Coal Deformation And Permeability Variations During The Heating Progress

As a kind of clean energy,strengthening the exploitation and utilization of coalbed methane can relieve the problem of resource shortage and reduce the risk of gas outburst explosion.The permeability of coal determines the exploitation effect of coalbed methane.The technology of coalbed methane exploitation by heat injection is a kind of efficient stimulation technology.In order to explore the evolution mechanism of coal permeability in the process of coalbed methane exploitation by heat injection,the high-temperature multi-functional triaxial test system is used.The thermal seepage test,thermal expansion test,pressure-reducing desorption test,temperature increasing desorption test of coal under different stress states and the seepage test of increasing pore pressure at different temperatures were carried out to explore the influence of thermal expansion,desorption deformation effect and slippage effect on permeability of coal during the process of temperature increasing at30-150℃.The main research of this paper is as follows:（1）Through the seepage test,thermal expansion test,pressure-reducing desorption test and temperature-increasing desorption test,the deformation law of coal under the action of temperature and desorption is studied,and it is found that:1)At 30-150℃,the influence of temperature on coal deformation is jointly determined by the thermal expansion deformation of the matrix and the shrinkage deformation of coal under external pressure due to the decrease of elastic modulus.2)Gas desorption will cause matrix shrinkage,and the desorption amount is positively correlated with desorption strain.At 30-150℃,the desorption capacity increases linearly with the increase of temperature,the desorption deformation rate increases gradually,and the desorption strain coefficient decreases first and then increases,reaching the minimum value in the range of 90-120℃.The desorption strain coefficient of coal is negatively correlated with the effective stress.When the temperature is 90℃,the radial desorption strain coefficient and the volume desorption strain coefficient of coal are negatively correlated with the pore pressure.At other temperatures,the desorption strain coefficient of coal is positively correlated with the pore pressure.3)In the process of thermal seepage,coal deformation is caused by thermal expansion deformation and desorption deformation,and is mainly affected by thermal expansion deformation,which is consistent with the law of thermal expansion deformation.4)There exists a critical desorption temperature from expansion deformation to contraction deformation during the process of heating and desorption at 30-150℃.With the increase of effective stress and pore pressure,the critical desorption temperature decreases,and the sensitivity of critical desorption temperature to effective stress is 1.875 times that of pore pressure.（2）Based on the deformation law of coal and the permeability evolution law of coal in the seepage test,the seepage law of coal under the effect of temperature and desorption is analyzed.The results show that:1)thermal expansion deformation will squeeze the seepage channel,leading to the decrease of permeability;The shrinkage deformation caused by coal dissolution will open the seepage channel and promote the increase of permeability.2)The influence of thermal expansion deformation on coal permeability is related to the initial permeability of coal.When the initial permeability of coal is greater than 43.08×10^-21m²,the permeability decreases due to thermal expansion deformation.When the initial permeability of coal is less than 1.77×10^-21m²,the inward expansion of thermal expansion deformation is limited and the effect on permeability is weakened.3)The lower the initial permeability of coal,the lower the temperature corresponding to the minimum permeability point at30-150℃with the increase of temperature.When the initial permeability is less than1.77×10^-21m²,the minimum temperature of permeability is 30℃.When the temperature is greater than 1.77×10^-21m²and less than 9.29×10^-21m²,the temperature is 60℃.When the initial permeability is greater than 9.29×10^-21m²and less than 31.27×10^-21m²,it is 90℃.When the initial permeability is greater than 43.08×10^-21m²,the permeability decreases monotonically,and the permeability is the lowest at 150℃.（3）Through the seepage test of increasing pore pressure at different temperatures,the relationship between pore pressure and slippage effect and the influence law of temperature on slippage factor are analyzed,and the evolution law of permeability of coal body considering slippage effect under temperature and desorption is explored.The results show that:1)The influence of pore pressure on slippage effect is mainly caused by the coal adsorption and desorption deformation caused by the change of pore pressure and the change of pore throat radius caused by the change of effective stress.With the increase of pore pressure,slippage effect gradually decreases.2)The influence of temperature on slippage effect is mainly caused by the change of coal pore throat radius caused by thermal expansion deformation and desorption deformation caused by temperature.With the increase of temperature,the slippage factor increases gradually at 30-60℃,decreases gradually at60-90℃,and increases rapidly at 90-150℃and begins to appear negative value.3)In the process of warming seepage,the effect of thermal expansion deformation,desorption deformation and slippage on permeability is affected by the initial permeability.When the initial permeability is less than 1.77×10^-21m²,the effect of desorption deformation and slippage on permeability is greater than that of thermal expansion deformation on permeability,and the permeability increases with the increase of temperature.When the initial permeability of coal is greater than 43.08×10^-21m²,the effect of desorption deformation and slippage on permeability is less than that of thermal expansion deformation,and the permeability decreases with the increase of temperature.