Quantifying fractured crystalline-rock properties using well tests, earth tides and barometric effects

SummaryCharacterization of fractured rock aquifers often requires the acquisition and analysis of diverse datasets obtained from various instrumentation configurations. In this investigation at the fractured rock research site in Floyd County, Virginia, a high-precision borehole extensometer and tiltmeter were used during pumping to monitor deformation in the vicinity of fractures identified from borehole logging. Strain data obtained from earth tide analyses were used with the extensometer and tiltmeter data to quantify hydromechanical properties, including fracture volumetric specific storage, porosity, Poisson’s ratio, the drained formation elastic modulus, and the effective dip direction of the fracture. Borehole tiltmeter data were used to estimate deformation caused by an aquifer test consisting of three pumping and recovery periods performed in well EX-1. During each period of the aquifer test the extensometer, located in W-03 and 27.7 m from the pumping well, was anchored over 2-m-long sections of (1) a fracture in hydraulic communication with EX-1, (2) a fracture that is not hydraulically connected with EX-1, and (3) an unfractured portion of bedrock directly above the hydraulically connected fracture. Results from the pumping tests yielded compressibilities of 1.3 × 10−11 Pa−1, and 1.7 × 10−11 Pa−1 for the lower and upper fractures, respectively. When coupled with areal strain calculated from earth tide analyses the volumetric specific storage values are 3.2 × 10−11 Pa−1, and 2.8 × 10−11 Pa−1 and the Poisson’s ratios are 0.26 and 0.31, respectively. Using this with a calculated barometric efficiency of 0.45 allows for porosity calculations of 0.02 and 0.03, respectively for the vicinity of fractures in well W-03.

► First aquifer test using both tiltmeters and borehole extensometers.
► Fracture characterization accomplished using multiple stress-strain signals.
► Fracture strain and tilt measured in response to cyclical pumping.
► Earth tides and axial strain allowed for calculation of Poisson’s ratio and porosity.
► Strain signals allow for calculation of axial and volumetric specific storage.