| Some fractured rock formations hold important resources, such as water, hydrocarbons or heat, whereas others are good candidates for waste disposal. Hydromechanical well tests have been developed to improve characterization of formation properties and parameter distributions by measuring displacement along with pressure while stressing a well. The displacement that occurs during a well test depends on the geometry of fractures or other sources of permeability, as well as the distribution of compliance or elastic modulus. Current methods of hydromechanical well testing, measures axial displacements along a wellbore, which may cause ambiguity in interpretations when 3-D components of deformation are present. The objectives of this research are to develop an instrument that can measure deformation of rock enveloping wells in three dimensions using technology that will facilitate applications over a wide range of conditions, and demonstrate the performance of this device during well tests in shallow wells in fractured rock. The approach is to use fiber optic strain gauges attached to a flexible coupling, which can move through 5 degrees of freedom. The device that was developed, which will be called the 3DX, has the capability to be lowered in a borehole, anchored into a desired section for testing, and deployed in a different location or removed from the borehole. Transverse displacement is calculated by taking the difference of strain measured between two opposing gauges, whereas axial displacement is calculated from the average displacement of the gauges. The RMS noise level of a 5-minute sample at 1 Hz is 80 nm in the axial direction, and 250 nm in the transverse direction based on field data and calibration factors determined in the lab. Field tests were conducted in Japan in the summer of 2011, and in Clemson, SC and Trenton, NJ during summer, 2012. The field tests show that the 3DX compressed axially approximately 3 to 6 μm per meter of drawdown and extended axially when the pressure increases. The transverse displacement was approximately 6 to 12 μm towards the south-southeast during the tests at Clemson in fractured gneiss at a depth of 25m. This is similar to the tilt direction from tiltmeters, providing a field validation of the measurements. The interpretation is that tilting is caused by a fracture that strikes roughly E-W and dips to the south. During ambient conditions in an open borehole the 3DX responds to changes in barometric pressure by closing approximately 0.15 with a 2 cm change in head when the water level rises and opening similarly when the water level drops.;The results suggest that 3D displacement can be measured during pumping and ambient monitoring in a rock enveloping borehole. The inclusion of the transverse displacement signal allows fracture orientation to be evaluated. |
