# Assessing risk to fresh wate r resources from long term CO 2 

# injection- laboratory and field studies

GCCC Di gital Pub lication Ser i es # 08- 03j

Rebecca C. Smyth Susan D. Hovork a Jiemin Lu Kath erin e D. Rom a nak Judson W. Partin Corrin e Wong Changbing Yang

Cited as: Smyth, R. C., Hovorka, S.D., Lu, J., Ro manak, K. D. , Partin, J.W., Wo ng, C., Assessing risk to fresh water resources from long term C O 2 

### inje ction- l a boratory and field studies: presented at th e 9th Intern ational C onference on Greenhouse Gas Control Technologies (GHGT-9), Wa shington, D.C., November 16-20, 2008. GCCC Digit a l Publication Series #08-03j. Keywo r ds : Geologic sequestration

, water resources

, laboratory experiments

, increased pH

, cations

, groundwater sampling

# Assessing risk to fresh wate r resources from long term CO 2 

# injection – laboratory and field studies

Rebecca C. Smyth a* 

## , Susan D. Hovorka a 

## , Jiemin Lu a 

## , Katherine D. Romanak a



## , Judson W. Partin b 

## , Corrine Wong b 

## , and Changbing Yang a

a Bureau of Economic Geology, Jackson School of Geosciences, The Un iversity of Texas at Austin, P.O. Box X, Austin, TX, 78713, US A

b Department of Geological Sciences, Jackson School of Geosciences , The University of Texas at Austin, Austin, TX,78705, USA

Abstract

In developing a site for geologi c sequestration, one must assess potential cons equences of failure to adequately contain injected carbon dioxide (CO 2 ). Upward migration of CO 2 or displacement of saline water because of increased pressure might impact protected water resource s 100s to 1000s of meters above a sequestration interval. Questions posed are: (1) Can changes i n chemistry of fresh water aquifers provide evidence of CO 2 leakage from deep injection/sequestration reservoirs containing brine and or hydrocarbons? (2) What parameters can we use to assess potential impacts to water quality? (3) If CO 2 leakage to freshwater aquifers occurs, will groundwater quality be degraded and if so, over what time period? Modeling and reaction experiments pl us known occurrences of naturally CO 2 -charged potable water show that the common chemical reaction products from dissolution of CO 2 into freshwater include rapid buffe ring of acidity by dissolution of calcite and slower equilibrium by reaction with clays and feldspars. Results from a se ries of laboratory batch reactions of CO 2 with diverse aquifer rocks show geochemical response within hours to days after introduction of CO 2 . Results included decreased pH and increased concentr ations of cations in CO 2 experimental runs relati ve to control runs using argon (Ar). Some cation (Ba, Ca, Fe, Mg, Mn, and Sr) concentrations increas ed over and an order of magnitude during CO 2 runs. Results are aquifer dependant in that experimental vessels containing different aquifer rocks showed different magnitudes of increase in cation concentration s. Field studies designed to improve unders tanding of risk to fresh water are unde rway in the vicinity of (1) SACROC oilfield in Scurry County, Texas, USA where CO 2 has been injected for enhanced oil recovery (EOR) since 1972 and (2) the Cranfield unit in Adams County, Mississippi, USA where CO 2 EOR is currently underway. Both field studies are funded by the U.S. Department of Energy (DOE) regi onal carbon sequestration partnership programs and industr ial sponsors. Preliminary results of groundwater monitoring are curre ntly available for the SACROC field study where researchers investigated 68 water wells and one spring during five field excurs ions between June 2006 and July 2008. Resu lts to date show no trend of preferentia l degradation below drinking water standards in areas of CO 2 injection (inside SACROC) as compared to areas outside of the
SACROC oil field.

