The 2008-20092009 Earthquakes Near Dallas-Fort Worth and Cleburne, TX See also paper in Leading Edge March 2010 Cliff Frohlich Institute for Geophysics University of Texas Eric Potter Bureau of Economic Geology University of Texas Brian Stump Chris Hayward Huffington Department of Earth Sciences Southern Methodist University
Questions: Are the DFW quakes related to natural gas development activity? If so: Are they caused by drilling, hydrofracture, gas production, or disposal of frac brines? Answers: Probably Disposal of frac brines Should we worry? What should be done?
2008-2009 Earthquakes: Related to Natural Gas activity? 10 small earthquakes (largest, M3.0) occur near DFW Oct 31- Nov 1 2008 4 small earthquakes (largest M3.3) occur near DFW May 16 2009 7 small earthquakes (largest M3.3) occur near Cleburne, June- July 2009 No previous historical i seismicity Barnett Shale (gray) Barnett Shale (gray) heavily produced since 2002
Background: Historical Seismicity in Eastern Texas Small natural quakes occur in NE Texas (1997, 1985) Small induced quakes have occurred in Texas (1932 Mexia, 1978 Snyder, 1993 Fashing) Largest magnitude is M4.6 (Snyder); most M3 or smaller Q: Are 2008-2009 earthquakes natural or induced? Q: If induced, caused by
No good-quality local seismograph records were available, but all felt and USGS-located quakes were recorded clearly on Oklahoma station WMOK USGS reported 14 DFW earthquakes WMOK cross-correlation finds ~180 quakes (S-P) times all ~30.7 sec (all from same general location) None occurred before 30 Oct, after 17 May only two (on 31 December) red: located by USGS green: located by UT-SMU
SMU borrows seismographs, installs network Nov-Dec 2008 Black triangles: SMU temporary stations Red circles: locations of quakes k as reported t d by b USGS Yellow square: 1-km square area where Nov-Dec quakes were located (see next slide)) Additional analysis strongly suggests that USGS reported USGS-reported quakes also occurred within or very near yellow square
Seismograms for different quakes not identical (so not all from identical location, but (S-P) arrivals time differences readable with 0.02 sec precision (S P) ti i ti t (S-P) time variations up to ~0.10 sec
one-km yellow square depths near 4.5 km
Our preliminary locations on GoogleMap picture AFMOM CPSTX AFDAD
DFW earthquakes, DFW airport, and RR Commission data yellow triangles: Our locations for Nov-Dec 2008 quakes. Our preferred focal depths: 4.5±1.0 km. white-red symbols, producing wells procedure: - wells drilled - hydrofrac fluids pumped into wells to increase permeability - brines (and gas) produced - bi brines pumped dinto safe, very permeable strata for disposal blue squares: salt water disposal well permitted for disposal in Ellenberger 10,752-13,729 ft (3.3-4.2 km).
Timing of quakes and injection at south SWD well
Main Results: No historically reported earthquakes near DFW or Cleburne for 150 years prior to October 2008 10 Sept 2008: Injection of 10,000 BWPD commenced at SWD well south of DFW airport 30 October 2008: small earthquakes began occurring with locations within ~1 km of SWD well bottom DFW and Cleburne quakes apparently induced by disposal [not drilling, frac, or production] [No earthquakes reported at vast majority of SWD or production wells]
Regional gas production and disposal wells Barnett shale:5% of US domestic natural gas production DFW quakes not related to drilling, frac, or production wells No earthquakes near vast majority of fswd wells
Why quakes at DFW and Cleburne and not at every SWD well? Regional tectonic stress: stressmax: vertical stressmin: NW-SE NW SE stressint : NE-SW Favors motion along normal f l aligned faults li d along l di direction i of stressint : NE-SW Fluids reduce normal stress on locked fault, allowing it to slip
Should We Worry About Hazard from Induced Earthquakes? In Texas there are thousands of waterflooding projects/disposal wells; only a few quakes caused; largest M3.3 in 2008-2009; largest historically M4.6. NOT a serious hazard. Largest at DFW and Cleburne are M~3.3 For fault dimension D: log 10 D(meters)~0.5M+0.7 ~ 2 cm. M~3.3; D ~ 225 m; slip NONE larger than natural regional seismicity i i NOT scary. WOULD NOT concern citizens in places where quakes are common, but NOT studied thoroughly
Should We Worry About Hazard from Induced Earthquakes? NOT studied d thoroughly; hl not well understood d Geothermal, tight gas production, and carbon sequestration ti all involve fluid injection Very few states have a regional seismograph network capable of detecting/locating ti ti M2.0 earthquakes At present drilling/producing companies are required to report only minimal information to Texas Railroad Commission; e.g., drilling locations, depths, monthly pressures and volumes. NOT discovered faults or recorded quakes. the
What should be done? The responsible (and practical) answer Routine monitoring i of small earthquakes Research to prove/disprove hypotheses: Induced earthquakes are never larger than largest regional natural earthquakes Large (M3) induced earthquakes represent a different physical phenomenon than microearthquakes induced by hydrofracture Large (M3) induced earthquakes don t occur unless there favorably oriented faults nearby SWD wells are Limited it exchange of information about faulting, etc that is now proprietary Public relations: small earthquakes are normal and Public relations: small earthquakes are normal and not dangerous
What should be done? The responsible (and practical) answer Routine monitoring i of small earthquakes The EarthScope transportable array: stations operating in The EarthScope transportable array: stations operating in Texas until 2013, but can be adopted for $35,000
What should be done? The responsible (and practical) answer Thus: We are seeking companies willing to join a joint UT/SMU consortium to support research concerning induced seismicity Petroleum Technology Transfer Council (PTTC) workshop in Houston on 2 June focusing on induced seismicity and describing our plans for consortium Research to prove/disprove hypotheses Limited exchange of information that is now proprietary about faulting, etc see http://www.beg.utexas.edu/pttc/workshops
The End Photo: AAPG Explorer