Energy Technologies: How They Work and Their Induced Seismicity Potential - Induced Seismicity Potential in Energy Technologies - page 57

Primary Oil and Gas Production
Although felt seismic activity known to be related to primary petroleum production is
uncommon relative to the large number of operating oil and gas fields worldwide, with-
drawal (extraction) of oil and gas has been linked to felt seismic events at 38 sites globally,
20 of which were in the United States (Appendix C; Box 1.1). These have included events
in Texas, Oklahoma, California, Louisiana, Illinois, and Nebraska, the majority of which
have been of M < 4.0 (Appendix C; see also Chapter 1); the well-documented events at
the Lacq gas field in southwestern France (see Box 2.5); and the large events in the Gazli
gas field in Uzbekistan (Box 3.4). Withdrawal of oil or gas from the subsurface can result
BOX 3.4
Induced Seismicity Related to Natural Gas Extraction:
A Case from Gazli, Uzbekistan
The Gazli gas field is located about 500 miles (800 km) east of the Caspian Sea in a generally aseismic
region of Uzbekistan. The gas deposits were discovered in 1956 and gas production began in 1962. The
gas field lies within a large (38 km by 12 km [22.8 mile by 7.2 mile]) asymmetrical anticline over crystalline
rocks. Large volumes of water were injected between 1962 and 1976 to enhance production, but sub-
sidence and reduced gas pressures were reported despite this injection; the initial pressure in the gas field
of about 70 atm (~71 bars or 1030 psi) in the 1960s decreased to about 30-35 atm (~30.4-35.5 bars or
435-515 psi) by 1976 and to about 15 atm (15.2 bars or 218 psi) by 1985. This pressure decrease indicates
a net removal of mass, even with injection of large volumes of water. Thus, although the field operators had
begun to use secondary recovery techniques (waterflooding), the cause of the earthquakes is attributed to
pressure decrease due to fluid withdrawal.
On April 8, 1976, a M ~ 7 earthquake occurred about 20 km (12 miles) north of the gas field boundary.
This was followed by another M ~ 7 earthquake on May 17, 1976. A third large earthquake (also M ~ 7)
occurred on March 20, 1984. All three earthquakes had epicenters 10-20 km (6-12 miles) north of the gas
field boundary, over an east-west distance of about 50 km (30 miles). Reported hypocentral depths of these
large earthquakes were 10-15 km (6-9 miles). Geodesic measurements indicated surface uplift of some
70-80 cm (~28 to 31.5 inches) north of the gas field at the epicentral locations of the three large earth-
quakes; this uplift is consistent with thrust movement on faults dipping to the north. However, source modeling
indicates that the ruptures progressed downward, which is uncommon for thrust mechanism earthquakes.
The locations and magnitudes of these large earthquakes were determined from worldwide seismographic
data and are therefore somewhat uncertain, leading to some uncertainty on the causal relationship between
gas extraction and earthquake activity. Nonetheless, observations of crustal uplift and the proximity of these
large earthquakes to the Gazli gas field in a previously seismically quiet region strongly suggest that they
were induced by hydrocarbon extraction.
SOURCES: Adushki et al. (2000); Grasso (1992); Simpson and Leith (1985).