Keywords: geologic sequestration

, water resources , laboratory experiments

, incr eased pH

, cations

, groundwater sampling1. Introduction

Site selection for geologic sequestration (GS) is the key element in assuring that carbon dioxide (CO 2 ) will be retained in an appropriate receiving formation fo r geologically significant periods of time. Restriction of fluid injection permits to properly selected sites is mandated by the Safe Drinking Water Act of 1974 (U.S. House of Representatives, 1974), and managed by the United States Environmental Protection Agency (USEPA) Underground Injection Control (UIC) Program. In a correctly se lected GS site there should be negligible transport of CO 2 out of the injection formation, and migration would be limited to diffusion into confining zones. Underground sources of drinking water (USDWs) near a correctly selected site would not be impacted by CO 2 injection, thereby meeting USEPA requirements to protect human drinking water supplies. However, it is prudent to assess potential impacts of (1) failure to contain injected CO 2 in the receiving formation, and (2) displacement of high-salinity fluid (brine) into USDWs. Potential errors in site selection cons idered here are presence of moderate to high permeability flow paths such as a vertically transmissive fault or improperly plugged well, along which CO 2 and/or brine could migrate. The possibility of impacts to USDWs is the focus of this brief paper. A USDW is defined in the USEPA Proposed Rule for Permitting Full-Scale Geologic Sequestration Projects (July 2008) as: An aquifer or portion of an aquifer that supplies any public water system or that contains a sufficient quantity of groundwater to supply a public water system, and currently supplies drinking water for human consumption, or that contains fewer than 10,000 mg/l total dissolved solids and is not an exempted aquifer . By regulation CO 2 would be injected below the deepest USDW and separated from it by a confin ing zone (i.e. low permeability seal).

In the following sections we present results from laborat ory experiments conducted to assess chemical by-products of reactions between injected CO 2 , aquifer host rocks, and fresh groundwater. We then present results from a fieldbased study of groundwater quality overlying the Scurry Area Canyon Reef Operators Committee (SACROC) oilfield in Scurry County, Texas, U.S.A. CO 2 has been injected at SACROC for enhanced oil recovery (EOR) since 1972 making this an ideal natural laboratory in which to observe potential impacts of CO 2 to groundwater. This work has been funded by (1) the Gulf Coast Carbon Center (GCCC) industrial sponsor program at the Bureau of Economic Geology (BEG) at The University of Texas at Austin, (2) the Department of Energy National Energy Technology Laboratory (DOE/NETL) through the Southwest Regional Partnership for Carbon Sequestration (SWP) administered by New Mexico T ech, and (3) Kinder Morgan CO 2 pipeline and production companies.

2. CO 2 -rock reaction fundamentals

Most studies of how CO 2 dissolves in water and reacts with the host rock have considered injection zones at greater than 1 km depth and focused on the trapping of CO 2 in mineral phases (Rochelle et al, 2004, Gunter et al., 2004, Worden and Smith, 2004). CO 2 -solubility dependence on pressure, temperature, and salinity is well known. Field observations at the Frio (e.g. Karaka et al., 2006 and Hovorka et al., 2006) and Weyburn (IEA, 2005) test sites, and simulations based on observations at the sites, have documented rock-brine-CO 2 reactions at 1,500 m depths in 100,000 mg/L basinal brines. Under these conditions, fluids are strongly reducing (methane saturated) and CO 2 is supercritical (Knauss et al., 2005 ). Under most circumstances CO 2 will dissolve in water and lower pH. In a system containing reactive mineral phases, the decrease in pH is buffered by dissolution of Ca/Mg/Fe-bearing silicate minerals (Gunter et al., 1993). High CO 2 content causes continued dissolution of silicates, and precipitation of clay minerals, carbonates, and quartz when the acidity has been reduced by silicate dissolution (Gunter et al., 1997; Hitchon et al., 1999). Dissolution or precipitation of carbonates depends upon rock mineralogy and water chemistry. In each case the dissolution and precipitation reactions are a function of thermodynamics and the rates of these reactions are controlled by chemical kinetics.

