Paradox Valley Unit Saltwater Injection Project - Induced Seismicity Potential in Energy Technologies - page 218

events. After analysis the injection rate was decreased by one-third from 345 gallons per
minute to 230 gallons per minute. The program of ceasing injection for 20 days twice per
year was also continued from June 2000 to January 2002 as were the lower injection rates.
In January 2002 the injection fluid was changed to 100 percent brine water from a mix-
ture of 70 percent brine with 30 percent freshwater, which was the injection mixture from
the start of the project. This heavier fluid increased the hydrostatic pressure measured at
the bottom of the injection well but no difference in the rate of induced seismicity resulted
from this change.
After monitoring injection into the Paradox Valley Unit injection well for almost
15 years, the Bureau of Reclamation has recorded over 4,600 induced seismic events.
The largest seismic event occurred on May 27, 2000, and had a magnitude of 4.3 (see
Figure K.1). After reviewing data on injection volume, injection rate, downhole pressure,
and percent of days injecting, the Bureau of Reclamation noted, “Of the four injection
parameters investigated, the downhole pressure exhibits the best correlation with the oc-
currence of near-well seismicity over time” (Bureau of Reclamation, 2009). The Bureau
of Reclamation also noted the record of seismic activity appears to be divided into three
distinct clusters occurring from 1997 to January 2000, 2003 to 2005, and July 2008 to the
present. The Bureau of Reclamation concludes, “There appears to be a gross correlation
between the three periods of increased near-well seismic activity and periods of increased
time-averaged injection pressures” (Bureau of Reclamation, 2010). These conclusions re-
iterate the results of other investigations into the cause of induced seismicity initiated by
underground injection.
The Bureau of Reclamation continues to inject saline fluids underground as part of
the Colorado River Basin Salinity Control Project, and it continues to control induced
seismicity by the biennial shutdown of injection activity and by limiting the volume of fluid
injected. Both of these actions minimize downhole injection pressure in an effort to limit
induced seismic events.