| Gas extraction is the basic measure and main way of gas prevention and utilization in mines at home and abroad.In the process of gas extracting in water-bearing coal seam,water will not only affect the mechanical properties of coal rock mass,but also affect the law of gas migration,so that the gas confluence process experiences complex gas-water two-phase seepage and gas single-phase seepage under different moisture content,and the broken coal rock mass around the borehole is the main flow path of gas entering the borehole,and the change of gas seepage characteristics by water is the key to affecting the effect of gas extraction.Therefore,the study on the gas-water coupling seepage characteristics of pressurized broken coal rock mass has certain engineering guiding significance for the efficient extraction of gas in water-bearing coal seam.In order to study the gas-water coupling seepage characteristics of broken coal rock mass,a gas-liquid coupling seepage test system for broken coal rock mass under pressure was developed,which can visualize and freely select the test parameters of the whole test process.Considering the influence of water on the deformation characteristics of broken coal rock mass under pressure,uniaxial compression tests of broken coal rock mass with different moisture content gradations were carried out,and the influence of moisture content,axial pressure on the deformation and re-crushing characteristics of the specimen was analyzed.The results show that with the increase of moisture content,the strain of the sample increases,and the secant modulus decreases,showing obvious softening effect;while with the increase of axial stress,the sample shows the characteristics of strain hardening,and the relative breakage of the sample increases rapidly at first and then slowly increases.The porosity of the sample decreases monotonously,indicating that water will aggravate the crushing and compaction of broken coal rock mass.The gas seepage test under different moisture content shows that the gas seepage of broken coal rock mass is non-Dacry flow,which conforms to Forchheimer formula.With the decrease of porosity,the permeability of the sample decreases,and the non-Dacry factor increases,and with the increase of moisture content,the permeability and non-Dacry factor of the broken coal rock mass decrease,indicating that the water film effect will reduce the seepage resistance and weaken the non-Dacry phenomenon.At the same time,according to the experimental results,the relationship between porosity and permeability,porosity and non-Dacry factor of broken coal rock mass was obtained,and based on the porosity evolution equation under the combined action of moisture content,axial stress,pore pressure and adsorption deformation,the gas permeability model of broken coal rock mass considering water content was established.The gas-water two-phase steady-state seepage test was carried out by using the gas-liquid coupling seepage test system of broken coal rock mass.The variation laws of porosity,water saturation and permeability with osmotic pressure and axial pressure were analyzed,and the gas-water relative permeability model of pressure-bearing broken coal rock mass was established.The results show that the relative permeability of gas decreases rapidly with the increase of water saturation.Under the combined action of axial pressure and pore pressure,the relative permeability of gas and water in broken coal and rock mass can be divided into three stages:gas seepage dominant,isotonic and water seepage dominant.In the early stage of extraction,the relative permeability of gas can be effectively improved by increasing the negative pressure of extraction to accelerate drainage and reduce the water saturation of coal.Based on the experimental results,the stress deformation field control equation and porosity evolution equation of broken coal rock mass in different water cut stage were derived.The seepage field control equations of gas single-phase seepage and gas-water two-phase seepage were given respectively.Combined with the gas permeability model and the gas-water relative permeability model,the fluid-solid coupling dynamic model of the gas-water coupling seepage in segmented broken coal rock mass was established,and the rationality and reliability of the model were verified by the experimental test results. |
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