Experimental Study On Nitrogen Foam Assisted Gravity Flooding In Fracture Reservoir

Gas assisted gravity flooding is an effective method to exploit the remaining oil at the top of the reservoir,but production rate is very low after the gas channeling occur.Foam assisted gravity flooding technology combines the advantages of gas assisted gravity discharge and foam flooding.The foam first enters the high permeability parts or cracks and accumulates,increases the flow resistance,inhibits the gas channeling and increases the sweep efficiency.In this paper,the interaction between crude oil and nitrogen under high temperature and high pressure were studied by experimental analysis and the related mechanism of nitrogen flooding was analyzed.Then the shape and size characteristics and flow resistance of the foam in the crack were studied under different injection conditions.The effect of nitrogen assisted gravity drive and nitrogen foam assisted gravity drive was contrasted.by two-dimensional visualization experiments and core experiments.The results of the study show that the solubility of nitrogen in crude oil is limited.At100℃and 7.87MPa,the solubility of nitrogen in crude oil was 6sm~3/m~3;Under the same temperature condition,the volume coefficient and viscosity reduction rate increased with the increase of solubility.The interfacial tension decreased with the increase of the pressure.When foam flowed in the fracture,with the increase of foam quality,the number of bubbles decreased,but the size of the bubble increased.The shape of bubbles gradually transformed into polygons.The flow resistance of foam in the crack increased first and then decreased with the increase of foam quality.When the foam quality was fixed,the flow velocity of the fluid had little effect on the shape and size of the foam.As the fluid velocity increased,the flow resistance increased.With the increase of surfactant solution concentration,the number of foam gradually increased,especially the number of small diameter foam increased,the stability of foam gradually increased,and the flow resistance of foam in cracks increased.As the crack width increased,the flow resistance of the foam in the crack decreased,and the effect on the shape and size of the foam is little.In the nitrogen assisted gravity drainage process,with the increase of pressure drop rate and extraction pressure,the oil recovery increased then decreased.As the dip angle increased,the oil and gas gravitational differentiation was enhanced,and the oil recovery was increased.With the increase of gas cap volume,the oil recovery first increased,then decreased and further increased.Nitrogen foam could effectively improve the flow resistance in the fracture,inhibited the gas channeling and increased the sweep efficiency.Surfactant solution could reduce the interfacial tension and improve the displacement efficiency,further improving the oil recovery.