Types and Causes of Induced Seismicity - Induced Seismicity Potential in Energy Technologies - page 31

With the cooperation of the Chevron Oil Company, which operated the field, USGS researchers carried out
a controlled induced seismicity experiment beginning in November 1970 and continuing to May 1974 (Raleigh
et al., 1976). One goal of the experiment was to quantitatively test the effective stress theory for activation of slip
on preexisting faults by pore pressure increases (Box 2.1). This portion of the experiment entailed a program
of careful measurements of the parameters involved in the Coulomb criterion (Box 2.1), including in situ stress
measurements, monitoring and modeling of changes of reservoir pore pressures, laboratory measurement of
the sliding resistance between rock surfaces in the reservoir formation where seismic events were occurring,
and detailed seismic monitoring to precisely locate the events and determine the fault orientation with respect
to the stress field. Together these measurements, when used with the Coulomb criterion expressed in terms of
the effective stress, predicted that a critical reservoir pressure of 257 bars was required to induce earthquakes
at an injection site within the cluster of earthquakes—a result that agreed with the observed and modeled pore
pressures. The second phase of the experiment turned seismic events “on” and “off” by cycling the pore pres-
sures above and below the critical reservoir pore pressure of 257 bars (25.7 MPa) (Figure 2). This experiment
proved that induced seismic events could be controlled by regulating the pore pressures.
Figure 2 Frequency of seismic events at Rangely. Stippled bars are seismic events within 1 km of the experi-
mental wells. The clear bars represent all other events. Pressure history in well Fee 69 is shown by the heavy line
and predicted critical pressure is designated by the dashed line. SOURCE: Raleigh et al. (1976).