Technology and equipment of sealed coring for accurate determination of coalbed gas content in ground well

Coalbed gas content is an important parameter of the characteristics of coal reservoirs and an important basis for the exploration and development of CBM resources,gas prevention and control in coal mines. In view of the fact that the coalbed gas content obtained by conventional coring method in surface wells is obviously lower than the actual content in coal reservoirs (especially in broken and soft coal seams),a sealed coring method for an accurate determination of coalbed gas content is proposed,which is suitable for the characteristics of broken and soft coal reservoirs. According to the characteristics of coal reservoirs (compared with oil-gas reservoirs,shallower burial,lower pressure,lower temperature and lower strength of coal body) and the requirement of an accurate determination of coalbed gas content,the sealed coring device,in which the outer barrel rotates and two inner barrels holds still,the ball valve is closed,the core barrel and the desorption tank are integrated,and the dual-diameter PDC core drilling bit has small-diameter at front and big-diameter at rear,have been are designed and trial-produced for broken and soft coal seams,which is suitable for the determination of coalbed gas content in surface wells. The diameter of the outer cylinder of the sealed coring device is 120 mm,the diameter of coal (rock) core can be up to 38 mm,the pressure-holding capacity is up to 10 MPa, and it is suitable for the sealed coring of surface wells for coal seams with a burial depth of 1 000 m. The dual-diameter PDC sealed coring bit reduces the flushing effect of drilling fluid on the coal core during coring drilling,and improves the recovery of coal core in broken and soft coal seams. The sealed coring equipment was used to carry out the comparative test of coalbed gas content determination for two surface wells in the broken and soft coal seams in Pansan coalmine of Huainan and Luling coalmine of Huaibei,which has got five sets of sealed coring coal samples and three sets of conventional coring coal samples. The test results of eight sets of coalbed gas content show that the coalbed gas content obtained by the sealed coring method is 20. 23% to 40. 34% higher than that obtained by the conventional coring method,and the mean value is 27. 22% ,which shows that the sealed coring technology can improve the testing accuracy of coalbed gas content. The successful development and application of the sealed coring equipment in surace well will provide more accurate parameters of coalbed gas content for CBM resource exploration and development,gas disaster prediction and precision control in coalmines.