Control Mechanism Of Water-gas Distribution To Pore Structure And Its Evaluation In Low-permeability Reservoir

In recent years,the development of low-permeability and tight sandstone reservoir has become an important area of exploration and development in China.Huagang Formation is characterized by low-permeability tight sandstone reservoir,which is rich in oil and gas resources in Xihu Sag,East China Sea.The tight sandstone reservoir in Xihu Sag is characterized by serious heterogeneity,high water saturation,low resistivity,and complex gas-water relationship.Because of these characteristics of tight sandstone reservoir,it's hard to do the evaluation.The petrology,physical properties,pore structure and the water-gas distribution of A,B and C were systematically investigated by integrated use of petrological,mineralogical,and geochemical methods.The characteristics and controlling factors of Huagang Formation were identified.The author analyzed the control mechanism of pore structure to water-gas distribution,clarified the response mechanism of parameters of Archie's formulas to pore structure,and established the evaluation method of gas-bearing properties based on characteristics of pore structure.Then its accuracy is verified by evaluating the gas-bearing properties of Huagang Formation.Huagang Formation with high compositional maturity and moderate structural maturity is dominated by feldspathic detritus quartz sandstone and mixed with a small amount of feldspathic detritus sandstone.The results showed that there were four types of pores:primary intergranular pores,intergranular dissolution pores,intragranular dissolution pores and micropores within clay aggregates.The contribution of intergranular dissolution pores and intragranular dissolution pores to the total face rate is over 86.13%which are mainly composed of secondary pores formed by feldspar dissolution.The initial physical properties are controlled by source and sedimentary conditions.Compaction is the main factor reducing the porosity of the reservoir,but the cementation is the key to the densification of reservoir and dissolution is the main factor increasing the porosity.Based on the data of high-pressure mercury intrusion(HPMI),nuclear magnetic resonance(NMR)and thin section observations,and using bimodal Gaussian density function to analyze the pore structure parameters,the reservoirs were divided into four types by fitting parameter?.Combined with centrifugal NMR,author clarified the control mechanism of pore structure to water-gas distribution.Logu₂(mid-value of large pore peak)and?*W₂(porosity of large pore peak)are the main factors controlling the microcosmic gas-water distribution.With the increase of centrifugal velocity,the gas saturation decreases,W₁(ration of small pore peak)increase,and W₂(ration of large pore peak),logu₂(mid-value of large pore peak),logu₁(mid-value of small pore peak),log?₂(standard deviation of large pore peak),log?₁(standard deviation of small pore peak)all decrease.The bound fluid resides in small pores and some pores which are controlled by fine throat.On the base of the classification of pore structure and the experimental data of rock electricity,it is found that the rock-electric parameters corresponding to different pore structures are quite different.The a(lithology coefficient)and m(cementation exponent)changed greatly,with a increasing from 0.64 of type I to 1.85 of type IV,m decreasing from 2 of type I to 1.37 of type IV.But the variation of b(lithology correlation constant)and n(saturation index)are small.This paper established the calculation formula of?with AC,DEN and GR,and identified the type of pore structure in the whole wells.Then the gas saturation of the reservoirs was calculated according to the different pore structure with various rock-electric parameters,and the evaluation of gas-bearing properties of Huagang Formation was completed based on characteristics of pore structure.According to the identification and calculation,Huagang Formation is mainly composed of type II and type III,and a small amount of type I locally.Compared with the water saturation test of sealing core,the validity of the method was verified.It was found that the water saturation which calculated by the classification of pore structure is more accurate than that by the conventional method,and the error is less than 8.35%,which is proved that this method is feasible and effective.Based on the gas-water profile,it is found that the gas-water distribution of Huagang Formation in Xihu Sag is controlled by tectonic characteristics and pore structure.