Study On Surface Well Sealed Coring And Fast Measurement For Coal Seam Gas Content

Coal seam gas content is one of important parameters to characterize coal reservoir characteristics,it is of great significance incoal seam gas exploration and development as well as gas control and management.In this paper,aiming at two challenges:coal seam gas content obtained by the existing surface well coring test is clearly lower than obtained by coal mining in broken,soft and low rank coal seams,and coal seam gas content testing time in meddle-high rank coal seams is quite long;relying on the project“Study on Estimation Method of Coal seam Gas Loss Quantity by Wireline Coring in Borehole”(4120212)supported by National Science Youth Fund and the“13th Five-Year”National Science and Technology Major Project“Technology and Device for Sealed Coring Gas Content Measurement in Surface Drilling Coal Seams”(2016ZX05045-002-002);applying theories and methods of coal geology,coal seam gasgeology,reservoir engineering,mathematical physics,mechanical design and drilling engineering.This study was performed using theoretical analysis,simulation test,sealed coring equipment and process research of coal seam gas desorption and diffusion for coal samples with different particle sizes under variable pressure or temperature conditions,and field engineering test application was performed in typical mining areas.The results and findings of this study are summarized as follows.(1)The study reveals the change law of coal seam gas desorption rate under variable pressure and atmospheric pressure.A coal seam gas diffusion-desorption model of coal samples under variable pressure has been established.simulates the process of wireline coring of surface wells,and a serious of coal rank samples with different particle sizes is tested for even and fast desorption after adsorbing high pressure methane gas.The Results show that the desorption rate of coal seam gas in simulated coring and exposed surface stage is respectively 9~116 times and 6~27times in the desorption tank.The measured simulation loss volume is 1.7~31.4 times that of US USBM direct method,and the smaller the size of coal samples,the greater the difference between them.This indicates that lower coal seam gas content obtained by the existing surface well core tests is mainly affected by inaccurate loss of gas estimation.(2)A technical solution ofcoal seam gas well sealed coring suitable for coal reservoir characteristics(compared with reservoirs,shallower burial,lower pressure and lower temperature)is proposed.For the first time,a sealed coring device which“outer barrel rotates with two inner barrels holds still,closed ball valve and integrated core barrels-desorption tank”is developed that is suitable for the measurement of gas content in coal seam gas wells.The main components include outer barrel,inner barrel,hydraulic drive barrel,ball valve,suspension assembly and accessories,etc;The length of sealed coring device is 2.15m,diameter of Outer cylinder is120mm,diameter of coal(rock)core can be up to 38mm,the pressure-holding capacity is up to 10 MPa,and it is suitable for sampling of surface wells with a depth of 1 000m in coal seams;in addition,it is featured by small geometry,simple structure,easy to use,economical and practical.(3)A dual-diameter PDC core drill bit for front small diameter drilled core-back large diameter drilling in broken and soft coal seams is invented.The drill bit reduces the flushing effect of drilling fluid on broken and soft coal seams during core drilling,and effectively improves the recovery of coal core in broken and soft coal seams.(4)The research develops a technology of sealed coring for coal seam gas content in surface wells,and the optimum parameters are:the drilling pressure,rotary speed,flow rate and mud density of middle-hard coal seams are18~30kN,70~100r/min,90~145L/min,1.0~1.1g/cm~3 respectively;the drilling pressure,rotary speed,flow rate and mud density in broken and soft coal seams are 12~16 kN,50~80 r/min,40~60 L/min,1.0~1.05 g/cm~3 respectively.(5)A fast measurement method of coal seam gas content with warming and natural desorption of coal samples is established.The results show that temperature has an important influence on desorption rate;coal samples is desorbed naturally when heated to 50?,and coal seam gas content measurement cycle is shortened from several weeks/months under conventional reservoirs temperature to several hours/days without affecting the accuracy of measurement of coal seam gas content.In the meantime,the test time is greatly shortened,and the work efficiency is significantly improved.(6)The results of this study were performed in a typical mining area in China,and achieved a great application effect.The results indicate that coal seam gas content obtained by sealed coring is 21.45%~42.8%higher than obtained by conventional coring method in broken and soft coal seams in Luling coal mine of Huaibei mining area and Pansan coal mine of Huainan mining area in Anhui Province,and the average is 31.90%;coal seam gas content obtained by sealed coring is 16.67%~41.67%higher than obtained by conventional coring method in middle-hard coal seams in Wen Jiapo coal mine of Binchang mining area in Shaanxi Province,and the average is 29.17%.The broken and soft coal seams in the Carboniferous-Permian coalfields in North China,South China and Southwest China are widely distributed,and the northwest Jurassic low rank coal seams is developed with rich coal seam gas resources.The popularization and application of research results of this paper is of great significance and has good application prospect for accurate and fast measurement for coal seam gas content in surface wells,improvement of coal seam gas resources exploration and development and coal mine gas disaster control.