在考虑煤层气的解吸和扩散效应及启动压力梯度的基础上,对低渗透煤层气藏流体输运特性进行分析,建立了低渗透煤层气水两相非线性渗流数学模型,推导出非线性渗流阶段气水两相的控制方程组。通过实例计算表明,液相拟启动压力梯度从0.001MPa/m增加至0.007MPa/m,煤层产气峰值下降约15%;不考虑启动压力梯度时的产气峰值量要比液相拟启动压力梯度为0.001MPa/m的产气峰值提升约7%。因此启动压力梯度的存在阻碍了裂隙中流体的流动,对煤层气开发影响较大。

The transport behavior of fluids in low permeability coalbed methane reservoirs is analyzed with considerations of the desorption and diffusion effects of the coalbed methane and the threshold pressure gradient. A mathematical model for gas-water two-phase nonlinear flow is established for low permeability coal reservoirs. The governing equations for the gas-water two-phase nonlinear flow are derived. The case study shows that, when the liquid phase threshold pressure gradient is increased from 0.001MPa/m to 0.007Mpa/m, the maximum gas production of coal reservoirs is reduced by nearly 15%; compared with the liquid phase threshold pressure gradient of 0.001MPa/m, the maximum gas production without the threshold pressure gradient is increased by nearly 7% and the cumulative gas production also has a certain extent of increase. Therefore the existence of the threshold pressure gradient makes the fluid in fractures more difficult to flow and leads to a low production rate. The case study shows that the mathematical model of the gas-water two-phase nonlinear flow could provide a theoretical basis for defining a reasonable gas-well deliverability of low permeability coal reservoirs.