There have been few studies of CO 2 introduced into a potable aquifer setting. Shallow aquifer conditions are unlike studies done at depth where (1) rocks have ge nerally been flushed by fresh water, (2) CO 2 will be a gas rather than a supercritical fluid, and (3) te mperature, pressure, and salinity are lower than in deep reservoirs. While potable waters that are high in CO 2 occur in nature (e.g. the Perrier region of France), these rock-water aquifer systems have had a long period in which to react and equilibrate with CO 2 . Hence, they may be an imperfect analogue for short tem reactions that could result fr om accidental leakage of CO 2 into a freshwater aquifer.3. Laborator y E x periments

B E G researc h e r s c o nd uct e dl abo rat o ry bat c h e x peri m e nt s to inv e stig ate t h e ex ten t t o wh ich inj ected CO 2 mi g r atin g up ward i n t o a shal l o w aqui fe r coul d m obi l i ze m a jor an d m i nor i o ns b o u n d t o aq ui fe r host r o c k s. A seri es of bat c h

expe rim e nts was designe d to react dive rse a qui fer roc k s wi th CO 2 in the medium of water. W e s e lected core sam p les from the Core Research Ce nter at th e B E G wi th t h e g o a l of re p r ese n t i ng vari ous a q ui fer s p r ese n t i n Texas an d throughout the Gu lf C o ast re gion. For eac h reactor 95 g of disa gg re gated, unwa she d roc k sam p le was com b ined with 5 0 0 m l o f d e ion i zed (DI) water in a wi d e m o u th 50 0 ml Erlen m e y er flask. Rubb er sto p p e rs facilitated airtigh t placem ent of infl ow a n d outflow t ubes

. T o minimize the co rrosive n ess of DI wate r, s odi um chloride (NaCl) was ad d e d to app r ox im a t e n a tu ral g r ou nd water activ ity (~4 0 ppm Cl - ). Th e exp e rim e n t was sp lit in to two period s wh ere in the first period argon (Ar) gas was bubbl ed through the reactor s for two weeks

. CO 2 w a s b ubb led th r oug h th e reactors for a second 2-week peri od immediately following. Whe n the gas supply was switche d from Ar to CO 2 at Day 14, 70 m l of DI water was adde d to each reactor to re place the water that was rem oved duri ng the Ar run (di l u t i o n fact o r1. 1 4 )

. Ten f l asks co nt ai ni ng sam p l e s of aqui fe r r o c k and a bl an k (DI wat e r o n l y ) we re ru n simu l t a neousl y . The o u t f l o w m a ni fol d sy st em was o p en t o r o om atm o sphe ri c p r ess u r e . D u ri ng A ri n ject i o n 2-m l

wat e r sam p l e s were t a ke n at 1, 5, 1 2 ho ur , 1 day

, 2, 4, 7

, 10 , 1 2 , a nd 1 4 day t i m e i n t e rval s f o r i o n a n al y s i s . W a t e r

was sam p l e d on th e sam e sched u l e d u ri ng th e C O 2 ru n. Aft e r e v ery sa m p li ng pe ri o d

, th e pH, oxi da t i on- red u ct i o n p o t en tial (ORP), d i sso l ved o x y g e n (DO), an d co ndu ctiv i t y were m easu r ed u s ing a mu ltip aram eter p r ob e (Hann a Inst rum e nt s 9 8 2 8 ) . C a t i o n anal y s i s was do ne at th e U n i v e r si t y of T e xas at Aust i n De part m e nt of Ge ol o g i c a l Sciences on a n Agilent 7500ce Qua d rupol e ICP-MS. T h e cation c once n trations m eas ure d from the reactors with

roc k sam p les were s u btracte d by the m easurem ents of th e b l ank reacto r to redu ce o relimin ate p r o c ed ural

cont ami n at i on effect s. S o di u m and chl o ri de were norm alized to the amount of NaC l adde d at th e b e gi n n i n g o f th e expe ri m e nt and s ubt ract ed.

