Research On Occurrence Status And Recoverability Of CBM Under The Restriction Of Coal Rank

The eastern edge of the Ordos Basin is one of the key bases for coalbed methane(CBM)development in China.However,due to the vast area and the development of CBM with different coal ranks in the region,the cognition of CBM occurrence status is unclear,and the evaluation of the recoverable potential is insufficient,which restrict the CBM exploration and development.In this thesis,the Baode,Liulin and Hancheng blocks with different coal ranks in the eastern margin of the Ordos Basin are taken as research blocks to carry out the research on the characteristics of pores and fractures,and to analyze the variation law of reservoir pores and fractures under the restriction of coal rank.Based on the methane isotherm adsorption experiment,combined with previous research results,a three-phase gas content calculation model was constructed to reveal the law of gas content change and evaluate the recoverable potential of the study area.In the study area,the proportion of micropores and transition pores are the most,followed by macropores,and the proportion of mesopores are the least.With the increase of coal rank,the proportion of micropores and transition pores decreases first and then increases;the proportion of mesopores increases first and then decreases;the change of macropores is not obvious.BET specific surface area is 0.09~16.84 m~2/g(averaging 2.81 m~2/g),and BJH pore volume is 0.53~21.27 ml/g(averaging 4.61ml/g).As the coal rank increases,the porosity and micropore content show a"high-low-high"change rule.The micropore content controls the specific surface and pore volume.With the change of micropore content,the specific surface and the pore volume also show the trend of"high-low-high",while the average pore size shows the trend of"high-low-high".The adsorption capacity of coal is restricted by the pore structure and shape.The low rank coal has large porosity,specific surface area,and pore volume,which provide large adsorption space for gas,but the pore morphology is mainly open connected pores,which makes the gas easy to diffuse and reduces the adsorption capacity;the middle rank coal has some ink bottle-shaped pores,and the high rank coal has a large number of ink bottle-shaped pores and a higher content of micropores,so that the adsorption capacity increases as the coal rank increases,and the adsorption gas content also increases.With the increase of coal rank,the change of porosity affects the storage space of free gas and dissolution gas.Under the same water saturation,the low rank coal and coal high rank have higher porosity and free gas content;the middle rank coal has lower porosity and less free gas content;the dissolution gas content also presents similar change law,but the dissolution gas content is far less than the adsorption gas and free gas.Based on the geological background,pore characteristics and three-phase gas simulation in the study area,the isotherm adsorption curve formula method was used to calculate the theoretical recovery rate and actual recovery rate of CBM with different coal ranks,and the recoverable potential of the study area was evaluated.The theoretical recovery rate in the study area is between 48%and 88%,and the actual recovery rate is between 18%and 76%.The average actual recovery rates of CMB in Baode,Liulin,and Hancheng areas are 36%,38%,and 46%,respectively;the recoverable resources are 66.11*10~8 m~3,248.42*10~8 m~3,394.80*10~8 m~3,respectively.As the coal rank increases,the recovery rates increase,and the recoverable potential of the CBM increases.