Optimization technology of efficient CBM productivity areas in Qinnanxi-Mabidong Block,Qinshui Basin,Shanxi,China

The southern part of the Qinshui basin is currently the largest CBM production base in China. With the grad- ual depletion of the shallow CBM resources,it will inevitably need to turn its attention to the deep CBM and the diffi- culty of mining will increase. Thus,it is particularly important to accurately locate the efficient CBM productivity areas as it is the case in the Qinnanxi-Mabidong block. Faced with this new technical challenge,the characteristics of deep coal reservoirs were analyzed. In combination with the production practice of coalbed methane wells in the early stage of the southern Qinshui basin,the three priority levels of resource base,gas production conditions and reservoir reform- ability were first divided,and then in terms of four properties such as gas-bearing properties,permeability,methane- leading ability and recoverability,the preferred standard and processes of the efficient CBM productivity areas are pro- posed,which forms the “ three-levels four-qualities” efficient CBM productivity areas optimization technology. The study indicates that the development potential of the productivity area is embodied as a combination of key geological conditions,including that coal reservoir gas content above the economic limit,microfracture width per unit length is greater than or equal to 50 μm,methane-leading index is greater than or equal to 30 nm,geo-stress is in the state of vertical stress≥the maximum horizontal principal stress≥the minimum horizontal principal stress or the maximum hor- izontal principal stress≥the vertical stress≥the minimum horizontal principal stress,the coal structure is dominated by intact coal-cataclastic coal and the local structure is relatively simple with the movable area is greater than or equal to 30% . Based on these criteria,three efficient production areas were selected in the Qinnanxi-Mabidong block. Five test production wells were deployed and more than 2 000 m3 of gas production per well was verified. Therefore,the reliabil- ity of the preferred technical method was verified and the success of Qinnanxi-Mabidong has provided a successful demonstration of technology for the selection of efficient deep CBM productivity areas in the Qinshui Basin.