Monday, February 4, 2013

Mining and Quakes

A study on the risk of man induced seismic activity has been carried out by the U.S. National Academy of Sciences. Environmental groups and special interest entities have focused on earthquakes as a possible result of hydraulic fracking where water and trace chemicals are pumped into hydrocarbon sites to facilitate petroleum extraction. There is a tremendous amout of information available from a surprising amount of mining efforts. This is from that study. 
A British government enquiry held fracking responsible for a series of tremors in April and May of 2011 at Preese Hall near Blackpool, the largest of which had a magnitude of 2.3 on the Richter scale, big enough to be felt by local residents.
Fracking has also been linked to a "swarm" of 50 small earthquakes in Oklahoma on Jan 18, 2011, ranging in magnitude from 1.0 to 2.8, which occurred at the same time as the fracking of a well in the Eola Oil Field. Some reported changes near Ohio fields have been suspected as well.

The study of effects of underground injection is not new. There has been considerable experience with this phenomenon in other areas, particularly in geothermal energy production and pumping CO2 underground for the purpose of carbon retrieval and storage projects. Seismic events have been traced to conventional oil and gas extraction, secondary oil recovery through waterflooding, geothermal energy, waste water injection and hydroelectric reservoirs. In some of these projects the volumes and pressures have been considerable.

The most famous example of induced seismic activity occurred in the 1960s at the Rocky Mountain Arsenal near Denver, Colorado, where the U.S. Army disposed of hundreds of thousands of gallons of contaminated water from chemical weapons production every day by pumping it into a well drilled on site. Between 1962 and 1967, more than 1,500 earthquakes were recorded in the area, some as high as magnitude 3 or 4, and noticed by local residents. "By November 1965, over 700 shocks had been recorded, and although 75 of these had been felt, no damage was recorded," according to previous research cited by the National Academy.  "Research indicated a strong relationship between injection volumes and earthquake frequency ... Although injection into the Arsenal well ceased in February 1966, earthquake activity continued for several more years. The strongest earthquakes actually occurred after injection into the well was discontinued."

Such events are also documented at The Geysers geothermal power project in northern California. The Geysers generates about 725 megawatts of electricity, enough to power a city the size of San Francisco, and supplies about 60 percent of the average power demand in the state's northern coastal region. Originally, the field produced power from steam flowing up naturally from 420 wells on the site. But as the underground pressure depleted, water injection commenced, which coincided with a significant rise in seismic activity, partly because of temperature differences between the injected water and hot rock formations. More than 1,000 seismic events with a magnitude of at least 1.5 are recorded every year, with around 25-30 reaching magnitude 3.0, and between one and three hitting magnitude 4.0 or more.

Between 1957 and 1983, aggressive gas production from the Lacq gas field in southwest France caused the pressure to drop from 660 bars to 160 (1 bar is roughly equal to normal atmospheric pressure). Some 800 seismic events with magnitudes ranging up to 4.2 were recorded as the pressure plunged.  These events are relatively rare compared with the large number of oil and gas fields around the world. Similarly, there are 151,000 injection wells licensed in the United States for waterflooding, enhanced oil recovery (EOR) or waste water disposal and documented instances of induced seismic activity remain very infrequent.

The most important risk factor is pressure changes linked to the net fluid balance (the total amount of fluid injected or withdrawn from the subsurface). The probability of triggering a significant seismic event increases with the volume of fluid injected (or withdrawn).

To trigger a magnitude 3 earthquake, a fault must rupture over an area of approximately 15 acres, increasing to 0.5 square miles for a magnitude 4 event, and 4.2 square miles for magnitude 5. The largest reported induced seismic events have all involved the injection of huge volumes that caused faults to rupture over a correspondingly large area. At Rocky Mountain Arsenal, the U.S. Army injected as much as 148-181,000 gallons (3,500-4,300 barrels) of water a day for years at a time. Massive injections into poorly selected rock formations induced 3 earthquakes of magnitude 5.0-5.5, the largest of which caused estimated damage of $500,000 in 1967.

Fracked wells simply do not inject a large enough volume of fluid over a big enough area to cause seismic events on this scale. In contrast, water injection at The Geysers has now hit more than 300 million barrels per year (partly offset by steam withdrawals). And carbon storage on utility-scale would involve the injection of truly staggering amounts of supercritical (gas/fluid) carbon dioxide into saline aquifers or unmineable coal seams.

I wouldn't rule out a Matt Damon movie, though.

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