沁水盆地南部煤层气井网排采压降漏斗的控制因素

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

以平面径向渗流理论及压降叠加原理为基础,依据沁水南部煤层气生产区块15口排采井生产数据,建立了井网排采条件下排水阶段至产气初期煤层气井压降漏斗的计算模型,应用Matlab数学软件实现了煤层气井压降漏斗的可视化模型.在分析了排水阶段至产气初期煤层气井压降漏斗的形状及演化特征的基础上,提出了单井压降漏斗形状的界定方法和压降漏斗叠加程度的评价方法,讨论了压降漏斗形状的控制因素.研究结果表明:排水阶段至产气初期,煤储层渗透率小于8×10-3μm2、地下水位高度高于680m、日排水量低于3m3/d时,产气量一般高于250m3/d,泄流半径与压降漏斗深度的比值一般小于40,泄流半径较小,煤储层压力降幅较大,压降漏斗属于第1种类型;煤储层渗透率大于8×10-3μm2、地下水位高度低于680m、日排水量小于3m3/d时,产气量一般低于250m3/d,泄流半径与压降漏斗深度的比值一般大于40,泄流半径较大,煤储层压力降幅较小,压降漏斗属于第2种类型;叠加区最大半径与井距的比值越大,压降叠加程度越高,叠加区域的面积越大,压降漏斗范围内煤储层压力的下降幅度也越大.排水阶段至产气初期,渗透率越低、地下水流体势越大、排水量越小、压降叠加程度越高,泄流半径与压降漏斗深度的比值就越小,越有利于产气初期煤层气产出.

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.