Resu lts sh ow th at catio n concen tration s were g e n e rally el ev ated in th e CO 2 ex p e rim e n t al ru n relative to th e Ar run.
R api d ge oche m i cal respo n se occ u r r edwi th i n h o u rs t o da y s aft e r i n t r o d u ct i o n o f C O 2 with sign ificant lib eratio n of the cations: Ca, Mg, K

, Si, Sr , Mn, Ba, B, F e , and Al. C o n centrations of som e cations ( e .g. Ca, M g , Mn, Fe , S r , Ba ) i n crease d ove r an or de r of m a gni tu de. Di ffe rent a q ui fer r o ck re act or s di spl a y e d di ffe re n t m a gni tude s o f i n c r ease i n cation conce n trations . Results of the r eact or containing Dockum aquifer roc k from Ra ndall County, Texas, U.S.A. are i n cl ude d a s an e x am pl e (F i g u r e 1). Fi g u r e 1 s h ow s rapi d el evat i o n i n conce n tr at i o ns of m a ny of th e cat i ons aft e r CO 2 was i n tr o duce d on day 14 . R e l ease ra t e of al l cat i ons decrease d t o war d s th e e n dof ex pe ri m e nt, whe r e th ei r

conce n tr at i o ns i n crease d sl ow l y or rem a i n ed at a c onst a nt l e vel

. Go o d c o r r el at i on bet w e e n th e c once n t r at i ons o f C a and M g su g g e s t dol omi t e as th ei r so urce mi neral

. K a n d Si m a y be rel e ased f r om K-f e l d spar . T w o ty pes o f cat i o n

co n c en tration p r o f iles were recogn ized aft e r in tro d u c tion o f CO 2 t o re act or ve ssel s ( 1 ) Ty p e I – c once n tr at i o ns i n crease d rapi dl y th e n i n c r e a sed sl owl y or rem a i n ed at c onst a nt c once n tr at i ons an d (2 ) Ty pe I I – c o ncent r at i ons i n crease d rapi d l y fol l o w e d by a gra d ual de cre a se. Ty pe I cat i ons i n cl u d e B

, B a , C a , C o , K, M g , M n , S ra n d Z n

. Ty pe II cations include Al, As, Cs

, Cu, F e , M o , Ni , Rb, U

, a n d V.

Figure 1. Cation concen trations through the process of experiment. CO 2 was introdu ced into th e s y stem a t Day 14. Figure 2. pH thr ough the exp e riment for all th e r eactors. CO 2 was introdu ced into the s y st em at

Da y 14

.Drop in concentration of Type II cations (e.g. Fe and Al) was unexpected and suggests that cation mobilization reached a maximum concentration followed by resorbp tion and/or reprecipitation. The decline in cation concentrations is tentatively interpreted as a result of re precipitation or sorption on reactive grain coatings. In the case of stagnant water, resorption suggests that some cation mobilization could be self-mediated by the system. This transience was also detected at the Frio, TX site. The elevation of concentrations of Type II cations, such as, Fe, Al, Mo, U, Cs, Cu, may be a short-term perturbation and ther efore a limited risk to water quality. As expected, pH dropped at the start of CO 2 flux for most of the reactors (Figure 2). In most cases, pH rebounded after abrupt decrease at the beginning of CO 2 run, then stayed relatively constant. In general, CO 2 reduced pH values up to 3 units relative to Ar gas. In Reactor 1, for example, pH dropped from over 7.9 to 4.8 within hours after introduction of CO 2 , then rebounded and remained at ~5.4. The pH rebound is probably due to mineral buffering, in this case, dissolution of dolomite and, to a lesser extent, potassium feldspar. The experimental results provide initial data for the degree and rate at which cations would be mobilized from aquifer host rocks if they are affected by CO 2 leakage. These data can be used for sensitivity analysis in design of field testing experiments to determine if any of the constituents elevated in the laboratory tests are detectable in groundwater that has or might have come in contact with injectate CO 2 . However, significance of elevated minor cations from samples in solution with minimally reactive gas (Ar) shows that the concentrations measured can not be transferred directly to aquifer conditions. Three hypothesis are proposed to explain overall high cation conc entrations (1) DI water with small amount of NaCl added (Cl ~40 ppm) was used in the experiment instead of aquife r water for each sediment, so some dissolution of reactive minerals occurred to reach rock-water equilibrium, (2) disagregation of th e rocks exposed more reactive surfaces than would be exposed in the aquifer setting, and (3) some contamination from the experimental design was noted and factored out, however contamina tion from sample handling (for example drilling mud introduced during coring or oxidation of core during storage) cannot be removed. If a geometry was created where the same volume of water was charged with excess gas and migrated though many rock volumes, cation concentrations might become quite high. Cation concentrations could also decrease as the reacted groundwater moved away from the CO 2 source and pH was buffered.

