Earthquake triggering and large-scale geologic storage ofcarbon dioxideMark D. Zobacka,1 and Steven M. Gorelickb

Departments of aGeophysics and bEnvironmental Earth System Science, Stanford University, Stanford, CA 94305

Edited by Pamela A. Matson, Stanford University, Stanford, CA, and approved May 4, 2012 (received for review March 27, 2012)

Despite its enormous cost, large-scale carbon capture and storage (CCS) is considered a viable strategy for significantly reducing CO2 emissionsassociated with coal-based electrical power generation and other industrial sources of CO2 [Intergovernmental Panel on Climate Change(2005) IPCC Special Report on Carbon Dioxide Capture and Storage. Prepared by Working Group III of the Intergovernmental Panel onClimate Change, eds Metz B, et al. (Cambridge Univ Press, Cambridge, UK); Szulczewski ML, et al. (2012) Proc Natl Acad Sci USA 109:5185–5189]. We argue here that there is a high probability that earthquakes will be triggered by injection of large volumes of CO2 into the brittlerocks commonly found in continental interiors. Because even small- to moderate-sized earthquakes threaten the seal integrity of CO2

repositories, in this context, large-scale CCS is a risky, and likely unsuccessful, strategy for significantly reducing greenhouse gas emissions.

carbon sequestration | climate change | triggered earthquakes

The combustion of coal for elec-trical power generation in theUnited States generates approx-imately 2.1 billion metric tons of

CO2 per year, ∼36% of all US emissions.In 2011, China generated more than threetimes that much CO2 by burning coal forelectricity, which accounted for ∼80%of its total emissions. (According to theEnergy Information Agency of the USDepartment of Energy, total CO2 emis-sions in China were 8.38 billion metrictonnes in 2011, with 6.95 billion tons fromcoal burning, nearly all of which is usedelectrical power generation.) Froma global perspective, if large-scale carboncapture and storage (CCS) is tosignificantly contribute to reducing theaccumulation of greenhouse gases, it mustoperate at a massive scale, on the orderof 3.5 billion tons (1) of CO2 per year,a volume roughly equivalent (2) to the∼27 billion barrels of oil currently pro-duced annually around the world. (Underreservoir conditions, one billion tons ofCO2 occupies a volume of ∼1.3 billioncubic meters, equivalent to 8.18 billionbarrels. Thus, 3.5 billion tons of carbondioxide would correspond to a volume ofapproximately 28.6 billion barrels. Thereare currently ∼850,000 wells producing oilaround the world.) Moreover, a leak ratefrom underground CO2 storage reservoirsof less than 1% per thousand years is re-quired for CCS to achieve the same climatebenefits as renewable energy sources (3).Before embarking on projects to inject

enormous volumes of CO2 at numeroussites around the world, it is important tonote that over time periods of just a fewdecades, modern seismic networks haveshown that earthquakes occur nearly ev-erywhere in continental interiors. Fig. 1,Upper shows instrumentally recordedearthquakes in the central and easternUnited States and southeastern Canada.Fig. 1, Lower shows instrumentally re-

corded intraplate earthquakes in southand east Asia (4). The seismicity catalogsare complete to magnitude (M) 3. Theoccurrence of these earthquakes meansthat nearly everywhere in continental in-teriors a subset of the preexisting faults inthe crust is potentially active in the currentstress field (5, 6). This is sometimes re-ferred to as the critically stressed nature ofthe brittle crust (7). It should also be notedthat despite the overall low rate of earth-quake occurrence in continental interiors,some of the most devastating earthquakesin history occurred in these regions. Ineastern China, the M 7.8, 1976 Tangshanearthquake, approximately 200 km east ofBeijing, killed several hundred thousandpeople. In the central United States,three M 7+ earthquakes in 1811 and 1812occurred in the New Madrid seismic zonein southeast Missouri.Because of the critically stressed nature

of the crust, fluid injection in deep wells cantrigger earthquakes when the injectionincreases pore pressure in the vicinity ofpreexisting potentially active faults. Theincreased pore pressure reduces the fric-tional resistance to fault slip, allowingelastic energy already stored in thesurrounding rocks to be released inearthquakes that would occur someday asthe result of natural geologic processes (8).This effect was first documented in the1960s in Denver, Colorado when injectioninto a 3-km-deep well at the nearby RockyMountain Arsenal triggered earthquakes(9). Soon thereafter it was shown experi-mentally (10) at the Rangely oil field inwestern Colorado that earthquakes couldbe turned on and off by varying the rate atwhich water was injected and thus modu-lating reservoir pressure. In 2011 alone, anumber of small to moderate earthquakesin the United States seem to have beentriggered by injection of wastewater (11).These include earthquakes near Guy,Arkansas that occurred in February and

