Laboratory Hydraulic Fracturing Experiments for Determining Reopening and Closing Pressures of Fractures

We carried out laboratory hydraulic fracturing tests, which revealed that the pressure at the reopening and closing of fractures during hydraulic fracturing can be detected by monitoring the strains in the borehole wall and the triggering of acoustic emissions.In the experiments, we measured the pressures during the reopening and closing of a fracture caused by elastic restitution in a block of rock not subjected to compressive load. The pressures are used for correcting those obtained in blocks of rock when biaxial compressive load is applied. The reopening pressure (Pr)is smaller than that obtained by the conventional method. This result expresses the following theoretical relationship.Pr=(3Sh-SH)/2where SH is the maximum horizontal stress, and Sh is the minimum horizontal stress.The corrected reopening pressure of fracture agrees to within 0.7 MPa with that obtained from the above equation.