Dynamic Evolution Of Coal Fracture Pore During Liquid Injection

Hydraulic fracturing is an effective method for producing coalbed methane(CBM),fracturing liquid has been injected in coal seams continuously,and the formation,propagation and gas permeability of macro-fractures that are formed by hydraulic fracturing have been studied.the coal that may constitute the macro-fracture must be effected by fracturing liquid,and the coal fracture pore is the key factor that affects the production of CBM.Therefore,it's significance to study the dynamic change of coal fracture pore that has effected by injected liquid.However,the previous research of coal fracture pore focuses on the influence of gas medium,and the conventional pore research methods(low pressure gas adsorption,mercury injection,scanning electron microscopy,etc.)could not work on the conditions of simulated in-situ stress and continuous injection,and monitor the real-time evolution.Therefore,the study of coal fracture pore under the conditions of simulated in-situ stress,continuous fluid injection and monitor the real-time evolution has become the key problem to research the dynamic evolution of coal fracture pore.In order to reveal the evolution laws and mechanism of coal fracture pore under continuous injection,this paper combines experimental research with theoretical analysis.A series of in-situ compression experiments under continuous injection liquid conditions and dynamically monitored the raw coal cores using nuclear magnetic resonance methods were carried out.The mechanism of fracture pore dynamic evolution is identified in terms of spherical expansion,and a dynamic evolution model of fracture pore of the coal is established based on pore compression model.The dynamic evolutions of coal fracture pore provide a reference for hydraulic fracturing construction.The main research results of this paper are as follows:(1)LI-NMR device could be used for monitoring of fracture pore under loading stress and continuous injection conditions dynamically.The loading-injection nuclear magnetic resonance(LI-NMR)device has successfully integrated loading and injection technology with NMR.The deviation range of LI-NMR with Mercury porositmetry(MIP)and(Low pressure gas adsorption)L-PGA for micro-pores(<10nm)and transition pores(10-100nm)is 0.3%-2.8%,fracture(>100000nm)and macro-pores(1000-100000)is 0.5%-2.6%.(2)There are three dynamic evolutions of coal fracture pore.Flowing Various: the total porosity increases 51.6%~79.9% within 10–120 minutes of the initial injection stage and pore channels form.Recombination Various: the number of micro-pores and transition pores(MP-TP)continues to increase,meso-pores(100-1000nm)(MP-TP),macro-pores and fractures(MP-F)decrease during the injection pressure reduction stage or confining pressure increases.U Various: the total porosity first decreases 17.0%~84.9% and then increases 0~30.6% when confining pressure either remains constant or increases.Flowing Various is caused by the flowing of injection liquid through the coal.Recombination Various is caused by a conversion of MEP and MP-F into MP-TP.U Various is caused by the support provided by the injection liquid and pore water to the broken pore walls such that new pores form.(3)According the evolution model of coal fracture pore under continuous injection: the compressibility of coal fracture pore increases with the increase of radius of pore and confining pressure,and tends to decrease with the increase of injection pressure.