The Permeability Model Of Coal Seam By Repeated Injection Pressure Drop Method And In Situ Test Study

As the depth of coal mining in China increases,the permeability of coal seams is becoming lower and lower,and gas disasters in mines are becoming more and more serious.As an important indicator of the ease of gas seepage and extraction,it is of great significance to determine the permeability accurately.At present,the radial flow method is commonly used in underground mines to calculate coal seam permeability,but this method has a long testing period and many parameters,which is difficult to meet the needs of intelligent and informative testing of gas parameters.This thesis proposes a rapid determination method for coal seam permeability by the gas injection and gas drop method,establishes a repeated gas injection coal seam permeability calculation model based on Darcy’s law,verifies the reliability of the method through numerical simulation and in-situ field testing,and draws the following main conclusions:(1)In this thesis,the simplified gas content equation and the proposed pressure equation are used to represent the gas content and the state of gas distribution in the coal body respectively,Darcy’s law is used to describe the state of gas percolation in the coal seam,and the radial unsteady flow control equation of the coal seam is established based on the law of mass conservation.Combined with the volume flow equation in the pressure drop process,an injection gas drop calculation model can be established to determine the coal seam permeability using the full interval pressure drop data.The model can obtain multiple sets of pressure drop curves by defining different injection pressures,and obtain multiple sets of permeability determination results for a single borehole,which can improve the accuracy of the test results while maintaining fast testing.(2)Establishing a percolation model based on single pore media with air injection pressure drop,and comparing the air injection-pressure drop process in the simulation with the actual measured data in the field,the pressure drop curves during the two tests are in high agreement with the calculated coal seam permeability.The results show that the gas injection duration and the length of pressure measuring chamber will have a great influence on the permeability measurement results of coal seam,while the discharge time before gas injection has no obvious influence on the pressure drop curve.When the length of gas injection is 8～10min and the length of pressure chamber is2～2.5m,the measured permeability is closer to the permeability defined in the model,and it is more reasonable to apply in the field.(3)Based on the numerical simulation results,a downhole cyclic gas injection pressure drop test system with three modules,namely "gas injection system","hole sealing system" and "slurry injection system",was set up in the field.The system is based on the following three modules The "gas injection system" provides compensating gas for the test system through an external high-pressure nitrogen cylinder with connection valve and shut-off valve pressure gauge;the hole is sealed and the pressure measuring space required for the radial flow of the test is sealed by the form of "bag sealing-cement mortar post-injection";the pressure drop test is carried out by The slurry is transported to the permeability test borehole through the grouting pipe by the mining pneumatic grouting pump under the joint action of the power cylinder and the suction and discharge valve set.(4)In situ test of permeability of coal seam 4,compensating gas under different pressures was injected twice at two measuring points of Hengyuan Coal mine by repeated injection pressure drop method.The results show that the drop rate of pressure in the gas chamber gradually slows down,and the gas injection pressure is positively correlated with the equilibrium time.By comparing the test results of the two methods,it is found that when the pressure in the pressure measuring gas chamber is slightly higher than the gas pressure in the coal seam,the permeability calculated by the pressure drop method is larger,which is more consistent with the measurement results of the radial flow method.As the gas injection pressure increases,the error of the measurement results increases.The four coal seams evaluated by the two methods are all difficult-can be extracted and the test results meet the field requirements.When the gas injection pressure is slightly higher than the gas pressure of the coal seam,the test period is obviously shortened,and the test results are accurate and the permeability of the coal seam is quickly measured.This thesis has 44 figures,26 tables and 119 references.