4. Field Studies

BEG researchers are currently involved in two field studies in which groundwater monitoring is designed to improve understanding of risk to fresh water from CO 2 injection operations. These studi es are located at (1) the SACROC oilfield in Scurry County, Texas, U.S.A. where CO 2 has been injected for EOR since 1972 and (2) the Cranfield unit in Adams County, Mississi ppi, U.S.A. where CO 2 EOR is currently underway. SA CROC offers a great opportunity to study potential impacts to fresh water resources because of the long time span over which to observe effects of engineered CO 2 injection. Since CO 2 EOR began at SACROC, approximately 50% of the 150 million metric tons (MMt) of injected CO 2 has been recovered. The majority of remaining CO 2 is likely trapped in the deep subsurface, but studies to demonstrate CO 2 has been sequestered at SACROC are still in progress. The SACRO C oilfield covers a ~90 mi 2 area in central Scurry County, TX (Figure 3). Geologic units present at the surface range from Permian to Quaternary in age. Two significant water-bearing units crop ou t at the surface within th e study area. The Triassicage Dockum Fm. (TrD in Figure 3) hosts the Dockum a quifer, which is designated a minor aquifer by the Texas Water Development Board (TWDB). The second significant water-bearing unit is the Ogallala Fm. (P-EOg in Figure 3). This erosional outlier of the Ogallala Fm. is isol ated from the Ogallala aquifer of the Texas High Plains, but provides significant fresh water resources in Scu rry and surrounding counties. Smaller quantities of fresh groundwater are produced from Permian-age units in eastern portions of the study area.

We have sampled freshwater from wells constructed in Ogallala Fm., Dockum Fm., Permian-age units, and combinations of these geologic units within an ~1,800 mi 2 area. Depth of wells sampled by BEG and TWDB ranges from 30 to 700 ft. Sampled wells are located over such a large region for several reasons. Limited data on groundwater quality prior to development of oil and gas fields, do not provide adequate background (i.e. prior to CO 2 injection) data against which to compare post-CO 2 -injection groundwater quality data. Water wells in the region are completed in numerous different and sometimes multiple geologic units; hence groundwater chemistry is highly variable. In order to determine if freshwater reso urces over the SACROC oilfield have been impacted by over 35 years of CO 2 injection, we must first define the regional variability in Dockum aquifer groundwater chemistry. In lieu of sampling before and after CO 2 injection, we have sampled inside and outside of SACROC.Figure 3. Surface geo l ogic units and water wells in vicinity of the SACROC oilfiel d

.

Du ri n g th e S W P SAC R O C gr ou n d wat e r stu d y B E G resear chers hav e (1 ) com p i l e d hi st ori cal gr ou n d wa t e r chemi s tr y d a ta fro m th e TWDB on lin e d a tab a se (196 fresh water wel l s) and (2 ) co llected mu lt ip le sam p les fro m 6 0 freshwater

w e lls, on e f r e sh w a terspr i ng , an d eigh t pr od u c tion- zon e -br i n e wells. W e co m p leted five sam p lin g even ts b e t w een Ju ne 2 006 an d Ju ly 200 8 w ithin a 1

, 2 7 5 mi 2 area (Figu r e 3). W a ter well sam p l i n g m e th o d o l og y in clud ed con tin uou s

measurem ent of field c h emica l param e ters (te m perature, pH, s p ecific conductivity, and di sso l v ed ox ygen ) in a f l ow cell an d m o n ito ri n g of d i sch arg e rate. We d i d no t co llect g r oun dwater sam p les u n til after field ch emic al p aram e ter s stab ilized to en sure th at samp les were from th e fo rm at ion a nd n o t st agna nt casi n g- v o l u m e -wat er. Ou r sam p l i ng pr ot oc ol i n cl u d ed fi el d al kal i ni t y t i tr at i ons i n No vem b er 20 07 , M arc h 20 0 8 , an d J u l y 200 8. La b o r a t o ry anal y t es measured in B E G groundwat er sam p les by Los Alam os Nati onal La borat o ry (L AN L) ar e: Al, A g , As

