Evolution Mechanism Of Coal Permeability Under The Synergistic Effect Of Intrinsic Water, Proppant And Effective Stres

Coal is a combustible mineral resource which is gradually formed from plant remains after hundreds of millions of years of geological processes,under the conditions of high temperature and high pressure and isolation from the outside air,and after peating and coalification.In the process of coal formation,coal is accompanied by coalbed methane,which is not only the dangerous gas that is most likely to cause accidents in the process of mine mining,but also a clean energy with commercial value and availability.Although the development and utilization of coalbed methane is challenging,it can alleviate the contradiction between energy supply and demand in production and life.The safe and efficient development and utilization of coalbed methane has attracted wide attention of researchers.With the increase of mining depth,mining operations will face complex geological conditions,and under the influence of in-situ stress,groundwater and high buried depth,coal generally have low permeability.In engineering practice,hydraulic fracturing technology is a commonly used anti-reflection method,and its mechanism is to fill fracturing fluid and propping agent into the coal fracture system,so as to make the fracture develop better and have strong anti-reflection performance.However,after the anti-permeability process is finished,the filled fracturing fluid will not be discharged completely,and the groundwater gushing will make some coal reservoirs full of water,which will affect the normal gas migration and weaken the hydraulic fracturing effect.Therefore,the change of permeability of coal in the process of hydraulic fracturing is extremely complex,and it is of great significance to find out the linkage between various influencing factors and permeability.In this paper,the water-bearing coal sample buried at a depth of about 1200 m in Zhaozhuang Coal Mine was tested,and the adsorption-deformation equation of water-bearing coal was deduced according to the experimental results.Finally,the coal permeability model under the synergistic effect of internal water and effective stress was established.Under the background of hydraulic fracturing,the evolution law of permeability under the synergistic effect of effective stress and proppant is studied.The following progress has been made:1)Aiming at water-bearing coal,the adsorption test and seepage test of effective stress change are carried out,the adsorption deformation characteristics and permeability evolution law of coal under different water contents are discussed respectively,and the linkage mechanism between water content and coal adsorption deformation and permeability is analyzed.2)Under the synergistic effect of internal water and effective stress,considering the difference of water film thickness and gas permeability between matrix pores and fissures,the dynamic water film calculation formula under the influence of stress and gas adsorption is deduced,and the permeability model under the synergistic effect of internal water and effective stress is established.The evolution law and seepage characteristics of water film under the influence of effective stress and gas adsorption are analyzed.Coal is a dual-pore medium composed of matrix and fissures,in which the gas permeability in fissures is far greater than that of matrix.When coal contains water,the greater the water saturation,the smaller the contribution of matrix permeability to synthetic permeability.Under high water saturation,there is almost no gas flow in matrix pores.3)Considering the influence of proppant embedment in hydraulic fracturing,the embedding depth of proppant was quantified by Hertz contact theory,and the coal fracture permeability model under the synergistic effect of effective stress and proppant was established.The influence mechanism of proppant embedment depth,proppant placement layers and coal fracture morphology on coal permeability was revealed.The compressibility of coal fractures under different water cut conditions and different embedding depths of proppants is quantitatively studied.