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作为典型的致密多孔介质,煤岩储层已被证实存在启动压力梯度.根据煤层气垂直裂缝井的双线性流动机制,综合考虑启动压力梯度和井筒储存效应的影响,建立了一个新的低渗透煤层气有限导流垂直裂缝井双线性流动数学模型,采用Laplace变换和Stehfest数值反演方法对数学模型进行了求解,并分析了无因次启动压力梯度等参数对无因次井底压力及其导数曲线的影响规律.分析结果表明:典型的低渗透煤层气垂直裂缝井双线性流动曲线可划分为早期续流段、双线性流段、煤层线性流段、过渡流段和煤层边界线性流段5个特征阶段,其中由于启动压力梯度存在的影响,无因次井底压力及其导数曲线自煤层线性流段开始出现明显上翘,且启动压力梯度值越大,曲线上翘趋势越明显;此外,煤层边界线性流段呈现为单位斜率的直线,而非1/2斜率的线性流段直线.这些结果表现出启动压力梯度对低渗透煤层气垂直裂缝井双线性流动的影响,可用于指导现场煤层气井试井分析.
As a typical dense porous media, coal seam rock formation has been proven to have a starting pressure gradient.According to the bilinear flow mechanism of the vertical fractured well in coalbed methane, considering the influence of starting pressure gradient and wellbore storage effect, a new low The bilinear flow mathematical model of the permeability fractured vertical fractured well of coalbed methane is studied. The mathematical model is solved by Laplace transform and Stehfest numerical inversion method. The effect of the dimensionless start pressure gradient and other parameters on the dimensionless bottom hole pressure And its derivative curve.The results show that the bilinear flow curve of a typical low permeability CBM vertical fractured well can be divided into early continuous flow section, bilinear flow section, coal seam linear flow section, transition flow section and coal seam Boundary linear flow segment has five characteristic phases. Because of the influence of the start-up pressure gradient, the dimensionless bottom hole pressure and its derivative curve begin to appear upward upward from the linear flow section of the coal seam, and the larger the starting pressure gradient value, the curve upturns The more obvious the trend is; in addition, the linear boundary of the coal seam presents a straight line with a unit slope instead of a straight line with a slope of 1/2 slope. Effect of the pressure gradient in low permeability coalbed methane wells with vertical fractures bilinear flow, can be used to guide the field of coalbed methane well test analysis.