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## General Comment

Over 3 trillion standard cubic f eet (TCF) or 150 million metric tons of CO2 have been injected for enhanced oil recovery (EOR) at the SACR OC oilfield in west Texas since 1972. Roughly half of the injected CO2 has been produced and reinjected as part of a recycling effort; an estimated 35 million tons is permanently sequeste red in the subsurface at SACROC. Clearly this is a large scale CO2 injection site. As with the other four CO2 injection sites referenced in

Section II of this Notice, th ere has been no documented leakage of CO2 at SACROC.

The attached document is a status report submitt ed to EPA Region 6 offices in Dallas, TX in April 2009. This report summarizes DOE/NE TL-funded, through the Southwest Regional Carbon Sequestration Partnership (SWP), research at SACROC. Objectives of the combined

research efforts of the Bureau of Economi c Geology, Los Alamos National Laboratory, New Mexico Tech, University of Pittsburgh, and Universi ty of Utah are to (1) assess potential impacts to the shallow subsurface over SACROC, (2) model th e fate of CO2 injected into the subsurface at SACROC, and (3) use geophysical methods to image the ~35 million tons of CO2 assumed to be trapped in the subsurface.1 

## Update on SWP Research Activi ties at the SACROC Oilfield, Scurry County, Texas an d Status of GCCC Studies on Risk to Aquifers from CO 2 

## Sequestration

Prepared by: Rebecca C. Smyth, Gulf Coast Carbon Center, Bureau of Economic Geology, J ackson School of Geosciences, The University of Texas at Austin

Prepared for: United States Environmen tal Protection Agency and Department of Energy National Energy Technology Laboratory Interim Data Request

April 26, 2009

Contents

Status of SWP Research Activities at SACROC .............................. 3

Intr oducti on ...................................................................................... 3

Ge ophysics ....................................................................................... 3

Gr oundwater ..................................................................................... 4 Reservoir/Inj ection Zone Modeling ................................................. 4

Surf ace Flux...................................................................................... 4 Status of GCCC Studies on Risk to Aquifers ................................... 4

Executiv e Summary ......................................................................... 4

Intr oduction ..................................................................................... 4

CO 2 

### -rock Reaction Fundamentals ............................................... 4

Laborator y Experime nts ................................................................... 7

SACROC Groundw ater Studies ..................................................... 11

Methods ..................................................................................... 11

Gr oundwater Data Analysis ...................................................... 12

Conclusions .................................................................................... 24

Acknowle dgments .......................................................................... 25

References ...................................................................................... 26 Appendix A. Federal Re quirements Under the Underground Injection Control (UIC ) Program for Carbons Dioxide (CO 2 

### ) Geologic Sequestration (GS) Wells, Fall 2008 Proposed Rule ..... 282 

### List of Figures Figure 1. Laboratory experi ment reaction vessels. ......................................... 8 Figure 2. Cation concentrations during laboratory experiment ...................... 9 Figure 3. pH values during expe riment for all reactors. ............................... 10 Figure 4. Locations of SACROC oilfield , local extent of Dockum aquifer,

and BEG and TWDB water wells used in study. .......................... 11 Figure 5. TWDB (1956-2008) and BEG (2007-2008) pH data. .................. 14 Figure 6. Contours of pH measured by TWDB in Scurry County between

1956 a nd 1980. ............................................................................... 14 Figure 7. Contours of pH measured by BEG and TWDB in Scurry County

between 1995 a nd 2008. ................................................................. 15 Figure 8. TWDB (1936-2008) and BE G (2007-2008) total dissolved solids (TD S) data. .......................................................................... 15 Figure 9. Scaled cross sections of selected BEG-sampled wells ................ 17

Figure 10. Locations of wells monitored for SWP SACROC groundwater study ........................................................................ 18 Figure 11. Piper diagram showing larg e variation in water chemistry

for BEG and TW DB wells ............................................................ 19 Figure 12. Piper diagram of BEG wells only ................................................ 19 Figure 13. Sulfate vs. chloride concen trations of all BEG samples .............. 20 Figure 14. Locations of transect plots relative to SACROC and potentiometric

surface co ntours ............................................................................ 21 Figure 15. Calcium, magnesium, a nd strontium concentrations along flow path ............................................................................ 22 Figure 16. Total dissolved solids, sodium, potassium, and chloride

c oncentrations along flow path .................................................... 22

List of Tables Table 1. Chemical analyte ranges and median values for 113 BEG freshwater sample s collected from privat e wells in Fisher,

Garza, Kent, and Scurry counties ..................................................... 13 Table 2 Stratigraphic units in the subsurface below the

SACROC oilfield. ............................................................................. 16 Table 3. Comparison of EPA drinki ng water standards with BEG

Do ckum aquifer we ll data. ............................................................... 233 

### Status of SWP Research Activities at SACROC

Introduction

One of three Southwest Regional Partners hip for Carbon Sequestration (SWP) Phase II research projects, being funded by the De partment of Energy National Energy Technology Laboratory (DOE NETL), is taking place at the Scurry Area Ca nyon Reef Operators Committee (SACROC) oilfield near the eastern edge of the Permian Basin in Scurry County, Texas. Dr. Brian McPherson of University of Utah and Dr. Reid Grigg of New Mexico Institute of Mining and Technology (NMT) are principal investigator and acting project manager, respectively, of the SWP. Rebecca C. Smyth of the Gulf Coast Carbon Center (GCCC) at the Bureau of Economic Geology (BEG) is site manager for SWP SACROC

research. Since 1972 over 150 million metric tons (MMt) (3 trillion standard cubic feet (TCF)) of carbon dioxide (CO 2 ) have been injected for enhanced oil recovery at SACROC. As of October 2008 only 75 MMt (1.5 TCF) of the injected CO 2 have been recovered, making this site a natural laboratory for research on subsurface CO 2 storage. SACROC is currently operated by Kinder Morgan Company (KM), which serves as the major industrial partner for this project. Schlumberger Ca rbon Services division is also providing project support. Two

