Measurement of Crack Opening Displacement during Hydraulic Fracturing Test and Crack Propagation

The measurement of the crack opening displacement during a hydraulic fracturing test is examined to estimate the crack propagation behavior. Hydraulic fracturing tests in granite were carried out under no confining pressure in the laboratory. During the test, the borehole deformation was measured by the clip–gauge–type sensor installed on the center of the injection unit. Three sensors were used to measure the deformation of three different orientations around a borehole at the same time. The pressure-displacement curve indicates the proportional relation initially, and abrupt increase in the deformation is observed at the breakdown. The elastic modulus is estimated from the initial tangent of the pressure displacement curve. The crack opening displacement, when the fluid pressure starts to act to the fracture surface, is determined from the deformation behavior. The results of the measurements of the displacements in different orientations around the borehole indicate the direction of the crack propagation induced by the hydraulic pressure. Finally, using the measured crack opening displacement, the final crack size is estimated approximately based on the linear elastic fracture mechanics.