Study On The Scope Of The "Three Zones" Of Gas Pressure Relief And Extraction In Coal Seam Group Mining

The coal reservoirs in China are characteristic of high metamorphism degree, low permeability, tiny pressure and low gas saturation generally., and there is great difficulty in gas extraction before coal mining, but coal mining can lead to the pressure relief and permeability improvement among surrounding strata, the permeability of coal seam will increase by dozens of times to hundreds of times, the flow velocity faster, and gas emission volume greater, which provides a favorable condition for gas extraction. Therefore, coal and gas simultaneous extraction by the use of pressure relief induced by mining is the optimal method to prevent and cure gas disasters in coal mine as well as protect the environment. Presently, from the standpoint of coal and gas simultaneous extraction, the research group in which the author studies proposed'three zones'of gas pressure relief and extraction in overlying strata of coal seam group, they are gas conductive fracture zone, gas pressure relief and desorption zone and difficult desorption zone. However, However, explicit scope dividing index of the"three zones"and its influencing factors were not studied. So, this dissertation has further studied these problems.The dissertation comes up with the calculating analytic formula for rock deformation under the condition of axial pressure unloading from the rock constitutive model of original fracture deformation in the process of rock triaxial unloading, and makes expansion ratio reaching 3‰as the deformation critical value for sufficient pressure relief. In addition, it also presents the calculative formula for critical value of sufficient pressure relief degree in different buried depths by calculating and fitting, and confirms the index of pressure relief degree of gas pressure relief and desorption zone. By the use of similar material physical simulation and numerical simulation, the influence of overlying key stratum structure, working face length and mining height on the scope of the"three zones"is studied. The results show that the essence of the influence of working face length and mining height on the scope of the"three zones"is their control effect on the failure of key stratum, and key stratum movement directly influences the scope of the"three zones". If there exists a key stratum within the fracture zone of overlying strata, the failure of the key stratum will lead to an uprush in the height of gas conductive fracture zone, which is higher than empirical formula result, and the gas conductive fracture zone height will reach the bottom of the upper key stratum. Gas pressure relief and desorption zone develops to the unbroken key stratum under which exist much bed separation space, and its maximum height reaches the bottom of the primary key stratum;In the condition of critical mining, only the stratum in"O"shape circle could keep sufficient pressure relief, and the width of sufficient pressure relief scope increases as the mining height increases. Based on the research results above, the dissertation propose the discrimination method of the"three zones"of gas pressure relief and extraction.Based on the discrimination of the"three zones"of gas pressure relief and extraction, the method for selecting gas pressure relief and extraction mode in coal seam group is presented. According to the discrimination of"three zones"scope of the fully mechanized caving face 9404 and K8206 in Yangquan mine area, gas pressure relief and emission status are forecasted, and the gas extraction techniques of"U+L ventilation & crossing layers gas extraction drilling"and"U+I ventilation & gas extraction of strike high-level gas extraction roadway"were selected to control gas. Gas extraction effect show that gas extraction rate of gas conductive fracture zone in the fully mechanized caving face 9404 and K8206reach 77.32% and 87.37% respectively, gas extraction rate of gas pressure relief and desorption zone in the fully mechanized caving face K8206 reaches 88.43%. The reliability and feasibility of the discrimination method of the"three zones"of gas pressure relief and extraction, and the select method of gas extraction technique have been proved by engineering practices.