Study Of Stress Percolation Patterns In Fractured And Crushed Carbonaceous Shale Of Longmaxi Formation,sichuan

Examining the stress percolation characteristics of shale under cyclic stress perturbation is crucial to the development of unconventional natural gas resources in coal seams,especially the permeability performance of fractured and crushed shale in the mining area is directly affecting the gas safety at the working face.Therefore,this thesis takes Longmaxi Formation carbonaceous shale as the experimental object,prepares fractured shale and crushed shale,and designs the percolation experiments of fractured and crushed shale under cyclic loading stress and cyclic pore pressure,starting from the comprehensive action law of effective stress and pore pressure on permeability,so as to obtain the stress percolation characteristics of shale under cyclic stress perturbation.The main research results of this thesis are as follows:(1)The effect of effective stress and pore pressure on the fracture structure of shale is an important factor in the variation of shale permeability: under the conditions of fixed perimeter pressure and fixed average effective stress,the permeability of three types of gases(nitrogen,methane and carbon dioxide)in fractured shale has the same trend,all showing a "U" shaped trend;under the conditions of constant inlet effective stress,the permeability of fractured shale decreases with increasing inlet pressure.Under the condition of constant inlet effective stress,the permeability of fractured shale decreases with increasing inlet pressure;when the effective shear stress is kept constant,the permeability of fractured shale decreases with increasing average effective positive stress;when the average effective positive stress is kept constant,the permeability curve shows a gradual increasing trend with effective shear stress.Therefore,the permeability can be increased by injecting nitrogen and carbon dioxide into the coal formation to increase the air pressure to cause the permeability breakthrough of the rock.(2)The permeability of fractured and crushed shale under cyclic loading stress shows a negative exponential relationship with effective stress;with the increase of cycle number,the permeability of fractured and crushed shale decreases as a whole;the cycle number has a similar effect on the absolute stress sensitivity and relative stress sensitivity of fractured and crushed shale,the smaller the cycle number,the stronger the stress sensitivity and relative stress sensitivity of fractured and crushed shale;at the same time,the smaller the cycle number,the greater the permeability damage rate and irreversible permeability damage rate of fractured and crushed shale.The smaller the number of cycles,the stronger the stress sensitivity and relative stress sensitivity of fractured and crushed shales;meanwhile,the smaller the number of cycles,the greater the permeability damage rate and irreversible permeability damage rate of fractured and crushed shales;the larger the grain size,the greater the permeability damage rate and irreversible permeability damage rate of crushed shales,and also the greater the initial stress sensitivity.The overall effect of cyclic loading stress on the structure of crushed shale is compaction,but the fractured shale has the phenomenon of permeability enhancement under cyclic loading stress.Therefore,cyclic stress can be applied to the coal formation through reasonable operations to achieve increased permeability.(3)The permeability of fractured and crushed shales under cyclic pore pressure tends to decrease with increasing number of cycles;the permeability damage rate and irreversible permeability damage rate of fractured and crushed shales decrease simultaneously with increasing number of cycles;the permeability damage rate and irreversible permeability damage rate of fractured shales are less affected by the number of cycles than that of crushed shales;the variation of uncaused permeability rate of fractured shales with increasing number of cycles is larger,and the variation of uncaused permeability rate of crushed shales with increasing number of cycles is smaller.The variation of the uncaused permeability rate of fractured shale with increasing number of cycles is basically insignificant;there is no absolute magnitude relationship between the pore pressure sensitivity of fractured shale and crushed shale,but in general the pore pressure sensitivity of both decreases with the increase of pore pressure.The cyclic pore pressure path aims to simulate the effect of air pressure on shale permeability in gas enrichment and extraction cyclic operation,and the results show that the increase of pore pressure will impede the flow of gas in shale,and the permeability of shale can be improved by increasing air pressure through gas injection and unloading pressure from surrounding rock.This thesis has 66 figures,6 table,and 94 reference.