微裂缝性特低渗透油藏单相流体渗流特征

Monophase porous flow characteristics in micro-fractured extra-low permeability reservoirs

微裂缝性特低渗透油藏具有特殊的渗流运动规律。通过对不同渗透率的微裂缝性特低渗透率岩心进行越流压力实验测试,发现岩心的导流能力基本由微裂缝提供,基质贡献极其微弱,基本没有窜流现象发生,其越流压力曲线形态接近于单一介质越流压力曲线形态。利用不同粘度的流体对不同渗透率的微裂缝性特低渗透率岩心进行单相渗流动态测试,结果表明,渗流曲线呈下凹型非线性曲线形态,但真实启动压力梯度比较大,且在渗透率较小时,启动压力梯度剧增,弯曲段比较长,拟直线段出现时间比较晚,流体的密度和粘度、油藏渗透率及启动压力梯度等均应考虑为压力的函数。在分析对比越流压力曲线和渗流曲线特征的基础上,建立了描述微裂缝性特低渗透油藏渗流特征的运动方程,该方程综合考虑了启动压力梯度和压敏效应等影响因素,是达西定律的推广形式。

Micro-fractured extra-low permeability reservoirs take on peculiar porous flow characteristics. Overflow pressure experiments on micro-fractured extra-low permeability sandstone cores with various permeabilities show that it is the micro-fracture instead of matrix that provides the flow conductivity mainly, and almost no crossflow phenomenon occurs, therefore the overflow curves are similar to those of single medium. The porous flow curves of micro-fractured extra-low permeability sandstone cores are tested with various-viscosity fluids in laboratory. It indicates that the curve is still of nonlinear and concave, but the real threshold pressure gradient is larger,which increases sharply with permeability decreasing,and the flections of the porous flow curve is longer, quasi-line emerges later. So the density and viscosity of the fluid, reservoir permeability and threshold pressure gradient are supposed to be the function of pressure. On the basis of the experiment results,a flow equation about porous flow in micro-fractured extra-low permeability reservoirs is presented. The equation is the popularized form of Darcy Law, which considers the effect of threshold pressure gradient, pressure sensitivity.