Shenjia Hydraulics

Release Date:

2023-01-21

The mechanism of nitrogen displacement is closely related to the displacement mechanism of gas‑flooded oil recovery. When gas is used to displace oil, the wettability of the reservoir rock may be water‑wet, oil‑wet, or heterogeneous. Compared with oil and water, the gas phase is non‑wetting. Under the influence of capillary pressure, gas‑flooded oil tends to flow more readily along the pore‑throat network of the rock. Due to its very low viscosity, gas exhibits a more pronounced preferential flow through larger pores and throats, resulting in a relatively low gas saturation at equilibrium. As rock permeability increases, pore‑throat heterogeneity becomes more pronounced; consequently, the gas‑flood recovery factor for high‑permeability rocks is lower than that for low‑permeability rocks. On the other hand, high‑permeability rocks contain a higher proportion of large pores and throats than low‑permeability rocks; under constant displacement pressure, these larger pores are more easily occupied by gas. Thus, the volumetric sweep efficiency of gas flooding in high‑permeability rocks exceeds that in low‑permeability rocks, leading to a higher recovery factor in the former.

Displacement device

　　The mechanism of nitrogen‑flooding displacement—where gas surges through the rock’s large pore‑throat network—determines the macroscopic distribution of residual oil: most of it remains in the relatively small pore‑throat system in its original state. This is the primary reason for the low oil recovery achieved by gas flooding. Meanwhile, owing to liquid‑blocking and gas‑blocking effects, a small fraction of residual oil persists as droplets within the gas‑swept large‑pore‑throat system; the amount of such residual oil depends on the magnitude of these blocking effects. As liquid‑blocking and gas‑blocking intensify, the residual oil increases, and the larger pores become progressively occluded, forcing the gas to enter the smaller pores to displace the remaining crude. Consequently, the resistance imposed by these blocking effects and the resulting gas‑channeling are the fundamental reasons for the narrow sweep efficiency of gas flooding.

　　Nitrogen displacement mechanism, water displacement mechanism

　　The mechanism of waterflooding depends on the wettability of the rock. For oil‑wet rocks, the waterflooding process is similar to gas flooding: injected water preferentially advances along the high‑permeability pathways with lower resistance. Residual oil remaining in these large pores adheres to the inner surfaces of the rock grains as an oil film. Due to liquid‑flow resistance, some residual oil becomes trapped in the throats in the form of droplets or oil columns. Because of the threshold pressure required for flow, waterflooding has little effect on relatively narrow oil‑wet pore channels, leaving the residual oil in its original state; consequently, waterflooding recovery factors are generally low in oil‑wet rocks.

　　The mechanism of nitrogen displacement: The waterflooding process is essentially an imbibition process. Due to capillary pressure, the driving forces in waterflooding significantly influence both the oil‑displacement mechanism and the distribution of residual oil. Under injection pressure, some water flows along high‑permeability pathways with low resistance, displacing crude oil, while other water, aided by capillary forces, advances through the pore network to mobilize oil. There are two primary mechanisms of oil displacement: one involves breakthrough along low‑resistance paths at the centers of the pore channels—similar to the gas‑drive mechanism—and the other entails the forward movement of a water film along the internal surfaces of grains, with newly injected water replenishing the film and stripping oil from the region immediately adjacent to it; this constitutes a stripping mechanism. Depending on the specific pore‑water–oil ratio, the prevailing capillary pressures, and the applied displacement pressures, the patterns of residual oil distribution vary markedly.