The Migration And Enrichment Law Of CBM In Close-distance Double Coal Seam Mining And Its Influence On Gas Production Of Surface Wells

The evolution characteristics of stress field,fracture field and flow field in close-distance double coal seam mining can promote exploration of the enrichment area of CBM（coalbed methane）,understanding of the law of migration and enrichment of CBM,and efficient extraction of CBM from surface wells were researched.The Zuoer panels of 28#and 30#coal seam in Dongyi mining area of Xinghua coal mine in Jixi mining area and the surface well HJD4 built above the working face is taken as the study area in this thesis which adopts the research methods of theoretical analysis,similar material simulation,numerical simulation and actual engineering effect analysis.The development characteristics of CBM storage space in closed gob are clarified.The evolution law of the fracture field and the migration and enrichment law of CBM under the action of close-distance double coal seam mining were studied.The drainage effect of surface wells in mining-gob period were analyzed.The reasons for the differences in gas production characteristics in each drainage period were explored.The evolution characteristics of the fracture field under the condition of close-distance double coal seam mining were analyzed,and the evolution law of the fracture field were summarized which adopts the research methods of similar material simulation and 3DEC numerical simulation.The fracture field which formed by mining is a shape of“high-level annular body”in three-dimensional space.The evolution of fracture field after the mining of close-distance double coal seams（underlying coal seams）is convergent.The evolution characteristics of stress,plastic strain and other indicators of the overlying strata in close-distance double coal seam mining were studied which adopts the research methods of COMSOL numerical simulation.The overlying strata stress can be divided along the strike direction into original rock stress area,stress concentration area,full pressure relief area and stress recovery area.The plastic strain area presents a“saddle-shaped”distribution.The change of indicators above will be caused by close-distance double coal seam mining（underlying coal seams）,which can be generally regarded as the enhancement effect on the basis of single coal seam mining.The distribution characteristics of CH₄ concentration after mining were analyzed,and the migration and enrichment law of CBM in each period were discussed.The CBM desorbed by the impact of mining will migrate upwards through the fractures in the fracture field,resulting in the continuous increase of the CH₄ concentration at the top of the fracture field over time;while the concentration of the CH₄ in the compaction area keeps at a low level.The migration and enrichment ability of CBM are affected by the changes of mining stress,porosity,permeability,and the range of fracture field,resulting in differences of CBM migration and enrichment ability in different periods.The gas production effect of well HJD4 in each period were analyzed,and the reasons for the differences in the gas production characteristics of the well in each period were clarified.The drainage effect of HJD4 well varies in different periods.The best drainage effect was achieved during 28#coal seam mining-drainage period,while the drainage effect during 28#coal seam gob-drainage period was the worst.The reason for the differences of gas production characteristics in different periods were affected by the continuous evolution of the fracture field and CBM migration and enrichment.The key to the effectiveness of gas production is determined by the establishment of a complete drainage channel between the“free gas enrichment zone”and the surface well.The gas production effect will be greatly improved by the effect of infiltration and flow enhancement under mining and pressure relief.The reason for the gradual increase in gas production efficiency is caused by the migration of CBM to the top of the fracture field and the formation of high CH₄ concentration zones around the wellbore,nevertheless the formation of high CH₄ concentration zones is limited by both time and space.This article is about 70,000 words,including 80 illustrations,21 tables,and 131references.