scales of SWP MVA research are ongoing in and around SACROC. Deep subsurface research components (geophy sics and reservoir zone modeling) are underway near a ~1 km 2 (~0.4 mi 2 ) injection experiment site centered on production well 56- 17; four water-alternatin g-gas (WAG) wells lie at corners of the nearly square-shaped-pattern. The 56-17 pattern is located at the southern en d of the North Platform area of SACROC in what is now designated the South Platfo rm production development area. Three SWP monitoring wells are within 100 to 300 ft from new injection wells located to the NE, SE, and SW of well 56-17. These monitoring wells are old production wells reentered and sidetracked through the 6,000 to 7,000 ft deep production/inje ction interval by KM in 2008 to benefit SWP research. WAG inject ion began in the 56-17 inject ion pattern in September and

October 2008. All four WAG wells were injecting CO 2 before the end of October 2008. Research groups from Los Alamos National Laboratory (LANL), NMT, University of Pittsburg and DOE NETL, and University of Utah are leading the deep subsurface research projects at SACROC. Shallow subsurface and surface SWP MVA research is taking place on a more regional scale looking for differences within SACROC versus outside of SACROC. Bureau of Economic Geology (BEG) researchers ha ve sampled water wells in a ~3300 km 2 (~1275

mi 2 ), multicounty area to look for potential impact s to freshwater resources from long-term

CO 2 injection at SACROC. Research ers from NMT are measuring CO 2 surface flux within and outside of SACROC. The multiple componen ts of SWP research are detailed in the following sections of this interim data report.



### Geophysics

Three parts of the ge ophysics component of MVA at SACROC SWP Phase II research are: (1) borehole geophysical logging, (2) surface reflection geophysical survey, (3) rock physics modeling, and (4) vert ical seismic profiling survey. The SWP partnership and KM are sharing th e cost of borehole ge ophysical logging at the 56-17 injection experi ment site; SWP is paying for pre- and post-CO 2 -injection in one of4 the monitoring wells and KM is paying for l ogging in the other two monitoring wells. KM has also agreed to share borehole logs coll ected in the new injection wells with SWP researchers. Schlumberger and Weat herford companies completed pre-CO 2 -injection borehole geophysical logging in three monitori ng wells in the 56-17 injection experiment pattern by July 2008. Types of cased hole logs run in the monitoring wells were cement bond, neutron, RST (sigma and CO modes), and sonic. The same suite of logs were run in injection wells after casing was installed; density, gamma , neutron, resistivit y, and sonic open hole logs were run in the injection wells before cas ing was installed. Repeat or post-injection cased hole logs were run in two of the monitoring wells in February 2009. Data processing and interpretation by Schlumberger, KM, and BEG are in progress. Multiple surface reflection geophysical surv eys cover the SWP injection experiment

site. In 2003 KM conducted a 3-D seismic surv ey of the SACROC North Platform area, the southern edge of which overl aps the SWP 56-17 site. BEG rese archer Dr. Bob Hardage and and Dr. William Harbert of University of P ittsburgh (UPitt) and DOE NETL have acquired pre-stacked, pre-migrated seismic data from th is survey courtesy of KM. Further processing of this pre-injection dataset is ongoing. Harb ert, using outside funding from DOE NETL and technical assistance from Hardage, has completed two smaller scale (~4 km 2 or 1.5mi 2 ) surface seismic surveys centered on injection we ll 59-2A at the northeastern corner of the SWP injection experiment pattern. Harbert and Hardage conducted the two surface seismic surveys in November 2008 and March 2009. Data processing and interpretation of surface seismic data are in progress. Data for rock physics modeling are being collected by Harbert’s research group at DOE NETL and UPitt. This group is collecting computed tomography (CT) and rock

mechanics data from SACROC reservoir zone core to aid in estimations of seismic velocities and interpretation of surface geophysical data. These data will also be used in BEG rock physics modeling to aid interpretation of SACROC reservoir zone pore-filling fluids. LANL is conducting vertical seismic profile (VSP) surveys in monitoring well 59-2, which is ~200 ft to the SW of injection well 59-2A in the SWP injection experiment site. The pre-injection survey took place in July 2008. The post-injection VSP survey was just completed in mid-April 2009. SWP SACROC V SP data processing is in progress.



### Groundwater

Groundwater studies being conducted by BEG re searchers under direction of Rebecca C. Smyth are detailed in a late r section of this report entitled : “Status of GCCC Studies on Risk to Aquifers.”



### Reservoir/Injection Zone Modeling

This work is being conducted by SWP research groups at the Univer sity of Utah under

direction of Dr. Brian McPherson and New Mexico Tech under direction of Dr. Reid Grigg. Progress reports on this compone nt of SWP SACROC research n eed to be provided by these

groups.



### Surface Flux

Measurement of CO 2 surface flux is being conducted by researchers at NMT and

BEG. The NMT team collected data in June 2008 and January 2009 within SACROC and at a “background” measurement site approximately 10 miles NW of SACROC. NMT researchers5 plan to repeat measurements at the same sites in June 2009 and January 2010 if project extensions are granted. Interpretation of NMT CO 2 surface flux data is in progress.

A second set of CO 2 surface flux measurements were collected at SACROC by BEG and Applied Nanotech Inc. (ANI) personnel in January 2009 using ANI experimental sensors. This work is being funded primarily th rough direct support to ANI from DOE.