March, where the largest earthquake wasM 4.7. In the Trinidad/Raton area nearthe border of Colorado and New Mexico,injection of produced water associatedwith coalbed methane production seemsto have triggered a number of earth-quakes, the largest being a M 5.3 eventthat occurred in August. Earthquakesseem to have been triggered by wastewaterinjection near Youngstown, Ohio onChristmas Eve and New Year’s Eve, thelargest of which was M 4.0. Although therisks associated with wastewater injectionare minimal and can be reduced evenfurther with proper planning (11), thesituation would be far more problematic ifsimilar-sized earthquakes were triggeredin formations intended to sequester CO2for hundreds to thousands of years.Deep borehole stress measurements

confirm the critically stressed nature of thecrust in continental interiors (12), in somecases at sites directly relevant to the fea-sibility of large-scale CCS. For example,deep borehole stress measurements at theMountaineer coal-burning power plant onthe Ohio River in West Virginia indicatea severe limitation on the rate at whichCO2 could be injected without the result-ing pressure build-up initiating slip onpreexisting faults (13). Because of thelow permeability of the formations atdepth, pore pressure increases would beexpected to trigger slip on preexistingfaults if CO2 injection rates exceedapproximately 1% of the 7 million tons ofCO2 emitted by the Mountaineer planteach year. Similarly, stress measurementsat Teapot Dome, Wyoming, the US gov-ernment-owned oil field where pilot CO2

Author contributions: M.D.Z. and S.M.G. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.1To whom correspondence should be addressed. E-mail:zoback@stanford.edu.

10164–10168 | PNAS | June 26, 2012 | vol. 109 | no. 26 www.pnas.org/cgi/doi/10.1073/pnas.1202473109

injection projects have been considered,show that very small pressure buildups arecapable of triggering slip on some preex-isting faults (14).Dam construction and water reservoir

impoundment produce much smaller pore

pressure changes at depth than are likely tooccur with CO2 sequestration, but manyhave triggered earthquakes at various sitesaround the world (15) (red dots in Fig. 1).Except for the much smaller pore pressureincreases at depth, reservoir-triggered

earthquakes are a good analog for thepotential for seismicity to be triggered byCO2 injection. Both activities cause porepressure increases that act over large areasand are persistent for long periods.Three reservoir impoundments in easternCanada (located in the ancient, stable coreof the North American continent) trig-gered earthquakes as large as M 4.1 andM 5 at the two sites (Fig. 1), despite thefact that the pore pressure increases atdepth were extremely small.

Triggered Earthquakes and SealIntegrityOur principal concern is not that injectionassociated with CCS projects is likely totrigger large earthquakes; the problem isthat even small to moderate earthquakesthreaten the seal integrity of a CO2 re-pository. In parts of the world with goodconstruction practices, it is unusual forearthquakes smaller than approximatelyM 6 to cause significant human harmor property damage. Fig. 2 uses well-established seismological relationships toshow how the magnitude of an earthquakeis related to the size of the fault thatslipped and the amount of fault slip thatoccurred (16). As shown, faults capable ofproducing M ∼6 earthquakes are at leasttens of kilometers in extent. (The fault sizeindicated along the abscissa is a lowerbound of fault size as it refers to the size ofthe fault segment that slips in a givenearthquake. The fault on which an earth-quake occurs is larger than the part of thefault that slips in an individual event.)In most cases, such faults should beeasily identified during geophysical sitecharacterization studies and thus shouldbe avoided at any site chosen for a CO2repository. (Faults in crystalline basementrocks might be difficult to recognize ingeophysical data. We assume, however,that any site chosen as a potential CO2repository would be carefully selected,avoiding the possibility of pressurechanges in the CO2 repository fromaffecting faults in crystalline basement.)The problem is that site characterizationstudies can easily miss the much smallerfaults associated with small to moderateearthquakes.Although the ground shaking from

small- to moderate-sized earthquakes isinconsequential, their impact on a CO2repository would not be. Most of thegeologic formations to be used for long-term storage of CO2 are likely to be atdepths of approximately 2 km—deepenough for there to be adequate sealingformations to isolate the CO2 from thebiosphere but not so deep as to encounterformations with very low permeability.Given large volumes of CO2 injectedinto selected formations for manydecades, if a small to moderate earth-

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Fig. 1. Upper: Instrumentally recorded seismicity and damaging historical earthquakes in the centraland eastern United States and southeastern Canada. Red dots indicate sites of reservoir-induced seis-micity. Lower: Seismicity of south and east Asia and sites of reservoir-induced seismicity. Both data setsare available from the US Geological Survey (4).

