Hydromechanical Well Tests in Fractured Rock

Hydromechanical well tests involve measuring and interpreting displacements along with pressures in pumping or observation wells.  Displacements occur in response to the distribution of formation properties so measurements of displacement can be used to augment the conventional approach of interpreting pressure signals during well tests. Hydromechanical well tests have been used for a half century, with early investigators employing pipe extensometers spanning the full length of a borehole.  Recent refinements use downhole extensometers and tiltmeters that temporarily anchor to the wall of a borehole.   A new tool combines an extensometer with a tiltmeter to measure axial displacements and rotations at the same location.  The tool can resolve displacements of +/- 5nm and tilts of less than +/- 30 nrad.  The device is isolated between packers and moved along the borehole to characterize the distribution of formation properties with depth.  Field data are analyzed using models that include fractures that deform in response to pressure changes, and some data require using analyses that include both deformable fractures and the poroelastic response of the matrix.  Key findings from this work show that 1.)  Fractures move measureable amounts during well tests and during ambient conditions; 2.) Apparent fracture stiffness (and specific storage) changes with time during well tests; 3.) Reverse water level response during slug tests in fractured rock is a hydromechanical effect;  4.)  Deformation is sensitive to heterogeneity within the fracture;  5.)  the matrix compresses when a fracture dilates;  6.)  Deformation is sensitive to the dip of a fracture, tilting up-dip during pumping and downdip during injection.