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 mixture 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 occurrence 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 reiterate 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.