, B, Ba, Be , Br

, Ca, Cd, Cl, C O 3 , C o , Cr, Cs, Cu, d 13 C, d D

, d 18 O, F, Fe, HCO 3 , Hg, K, L i , Mg, Mn, M o , Na

, Ni, N O 3 , Pb, P O 4 , Rb

, Sb, Se , Si, Sn, SO 4 , Sr , Th, Ti, Tl, U, V, and Zn. La boratory analytes measure d in Th e Univ ersity o f Texas at Au stin Depart m e nt of Geol ogi cal S c i e nces ( U T DG S) ar e di ss ol ve d i n or gani c carb on (D IC ), di ss ol ve d or gani c carb o n (D OC), m e thane (C H 4 ) a n d CO 2 from headspace gas in se lected sam p les. The c h emistry of Doc kum groundwater is hi g h l y vari abl e . Fi g u r e 4 i s a Pi per pl ot o f B E G Do ckum aqui fer fre sh wat e r sam p l e s col l ect ed i n si de a n d out si de of SAC R O C c o mb i n e d wi th dat afrom wel l s com p l e t e d i n unde rl y i ng z one s. Permi a n wel l dat aare fr om bot h dee p er wells near SACROC and we lls located east o f SACROC wh ere Permian u n its cr op ou t at th e su rface (Figure 4). Sam p l e s of p r o duce dwat e r ( b ri nes ) ar e fr om SAC R OC oi l f i e l d wel l s c o mp l e t e d at de pth s bet w een 6

, 00 0 a n d 7, 00 0 ft. Th ere is no d i stin ct group in g of BEG well sam p les in si de ve rsus o u t s i d e of S A C R O C (Fi g ure 4 )

. Pr o duct i o n z o n e

sam p l e s are clearl y sho w n t o be s odi um -chl ori d e-t y pe wat e rs d i stin ct from m o st o f th e BEG sam p les i n all p o rtion s

o f

Figu r e 4.

Figure 4. Piper p l ot of all B E G fr eshwater s a mp les, BEG and Kin d er Morgan prod uced water

.Tren ds of m a jor el em ent concent r at i ons s u gge st mi xi ng of Doc kum aq ui fer gr o u n d w at er and wat e r wi th Permi a n and pr o duct i o n zo ne g e oc h e mi cal si gnatu res

. M i xi n g m odel s fr o m Dr. C h a n g b i n g Ya n g ( 2 00 8, per s o n al

co mmu n i catio n ) at BEG predict th at <1 % p r odu ced o ilfield b r i n e or 12 % Permian water co u l d mix with th e lo west TDS BEG Docku m freshwat e r sam p le to p r odu ce th e h i gh est TDS Do cku m sa m p le. Sev e ral m ech a n ism s can b e

use d t o ex pl ai n suc h mi xi ng, e s peci al l y i n an oi l fi eld t h at ha s bee n operating si nce the earl y 1950s.

The m a i n que st i on rem a i n s: Has Doc kum aqui fer wat e r been i m pact ed by th e l o ng hi st ory of C O 2 inj ectio n at SACROC ? If co ndu it flo w alo n g leak ing well b o res is re sponsi b le for increase d TDS in Dockum aqui fer water

ove rl y i ng SA C R O C

, we w oul d al so e x p ect t o s ee impacts to Dockum wa ter from injectate CO 2 . La borator y

ex p e rim e n t s a n d field ob serv atio ns sho w th at p H will decrease and catio n con cen t r atio n s will in crease in flu i d reservoirs e x pose d to i n ject ed CO 2 . Metho d s fo r in terpretin g Do ckum g r o u n d water ch emistry in clu d e sp atial

p l o ttin g o f concen tration co n t o u rs. Fo r an al ytes su ch as pH, we h a v e v a lid d a ta sp an n i n g pre- an d po st-inj ection peri ods . P r e- 1 9 8 0 pH

- d at a fr om th e TWDB dat abase ar e s h o w n i n Fi gu r e 5. Doc kum aqui fer wat e r p H ra n g ed fr om ~6 .5 to 9.0 in sa m p les co ll ect ed bet w ee n 19 36 an d 1 9 7 8

. (a). ( b ).

