Hydraulic impedance technique for the characterization of unsaturated fractured rock

Abstract

A hydraulic impedance technique was developed by using an impeding layer and conductance layer. Laboratory experiments show that the developed technique is promising for the in situ characterization of unsaturated hydraulic properties of fractured rock, such as fracture flow and effective transmissivity as a function of matrix suction. An estimated suction boundary condition at a borehole surface can be imposed using this impedance technique and flow into the fractured rock can be measured. Because the conducting properties of saturated fractures are normally much higher than the rock matrix, an estimation of saturated fracture transmissivity is possible. Then as the suction at the borehole surface is increased, the fracture transmissivity decreases, yielding information about the drainage characterization of the fracture. The suction at which fracture flow becomes insignificant is critical to understanding fracture versus matrix flow in fractured rock. C125 cellulose tubular membrane with an average conductance of 2.64x10⁻⁸ s⁻¹ was selected as the impeding membrane, the conductance of the membrane decreases when the initial saturation decreases. To minimize the variation in conductance, the membrane must be presoaked for 100 hours. 10-pound canvas was selected as the conductance layer, the transmissivity (T, cm⁻²/s) of the canvas is an exponential function of the confining pressure (P, kPa), expressed as T(P)=0.0274e⁻⁰•⁰²⁹⁶ᴾ. The canvas layer can yield a relative constant suction along the membrane/rock interface as shown by modeling. A 1-D radial analytical flow model can be used on a small scale to simulate flow if the boundary conditions can be specified for the technique and the effect of gravity is insignificant.