HydroMechanics of Fractured Rock

Overview

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HYDROMECHANICAL (HM) WELL TESTS
Conventional hydraulic well tests involve stressing a well and interpreting the resulting changes in pressure in the vicinity. Thepressure changes cause small displacements in the porous or fractured medium, and this provides a potentially useful signal that can be interpreted along with the pressure transients. We have been developing methods for conducting and analyzing hydromechanical well tests in an effort to improve understanding of fractured rock.

THEORETICAL ANALYSIS
Fractures either dilate or contract in response to head changes during hydraulic well tests. Dropping the hydraulic head will increase effective stress, compressing asperities on fracture surfaces and causing aperture to diminish. Increasing hydraulic heads during an injection test, or during the recovery following pumping, will relieve the effective stress on asperities and dilate fracture aperture. Significant increases in head during injection into a well may cause the fracture walls to separate completely and no longer be supported by asperities. Continued injection may elevate the stress intensity enough to cause propagation and the creation of new fracture surface by hydraulic fracturing. Field data are interpreted using a model that couples fluid flow and deformation of a flat-lying, circular fracture.

FIELD TESTS
A distinctive characteristic of hydromechanical well tests is the hysteretic relationship between displacement and well bore pressure. This is shown below in the data from a constant rate pumping test (lower left) and from a slug test (upper right). Interestingly, the loop-like shape of these data appears in nearly all the field tests we have conducted thus far, and in the theoretical analyses. The shape of the loop differs between these tests, however, and the plots create a unique fingerprint sensitive to formation properties.

REMOVABLE BOREHOLE EXTENSOMETER
The field technique requires a borehole extensometer for measuring the small axial displacements accompanying HM well tests. We built our first working prototype in 2002, and new designs were developed in 2003 and 2004 that have been tested to submicron repeatability.