Study On Indirect Rapid Determination Method Of Coal Seam Gas Content

Gas content in coal seams is a key parameter in coalbed gas control and recovery.There is a difficult problem of long measuring time and lots of measurement per day.A comprehensive approach was used,including theoretical analysis,experimental research and field experiments,to study the rapid indirect gas content measurement method.Firstly,based on the Fick’s law of diffusion,a model for indirectly calculating gas content was developed.In this model,the characteristic parameters of initial gas desorption rate were used.Theoretical and experimental studies were conducted to analyze the effects of particle size,shape,temperature and exposure time on gas desorption.It is found that the effect of particle size on gas desorption and diffusion mainly lies in the stage of internal diffusion dominant control,where the diffusion rate is inversely proportional to the square of particle size.Also,different particle shapes lead to different effective diffusion area,resulting in a significant difference in the initial desorption rate,where the initial desorption rate of flaky is the highest,followed by cylindrical and spherical in sequence.With the increase of desorption time,the influence of particle shape on the desorption and diffusion gradually reduced.The temperature also plays an important role on the diffusion forms(gas-phase diffusion and surface diffusion).That is attributed to the effect on diffusion coefficients and desorptionrate.The research results mentioned above can pave a theoretical foundation for rapid determination of gas content in coal seams indirectly.Based on the influence of each factors on desorption,parameters of particle size,determination time and verification basis were obtained in the rapid determination of gas content.Results show that coal powders with 1-3 mm in size are more suitable for this method within the time of 15 minutes from the sample exposure.The average particle size determined by image scanning method was stable at 1.75 mm when the mass of 1-3 mm coal sample larger than 15 g.This trend conforms to Logarithmic Weibull distribution.Accordingly,the mass of coal sample was suggested to be 20 g for rapid determination.This study improved the deep-hole fixed-point rapid sampling technology by developing multi-stage jet boost water tail.This technology can realize rapid fixed-point sampling within 2 minutes,solving the problem of accurate timing of the coal sample exposure.A relationship between the sampling depth and input power was established.At the same time,it innovated the DGC gas concentration measurement device,confirming its accuracy through experiments,and solved the problem of on-site verification of indirect gas content determination.This study advanced the rapid indirect measurement technique of coalbed gas content at mine sites.The rapid indirect determination of gas content for a specific coal seam is divided into two stages: learning and application.The learning phase corrects the model relationship between the desorption characteristic parameters and the gas content;while the application phase measures the desorption law and calculates gas content by the modified model.A gas content rapid tester was developed and field experiments were conducted at Dawan Coal Mine.The results show that the maximum relative error between the gas rapid measurement results and the DGC measurement results is only 5.84%,which meets the industrial application requirements.Packaged technology of indirect rapid determination for gas content was finally formed that include the sampling,determination method and verification basis from the field.