Interpretation of Field Tests in Low Permeability Fractured Media. Recent Experiences

We have been treating fractured media as the result of embedding conductive 2D fractures in a 3D continuum medium. Automatic calibration has normally succeeded in producing models capable of predicting independent data sets. However, in recent times, we have faced a set of tests performed in very low conductive, highly heterogeneous granite where using exclusively the standard procedure has not produced good results. Difficulties include the following: (1) complex fracture geometry; (2) spurious effects caused by ill-shaped tetrahedra; (3) weak responses to pumping; (4) superposition of natural head data (controlled by boundary conditions) with drawdowns caused by pumping; (5) coupling these two flow conditions to produce the flow field needed for the tracer test is extremely sensitive to small errors. Finally, these difficulties are aggravated by the large computer times required by a fully 3D medium. Suggestions for overcoming these problems are outlined.