Characterization Of Shale Pore And Fracture And Shale Gas Enrichment In Da'Anzhai Member In Yuanba Area

The pore and fracture characteristics of shale are important factors affecting the occurrence and enrichment of shale gas.This paper comprehensively characterizes the pores and fractures of the shale in the Da'anzhai member of the Yuanba area through various experiments such as thin section,micron CT,scanning electron microscopy,uniform temperature measurement,acoustic emission,high-pressure mercury intrusion,nitrogen adsorption,and carbon dioxide adsorption.Based on this,the rule of shale gas enrichment and high production was analyzed,the main understanding is as follows:(1)The pore types in shale are mainly mineral matrix pore,organic matter pore and micro-fracture.According to the quantitative statistics of scanning electron microscope images,the average porosity ratio of inorganic materials in shale is 91.66%,and the average porosity ratio of clay minerals in shale is 51.33%,both of which show that inorganic pores are the main part of shale.(2)The peak pore radius of shale was concentrated in 1?10nm(46.99%)and10?100nm(30.31%),and obvious macropores were developed(15.27%?21.11% on average).The pore characteristics(mainly inorganic pore,medium-macro pore)and the temperature and pressure conditions(high temperature and high pressure)of Da'anzhai member in Yuanba area determine that the occurrence state of shale gas is dominated by free gas.(3)Structural fractures developed in shale include structural shear joints and structural micro-fractures,and mainly structural micro-fractures.The late yanshanian,early xishan and early yanshanian periods were the main periods for the formation of micro-fractures.(4)According to the study,the micro-fracture porosity in shale is about 20%?30%,which is an important factor for high yield of shale gas.Full-field splicing of rock slices and scanning electron microscopy were used to characterize nano-micron full-scale micro-fractures in shale.The contribution rate of micro-fractures to the total porosity is10%?30%,with an average of 28%,which is close to the proportion of micro-fractures calculated by micron CT experiment(average 23.28%)and the percentage of macropores(21.11% on average).(5)The enrichment and high yield of shale gas in this area are controlled by sedimentary facies,micro-fractures and preservation conditions.The shallow lake—semi deep lacustrine zone organic-rich shale is the basis of shale gas enrichment,the development of micro-fractures is an important condition for shale gas enrichment,The preservation condition is an important guarantee for shale gas enrichment and high yield.The wells YB21-YB11 in the hydrocarbon-rich sag with gentle structure in the west of Yuanba were selected as shale gas-rich and high-yield areas.(6)The Yuanba area has shale pore and fracture conditions and enrichment and high production conditions for the exploration and development of shale gas in the Da'anzhai member.