Types and Causes of Induced Seismicity - Induced Seismicity Potential in Energy Technologies - page 35

of these kinds of rocks, the potential exists to induce pore pressure increase at considerable
distances from the injection well and thus trigger slip on faults that are located kilometers
away from the injection source.
Seismicity induced by fluid withdrawal cannot be explained without taking into account
the accompanying stress changes, which are associated with the large-scale contraction of
the reservoir caused by pore pressure reduction or uplift caused by removal of a significant
mass of hydrocarbons. The magnitude of the events can be potentially large, because the
stress change takes place over areas that are similar in size to the reservoir. However, to
trigger an earthquake requires the initial state of stress to be very close to critical, because
the perturbation of the stress is minute compared to the magnitude of the pore pressure
reduction. For example, in the well-documented Lacq gas field (France) the increase of the
maximum shear stress was estimated to be about 0.1 MPa (1 bar) in regions surrounding
the reservoir for a pressure drop of 30 MPa (300 bar) in the reservoir.
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