Research On Micro-pore Structure And Micro-flow Mechanism Of Ultra Low Permeability Sandstone Reservoir

Micro-flow mechanism of Ultra low permeability sandstone reservoir is different from traditional reservoir because of the poor physical property, fine pore throat and strong pore structure heterogeneity. Micro-pore structure is understood by the numbers and key elements which affect on Micro-flow mechanism are revealed essentially through this study, micro-pore structure characteristics, oil/water micro-displacement characteristics, micro-flow rule and oil/water relative permeability curve characteristics of Chang8, Chang6, Chang4+5 reservoir in Yanchang Formation of Ordos Basin are studied in depth based on the results of high pressure mercury penetration, constant speed mercury penetration, NMR, true sandstone micro-displacement, kickoff pressure gradient test, oil/water relative permeability test and geoanalysis In this dissertation. The main conclusions are as follows:(1) Physical property of ultra low permeability reservoir is influenced by pore and throat attributes. Physical property difference is the reflection of pore structure difference. Contribution value of permeability is affected by the manifold distribution of throat radius, pore structure difference is reflected in the size and distribution of throat radius. Quality of ultra low permeability reservoir is controlled by throat, permeability is contributed by little portion greater throat. In four studied areas, the permeability of samples is higher when the separation factor value distribute within 2.0~3.0 and the variation coefficient value distribute within 0.15～0.3. Capillary pressure curve is mainly influenced by pore in the low input mercury pressure. Variation tendency of capillary pressure curve mainly depends on throat alteration when the input mercury pressure increases to certain extent.(2) Pore structure uniformity coefficient (Ep) can evaluate pore structure inhomogeneous degree preferably. Ep average values of four areas distribute within 1.144～1.607. Distribution curves of Ep present upwarp, smooth and ward triple section.(3) Mobile fluid percent is the comprehensive reflection of micro-attributes of ultra low permeability. Difference of mobile fluid percent of laboratory sample is greater in studied areas for the complex and versatile micro-pore structure. The mobile fluid percent mainly spread within 14.04%～72.15%, average value is 47.38%.(4) The results of oil/water micro-displacement experiment show that experimental models of studied areas display six kinds of displacement passageways, four kinds of displacement types and three kinds of irreducible oil modes. Heterogeneity of micro-pore structure and wettability are the main reasons that affect the oil/water displacement attribute and remaining oil distribution. Displacement pressure only can improve the displacement efficiency to some extent. Inject multiple from 1PV to 2PV impact on displacement efficiency greater. Difference between displacement efficiency and Mobile oil percent which is the upper limit of displacement efficiency indicates the greater potential after waterflood development of ultra low permeability reservoir.(5) Kickoff pressure gradient is the particular performance of fluid flow through porous medium in ultra low permeability reservoir, also is the essential reason why the critical threshold permeability exists. Displacement pressure system established to decrease the critical threshold permeability can contribute to improve development effect.(6) Two-phase flow range of oil/water relative permeability curves is narrow in ultra low permeability reservoir and average value of laboratory sample is 28.56%. Alteration of micro-pore structure and wettability of reservoir can create influences on attribute of relative permeability curve.