### Status of GCCC Studies on Risk to Aquifers

Executive Summary

The Gulf Coast Carbon Center (GCCC) of the BEG has interim results from two studies related to the potential for damage to fr eshwater resources as a consequence of failure to adequately sequester injected carbon dioxide (CO 2 ). Questions posed by studies detailed

here are: (1) Can changes in chemistry of freshwater aquifers provide evidence of CO 2 leakage from a deep-injection/sequ estration reservoir? (2) What parameters can we use to as sess potential impact s on water quality? (3) If CO 2 leakage to freshwater aquifers occurs, will groundwater quality be degraded? One study is a series of laboratory batch experiments to test what happens when aquifer rocks and freshwater react with CO 2 . Reactions occurred wi thin a day, including decreased pH and increased dissolved constituen ts. In some samples, cations Ba, Ca, Fe, Mg, Mn, and Sr increased significan tly. Different aquifer-rock–fresh water reaction experimental vessels showed different magnitudes of increase in cation concentration. From this result we conclude that simple but aqui fer-specific laboratory tests would help in the design of earlywarning freshwater monitoring plans. In ad dition, such tests woul d allow a realistic

determination if a constituent of concern would be released in the event of an accidental major CO 2 leak to freshwater. This study wa s funded by GCCC (gulfcoastcarbon.org). The other study is on fresh groundwater above the SACROC oil field in Scurry County, Texas, USA, where CO 2 has been injected to a depths between 6,000 and 7,000 ft for enhanced oil recovery (EOR) since 1972. From 2006 through 2008, BEG researchers sampled 68 water wells and 1 spring and analyzed major, minor, and trace-element chemistry of water samples taken over a 3300 km 2 (1275 mi 2 ) area. Work completed to date has used pH and

dissolved constituents identified in the labo ratory study as indicators of impacts to groundwater. We find no diagnostic patterns of changes through time or over the area above the injection. This result lead s to a preliminary conclusion, subject to limitations of the approach, that no leakage of injectate-CO 2 into the aquifer has occurred. However, the groundwater system is complex and includes numerous water-bearing zones with diverse rocks and water types. In addition, na tural recharge may be contributing CO 2 from organic/microbial sources and salinization of is olated portions of the aquifer has resulted from historic oil-field practices. Work on this extensive, complex database continues to assess whether any other indicators of leakage of CO 2 from depth can be identified. This study was funded by the U.S. Department of Energy ( DOE) through the Southwest Regional Carbon Sequestration Partnership through NMT. Additiona l financial and logistical support are being provided by Los Alamos National Laboratory and SACROC operator Kinder Morgan.6 

### Introduction

Site selection for geologic sequestration (G S) is the key element in ensuring that carbon dioxide (CO 2 ) will be retained in an appropriate receiving formation for geologically significant periods of time. Restri ction of fluid-injection permits to properly selected sites is mandated by the Safe Drinking Water Act of 19 74 (U.S. House of Representatives, 1974) and managed by the U.S. Environmental Protec tion Agency (EPA) Underground Injection Control (UIC) Program. In a correctly selected GS site, transport of CO 2 out of the injection formation should be negligible, and migration would be limited to diffusion into confining zones. Underground sources of drinking water (U SDWs) near a correctly selected site would not be impacted by CO 2 injection, thereby meeting EPA requirements to protect drinkingwater 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 highpermeability flowpaths, such as a vertically transmissive fault or improperly plugged well, along which CO 2 and/or brine could migrate. The possi bility of impacts to USDWs is the

focus of this brief. A USDW is defined in the EPA Proposed Rule for Permitting Full-Scale Geologic Sequestration Pr ojects (July 2008) as: An aquifer or portion of an aqui fer that supplies any public water system or that contains a sufficient quantity of groundwat er to supply a public water system, and currently supplies drinking water for human c onsumption, or that contains fewer than 10,000 mg/l total dissolved solids an d is not an exempted aquifer. In the proposed rule, CO 2 would be isolated from USDWs by a confining zone (i.e., lowpermeability seal). In the following sections we present resu lts of laboratory expe riments conducted to assess chemical byproducts of reactions between injected CO 2 , aquifer host rocks, and fresh groundwater. We then present results from a field-based study of groundwater quality overlying the Scurry Area Canyon Reef Operator s Committee (SACROC) oil field in Scurry County, Texas. CO 2 has been injected at SACROC for enhanced oil recovery (EOR) since 1972, making it an ideal natural laboratory for observing potential impacts of CO 2 to groundwater. This work has been funded by the Gulf Coast Carbon Center (GCCC) industrial sponsor program at the Bureau of Economic Geology (BEG), the Department of Energy National Energy Technology Laboratory ( DOE/NETL) through the Southwest Regional Partnership for Carbon Sequestra tion (SWP) administered by New Mexico Tech, and Kinder Morgan pipeline and production companies.



### 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 depths >1 km and focus on trapping of CO 2 in mineral phases (Gunter et al., 2004; Rochelle et al., 2004; Worden and Smith, 2004). CO 2 -solubility dependence on pressure, temperature, and salin ity is well known. Field observations at Frio (e.g., Karaka et al., 2006 and Hovorka et al ., 2006), Weyburn (IEA, 2005) test sites, and

simulations based on observations at th e sites, have documented rock-brine-CO 2 reactions at 1,500-m depths in 100,000 mg/L basinal brines. U nder these conditions, fluids are strongly reducing (methane saturated), and CO 2 is supercritical (Knauss et al., 2005).7 Under most circumstances, CO 2 will dissolve in water and lower pH, which is a process represented by the following chemical reactions:

CO 2(g) + H 2 O ↔ H 2 CO 3(aq) ; H 2 CO 3(aq) ↔ HCO 3(aq) – + H + ; HCO 3(aq) – ↔ CO 3(aq) 2– + H + (aq).