Figure 5. (a) pH values measur edin fr eshwater w e ll samples befo re 1980 (TWD B). Data po ints d e noted b y green tr iangles. (b) pH values measured in freshwater wells be tw een 19 95 ( T W D B) and 20 08 ( B EG &TW D B )

.

Doc kum aqui fer p H m easured i n sam p l e s c o llected f r o m 1 995 to 200 8, af terCO 2 i n je ct i on ha d bee n on goi ng fo r

ove r 2 0 y ears, i s sho w n i n Fi gu re 5

. C aref u l com p ari s on w i th pre- 1 9 8 0 c ont ou rs re veal s ei th er no c h an ge or a sl i g ht increase i n pH near SACR OC betwee n th e t w o t i m e peri ods ( F i g ure 5)

. We w o ul d e xpect to see pH fall if injectate CO 2 had l eak ed i n t o D o c kum freshwat e r zones or i f re servo i r brine with lo wered pH and in creased cation

conce n trations (as doc um ented at SACROC by Ha n, 2008) had bee n dis p laced upward into Doc kum fre shwate r.

If pH c h a nge has bee n bu ffe red by di ss ol u t i on o f si l i cat e mi neral s i n th e D o c kum aqui fer

, cat i on c once n tr at i o ns

wo ul d be hi gh er o v er S A C R OC

. W e d o n o t see sy st em at ic increases in cation conce n trations m easured in Doc kum

aq u i fer sam p les in sid e v e rsu s o u t sid e of SACROC (figu res 5 an d 6 )

. Howev e r, we are still ev alu a tin g th e d a ta fro m
th e pers pect i v e of regi on al fl o w path s a nd re act i v e tr ans p o rt m odel i ng. A n othe r way t o eval uat e dat afr om Dockum

g r ou nd water sa m p les is to assess d e gradation o f water q u a l ity by com p aring analyte con cen tration s t o EPA drink i ng
water stan d ards. Data su mm a r ized i n Tab l e 1 are fro m BE G sa m p les th at were filtered (0 .2 to 0.45 μm ) in t h e field

. C a t i on sam p l e s were pre s er v e d wi th ni tr i c aci d t o p H 2. No pre s er vat i v e was a dde d t o th e a n i o n sa m p l e s. Th e perce n t a ge o f B E G D o c kum aqui fer sam p l e s wi th a n al y tes excee ding USEPA prim ary max i mu m co n t a min an t lev e ls (M C L s) an d s econ d ar y dri n ki n g wat e r st a ndar d s are hi g h l i ght e d i n y e l l o w i n Tabl e 1. W i th o u t except i o n, t h e perce n tage of s a m p les with analytes in excess of USEPA st an d ard s is h i gher ou tsid e th an in sid e of SAC R OC (Tab le 1). Arsenic (As) and lead (Pb) are liste d as cont ami n ant s o f part i c ul ar c o ncer n i n t h e Ju l y 200 8 E P A p r o p o sed ru l e because these constituents might be leach e d from rocks i n GS reservoirs and m obilized to im pact U S DWs. Of the 9

. 8 p e rcen t o f wells with As lev e ls abov e MCLs, on ly 1

. 6 perce n t of thes e are inside SACROC (Tab l e 1 ) . On ly ~1 2

p e rcen t of th e filtered BEG Do cku m sa m p le s h a d Pb lev e ls ab ov e a d e tectio n limi t o f 0

. 00 2 m g /L. Fo rty two p e rcen t of these sam p les are from wells insi d e v e r s us 58 % ou tsid e o f SA CR O C

.Table 1

. Comparison of EPA drin king water st and a rds with B E G Dockum aquifer well d a ta

.

