Features Of Low Porosity And Permeability Reservoir Of Huagang Formation Of Western Sub-depression In Xihu Depression

Based on the techniques of blue epoxy resin-impregnated thin sections, ESEM, porosity and permeability test, NMR, high pressure Hg injection, water flooding gas experiment, productivity simulation experiment, etc., the features of low porosity and permeability reservoirs of Huagang Formation of the two prospective oil-gas-bearing structures (Ningbo31-1and Huangyanl-1) in western sub-depression in Xihu depression are studied. It is aimed to discuss the porosity and permeability, micro-pore texture, percolation, productivity, and their relationships of the low porosity and permeability reservoirs, and provide some scientific basis for the reasonable highly-effective development of the gas pools. The conclusions are as the followings:(1)With the increasing of buried depth, the physical properties of the reservoir are more and more poorer, and the correlation between the porosity and the permeability in these poorer physical property reservoir become worse. In addition, under the combined influence of sedimentary and diagenesis which are mainly feldspar dissolution, the physical property of feldspathic litharenite is the best, the lithic arkose is middle, and the feldspar quartz sandstone is the worst. Furthermore, the differences among the permeability are larger than those among the porosity in different rocks, which suggests that the types of the reservoir space are various and complicated.(2) The most pores of the cores are intergranular solution pore and primary intergranular pore (the percentage of them is3:2) with a small amount of grain dissolve inside hole, molds dissolve inside hole, kaolinite intercrystal pore and other rare micro-fractures. Throat types are dominated by the reduced-neck, the cluster pore throat, and some reduced-neck, flaky and curved lamellar throat.(3)The percolation of the reservoir is affected by several factors, such as the physical property, the pore structure, the original water saturation, the wettability, etc. Among which, the pore structure is the greatest impacting factor. The connected pores of the reservoirs are mostly the pores with low frequency distribution and bulky pore radius. When the gas pools are umder production pressure, the percolation always exist only in the bulky radius throats, there may exit no percolation in tiny throats.(4)The Huagang Formation is a typicaly low porosity and permeability, hydrophilic reservoir. Water flooding gas experiment and NMR show that the original water saturation is relatively higher. In the Water flooding gas experiment, gas relative permeability reduce rapidly with the increasing of the water saturation until the gas relative permeability equals water relative permeability. This characteristic led to the simultaneous production of gas and water soon after the reservoir went into production. At this moment, the efficiency of water drive gas also reduced significantly which is an unfavorable factor for the development of gas reservoir. In addition, anhydrous recovery accounted for a large proportion of the ultimate recovery, As a result, full attention should be paid to the gas exploit before water breakthrough in the reservoirs.(5)The predicted daily gas production of the H5(3538.9m～3562.0m) and H9+H10(3975.lm～3988.lm) Layer of well2, H11Layer of well1(4289.0m～4299.0m), the3th subzone of H7Layer of well4(3962.0m～3977.0m) are all high, and the physical property and pore structure are favorable for the percolation, which shows a favourble gas exploration prospectation.