In a system containing reactive mineral phases, decrease in pH is buffered by dissolution of carbonate or Ca/Mg/Fe-bearing silicate mi nerals (Gunter et al., 1993). High CO 2 content causes continued dissolution of silicates and pr ecipitation of clay minerals, carbonates, and quartz when acidity has been reduced by sili cate dissolution (Gunter et al., 1997; Hitchon et al., 1999). Dissolution or precipitation of carbonates depends on mineralogy and water chemistry. In each case, dissolution and pr ecipitation reactions are a function of thermodynamics. Rates of these reactions are controlled by chemical kinetics (e.g., Wang and Jeffe, 2004).

Studies of CO 2 being introduced into a potable aquifer setting are few. Shallow aquifer conditions are unlike st udies done at depth. Rocks have been flushed by freshwater,

CO 2 is present as a gas rather than a supercriti cal fluid, and temperature, pressure, and salinity are lower than in deep-reservoir conditi ons. Whereas potable waters high in CO 2 occur in nature (e.g., the Perrier region of France), these rock-water a quifer systems have had a long period in which to react and equilibrate with CO 2 . Hence, they may be an imperfect analog

for short-term reactions that could result from accidental leakage of CO 2 into a freshwater aquifer.



### Laboratory Experiments

BEG researchers conducted laboratory batch experiments to inves tigate the extent to which injected CO 2 migrating upward into a shallow aquifer could mobilize major and minor ions bound to aquifer host rocks. A series of batch experiments was designed to cause diverse aquifer rocks to react with CO 2 in the medium of water. We selected core samples from the

Core Research Center at BEG in order to re present various aquifers present in Texas and throughout the Gulf Coast region. For each reactor, 95 g of disaggregated, unw ashed rock sample was combined with 500 mL of deionized (DI) water in a wide-m outh 500-mL Erlenmeyer flask (Figure 1). Rubber stoppers facilitated air-tight placement of inflow and outflow tubes. To minimize the corrosiveness of DI water, sodium chloride (NaCl) was added to approximate natural groundwater activity (~40 ppm Cl - ). The experiment was split in to two periods. In the first period, argon (Ar) gas was bubbled thr ough the reactors for 2 weeks. CO 2 was bubbled through the reactors for a second 2-week period immediately following. When the gas supply

was switched from Ar to CO 2 on Day 14, 70 mL of DI water wa s added to each reactor to replace the water that had been removed durin g the Ar run (dilution fa ctor 1.14). Ten flasks containing samples of aquifer rock and a bla nk (DI water only) were run simultaneously. The outflow manifold system was open to room atmospheric pressure.8 Figure 1. Laboratory experiment reaction vessels.

During Ar injection, 2-mL water samples were taken at 1-, 5-, 12- hour, 1-, 2-, 4-, 7-, 10-, 12-, and 14-day time intervals for ion analysis. Water was sampled on the same schedule during the CO 2 run. After every sampling period, pH, oxidation-reduction pot ential (ORP), dissolved oxygen (DO), and conductivity were measured using a multiparameter probe (Hanna Instruments 9828). Cation analysis was done at The University of Texas at Austin Department of Geological Sciences on an Agilent 7500ce Quadrupole ICP-MS. Cation

concentrations measured from the reactors with rock samples were subtracted by

measurements of the blank reactor. Procedural contamination effects were thus reduced or

eliminated. Sodium and chloride were norma lized as the amount of NaCl added at the beginning of the experiment and subtracted.

Results show that cation concentrations were generally elevated in the CO 2 experimental run relative to th e Ar run. Rapid geochemical response occurred within hours to days of introduction of CO 2 with significant liberation of ca tions Ca, Mg, K, Si, Sr, Mn, Ba, B, Fe, and Al. Concentrations of some cati ons (e.g., Ca, Mg, Mn, Fe, Sr, Ba) increased over an order of magnitude. Different aquifer rock reactors displayed different magnitudes of

increase in cation concentrations. Results of the reactor containing Dockum sandstone aquifer rock from Randall County, Texas, are in cluded as an example (Figure 2). Figure 2 shows rapid elevation in con centrations of many cations after CO 2 was introduced on day 14. Release rate of all cations decreased toward the end of the experiment, at which point their concentrations increased sl owly or remained at a constant level. Good

correlation between concentrations of Ca and Mg suggests dolomite as their source mineral. K and Si may be released from K-feldspar. Two types of cation concentration profiles were recognized afte r introduction of CO 2 to reactor vessels: (1) Type I—concentrations increased rapidly then increased slowly or remained at constant concentrations and (2) Type II—concentr ations increased rapidly then gradually decreased. Type I cati ons include B, Ba, Ca, Co, K, Mg, Mn, Sr, and Zn. Type II cations include Al, As, Cs, Cu, Fe, Mo, Ni, Rb, U, and V.9 Figure 2. Cation concentrations throughout the e xperiment on a sandstone sample from the Dockum aquifer. CO 2 was introduced into the system on Day 14.

Drop in concentration of Type II cations (e.g., Fe and Al) was unexpected and suggests that cation mobilization reached a maxi mum concentration before resorption and/or reprecipitation. Decline in cation concentrations is tentatively interp reted as a result of reprecipitation or sorption on reactive grain coati ngs. 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 Fri o, Texas, site (written communi cation 2008; Y. Kharaka, USGS).

