Control factors of coalbed methane well depressurization cone under drainage well network in southern Qinshui basin

Abstract：Under the condition of drainage by well network, this study established a mathematic model of coalbed methane (CBM) well depressurization cone from drainage stage to initial pro- duction-gas stage on the basis of the radial fluid flow theory, superposition principle of pres- sure drop, and the production data of 15 CBM wells in southern Qinshui basin. Visualization of the depressurization cone was realized using Matlab. Based on the analysis of the shape and e- volution characteristics of depressurization cones, this study proposes a defining method for single well depressurization cone and evaluation methodology for superposition degree of composite depressurization cone, and discusses the control factors of the shape for depressurization cone. The results show that: from drainage stage to initial production-gas stage, when the per- meability is bigger than 8 X 10.3 ptm2 , ground water level is higher than 680 m and daily aver- age displacement is less than 3 ma/d, daily average gas production of CBM wells are usually larger than 250 mS/d, the ratio of flush-flow radius to depth of depressurization cone is gener- ally less than 40, and the flush-flow radius is relatively small, accompanied by a rise in decrea- sing amplitude of pressure, and the depressurization cone belongs to the first type; when the permeability is bigger than 8 × 10-3μm2 , ground water level is lower than 680 m and daily av- erage displacement is less than 3 ma/d, daily average gas production of CBM wells are usually less than 250 ma/d and the ratio of flush-flow radius to depth of depressurization cone is gener- ally more than 40, and the flush-flow radius is relatively big, replaced by a drop in decreasing amplitude of pressure, and this depressurization cone belongs to the second type; the ratio of maximum radius of superposed area to well spacing is higher, the degree of superposition improves, the area of superposition increases, and the decrease amplitude of the coal reservoir pressure in the range of depressurization cone also rises. From the drainage stage to initial pro- duction-gas stage, lower permeability, larger underground water, smaller drainage or higher degree of superposition results in smaller ratio of flush-flow radius to depth of depressurization cone, which is beneficial to output of CBM in the initial production-gas stage.