation, and improving oil displacement efficiency.
Oil soluble viscosity reducer, steam and nitrogen work together to reduce surface tension of rock and interfacial tension, improve wettability of rock, plug big pore channels, cause Jamin effect, and ultimately reduce water phase permeability.