Elevation of concentrations of Type II cations, such as Fe, Al, Mo, U, Cs, Cu, may be a shortterm perturbation and, therefore, a limited risk to water quality. As expected, pH dropped at the start of CO 2 flux for most reactor s (Figure 3). In most cases, pH shows a quick rebound after the abru pt decrease at the beginning of the CO 2 run and then keeps 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 CO 2 was added; it 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. 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 as a sensitivity analysis to design field-testing experiments to determine whether any constituents elevated in the laboratory te sts 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 mi nimally reactive gas (Ar) shows that the10 concentrations measured cannot be transfe rred directly to aquifer conditions. Three hypotheses are proposed to explai n overall high cation concentr ations: (1) Because DI water with a small amount of NaCl added (Cl ~40 pp m) was used in the experiment instead of aquifer water for each sediment, some dissolutionof reactive minerals occurred to reach rockwater equilibrium. (2) Disaggregat ion of rocks exposed more reactive surfa ces than would be exposed in the aquifer setti ng. (3) Although some contamina tion from the experimental design was noted and factored out, contaminat ion from sample handling (e.g., drilling mud introduced during coring or oxidation of core during storage) cannot be removed.

Figure 3. pH through the experiment for all reactors. CO 2 was introduced into the system on day 14.

Note that because water was not flushed th rough the system, the water analyzed was representative of what would be mobilized per unit of water under stagnant conditions. As follows, water interacted with only one volum e of rock, which begs the question of how concentrated a unit of water might become up on subsequent interactions with additional

volumes of rock. The concentration obtained in an aquifer setting would depend on the geometry of CO 2 flow rate and water migration. If th e reactive fluids moved through the same rock volume repeatedly along a flowpath, the reactive constituents in the rock might be quickly depleted. However, if a geometry were created in which the same volume of water were charged with excess gas and migrat ed through 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. Additional reactive transport studies are require d to understand these relationships.11 

### SACROC Field Study

A field study designed to improve understandin g of risk to freshwater is under way in the vicinity of the ~90-mi 2 SACROC oil field in north centr al Scurry County, Texas, USA (Figure 4), where CO 2 has been injected for enhanced oi l recovery (EOR) since 1972. Most of Scurry County is underlain by the Dockum aqui fer, which is designated one of the minor aquifers of Texas by the Texas Water Deve lopment Board (TWDB). To assess whether

freshwater resources over the SACROC oil fiel d have been impacted by over 35 years of CO 2 injection, we must first define the regional variability in Dockum aquifer groundwater chemistry. Regionally, throughout the Texas pa nhandle and southeastern New Mexico, the Dockum aquifer has been recognized as be ing chemically heterogeneous (Dutton and Simpkins, 1986). We have gained understa nding about why Dockum groundwater is chemically heterogeneous, and we recogni ze no systematic differences in Dockum groundwater overlying SACROC versus groundwat er overlying areas outside of SACROC.

Figure 4. Locations of SACROC oilfield, local exte nt of Dockum aquifer, and BEG and TWDB water wells used in study. Location of Scurry County is shown by black-square in insert map of Texas.



### Methods of SACROC groundwater study Because data on groundwater quality prior to CO 2 injection at SACROC are spatially and temporally limited, there is not a good “b ackground” dataset against which to compare post-CO 2 -injection groundwater quality. For this reason, and to under stand why Dockum groundwater is chemically hete rogeneous, our study covers 3,300 km 2 (1,275-mi 2 ) (area covered by black dots in Figure 4). In lieu of samp ling before and after CO 2 injection, BEG sampled both inside and outside SACROC duri ng two consecutive year s to assess spatial variation in groundwater chemistry.12 During the SWP SACROC groundwater study BEG researchers:

• compiled historical (dating back to 1936) to recent (2008) groundwater chemistry data for eight counties (Figure 4) from the TWDB online database (TWDB, 2009),

• collected multiple freshwater samples from 60 private water supply wells (113 samples) and one spring (1 sample), and brine from eight production/injection wells (10 samples);

• compiled chemical data from SACR OC production/inject ion zone brine analyses; data from brine samples are being used in geochemical mixing

models and are not discusse d in this document,

• conducted a study of shallow subsurface stratigraphy, and

• measured water levels to construc t potentiometric surface maps and identify groundwater flow paths in the Dockum aquifer.

BEG researchers completed six water quality sampling and water level monitoring trips between June 2006 and November 2008. Water well sampling methodology included

continuous measurement of field chemical parameters (temperature, pH, specific conductivity, and dissolved oxygen) in a flow cell and monitoring of discharge rate. To ensure that samples were from the formation and not stagnant casing- volume-water, we did not collect groundwater samples until after fi eld chemical parameters stabilized. We performed alkalinity titrations in the field using filtered, unpreserved water samples. Other sampling protocol included: (1) field filtering and acid preservation of cation samples and (2) storage of all samples at temperature belo w 4°C immediately afte r sampling and during shipping. Laboratory analytes measured in BEG groundwater samples by LANL are: Al, Ag, As, B, Ba, Be, Br, Ca, Cd, Cl, CO 3 , Co, Cr, Cs, Cu, δ 13 C, dD, δ 18 O, F, Fe, HCO 3 , Hg, K, Li, Mg, Mn, Mo, Na, Ni, NO 3 , Pb, PCO 2 , PO 4 , Rb, Sb, Se, Si, Sn, SO 4 , Sr, TDS, Th, Ti, Tl, U, V, and Zn. Laboratory analytes measured by The Un iversity of Texas at Austin, Department of Geological Sciences (UT DGS), are: dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), methane (CH 4 ), and CO 2 from headspace gas in selected samples.



### Groundwater data analysis Ranges and median values of 40 chemical an alytes, total dissolved solids (TDS), and well depths for BEG freshwater samples are sh own in Table 1. This table includes results from all 60 BEG-sampled water wells in Fisher, Garza, Kent, and Scurry counties (black dots in Figure 4). Analytes of particular concern in proposed EPA rules for geologic sequestration (Appendix A) are pH, arse nic, lead, mercury.13 Table 1. Chemical analyte ranges and medi an values for 113 BEG freshwater samples collected from private wells in Fisher , Garza, Kent, and Scurry counties.14 GCCC laboratory tests and results from other studies show that pH will decrease and dissolved solids (TDS) will increase when CO 2 concentration is in groundwater is increased. However, our analysis of TWDB and BEG pH data over time (1956-2008) does not reveal anomalous pH or TDS values associated with SACROC (figures 5 thro ugh 8). Plots of other analytes show similar consistencies in time and space.

