The Colorado River Basin Salinity Control Project is located in Montrose County, on the western border of Colorado. The project diverts naturally occurring seepage of salt brine that would normally flow into the Delores River (and then into the Colorado River) and injects the brine underground. The project is operated by the U.S. Department of the Interior, Bureau of Reclamation. Due to concerns of induced seismicity, seismic data for this project have been continuously recorded and analyzed since the project began in 1996 in order to understand and mitigate the effects of any induced seismic events.
The Paradox Valley Unit (PVU) is a group of wells that are part of this project. The brine is produced from nine extraction wells before it can flow into the Delores River. The brine is then injected into one disposal well. The well is located near the town of Bedrock, Colorado, approximately 1 mile southwest of the extraction wells. The well injects the brine into a limestone formation at a depth of approximately 14,100 to 15,750 feet. The project began in July 1996 with an initial injection rate of 345 gallons per minute at a pressure of 4,900 psi. Current injection rates are approximately 230 gallons per minutes at a pressure of 5,300 psi.
The possibility of induced seismicity was addressed during the planning stages of the PVU injection program because the Paradox Valley Unit injection program was comparable to both the injection programs at the Rocky Mountain Arsenal northeast of Denver and the water injection program for improved oil recovery at Rangely, Colorado. Eight years before injection was begun at the PVU site, the Bureau of Reclamation commissioned a seismic monitoring network to measure the seismic activity in the Paradox Valley region. The original network consisted of 10 seismic monitoring stations. The system was upgraded to 16 stations after the injection began in 1996 and currently totals 20 stations.
Earthquakes were recorded almost immediately after the beginning of injection in July 1996 with the first seismic event measured in November 1996. Minor earthquakes continued through mid-1999, and two magnitude 3.5 events occurred in June and July 1999. In response to the higher-magnitude earthquakes, the Bureau of Reclamation initiated a program to cease injection for 20 days every 6 months. Prior to these events they had noted the rate of seismicity had decreased during the shutdowns following unscheduled maintenance. The Bureau of Reclamation hoped stopping injection twice yearly would allow time for the injection fluid to diffuse from the pressurized fractures into the rock matrix.
After a magnitude 4.3 earthquake occurred in May 2000, PVU stopped injection for 28 days to allow evaluation of the injection program and its relationship to induced seismic
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.
FIGURE K.1 Twenty-year data set collected by the Bureau of Reclamation for the Paradox Valley project. Upper figure shows the average daily injection flow rate in gallons per minute. Lower figure shows all induced events and their magnitudes over the same period with distance from the injection well. SOURCE: Block (2011).
Block, L. 2011. Paradox Valley Deep Disposal Well and Induced Seismicity. Presentation to the National Research Council Committee on Induced Seismicity Potential in Energy Technologies, Dallas, TX, September 14.
Bureau of Reclamation. 2009. Overview of PVU-Induced Seismicity from 1996 to 2009 and implications for Future Injection Operations. Technical Memorandum No. 86-68330-2009-22.
Bureau of Reclamation. 2010. 2009 Annual Report Paradox Valley Seismic Network, Paradox Valley Project, Colorado. Technical Memorandum No. 86-68330-2010-07.