Triggered Earthquakes in North-Central Arkansas and their Relationship to Class II UIC Wells Scott M. Ausbrooks, The Arkansas Geological Survey Steve Horton, Ph.D., CERI at University i of Memphis Contributors: AGS: David Johnston, Susan Horvath, Jerry Clark, and Mike Hanley CERI: M. Withers, H. Withers, J. Bollwerk, C. McGoldrick, D. Steiner, and K. Tucker USGS: NEIC locations and notification; two seismic stations
Historic and Recent Seismicity
G-G G Swarm Overview The Guy-GreenbrierGreenbrier fault, apreviously unknown fault, is illuminated by over a 1,300 earthquakes (M 4.7) that occurred between September, 2010, and present. The fault is theoretically capable of potentially damaging M5.6 6.0 earthquake. producing a Two well-documented d cases - Rocky Mountain Arsenal, Colorado, in the 1960s and Paradox Valley, Colorado, in the 1990s - demonstrate that fluid injection into the subsurface can ti trigger earthquakes. A plausible hydraulic connection exists between the injection depths at a waste-disposal well and the nearby Guy-GreenbrierGreenbrier Fault.
Number of events per year: ~ 200 = M>2.5 2010 = 624 ~ 40 = M>3.0 2011 = 724 4 = M>4.0 2012 = 8 2013 = 2 Total: 1,358 (>M1.0)
Sep 2010 to Mar 2011
Some Potential Factors contributing to Triggered/Induced Seismicity and what we know... Faults that are critically stressed (near failure) and are favorably oriented to the regional stress. Tight injection zone rocks with low primary porosity/permeability between the grains. This pressures up the injection reservoir/aquifer quickly. Most likely the pressure front will take the path of least resistance via the joints and fractures (secondary porosity/permeability). The injection of fluids increases the pore pressure within the reservoir/aquifer of injection. Plausible hydraulic connectivity between the well(s) and the fault(s) via orthogonal joint sets and fractures. When there is a hydraulic connection between the injection reservoir/aquifer e and nearby fault(s), the potential t exists in increasing the pore pressure in the fault zone. This could change the Mohr-Coulomb criterion resulting in movement.
Structural Cross-Section SOURCE: AAPG Bulletin V. 74 (July, 1990), P. 1030-1037, 4 Figs., 1 Table
Guy-Greenbrier Fault
Orthogonal Fracturing in the Earth s Continental Crust S. Parker Gay, Jr., 1973: summarized some 30 different studies in the literature pertinent to crustal fracturing: The earth s crust is cut by a number of set of parallel to sub-parallel deep fractures that occur pervasively throughout the globe most if not all occurred in Precambrian time these are typically paired with another set orthogonal to each other pairset and may have been reactivated numerous times (both in the Precambrian and in the Phanerozoic time. Sedimentary rocks become jointed early in their history as aresult of minor vertical movement along the basement pairsets by a bridging mechanism - this mechanism results in forming orthogonal tension joints in the overlying sedimentary rock mirroring the causative basement fractures below.
Plausible Hydraulic Connectivity Source: Imes, J. L. and Emmett, L.F., USGS, 1994 Tectonic activity and erosion results in uplift of the Ozark Plateau above sea level creating faults and fractures Major rift (New Madrid) forms on the southeast flank also contributes to regional faulting and fracturing preferential northwest- Numerous orientation southeast faults to the and fractures exhibit northeast-southwest southwest and These faults and fractures provide avenues for ground water movement through virtually impermeable rock Many of the faults and fractures in the younger overlying Paleozoic rocks in the eastern portion of the Ozarks are the result of repeated differential movement across weak zones associated with the faults in the underlying basement faults.
STRATIGRAPHIC SECTION GEOHYDROLOGIC UNITS BOONE Ozark Confining Unit is thin and predominately sandstone HUNTON in the Study Area ARBUCKLE St. Francois Confining Unit is missing in the Study Area (Caplin, 1960)
Disposal Well Comparisons Moore Estate: Total Depth: 10,500 feet Sedimentary Rock: Injection Zone: 0 to 10,500 feet 7,480 to 7,540 feet (Boone) Injection Zone: 9,300 to 10,500 feet (Arbuckle) SRE: Total Depth: 6,460 feet Sedimentary Rock: 0 to 6,460 feet Injection Zone: 5,975 to 6,460 feet (Boone and Hunton) Edgmond: Total Depth: 12,160 feet Sedimentary Rock: 0 to 12,090 feet Igneous Rock: 12,090 to 12,160 feet Injection Zone: 7,806 to 10,970 feet (Arbuckle) Trammel: Total Depth: 7,160 feet Sedimentary Rock: 0 to 7,160 feet Injection Zone: 6,503 to 6,590 feet (Boone)
Were the Guy-greenbrier Earthquakes Triggered by fluid injection? The Guy-Greenbrier Greenbrier fault was critically stressed prior to the start t of injection. Therefore, the Mohr-Coulomb criterion must have been changed incrementally (naturally or by human activity) shortly before or coincident with the earthquakes. The earthquakes along the Guy-GreenbrierGreenbrier fault began after the start of injection at well #1 with intense seismic activity following the start of injection at well #5. Earthquake frequency in the study area shows a strong correlation with the volume of injection at well#1 and well#5.
Were the Guy-greenbrier Earthquakes Triggered by fluid injection? Continued The injection of fluids increased pore pressure in the Ozark aquifer, and because of the hydraulic connection between the Ozark aquifer and the Guy-GreenbrierGreenbrier fault, pore pressure most likely increased in the fault zone. Given the spatial and temporal correlation between the UIC wells and activity on the fault, it would be an extraordinary coincidence if the earthquakes were not triggered by fluid injection.
References and Sources: Arkansas Geological Survey Arkansas Oil and Gas Commission Burroughs, R., K., 1988, Structural Geology of the Enola Arkansas Earthquake Swarm: Master Thesis, University of Arkansas. Cox, R.T., 2009, Tectonophysics, 474, p 674-683. Gay, Jr., Parker, S., 1973, Pervasive Orthogonal Fracturing in the Earth s Continental Crust. Harrison, R., 2009, USGS PowerPoint: History and Fabric of the Region Surrounding the Northern Mississippi Embayment and the New Madrid Seismic Zone: Midcontinent U.S. Imes, J. L. and Emmett, L.F., 1994, Major geohydrologic units in and adjacent to the Ozarks Plateaus Province, Missouri, Arkansas, Kansas, and Oklahoma, U. S. Geological Survey Hydrologic Investigations Atlas, HA 711-A VanArsdale, R.B. and Schweig, E.S., III, 1990. Subsurface structure of the Eastern Arkoma Basin, The American Association of Petroleum Geologists Bulletin, V. 74, No. 7 (July 1990), P. 1030-1037, 4Figs., 1 Table.
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