Figure 5. TWDB (1956-2008) and BEG (2007-2008) pH data from Borden, Fisher, Howard, Kent, Mitchell, Nolan, and Scurry counties, TX

Figure 6. Contours of pH measured by TWDB in Scurry County between 1956 and 1980. Contour interval = 0.5 pH units.15 The pH measured in Dockum aquifer sa mples collected from 1995 through 2008, after

CO 2 injection had been ongoing for ove r 20 years, is shown in fi gure 7. Careful comparison with pre-1980 contours reveals either no change or even a slight increase in pH near SACROC between the two time periods (figures 6 and 7). We would expect to see the reverse trend in pH if either (1) injectate CO 2 had leaked into Dockum freshwater zones or (2) reservoir brine with lowered pH and increased cation concentrations had been displaced upward into Dockum freshwater zones.

Figure 7. Contours of pH measured by BEG and TWDB in Scurry County between 1995 and 2008. Contour interval = 0.5 pH units.

Figure 8. TWDB (1936-2008) and BEG (2007-2008) total dissolved solids (TDS) data from Borden, Fisher, Howard, Kent, Mitchell, Nolan, and Scurry counties, TX16 Water wells in the region are comple ted in numerous different and sometimes multiple geologic units. The TWDB is a State agency that studies groundwater resources in Texas. They have delineated major and minor aquifers throughout Texas based on estimates of groundwater quantity and quality. One of th e minor aquifers of Texas, the Dockum,

underlies the SACROC oilfield in Scurry County and surr ounding areas (figure 4). The majority of water wells in Scurry Co. are classified as Dockum wells by the TWDB. During the BEG study of groundwater resources in th e vicinity of SACROC we identified large variations in well depth, static water level, a nd water chemistry of freshwater wells. The base of fresh water is at approximately 500 ft be low ground surface at the base of the Triassic Dockum Formation. The SACROC produc tion/injection zones are in the Pennsylvanian/Periman age Canyon and Cisco formations at approxi mately 6,500 ft below surface (table 2).

Table 2. Stratigraphic units in the subsurface be low the SACROC oilfield (multiple sources for stratigraphic nomenclature)17 BEG sampled freshwater from wells constructed in the Ogallala Formation, Dockum Formation, Permian-age units, and combinations of these geologic units. By plotting depth of water wells and depth to top a nd bottom of the Dockum Santa Ro sa (figure 9), we classified wells by stratigraphic unit in which they are completed. Figure 10 shows BEG wells classified by geologic formation in which they are completed. We are currently evaluating groundwater chemistry trends by grouping data according to stratigraphic unit in which they are completed.

Figure 9. Scaled cross sections of selected BEG-sa mpled wells relative to top and bottom of Dockum Santa Rosa subunit of Dockum aquifer.

Geologic units present at the surface with in our study area range from Permian to Quaternary in age. Two signi ficant water-bearing units crop out at the surface within the study area. The Triassic-age Dockum Formation (Fm.) (TrD in Figure 10) hosts the Dockum aquifer. The second significant water-bearing uni t is the Ogallala Fm. (P-EOg in Figure 10). This erosional outlier of the Ogallala Fm. is isolated from the Ogallala aquifer of the Texas High Plains but provides signifi cant freshwater resources in Scurry and surrounding counties. Smaller quantities of fresh groundw ater are produced from Permia n-age units in the east parts of the study area (Figure 10).18 Figure 10. Locations of wells monitored fo r SWP SACROC groundwater study superimposed on surface geologic units. Geologic unit abbreviations : Q- undifferentiated Quaternary units; P-EOg –

Paleocene-Eocene Ogallala Fm.; TrD – Triassic Dockum Fm.; P – undifferentiated Permian units.

Chemistry of Dockum aquifer groundwater is highly heterogeneous. The chemistry of Dockum aquifer groundwater varies greatly because (1) the Dockum Formation is heterogeneous, (2) the history of oil and ga s activity in the region is long, and (3) wells designated as Dockum aquifer by TWDB are comp leted in different stra tigraphic intervals. Wells completed within the Ogallala outcrop are designated by TWDB as Dockum aquifer wells even though not all of them extend into the underlying Dockum Formation, which is subdivided into different depositional units with depth (McGowan et. al., 1979), not all of which are hydraulically connected. A few Doc kum aquifer wells penetrate multiple Dockum subunits and extend into underlying Permian-age strata. Ranges of analytes measured in all freshwater wells sampled by BEG A Piper diagram is a graphical display used to show variations in large groundwater chemistry datasets. Milliequivale nt percentages of major cati ons (Ca, K, Mg, and Na) and anions (Cl, HCO 3 , and SO 4 ) are plotted on trilinear diag rams, and combined values are projected onto a central quadral inear plot. A Piper diagram of TWDB database samples from Dockum aquifer wells in Scurry and immedi ately surrounding counties (figure 11) shows tremendous variation in chemical composition, es pecially for a single aquifer. Most of the TWDB historical samples are from areas outsi de SACROC, but there is no clear distinction between samples collected inside versus outside the SACROC oil field (figure 12).19 Figure 11. Piper diagram showing large variati on in water chemistry for BEG and TWDB wells shown in figures 4 and 10.

