| The pore throat scale of extra-ultra-low permeability porous media reservoir is small,and the capillary force is large,the existence of fluid boundary layer and the threshold pressure gradient and the role of wettability and wettability hysteresis are important factors for multiphase seepage.Clarifying the mechanism of above factors and their influencing characteristics can help improve the understanding of seepage in extra-ultralow permeability reservoirs,which is of guiding significance for formulating reservoir development plans to improve reservoir recovery.In this paper,firstly,the macroscopic and microscopic distribution of wettability in extra-ultra-low permeability reservoir of LPY block target studied area is analyzed.Secondly,the dynamic effect of capillary pressure and its influencing factors in extraultra-low permeability reservoir during water flooding process are deeply analyzed,and the exponential model and power function model of capillary force dynamic effect coefficient with water saturation are established.And then,the relative permeability characteristics and formation mechanism of water-wet,oil-wet and mixed-wet reservoirs were evaluated,and an oil-water relative permeability model considering the initiation pressure gradient,capillary force and end effect was established,and the effect of the displacement pressure drop gradient on the relative permeability curve of the extra-ultralow permeability oil reservoir was studied by using the unsteady relative permeability test method.Finally,numerical model of the extra-ultra-low permeability reservoir in the target study area is developed by combining the results of capillary pressure and relative permeability curves.The main work and conclusions of the paper are given as follows:(1)The characteristics of the effects of displacement pressure difference,reservoir physical properties,fluid properties and wetting phase saturation on the dynamic effect of capillary pressure in extra-ultra-low permeability reservoirs were evaluated.The dynamic effect coefficient of capillary pressure increased with increasing displacement pressure difference,decreasing reservoir permeability,increasing non-wetting phase density,increasing wetting phase viscosity and decreasing wetting phase saturation.The exponential model and power function model between dynamic wetting coefficient of capillary pressure and water saturation were established by dimensionless analysis.The results show that the power function model predicts the results with small errors at low wetting phase saturation,while it is necessary to choose a suitable model for prediction according to the specific situation at medium and large water saturation.(2)An oil-water relative permeability solution model considering the threshold pressure gradient and capillary pressure was developed,and the oil relative permeability increased considering the effect of the threshold pressure gradient and capillary pressure.Relative permeability experiment of unsteady test was carried out in two ultra-low permeability cores in the target studied area of the LPY block,and the effect of displacement pressure drop gradient on the relative permeability curve was evaluated.As the displacement pressure drop gradient increases,the relative permeability of both oil and water phases increases,and the range of co-permeability increases,and the copermeability point moves up to upper and right,and the remaining oil saturation decreases.(3)A mathematical model of one-dimensional displacement oil-water seepage was established considering threshold pressure gradient and dynamic effect of capillary pressure,and the influence of capillary pressure dynamic effect on oil-water two-phase seepage was evaluated.The predicted water breakthrough time of reservoir delay and the reservoir recovery degree is improved at water breakthrough time considering the dynamic effect of capillary pressure,while the dynamic effect of capillary pressure has little influence on the reservoir before water breakthrough.(4)A numerical model of extra-ultra-low permeability reservoir was established considering the dynamic effect of capillary pressure and relative permeability zoning,and the impact characteristics of the dynamic effect of capillary pressure on production prediction ware evaluated,the necessity of relative permeability zoning simulation is analyzed as well as.When the wettability of oil reservoir is water-wet,the predicted breakthrough time of production well delayed and the reservoir recovery increased considering the dynamic effect of capillary pressure,which is due to capillary pressure is the driving force for waterflooding process.Considering the dynamic effect of capillary pressure in oil-wet oil reservoir,the predicted breakthrough time of production well was advanced,and the increase rate of water cut decreases compared with the static capillary force after water breakthrough,and the productivity decrease rate slows down.The prediction of reservoir water saturation distribution,and the characteristics of waterflooding sweep,and productivity of production wells with different distance between production well and injection well,and water breakthrough time by the numerical model of relative permeability zoning is more consistent with the actual seepage characteristics of ultra-ultra-low permeability reservoirs. |
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