Study On Micro-scale Pore Throat Heterogeneity And Fluid Occurrence Law Of Tight Sandstone Reservoir

In recent years,unconventional oil and gas resources have attracted much attention.The tight sandstone reservoir has dense lithology and strong pore throat heterogeneity,which seriously restricts the exploration and development of tight oil and gas.In this study,the Chang 8 tight sandstone reservoir of Yanchang Formation in an oil area of Ordos Basin was taken as the research object.Based on conventional physical propertytest,scanning electron microscope,casting thin section,particle size analysis,high pressure mercury injection,nuclear magnetic resonance and other experiments,the information entropy theory was introduced to study the micro-scale pore throat heterogeneity and movable fluid occurrence state of Chang 8 reservoir.The main conclusions are as follows:（1）The Chang 8 reservoir is dominated by lithic feldspar sandstone with low comp-ositional maturity.The reservoir space is mainly intergranular pore,dissolved pore and intercrystalline pore.The reservoir physical properties are poor,and the average porosi-ty and permeability are 11.58%and 0.254×10^-3μm²,respectively.（2）Chang 8 reservoir is divided into three categories.The pore throat distribution width of type I,II and III reservoirs becomes narrower in turn,and the main peak gradually shifts to the left.The pore throat radius corresponding to the main peak is 1.0μm～10.0μm,0.5μm～5.00μm,0.01μm～1.0μm,respectively.（3）By introducing the slope extreme value of the pore size distribution curve,the pore throat interval is divided into large pore throat and small pore throat.The overall pore throat development of Chang 8 reservoir in the study area is dominated by large pore throat.The average content of macropore throat in type I,II and III reservoirs is 69.11%,67.75%and57.47%respectively.At the same time,there was a significant positive correlation between macropore throat content and intergranular pore content,R² was 0.8643.（4）The pore throat heterogeneity of Chang 8 reservoir is strong,and the average entropy of pore throat radius is 3.28.Macropore throat has strong heterogeneity and good correlation with porosity and permeability.The heterogeneity of small pore throat is weak,and the correlation with porosity and permeability is poor.（5）The movable fluid saturation of Chang 8 reservoir is 28.65%～60.88%,with an average of 42.87%.Macropore throat is the main occurrence space of movable fluid,and the average saturation of movable fluid reaches 35.68%.The occurrence characteristics of movable fluid in different types of reservoirs are significantly different.The average movable fluid saturation of Class I,II and III reservoirs is 48.88%,44.54%and 36.02%respectively.The connectivity of pore throat have a great influence on the occurrence state of movable fluid.The influence of clay mineral content on the occurrence of movable fluid is the result of the interaction of various clay minerals.（6）There is a negative correlation between pore throat radius entropy and movable fluid saturation.The stronger the reservoir heterogeneity,the less the movable fluid.The influence of pore-throat heterogeneity at different scales on the occurrence of movable fluid is obviously different.Compared with small pore throat,the negative correlation between large pore throat heterogeneity and movable fluid saturation is more significant,which is the key factor affecting fluid mobility.