Figure 12. Piper diagram of BEG wells only showing no clear distinction between BEG groundwater samples collected inside SACROC versus outside SACROC.20 A plot of sulfate (SO 4 ) versus chloride (Cl) anions (Figure 13) s hows a more distinct grouping of samples from inside versus outs ide SACROC. BEG freshw ater samples from inside have higher Cl values than those coll ected outside SACROC. Samples with higher Cl

and SO 4 concentrations (e.g., Permia n and produced water sample s) have much higher total dissolved solids (TDS) than BEG freshwater samples. Trends of major element concentrations in Figures 11 and 12 suggest mixing of Dockum aquifer groundwater and water with Permian and production-zone geochemi cal signatures. Mixing models tested using PHREEQC code at BEG predict that <1% pro duced oil field brine or 12% Permian water

could mix with the lowest TDS BEG Dockum fr eshwater sample to produce the highest TDS Dockum sample.

Figure 13. Sulfate vs. chloride concentrations of all BEG samples (five sampling periods, inside and outside of SACROC) and BEG and KM produced water samples.

Several mechanisms can be used to explain fluid mixing, especially in oil fields that have been operating since the early 1950s. Evidence of fluid mixing over SACROC should not be taken as proof of brine (or associated CO 2 ) leakage from compromised production and/or injection well casings. Historical oil-field practices did not take the environmental impacts of co-produced brine disposal into acco unt. Use of unlined surface pits for disposal of co-produced oil-field brine was routin e practice up until the mid-1960s and was not

abandoned in Texas until the mid-1970s. In the US, many cases have been documented of oilfield brine contamination of groundwater resu lting from brine discharge into streams or ponds, pipeline leaks, and inf iltration pits (e.g., Reed, 1961; Fisher and Sublette, 2005; Kharaka et. al., 2005). Incidents of casing leaks have most lik ely occurred over the ~60-yr history of oil-field activity at SACROC. However, current KM operational practices,21 including use of protective su rface casing and extensive cat hodic protection networks, make conduit flow between the ~7,000-ft-deep producti on and freshwater z ones highly unlikely. The main question pertinent to this study remains: Has Dockum aquifer water been impacted by the long history of CO 2 injection at SACROC? If conduit flow along leaking well bores is responsible for increased TDS in Dockum aquifer water overlying SACROC, we would also expect to see impacts from high CO 2 concentrations. Given the complexity of the natural system and the likely signal from early oil-field activities, it is unrealistic to try to prove that no upward vertical communication of fluids has occurred in SACROC oil field. Evidence presented here allows us to say th at no obvious impacts to groundwater are found, and that the impacts observed are not a result of interaction of freshwat er with large volumes of injected CO 2

. More detailed description of the sh allow subsurface stratigraphy underneath SACROC allowed us to construct a potentiom etric surface map using water levels from only those wells completed in the Dockum Santa Ro sa subunit of the Dockum aquifer. This map reveals a groundwater mound over SACROC (insert with cross section lo cation lines in figure 14). One way to assess if shallow freshwa ter resources have been impacted by CO 2 injection at SACROC is to look at profiles of chemical constituents along Dockum aquifer groundwater flow paths in vici nity of SACROC (figures 14 through 16). There are no clear trends of lower pH, higher TDS, or higher con centrations of other analytes over SACROC versus areas downgrad ient from SACROC.

Figure 14. (a) Locations of tran sect plots relative to SACRO C and potentiometric surface contours of Dockum Santa Rosa and (b) pH alo ng gradient-paralle transects W, X, and ZY.22 Figure 15. Calcium (Ca), Magnesium (Mg), and Strontium (Sr) concentrations along flow path transect line X.

Figure 16. Total dissolved solids (tds), sodium (Na), Potassium (K), and Chloride (Cl) concentrations along flow path transect line X.

Another way to evaluate data from Dockum groundwat er samples is to assess degradation of water quality by comparing anal yte concentrations with EPA drinking water

standards. Data summarized in Table 3 are fr om BEG samples that were filtered (0.2 to 0.45 μm) in the field. Cation samples were preser ved with nitric acid to pH below 2. No preservative was added to anion samples. Some Dockum freshwater samples have constituents that exceed drinking water samp les. The percentage of BEG Dockum aquifer samples taken from 60 wells and 1 spring w ith analytes exceeding EPA primary maximum contaminant levels (MCLs) and secondary drin king water standards are highlighted in yellow in Table 3. Without exception, the percentage of samples with analytes in excess of EPA standards is higher outside than inside SACROC (Table 3). Arsenic (As) and lead (Pb) are listed as cont aminants of particular concern in the July 2008 EPA proposed rule. The concern is that such constituents will be leached from rocks in the appropriate GS reservoir and mobilized to impact USDWs. Of the 9.8 percent of wells with As levels above MCLs, only 1.6 percent of these are inside SACROC (Table 3). Only ~12 percent of the filtered BEG Dockum samples had Pb levels above a detection limit of 0.002 mg/L. Forty-two percent of these samp les are from wells inside SACROC.23 Table3. Comparison of EPA drinking water stan dards with BEG Dockum aquifer well data.

BEG researchers are studying stable carbon isotopes of fresh groundwater, produced water, injectate CO 2 , solid carbonate phase, and plant ma tter, which forms the substrate for

microbial oxidation of CO 2 . Potential contributions to th e carbon isotope signature include mixing with Permian and produced water, microbially produced CO 2 , and injectate CO 2 . A plot of carbon isotope ratios versus dissolved inorganic car bon (DIC) shows an increase in light carbon with increasing DIC. This datum is consistent with i nput from microbial CO 2 but does not rule out the pr esence of injectate CO 2 . Modeling of the system is underway to test if it is sensitive to injected CO 2 and if so, if any injected CO 2 can be identified. BEG researchers are currently collecting water levels in Dockum aquifer wells throughout Scurry County. Our objective is to ge nerate potentiometric surface maps for the various stratigraphic intervals that compose th e Dockum aquifer. These maps will aid in reactive transport modeling and further inte rpretation of Dockum aquifer groundwater chemistry.



