High-pressure fluid injections cause transient pore pressure changes
over large distances, which may induce seismicity. The zone of influence
for such an injection was studied at high spatial resolutions in six
decameter-scaled fluid injection experiments in crystalline rock. Pore
pressure time series revealed two distinct responses based on the lag
time and magnitude of pressure change, namely, a near- and far-field
response. The near-field response is due to pressure diffusion. In the
far-field, the fast response time and decay of pressure changes are
produced by effective stress changes in the anisotropic stress field.
Our experiments prove for the first time that fracture fluid pressure
perturbations around the injection point are not limited to the
near-field and can extend beyond the pressurized zone.