Study On The Method Of Freezing Sampling For Measuring Gas Content In Coal Seam

The gas content of coal seam is an extremely important and basic parameter of methane gas,which provides scientific reference for the exploration,evaluation and development of coalbed methane resources,including the prevention and control of mine gas disasters.At present,the method for determining the gas content of coal seams with the widest range and the highest relative accuracy is the direct measurement method.However,during the sampling process,a large amount of gas desorption will cause a loss of gas content,which may cause a large error in the measurement result.Related simulations and low temperature tests show that the suppression of gas desorption by use of low temperature method can greatly reduce the amount of gas loss,making the measurement results more accurate and more reliable.Therefore,it is of great practical significance to use the frozen coal body to measure the gas content of the coal seam.In this paper,the design of coal-containing frozen core drilling tools is carried out,and the physical and mechanical properties of coal after freezing and the adsorption-desorption-diffusion characteristics of coal after the action of freezing are studied;a cryogenic core-forming technology containing gas is proposed,which is to use evaporative cooling and freezing.The coal body further inhibits the gas desorption in the coal core during the coring process,reduces the gas loss and improves the gas content determination accuracy.Through the numerical simulation and laboratory experiments,the refrigeration performance of different refrigerants and the distribution characteristics of the core temperature field in the evaporator were compared and analyzed under freezing conditions.The specific research conclusions are as follows:(1)Freezing promotes the development of pores in coal pores,simultaneously increases the pore size and pore number of adsorption pores and seepage pores.The porosity change rate of frozen coal is positively correlated with freezing temperature and water content,besides the permeability and freezing temperature T satisfy the exponential function relationship.(2)The full stress-strain curve of the frozen coal specimen is obtained.With the decrease of freezing temperature,the crack compaction stage of the coal body increases,the elastic stage shortens,and the compressive strength decreases.The compressive strength of the coal body before freezing is positively correlated with the water content,and the compressive strength and water content of the coal is negatively correlated after freezing;under different freezing temperatures,the compressive strength of coal increases with temperature.(3)The adsorption constant-a of the Langmuir model increases linearly with decreasing temperature,while the adsorption constant-b increases exponentially with decreasing temperature.Under the action of the adsorption pressure P,the gas adsorption amount increases as the temperature decreases.The gas desorption process in low temperature environment can be divided into three stages: the desorption rate is approximately linear reduction phase,the slow reduction phase and the horizontal linear reduction phase.Under the same temperature conditions,the gas diffusion coefficient and the adsorption equilibrium pressure of coal are weaker;while the gas diffusion coefficient of coal decreases with the decrease of adsorption temperature.(4)The temperature-dependent curves of the coal samples under the phase-change cold source are U-shaped.The lowest temperature of dry ice-alcohol refrigeration is high,the lowest temperature of liquid nitrogen refrigeration is low,and the cold source consumes fast,the simulation is exhausted only for 5s,and the coal sample can be kept at a constant low temperature for a long time under evaporative compression refrigeration.According to the accurate determination of the gas content,it is determined that the evaporative cooling is the best cold source.(5)Numerical simulation and experimental tests show that the temperature difference between different measuring points in the coal core varies significantly with time.The temperature of the outer wall of the coal core decreases rapidly in the radial direction,and the temperature at the axial center decreases the slowest.The axial direction is in the evaporator.The axial centerline of the tube is bounded,and the cooling rate to both sides is gradually reduced;the fitting curve of the measured data indicates that the cooling rate is logarithmically related to time.(6)Exploring the principle of sampling technology of frozen coal.Drilling to the bottom of the hole during construction,freezing the coal sample to low temperature in a short time,suppressing gas desorption by low temperature,promoting free gas adsorption,reducing gas loss of coal body,improving the accuracy of gas loss calculated by coal body,thereby obtaining more accurate coal sample raw gas.The simulation test verified that the gas-containing frozen core drilling tool can meet the requirements of the core-finishing process.