Research On The Mechanism And Field Implication Of Fracturing Coal To Enhance Permeability With Fluid

As shallow coal seam resources almost exhausted because of the continuous exploitation today,the exploitation gradually develops to deep coal seam.With the increase of mining depth,the in-situ stress and gas pressure of coal seam increase gradually,and the gas disaster becomes more and more serious.How to improve the permeability of coal and rock and realize safe and efficient co-mining of coal and gas is always a difficult problem.In this thesis,the mechanism of fluid-induced fracturing of coal and rock mass is taken as the research object,and the theory and effect of fluid-induced fracturing and seepage enhancement of coal and rock are systematically studied by means of theoretical analysis,laboratory and field tests.Based on theories of elasticity,fracture mechanics and stress wave propagation,the mechanism of fluid-induced fracture of coal and rock is analyzed.Based on the distribution law of in-situ stress in coal mine,considering the influence of in-situ stress and mining loading and unloading on raw coal,the influence of water content on mechanical properties and seepage law of raw coal is studied.In this thesis,a field test of liquid carbon dioxide fracturing in a mine is carried out.The effective influence radius and aging characteristics were further analyzed.The gas extraction efficiency of coal seam is improved by fracturing during the phase transition of liquid carbon dioxide.A conceptual model of liquid carbon dioxide fracturing in coal seams was established.Hydraulic fracturing was performed in underground mining at a coalmine.The fluid pressure,injection rate,deformation of roadway after fracturing and gas extraction in different geological conditions are studied.The influence of geological factors and physical and mechanical properties of coal and rock on hydraulic fracturing effect is analyzed.The main conclusions are as follows:(1)The mechanism of coal rock fractured by high pressure fluid under quasi-static and that fractured by dynamic load high pressure fluid is quite different.Quasi-static high-pressure fluid fracturing changes the stress state of borehole inner wall through tension or shear fracture caused by high pressure fluid.The dynamic load high-pressure fluid fracturing impacts coal and rock through ultra-high pressure fluid,resulting in compression fracture of ultra-high pressure fluid and tensile fracture caused by stress wave propagation.According to the stress state analysis of borehole wall,the tensile strength is much lower than triaxial compressive strength of coal and rock in quasi-static high pressure fluid fractured coal and rock.Therefore,in the process of fracturing,the stress condition of shear fracture is more stringent,and coal and rock are more prone to tensile fracture.(2)Whether under quasi-static or dynamic loading conditions,fracture propagation can be analyzed by fracture mechanics after coal and rock fracturing.The fracture propagation is affected by the stress state of the crack tip.When the crack tip is only affected by the tensile stress,the crack expands in the form of type I crack.When that is affected by both shear stress and tensile stress,the crack may expand in the form of type II crack or I-II composite crack.The interaction between artificial fracture and natural fracture is mainly affected by the permeability and stress state of natural fracture.Natural fractures need to sufficient permeability to fluid-intrude and change their original stress state.The change of stress state of natural fractures caused by fluid pressure must meet the requirement of its expansion.Otherwise,artificial fractures will pass through natural fracture directly.(3)With the increase of water content,the center of the Mohr stress circle moves to the left,and the ultimate strength and peak strain decrease gradually.The peak strength and peak axial strain both have a quadratic polynomial relationship with the water content.With the increase of loading and unloading confining pressure times of post-peak cycles,the radial strain decrement and increment of raw coal sample after loading and unloading confining pressure gradually decrease.During the process of post-peak loading-unloading confining pressure,the permeability gradually decreases with loading confining pressure,and as the unloading confining pressure gradually increases.The permeability of the coal sample increases in wave shape with the increase of axial strain.The permeability increases after each loading and unloading confining pressure compared with that after the previous loading and unloading confining pressure.The permeability and effective stress of the sample are subject to the exponential function distribution.The gas permeability decreases exponentially with the increase of the effective stress,and increases exponentially with the decrease of the effective stress.(4)Different distances from the observation boreholes to the two fracturing boreholes were designed during the liquid carbon dioxide fracturing.By comparing the variations in two streams,the effective damage radius was determined to be 8 m.There was no significant decrease in pure gas flow rate of observation boreholes after fracturing for a period of two weeks.The pure gas flow rate increased by factors of 0.82–1.90(average 1.17)in comparison with the pre-fracturing data.The relatively high values of gas pure flow rate could guarantee the actual effect of gas drainage as the rapid decrease of gas pure flow rate is common for the low permeability coal seams.There is a close linear negative correlation relationship between the radius and the time.It can provide a theoretical basis for the optimization of the layout of subsequent fracturing and pumping holes in the coal seam.(5)A conceptual model was proposed to further illustrate the formation of newly-developed complex fractures and displacement of gas after liquid carbon dioxide fracturing.Both of these two effects were beneficial for gas drainage and shortening the time to extract high content gas in coal seams.When the thickness of coal seams and geological conditions were varied,the gas drainage efficiency could be further optimized by adjusting the numbers of fracturing tubes,orientations of fracturing,and layouts of boreholes.(6)Geological conditions have significant influence on permeability enhancement of fractured coal seam.When geo-stress is high,the permeability enhancement effect of hydraulic fracturing coal seam is poor.The thesis confirms the existence of a methane enrichment circle after hydraulic fracturing.Because of the high stress sensitivity of coal,the induced artificial fractures in coal seams closed with the decline of fluid pressure.High geo-stress could lead to a sharp decline of the methane extraction rate.