The Fine Reservoir Description And Development Dynamic Analysis About Low/ultra-low Permeability Reservior

Development contradictions in low or extra low permeability reservoir have occurred at its complexity on the distribution of oil and water, micro pore-throat net distribution, flow principles and poor displacement efficiency. So studies about qualitative description of micro pore structure and flow principle are necessary to be done based on the sedimentary and diagenesis and then combining the flow zone character allows a further detailed reservoir description and performance analysis both from macroscopic and microscopic aspects.With the help of cast thin section, SEM, high pressure Hg injection, constant speed Hg injection, oil-water relative flow, NMR and true sandstone micro model for water-oil displacement et.al, deep analysis have been made on the micro pore structure, micro flow character of Chang2and Chang6of Yanchang group at Ordos basin and then on the combination of various flow unit characteristics there have been made analysis about the reservoir fundamental geological feature and coupling mechanism of micro features. Furthermore on the basis of performance dynamic data the dynamic production rules of various flow units have been figured out along the following conclusions:(1) Stratum thickness and tectonics is relative steady at two areas and there develop a few nose-like structures which succeed well one by one. There are also other geological features such as arkosic sandstone, poor reservoir quality, serious heterogeneity and diagenesis.(2) High pressure Hg injection and constant speed Hg injection experiments have found that low or extra low permeability reservoir has many features:complex pore throat structure, strong micro heterogeneity and small discrepancy at pore radius which is averagely small and various then suggesting that reservoir quality is mainly controlled by throat and the configuration relationships among pore and throat. Average throat radius and the numbers of efficient throat at unit volume are the main two parameters mostly affecting reservoir quality.(3) The discrepancy of permeability contribution rate by pore-throat controlling interval is the main factor resulting in the degree of oil and water differentiation at low, extra low reservoir and more serious the discrepancy is, poorer the differentiation is.(4)Experimental data of Oil-water relative permeability, NMR and water-oil displacement demonstrate that low, extra low permeability reservoir has low oil relative permeability, narrow two phase flow interval, strong jam of two phase flow, low moveable fluid saturation and poor waterflood efficiency. Micro pore structure heterogeneity mainly affects the oil-water displacement feature and distribution of residual oil.(5) Under a certain injection pressure, increasing water displacement times and injection pressure can enhance the displacement efficiency. While after flushing for a long time many changes have occurred as follows:①the formation of favorable flow path will further exacerbate micro heterogeneity and additionally increasing pressure just speeds up waterflood in the favorable path which is meaningless for displacement efficiency;②lay minerals will disperse or swell because of water and then block up part of pores, which on the other hand decrease moveable saturation and damage the reservoir with displacement efficiency declining because of the accretion of surface area of clay minerals.(6) There are four kinds of flow units being classified as E、G、M、P at low or extra low permeability reservoir and corresponding to the sequence of flow units above the feature of micro pore structure, permeability, moveable fluid saturation declines gradually one by one. Furthermore, E type flow unit has high initial oil output, high water contention percentage and at the initial stage of water injection the oil productivity rises a lot and with the continuation of water injection water content percentage ascends quickly while the accretion of oil productivity is not prominent; G type flow unit has a moderate initial oil output, moderate water content percentage and at the initial stage of water injection, oil productivity increases slowly and with the continuation of water injection, both water content percentage and oil productivity ascend.(7) Number percentage of current throat is one kind of reflection of the accessibility of the favorite pathway’s formation during the water injection and larger the value is, the easier favorable flow path forms.