Characteristic And Quantitative Evaluation On Remaining Oil Distribution In Tight Sandstone Reservoir

As an important global unconventional oil and gas resources,tight oil exploration and development has been extensively studied recently.Tight sandstone reservoir possesses the characteristics of poor physical property,complicated micro-pore structure,and heterogeneously seepage,which deviates much from conventional reservoir.In this study,micro-pore structure,distribution of micro residual oil,displacement mechanism and the macroscopic and microscopic distribution of remaining oil have been conducted.Based on the fine description of tight sandstone reservoir characterization,many technical methods,such as scanning electron microscopy(SEM),field emission scanning electron microscope(FESEM),cast thin section,image pore,high pressure mercury,nano CT scanning,T2 NMR spectra,NMR core water displacement experiment,and real sandstone micro model experiment of water displacing oil etc.,are applied to study the Ycharacteristics of anchang Group Chang 8 reservoir.The following results and conclusions are being exposed.In this study,The classification system of tight sandstone pore throat was established,and the classification of micro-pore throat characteristics was evaluated quantitatively.The rock properties(such as porosity,and permeability),pore type(such as facial porosity,and average pore throat radius),pore structure(such as mercury pressure curve classification feature,separation coefficient,maximum pore throat radius,mercury withdrawal efficiency,and the relationship between the peak value of the pore throat and mercury)were investigated by methods of scanning electron microscopy,field emission scanning electron microscope,cast thin section,image pore,high pressure mercury technology,respectively.It has been found the physical property of tight sandstone reservoir and the correlation with the characteristic parameter of pore throat were poor.The throat size can control the seepage characteristics of the reservoir in great extent.Sorting coefficient,variation coefficient,median pressure,median radius,displacement pressure,maximum pore d iameter have great influence on the permeability,while the maximum mercury injection saturation,mercury withdrawal efficiency,mercury withdrawal saturation and pore throat ratio have more obvious influence on porosity.The correlation between the pore throat parameters and separation coefficient and the efficiency of mercury withdrawal is not good,while the correlation of mercury withdrawal efficiency is not good.Based on the properties,pore type and pore structure,the pore throat classification system of Yanchang Group Chang 8 reservoir was established.The classification was divided into three parts.The distribution law of the remaining oil in the tight sandstone reservoir was revealed.On the basis of classification system of pore throat in in the tight sandstone reservoir,the experiments of nano-scale CT-scanning,nuclear magnetic resonance(NMR)were conducted to analyze the production degree of water drive hole throat,three dimensional structure of micro nano-scale pore and throat,pore throat configuration relationship,and the percentage of movable fluid.Herein,a quantitative evaluation on the distribution of microscopic residual oil in tight sandstone reservoir was eveluated by combining the core water displacement NMR experiments and real sandstone micro model of water flooding experimental results.The research shows that the percentage of movable fluid in tight sandstone reservoir is mainly distributed between 29.91% and 79.97%,with an average of 56.78%.the porosity of movable fluid is mainly distributed between 0.83% and 8.65%,with an average of 3.92%.Under station of saturated formation water,core samples of nuclear magnetic resonance T2 spectrum shows bimodal characteristics,individual shows multiple kurtosis characteristics.The correlationship between movable fluid percentage and properties is bad.The distribution of microscopic remaining oil depends on the primary pore and secondary pore development degree,and the remaining oil mainly distributes in the relatively narrow pore throat in the reservoir of developed intergranular pore.The remaining oil mainly distributes in the small range among the corners in the reservoir of developed intergranular pores and secondary pores.A set of prediction techniques for residual oil distribution in tight sandstone reservoirs was proposed in this thesis.The real standstone microscopic model and the single well,the displacement pressure system of typical well group and production dynamics,the technology of predicting the microscopic rema ining oil distribution and macro type were combined to established a different model,to study the difference between the water flooding effect and remaining oil distribution caused by plane and vertical heterogeneity the research shows that the combinatio n model can well simulate the effect of well pattern and the displacement pressure system on the oil displacement effect,and the effects of vertical and horizontal heterogeneity on water displacement efficiency and sweep coefficient.The starting pressure is small in the high permeability model while the injected water volume reached at about 1 times,2 times and 3 time,and the required displacement pressure is relatively small.Water flooding effect will be better,and the different injection volume of water displace oil increase rate is higher.The better of micro-pore throat structure,the higher the displacement efficiency will be.The increasing displacement pressure is not good for improving oil displacement efficiency.When the displacement pressure increases to a certain degree,the displacement efficiency can be reduced if the pressure continues to increase.