利用瞬态法提取岩样非Darcy流渗透特性

建立了一种测定岩样非Darcy流渗透特性试验系统的动力学模型.基于岩样两端孔隙压差的时间序列,利用差分计算或曲线拟合得到渗流速度及其变化率的时间序列.通过线性回归,得到岩样的Forchheimer非Darcy流渗透特性,即渗透率、非Darcy流β因子和加速度系数.试验结果表明,无论岩样处于峰前还是峰后应力状态,岩样中的渗流都不服从Darcy定律;当非Darcy流β因子为正时,非Darcy流的渗透率κ小于Darcy流的渗透率κD;渗透率(κ和κD)、非Darcy流β因子和加速度系数可近似表示为应变的幂指数函数;三种渗透特性中的每两种整体上存在幂指数关系;在加载的初始阶段,由于孔隙和微裂隙的压缩和闭合,渗透率随应变减小,而非Darcy流β因子和加速度系数随应变增大;随后,由于裂隙的扩展,岩样的渗透率迅速增大,而非Darcy流β因子和加速度系数迅速减小,并在峰值应力附近达各自的极值;由于围压的作用,峰后应力状态下岩样的渗透特性随应变的变化缓慢.图3,参18.

Pick-up of the Permeability Parameters of Non-Darcy Flow in Rock by Transient Permeating Method

A dynamic model of the test system which was used to measure the permeability parameters of non-Darcy flow in rock specimen was established. On the basis of a single time-series of pressure difference between the two ends of the rock specimen, the time-series of the seepage velocity and its rate of change were calculated by the finite difference or curve fitting. By the linear regression with three variables, the permeability,factor, and the acceleration coefficient of the Forchheimer's non-Darcy flow in rock specimen were obtained. Results show that the seepage in rocks does not obey the Darcy's law whether the rock specimen is under a failed state or under a non-failed state. The permeability of rock specimen obtained by the analysis on a dynamic process of non-Darcy flow is less than that obtained by the analysis on the transient process of Darcy's flow kD. The permeability (both k and kD), the factor of non-Darcy flow and the acceleration coefficient can be approximately expressed as a power exponential of the strain of rock specimen. Each two of them satisfies power exponent function on the whole. Due to the compression and closure of the porous and the micro-crannies in the rock specimen during the initial stage of loading, the permeability decrease with the increasing of the strain, contrarily, the actor of non-Darcy flow and the acceleration coefficient increases with the strain. Subsequently, due to the expansion of cranny and the connection of porous path, the permeability of the rock specimen varies rapidly with the increasing of strain, and reaches its extreme value near the peak stress state. Under the action of confining pressure, the permeability parameters of rock specimen under post-failure state changes slowly. 3figs.,18refs.