Features Of Low Porosity And Permeability Reservoir Of The Paleogene Pinghu Formation In Xihu Depression

Based on the techniques of blue epoxy resin-impregnated thin sections,ESEM,porosity and permeability test,NMR,high pressure Hg injection,sensitivity test,water flooding gas experiment,productivity simulation experiment etc.,this papers studies the features of low porosity and permeability reservoir of Pinghu Formation in Xihu depression.The aim is to discuss low porosity and permeability reservoir’s features of the porosity and permeability,micro-pore texture,percolation,productivity,and controlling factors of the reservoir development,and provide some scientific basis for the reasonable highly-effective development of the gas pools.The conclusions are following:(1)The physical properties of the reservoir in Pinghu formation of A and B zone are better than that of X and Y zone.The reservoir with poorer reservoir physical property in A and B zone is reported as a typical tight reservoir with low porosity and low permeability or the extra-low porosity and extra-low permeability reservoir.The gas reservoirs in X and Y zone are typical tight reservoir with low porosity,low permeability,strong hydrophilicity and relatively high initial saturation of irreducible water.The reservoir physical properties of T zone is poor,it is a typical tight reservoir with low porosity and low permeability or the extra-low porosity and extra-low permeability reservoir.(2)The reservoir capillary of Pinghu formation in A and B zone is mainly high mercury injection saturation-low displacement pressure-bold skewness-well sorted,which shows that the reservoir storage space is large,and where the fluid flows easily.The reservoir capillary of Pinghu formation in A and B zone is mainly low mercury injection saturationlower displacement pressure-slightly bold skewness-better sorted,which shows the small reservoir space and lower permeability.(3)The NMR experiments to the reservoir of Pinghu formation in B zone shows a large difference of the initial saturation of irreducible water,ranging from 34.61% to 61.2%.Most of the reservoirs with a initial saturation of irreducible water of 50% are reported as the conventional low permeability reservoir,and a small part of the conventional low permeability reservoir are low porosity and slightly tight reservoir with a low permeability type Ⅰ or Ⅱ.The reservoir sensitivity experiment analysis shows that alkali sensitive type of A zone are lower flow rate sensitivity,low-medium salt sensitivity,non-low acid sensitivity,low-medium alkali sensitivity,and the type of A zone are non-flow rate sensitivity,low salt sensitivity,non-acid sensitivity and low alkali sensitivity.(4)Water flooding gas experiment and productivity simulation experiment results show that: Anhydrous recovery accounted for a large proportion of the ultimate recovery,and therefore should pay full attention to the gas exploit before water breakthrough in the reservoir.Comprehensive analysis that: Gas reservoirs with permerbility > 10 mD is conventional reservoirs,10-1mD is conventional low permeability reservoirs,1-0.5mD is type I sightly tight reservoirs with low permeability,exploration proved that gas reservoir with a permeability over 0.5mD has a natural capacity;gas reservoir with a permerbility ranging 0.5-0.1mD is type II slightly tight reservoirs with low permeability,is extra-low-low porosity and low permeability,strongly hydrophilic reservoir,and can get a certain capacity after fracturing.Gas reservoir with permerbility lower than 0.1mD is a tight reservoir with extra-low porosity and extra-low permeability and exceptionally strong hydrophilicity,which may not get an industrial capacity under this stage even after fracturing.(5)The development of low porosity and permeability reservoir is affected by deposition,diagenesis and other factors: Provenance and sedimentation direct control of reservoir compositional maturity and textural maturity,indirectly control of reservoir properties,is the congenital factors of the development of low porosity and permeability reservoir.Diagenesis is acquired controlling factors of reservoir development,significantly affect the quality of the reservoir,which compaction,cementation and dissolution are the main diagenesis influencing the porosity formation and evolution of reservoir sandstone.Compaction and cementation is typical destructive diagenesis,and dissolution is the main controlling factor of the relatively high-quality reservoir development in the low porosity and permeability reservoir in Xihu Depression.