Interactions Between Fluid and Fractures During Well Tests in Fractured Rock

Deformation during hydraulic well tests causes basic aquifer properties to change, and the displacement signal can be interpreted to improve characterization of fractured aquifers. These conclusions are based on results and analyses of hydromechanical well tests, which involve measuring and interpreting displacements of rock along with the transient pressure signal resulting from hydraulic well tests. We used a precision extensometer between packers to measure axial displacements during slug and pumping tests in fractured biotite gneiss. The field data from several 100 tests show that fractures typically open or close with an apparent normal compliance of 1 to 5 microns of displacement per m of head change in the wellbore, although some fractures were stiffer than this. The displacement is a hysteretic function of the well bore pressure; that is, displacements are smaller earlier in the test than they are at the same pressure late in the test. This hysteretic behavior can be predicted using a discrete fracture model that considers coupled fluid flow and elastic deformation. Both field and theoretical data indicate that during well tests the apparent compliance of a formation can increase by a factor of 10 or more. Compliance is proportional to storativity, so a 10-fold increase in compliance means that the storativity increases by a factor of 10. During slug tests, the fracture continues to open while the wellbore pressure falls early in the test, which produces the peculiar result of a negative storativity. These changes in S stabilize and approach the value determined by interpreting typical hydraulic well tests at late times when relatively isolated fractures are tested. However, a different result occurs when the primary fracture being tested is cross-cut by other fractures roughly parallel to the borehole. The cross-cutting fractures cause water to leak out of the primary fracture, changing the pressure distribution within it, and reducing the resulting displacement. The transmissivity of a fracture is proportional to the cube of its aperture, so relatively small changes in aperture can cause significant changes in T. For example, normalized transmissivity of biotite gneiss is sensitive to pressure by approximately 0.01/m to 0.04/m (this means that T varies by 1 to 4 percent per m of drawdown). Pressure sensitive T has been reported in soft sedimentary formations, but it appears to be significant in some situations in fractured gneiss. Displacements during hydromechanical well tests are sensitive to the properties and geometries of fractures in the vicinity of the well, so inverse methods can be used to estimate characteristics of fracture networks. Recent analyses have predicted the occurrence of leakage and blockages in primary fractures, which appear to be confirmed with interference tests in nearby boreholes. Preliminary results also suggest that it may be possible to identify fractures that are softened by weathering or stiffened by mineralization.