Coal multi-scale pore fracture variations and permeability sharp increase characteristics induced by in-situ hydraulic injection
Hydraulic injection in coal is beneficial to improve gas extraction flow.Multi-phase and multi-field true triaxial dynamic seepage experimental system of coal rock was developed,of which,the boundary sealing and three-dimensional stress loading stability of high-pressure seepage test of square coal samples were verified and analyzed.Characteristics of pore-fracture development,coal damage and permeability sharp increase of in-situ coal hydraulic injection af-fected by different stress loading and water injection rate were studied.Brunauer-Emmett-Teller(BET),Barrett-Joyner-Halenda(BJH)and Frenkel-Halsey-Hill(FHH)theoretical mod-els were adopted to quantitatively analyze specific surface area,pore size distribution and pore fractal characteristics.Based on acoustic emission system,coal fracturing damage characteristics was monitored.The results show that hydraulic injection could promote development of frac-tures,thus reducing ultrasonic velocity in coal.After hydraulic fracturing,the hysteresis loop ranges of coal samples rise.Main peak area of pore size expands from 0.7-2 nm to 0.7-30 nm.Meanwhile,total pore volume grows.The percentages of micropore and mesopore volumes of 1 # coal sample increase by 10.57％and 3.06％,respectively.Specific surface areas of three coal samples rise by 276.17％,224.59％and 283.18％,respectively.Fractal dimension D]value decreases while the D2 value gets larger.Pore surface becomes smoother.Coal-breakage water pressure values of three coal samples are 14.49,19.26 and 22.40 MPa,with corre-sponding time being 164.7,107.0 and 205.5 s,respectively.This indicates that under same conditions,the coal breakage pressure of unequal three-direction stress loading is smaller than that of equal three-direction loading.The faster the water injection rate,the shorter the time re-quired for coal breakage.Acoustic emission positioning point of 1 # coal sample is the most(with maximum amplitude being 83 dB),while that of 2 # coal sample is the least.Projec-tion of acoustic emission positioning points are basically consistent with the cracks on XY and XZ end faces of coal sample.After hydraulic injection,permeability of 1 # coal sample in-creases by 6 3.07 times,while permeability of 2 # coal sample rises by 11.91 times.In con-trast,permeability of 3 # coal sample only grows 2.33 times.Changes of porosity,fracture,damage and permeability of each coal sample are consistent with each other.1 # coal sample with unequal three-direction loading and 3 mL/min water injection rate has the best improve-ment effect.
efficient gas extractionhydraulic injectionpore fracturesharp increase in perme-abilityexperimental study
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