Project Report

Oil, natural gas, heat, and water can be recovered from subsurface formations and CO2 can be stored there, so improving the understanding of storage characteristics in porous or fractured materials has wide ranging applications in underground laboratories or other settings. Pumping water from a well and measuring the pressure change with time is one method used to characterize aquifers. Storativity is a common parameter that describes volume of water released from storage per unit decline in head per unit area, but it weakly affects the pressure signal during pumping. This creates a non-uniqueness problem wherein different storativity values can produce the same pressure signals. A hydromechanical well test involves measuring fracture displacement during pumping, and can solve the non-uniqueness issue because its signal depends on the storativity of the aquifer. Another considerable problem is if the fractures are dipping then the axial displacements observed will be an underestimate of the actual displacement of the fracture. Including the transverse components of displacement promises to markedly reduce the non-uniqueness of interpretations when dipping fractures occur in the subsurface. We hypothesize that fractures displace in three dimensions and being able to correctly measure axial and transverse displacement of fractures will more accurately characterize of subsurface formations. The ultimate goal will be to advance the recovery of resources or the sequestration of wastes, like spent nuclear fuels or CO2. With this in mind, we have developed a device to measure the deformation of boreholes in three dimensions in order to improve understanding of how storage changes occur during pumping from a well. The device uses two anchors that grip the borehole wall and are separated by a rigid rod and a flexible coupling. Strain caused by relative displacement of the anchors is concentrated in the flexible coupling, and a suite of fiber bragg grating strain gauges is mounted on the coupling to measure how it flexes. Those data are then used to calculate the deformation in three orthogonal directions (including the borehole axis) as the borehole deforms in response to changes in water pressure. A prototype device, which we call a 3DX (3D extensometer), was developed to be used in an open borehole with diameters between 86 mm (3.4 inches) and 99 mm (3.9 inches) in diameter. The main accomplishment from my research experience is that I demonstrated the feasibility of measuring 3D deformation in a well during pumping. This is one of the first times this measurement has been made, and it is first time it was made with a simple, portable device. The experimental data are similar in magnitude and trend to previous findings, so we are encouraged that the data are accurate. The device was first tested at a site underlain by fractured granite in Tsukuba, Japan, where a total of six field tests were conducted, with modifications in the equipment and procedures made after each test. The best results occurred during the fifth test. The 3DX was left in the borehole overnight, which allowed it to equilibrate with the surroundings. First a slug was inserted into the borehole, which raised the head by a meter and expanded the fracture by about 0.2 microns. After this the head was dropped four meters and we saw 0.8 microns of compression. The axial compression of the borehole was found by averaging the displacement experienced by the four gauges. The magnitude of the transverse displacement was 12 microns across one set of opposing gauges, and less than 1 micron of transverse displacement occurred across the other set of opposing gauges. The difference in displacements measured in opposing gauges shows that we were able to measure the transverse displacement of the fracture, while at the same time measuring average compression leads us to concluding that we can measure fractures in 3D.

National Science Foundation (NSF)
Office of International and Integrative Activities (IIA)
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Carter Kimsey
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Skawski Glenn M
United States
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