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PERSPECTIV

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quake were to be triggered in a geologicformation at approximately 2 km depth,it could jeopardize the seal integrity ofthe storage formation. For example, if aM ∼4 earthquake were to be triggered byCO2 sequestration (17)—an event thatwould be widely felt in a populated areabut for which shaking would be unlikely tocause harm or damage—it would be as-sociated with several centimeters of slipon a fault several kilometers in size. Be-cause laboratory studies show that justa few millimeters of shear displacementare capable of enhancing fracture andjoint permeability (18), several centi-meters of slip would be capable of creatinga permeable hydraulic pathway that couldcompromise the seal integrity of the CO2reservoir and potentially reach thenear surface.

Safe SequestrationIt is important to emphasize that CCS canbe valuable and useful for reducinggreenhouse gas emissions in specific sit-uations. A good example is the injection ofCO2 into the Utsira formation (19) at theSleipner gas field in the North Sea, wherea significant amount of CO2 is coproducedwith natural gas. After separating the CO2from the produced gas, approximately 1million tons of CO2 per year has been in-jected over the past 15 y without triggeringseismicity. Assuming isolation from thenear surface, injection into highly porousand permeable reservoirs that are laterallyextensive would produce small increases inpressure in response to CO2 injection.Moreover, weak, poorly cemented sand-stones are expected to deform slowly inresponse to applied geologic forces. Insuch reservoirs, the stresses relax overtime, and such formations are not prone tofaulting (20). In this regard, the Utsiraformation is ideal for CO2 sequestration.

It is isolated from vertical migration byimpermeable shale formations, and it ishighly porous, permeable, laterally exten-sive, and weakly cemented.To contribute significantly to green-

house gas emission reductions (2), roughly3,500 sites similar to the Utsira formationwould have to be found at convenient lo-cations around the world, assuming com-parable injection rates of approximately1 million tons of CO2 per year. In fact, itwould take approximately 85 such sites

coming on line each year to reach a goal ofstoring approximately 1 billion tons ofCO2 by midcentury. Clearly this is anextraordinarily difficult, if not impossible,task if only highly porous and permeableand weakly cemented formations are tobe used.Of course, rather than using potentially

problematic geologic formations close tocoal-burning power plants for sequestra-tion (as illustrated by the Mountaineercase study cited above), relatively idealformations for CO2 storage could besought on a regional basis to accommo-date emissions from a number of plants.One example of this is the potential use ofthe Mt. Simon sandstone in the Illinoisbasin. The Mt. Simon is porous, perme-able, and regionally extensive. However,models of injection of 100 million tons ofCO2 per year for 40 y predicts (21)increases in pore pressure of severalmegapascals over a region of ∼40,000 km2.The approximate area of significantlyincreased pore pressure resulting frominjection is shown as the blue-shaded areain Fig. 3, essentially adjacent to theWabash fault zone, where a series ofmoderate natural earthquakes occurred inthe spring of 2008, the largest being M 5.2.Paleoseismic data indicate the occurrenceof much larger nearby earthquakes (somegreater than M ∼7) in the recent geologicpast (22). Importantly, the 100 million tonannual CO2 injection rate used in the

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Fig. 3. Instrumentally recorded seismicity in the New Madrid and Wabash Valley seismic zones (modi-fied from ref. 23). Red circles indicate earthquakes that occurred from 1974 to 2002 with magnitudeslarger than 2.5 located using modern instruments. Green circles denote earthquakes that occurred be-fore 1974. Larger earthquakes are represented by larger circles. The area shown in blue corresponds tothe area where a pressure increase of several megapascals would result from injecting 100 million tonsper year of CO2 into the Mt. Simon sandstone in the Illinois Basin for 40 y (19).

10166 | www.pnas.org/cgi/doi/10.1073/pnas.1202473109 Zoback and Gorelick

modeling only represents approximatelyone seventh of the CO2 generated by thecoal-burning power plants in the OhioRiver Valley alone.Because of the need to carefully monitor

