Development of a 3d Fbg Extensometer for Hydromechanical Well Testing

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. 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 99 mm (3.9 inches) in diameter. The device was first tested at a site underlain by fractured granite in Tsukuba, Japan, where pumping tests were performed in an open borehole with the 3DX deployed at a depth of 21 meters. Drawdown of approximately 10 m during constant-rate pumping tests was accompanied by both axial and transverse displacements on the order of microns. The axial displacement signal is similar in magnitude and pattern to signals obtained at other sites using a uniaxial extensometer. Both the new and previous signals indicate tenths of a micron of displacement occur per meter of drawdown, for example, and both indicate that displacement is a hysteretic function of head in the wellbore. These similarities partly validate the response of the new device. The results are encouraging because including transverse components of displacement promises to markedly reduce the non-uniqueness of interpretations when dipping fractures occur in the subsurface, and moreover, the 3D displacement sensor is relatively simple so it should be readily deployed in a variety of applications.