| Tight-low permeability reservoirs have the characteristics of low matrix permeability,strong stress sensitivity and small pore throat size,which makes the driving of remaining oil and residual oil in tight matrix very difficult,and the recovery effect is generally poor.In this paper,based on the differential pressure driving conditions of tight-low permeability reservoirs,the study on drivability and driving effect of oil in the matrix has been carried out.The threshold pressure gradient?TPG?of crude oil in different permeability cores are measured by experiments,which provide basic parameters for subsequent research.The effect of reservoir matrix wettability on TPG of crude oil is studied by using global quantitative control of wettability artificial core,it is found that TPG of crude oil decreased monotonously with the increase of wettability index,the lower the core permeability,the larger the variation of TPG of oil with the wettability index.The stress sensitivity test for single phase flow of crude oil in tight-low permeability core is carried out.According to experimental results,a power function decline rule between TPG of crude oil and effective stress in tight-low permeability core is obtained.Based on this,a seepage model considering the influence of effective stress on permeability and TPG of crude oil is established.Based on this model,the effects of effective stress changes on productivity and reservoir pressure distribution during the depletion development process of tight-low permeability reservoir are studied,both ultimate drainage radius and elastic recovery factor of tight-low permeability reservoirs are calculated.The results show that well productivity considering the influence of effective stress on TPG is less than that considering TPG is constant.When initial formation pressure and bottom hole pressure are the same,the higher the overburden pressure is,the lower the well productivity is.The relationship between the average water saturation and the two-phase critical driving pressure gradient in the one-dimensional water flooding process of different permeability cores is studied.It is found that two-phase critical driving pressure gradient increases first and then decreases with the increase of average water,reaches a maximum when the oil water front is near the outlet end.The influence of core wettability on two-phase critical driving pressure gradient is determined,and the non-monotonic variation of the maximum oil-water two-phase critical driving pressure gradient decreases first and then increases with the increase of wettability index is obtained.Considering the existence of oil-water two-phase zone and original oil-bearing zone in the actual reservoir,the flow-integration method is used to calculate the sweeping efficiency of the water injection area of the five-point well pattern in tight-low permeability reservoir.The results show that when permeability is lower than5×10-3?m2,it is difficult to further improve the sweep efficiency by increasing the pressure difference between injection and production.The difference of the critical driving pressure gradient between surfactant with ultra-low interfacial tension and strong emulsifying ability in tight-low permeability cores is compared,it is proved that the critical driving pressure gradient of crude oil in compact-low permeability matrix can be significantly reduced by reducing the oil-water interfacial tension,and the surfactant with strong emulsification ability can block the flow channel.Through the oil displacement experiment of one-dimension cylindrical core with different performance surfactants,it is found that when core permeability is2×10-3?m2?50×10-3?m2,injecting strong emulsification ability surfactant after water flooding can greatly improve the oil displacement efficiency.When core permeability is0.1×10-3?m2?2×10-3?m2,injecting ultra-low interfacial tension surfactant before water flooding has a higher improvement in oil displacement efficiency.Combined with the experimental results and the actual development needs of the reservoir,the applicable range of surfactants with different properties is given. |
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