CO2 repositories with observation wells,geophysical and geochemical monitoringsystems, etc., it is likely that most sites willhave to be located on land or very nearshore. Otherwise, highly porous reservoirslocated offshore, like those adjacent tosalt domes along the US Gulf Coast,would be relatively ideal sites because saltformations are known to be excellent sealsfor hydrocarbons.Depleted oil and gas reservoirs are

potentially suitable for CO2 storage fora variety of reasons—an infrastructure ofwells and pipelines exist, and there is a greatdeal of geologic and subsurface propertydata available to characterize the sub-surface from decades of study. In addition,from an earthquake-triggering perspective,depleted reservoirs are attractive becauseat the time injection of CO2 might start,the pore pressure would be below the valuethat existed before petroleum production.Thus, there could be significant injection ofCO2 before pressures increase to pre-production values, thereby reducing thepotential for triggering earthquakes.There are a number of potential issues to

consider before using depleted oil andgas reservoirs for CO2 storage, the mostimportant of which are capacity andgeographic distribution. The reasons thatthere is such interest in using salineaquifers for CO2 storage is that they arepotentially well distributed with respect tolikely sources of CO2, and they couldpresumably accommodate the enormousvolumes of CO2 that need to be stored. Ifone were only to consider the UnitedStates, storing the 2.1 billion tons of CO2currently generated annually by coal-burning power plants in depleted oil andgas reservoirs would require injection ofCO2 at a rate of approximately 17 billionbarrels per year; a rate equivalent to eighttimes current US annual oil productionand more than four times US peak annual

oil production that occurred in the early1970s. In addition, it is important to makesure that production-related activities,such as water flooding during secondaryrecovery, did not compromise the sealcapacity of the reservoirs. There alsoneeds to be careful study of the wells inthe depleted oil or gas field to makesure that poorly cemented well casings,especially in older wells, will not be path-ways for release of stored CO2 (23).Finally, there are likely to be complicatedlegal questions concerning ownership andliability that will need to be worked out ona case-by-case basis.Although enhanced oil recovery (EOR)

using CO2 (in which CO2 is injected todissolve in oil and reduce its viscosity)would be a beneficial use of CO2, it isimportant not to confuse this with CCS. InCCS the goal is to inject large quantitiesof CO2 into available pore space and storeit there for hundreds to thousands ofyears. When CO2 is used for EOR, the CO2dissolved in the oil is separated and cap-tured from produced oil and then re-injected. Thus, smaller volumes of CO2 areused, and the long-term storage capacity ofthe reservoir is not an issue.Many CCS research projects are cur-

rently underway around the world.Much ofthis work involves characterization andtesting of potential storage formations andincludes a number of small-scale pilot in-jection projects. Because the storage ca-pacity/pressure build-up issue is critical toassess the potential for triggered seismicity,small-scale pilot injection projects do notreflect how pressures are likely to change(increase) once full-scale injection is imple-mented. Moreover, even though limitationson pressure build-up are among the manyfactors that are evaluated when potentialformations are considered as sequestrationsites, this is usually done in the context ofnot allowing pressures to exceed the pres-sure at which hydraulic fractures would beinitiated in the storage formation or cap-rock. In the context of a critically stressedcrust, slip on preexisting, unidentified faultscould trigger small- to moderate-sized

earthquakes at pressures far below that atwhich hydraulic fractures would form.As mentioned above, sequences of small

to moderate earthquakes were apparentlyinduced by injection of waste water nearGuy, Arkansas, Trinidad, Colorado, andYoungstown, Ohio in 2011 and on theDallas-Ft. Worth airport, Texas. Althoughthese earthquakes were widely felt, theycaused no injury, and only the Trinidadearthquake resulted in any significantdamage. However, had similar earthquakesbeen triggered at sites where CO2 wasbeing injected, the impacts would haveraised pressing and important questions:Had the seal been breached? Was it stillsafe to leave previously injected CO2in place?In summary, multiple lines of evidence

indicate that preexisting faults found inbrittle rocks almost everywhere in theearth’s crust are subject to failure, oftenin response to very small increases in porepressure. In light of the risk posed toa CO2 repository by even small- tomoderate-sized earthquakes, formationssuitable for large-scale injection of CO2must be carefully chosen. In addition tobeing well sealed by impermeable over-laying strata, they should also be weaklycemented (so as not to fail through brittlefaulting) and porous, permeable, andlaterally extensive to accommodate largevolumes of CO2 with minimal pressureincreases. Thus, the issue is not whetherCO2 can be safely stored at a given site;the issue is whether the capacity exists forsufficient volumes of CO2 to be storedgeologically for it to have the desiredbeneficial effect on climate change. Inthis context, it must be recognized thatlarge-scale CCS will be an extremely ex-pensive and risky strategy for achievingsignificant reductions in greenhousegas emissions.

ACKNOWLEDGMENTS.We thank Owen Hurd andRandi Walters for their help in preparation ofthe figures, and Mary Lou Zoback, Greg Beroza,Doug Arent, Sally Benson, Franklin Orr, JohnBredehoeft, and Larry Goulder for theirhelpful comments.

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