BEG r e sear ch er s ar e stud ying stab le carbon i s o t op es of fres h water, produced water, inje ctate CO 2 , an d p l an t m a tter

, whi c h f o rm s th e s ubst r at e f o r mi crobi al ox i d at i on of C O 2 . So lid carb onate p h a se is al so im p o r tan t bu t an alytica l

resu lts are no t yet av ailab l e fro m LANL. Po t e n tial co n tri b u tions t o the carbon is otope signature include mixing wit h
Permi a n and pr od uce d wat e r, m i crobi al l y -pr o d u ce d C O 2 , c arbonated diss olution, a nd i n jectate CO 2 . A p l o t of carbo n i s ot o p e rat i o s vers us di ss ol ve d i n o r ga ni c car b o n ( D IC ) s h o w s a n i n crease i n l i g ht carb o n wi th i n creasi n g D I C

. Thi s is co n s isten t with in pu t fro m micro b i al CO 2 but d o es not ru l e o u t th e pre s ence of inject ate CO 2 . Fu rth e r stu d ies are in progress at BEG.

5. Summ a ry

Labo ratory exp e rim e n t s an d field ob serv ati o n s show th at p H will d ecrease and catio n c o n c en tration s will in crease in fl ui d re ser voi r s exp o se d t o i n ject ed C O 2 . Thi s has be en sh ow n t o be tr ue i n th e 6,0 0 0 t o 7, 00 0 ft dee p in

j ection / pr oductio n zon e s of th e SA CRO C o ilf ield ( H an , 2 008 ). U n d e r g r oun d so ur ces o f dr ink i ng w a tero v e r l yin g SACROC are co n t ain e d wi th in mu ltip le stratig rap h i c ho riz o n s o f th e Do cku m aq uifer. Ch emical co n s titu en ts

m easuredi n sa m p l e s of D o c kum gro u n d w at er s u g g est mi nor mi xi ng wi th m o re sal i n e ( h i ghe r T D S ) Pe rmi a n wat e r

or c o

- p r o duce doi l f i e l d bri n es. Ho weve r, th i s doe s n o t im pl y th at C O 2 in j e cted for EOR at SACROC h a s mix e d with
Doc kum groundwate r. Analy s is of BE G/L ANLstable carbon isotopic data to trace various s o urces of C O 2 in t h e subsurface a bove SACROC i s in progress. Doc kum aquifer chemical con cent r ations i n side of SACROC fall within the range of T W DB groundwate r chemic al an alyses in sa m p les tak e n f r o m a mu lt i- cou n t y r e g i on surr ou nd ing SAC R O C

. We do not see e v i d e n ce o f As or P b ha vi n g been m obi l i zed fr om i n ject i on zo nes at S A C R O C i n t o

o v e rlyin g USDW s (Do c kum aq u i fer zon e s). Th ere are Do cku m wells with ch emica l co nstitu en ts i n excess o f EPA dri nki ng wat e r st andard s wi t h i n ou r stu d y area. Ho we ver , a l arge r perc ent a ge of t h ese wel l s are l o ca t e d ou tsi d e of th e SACROC o ilfield . Th e main co n c lu si o n is th at no wid e sp read d e g r ad ation o f water q u a lity ex ists in sid e th e SACROC o ilfi l ed as a resu lt of CO 2 -inj ectio n. Work is in prog ress to determin e if C O 2 inj ection at SACR OC is chan gi n g re gi o n al D o c kum aq ui fer g r ou n d wa t e r fl o w pat t e r n s.

6. Ackn owledge ments

We g r at ef ul l y ackn o w l e d g e sup p o rt f

o r mu ch of th i s wo rk fr om th e Depart m e nt of E n er gy N a t i onal Ene r gy Tech nol ogy La bo rat o ry and K i nde r M o r g a n C O 2 -pi p el i n e a nd pr o duct i o n com p ani e s. L ANL per f o rm ed m o st of th e chemical analyses on groundw ater sam p les collected by B E G as part of t

h e S W P pha se 2 p r oject at

S A C R O C

. B a t c hreactor experiments were funded by the industria l sponsors of the Gulf Coast Carbon Center ( www.gulfcoastcarbon.org ). We thank Bob Reedy for advice in setting up the batch reactor technique and Bridget Scanlon for assistance with ion analysis.

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