Study On The Crack Propagation Evolvement And Antireflection Mechanism Of Hydraulic Fracture In Low Permeable Coal And Rock Mass

Coal bed methane (CBM), generated by coal seams during the coalification process, is a sort of unconventional nature gas which lies in coal bed as a form of physical adsorption. It is also the most important supplementary energy of conventional gas. The production and utilization of CBM, on one hand, compensate for the shortage of nature gas in our country and at bottom reduce the disasters of gas explosion and outstanding; on the other hand, it can also reduce gas emissions and environmental pollution and most importantly improve the energy structure. Because of geological structure damage in the later stage of coal forming, the coal bed is fractured and the storage structure of coal is destroyed, and eventually the solid skeleton is changed. These complicated geological conditions result in "three low" features—low pressure, low saturation and low permeability. Besides, the geological environment of coal stage in our country is not uniformity. Therefore, choosing a measure, with ideal effect of antireflection, which can polish up the solid skeleton of coal and rock mass and improve the gas permeability of coal bed is important for the coal bed methane exploitation. Antireflection technology of hydraulic fracture is an effective measure which has a simple construction process, low cost and low requirement of technology. The law of crack developing and the antireflection technology of hydraulic fracture in low permeable coal and rock mass are researched by using similar simulation experiment, theoretical analysis and similar simulation experiment based on the engineering practice.(1)The similar simulation study on hydraulic fracture in low permeable coal and rock mass is conducted through true triaxial experimental system. The mechanical conditions on the crack growth, development and stretch as well as the starting-crack conditions of secondary fissure and influencing factors of crack turning are obtained. The law of crack propagation of hydraulic fracture is given.(2) The frequency characteristics of acoustic emission(AE) of coal and rock mass in the hydraulic fracture process are analyzed via the true triaxial experiment. The results show that there have been lots of AE signals before the most active stage of AE. It is proved that there exist many original cracks in the inner of coal or rock samples and such damages are easier to lead to the phenomenon of AE during the evolution of original cracks. Therefore, the fracture orientation is influenced by the characters of cracks as well as the state of the initial rock stress field.(3) According to the study on the antireflection mechanism of hydraulic fracture in low permeable coal and rock mass via true triaxial experiment. It can be proved out that the permeability of fracture increases with increase of the cracks, following the law of exponential decay. It can be described as y=0.0015e0.0942x.The number of injecting water pressure and ground stress deviation have deep influence on the fracturing effect evaluation through the analysis of influence factors of fracturing effect evaluation of hydraulic fracture in low permeable coal and rock mass. The research provides theoretical basis for numerical simulations in chapter4.(4) The numerical simulation experiment on the hydraulic fractur e under different stress and with different number of injecting water hole s is conducted by using RFPA2D. The effects of permeable fracture under different circumstances are analyzed. The following results are presented:①The fracturing process is dominated by tensile failure while there are a few of shear failure with the increase of injecting water pressure. Besid es, with the increase of stress difference, shear failure phenomenon is mo re serious.②he hydraulic fracturing effects and antireflection effects are more notable with larger stress difference, more injecting water holes an d little initiation pressure, more AE signals. Most importantly, the permea bility of coal bed is improved.(5)The construction parameters of hydraulic fracture are selected for XX coal mine on the basis of the study above. It will guide the wor k on the construction sites by analyzing the effect of hydraulic fracture.