Interpreting Mechanical Displacements During Hydromechanical Well Tests in Fractured Rock

Hydromechanical well tests involve measuring and interpreting mechanical displacements of an aquifer that accompany the transient pressure signal during hydraulic well tests. The displacement signal can be analyzed along with the transient pressure record to estimate characteristics of the aquifer. We have used a precision extensometer between straddle packers to measure axial displacements during both slug tests and pumping tests in fractured crystalline rock. The field data show an apparent normal compliance of hydraulically active fractures in the range of 1 to 5 microns of displacement/(m of head change in the wellbore). However, during the more than 200 hydromechanical pumping and slug tests we have conducted, the displacement has always appeared as 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 is also typical of results from theoretical analyses using a discrete fracture model that considers coupled fluid flow and deformation. As a result, 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, and the theoretical analysis indicates the compliance approaches the inverse of the fracture normal stiffness at late times. This hydromechanical effect occurs because opening displacements are relatively small when the pressure change is limited to a small region of the fracture early in a test. This causes the apparent compliance to be less than it is later in the test when the pressure change has spread over a larger area of the fracture. Compliance is closely related to storativity, a basic aquifer parameter commonly assumed to be constant. These results indicate that storativity may actually change during a well test. One consequence is that variations in drawdown curves that are interpreted as resulting from variations in aquifer properties or transient changes in flow dimension may instead result from transient changes in storativity produced by this hydromechanical effect.