### Conclusions

Geochemical modeling and reaction experime nts plus known occurrences of naturally

CO 2 charged potable water show that common ch emical reaction products from dissolution of

CO 2 into freshwater include rapid buffering of acidity by dissolution of calcite and slower equilibrium by reaction with clays and feldspar s. Results from a series of laboratory batch

reactions of CO 2 with diverse aquifer rocks show geoc hemical response with in hours to days

after introduction of CO 2 . Results include decrease in pH and increased concentration of

cations in CO 2 experimental runs relative to control runs using argon (Ar) . Some cation (Ba, Ca, Fe, Mg, Mn, and Sr) concentrations incr eased over an order of magnitude during CO 2 runs. Results are aquifer dependa nt in that experimental vesse ls containing different aquifer rocks showed different magnitudes of increase in cation concentrations. Laboratory experiments and field observations show that pH will decrease and cation

concentrations will increase in rock-w ater systems exposed to injected CO 2 . Underground sources of drinking water contained within multiple stratigraphic horizons of the Dockum24 aquifer provides a test case to see if these changes can be detected overlying 6,000- to 7,000- ft-deep CO 2 injection zones of the SACROC oil fi eld. An intensive water sampling program was undertaken and shows that Dockum groundwat er has both natural and oil-field related variability. Our approach is to compare com positions inside SACROC and outside SACROC looking for variability that might correspond to the changes expected to result from

introduction of CO 2 into the aquifer. Chemical constituents measured in samples of Dockum groundwater throughout the study area suggest minor mixing with more sa line (higher TDS) Permian water or coproduced oil-field brines. In many oil fields such mixing is the result of former production practices, and this mixing is ther efore not attributable to the CO 2 injection process. Past practices preclude attempting to determine the ef fectiveness of current practices such as brine re-injection, protective surface casing, and ex tensive cathodic protection networks employed to protect USDWs. One key question can be answered in th is progress report – did injection of CO 2 at depth damage water quality in Dockum USDW? Results show no systematic or widespread degradation of water quality inside SACROC as compared to groundw ater outside SACROC. Dockum aquifer chemical concentrations insi de SACROC fall within the range of TWDB groundwater chemical analyses of sample s taken from a multicounty region surrounding SACROC. Dockum wells with chemical constituents in excess of EPA drinking water standards are present within our study area. However, a larger percentage of these wells are located outside the SACROC oil field. In particular , we see no evidence of As or Pb increase in Dockum aquifer zones. Analysis is underway of all the factors needed to assess whether any geochemical evidence suggests that CO 2 injected for EOR at SACROC has mixed with Dockum groundwater. Work in progress includes stable is otopic studies, flow path studies, and rockwater interaction modeling. At this time we identify no obvious indications that CO 2 from the injection zone has inte racted with groundwater ; however, a definitive conclusion requires additional analysis to separate complexities of natural a quifer variability from any CO 2 injection-related change.



### Acknowledgments

We gratefully acknowledge support of much of this work from the Department of Energy National Energy Technology Laboratory and Kinder Morgan CO 2 pipeline and production companies. LANL performed most chemical analyses on groundwater samples collected by BEG as part of the SWP phase-2 project at SACROC. Batch reactor experiments were funded by industrial sponsors of the Gulf Coast Carbon Center ( www.gulfcoastcarbon.org ). We thank Bob Reedy for advice in setting up th e batch reactor technique and Bridget Scanlon for assistance with ion analysis. The text and figures in this document were prepared by

numerous members of the GCCC research te am including: Rebecca C. Smyth, Susan D. Hovorka, Jiemin Lu, and Katherine Romanak.25 

### References

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Kharaka, Y.K., Thordsen, J.J., Kakouros, E. and Herkelrath, W.N, 2005, Impacts of petroleum production on ground and surface waters: results fr om the Osage-Skiatook petroleum e nvironmental research A site, Osage county, Oklahoma. Environmental Geoscience, v. 12, p. 127-138.

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McGowan, J.H., Granata, G.E. and Seni, S.J., 1979, Depositional framework of the Lower Dockum Group (Triassic) Texas Panhandle: The Un iversity of Texas at Austin, Bureau of Economic Geology Report of Investigations No. 97, 60 p.

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### Appendix A.

Federal Requirements under th e Underground Injection Control (UIC) Program for Carbon Dioxide (CO 2 

### ) Geologic Sequestration (GS) Wells, Fall 2008 Proposed Rule

From the EPA website for public comments: EPA is proposing Federal requirements under the Safe Drinking Water Act (SDWA) for underground injection of carbon dioxide (CO 2 ) for the purpose of geologic sequestration (GS). GS is one of a portfolio of options that could be deployed to reduce CO 2 emissions to the atmosphere and help to mitigate climate change. This proposal applies to owners or operators of wells that will be used to inject CO 2 into the subsurface for the purpose of long-term storage. It proposes a new class of well and minimum technical criteria for geologic site characteri zation, fluid movement, ar ea of review (AoR) and corrective action, well construc tion, operation, mechanical integr ity testing, monitoring, well plugging, post injection site ca re, and site closure for prot ecting underground sources of drinking water (USDWs). Elements of this proposal are based on th e existing Underground Injection Control (UIC) regulatory framework, w ith modifications to address the uniqueness

of CO 2 injection for GS. If finalized, this pr oposal would help ensu re consistency in permitting underground injection of CO 2 at GS operations across the U.S. and provide requirements to prevent endangerment of USDWs in anticipation of the eventual use of GS to

reduce CO 2 emissions

Summary of Proposed Rule: EPA proposes amending the Safe Drinking Water Act (SDWA) by using its authority through the Underground In jection Control (UIC) program to regulate geologic sequestration (G S) of carbon dioxide (CO 2 ). It plans to do so by establishing Class

VI wells. Class VI GS well rules contain minimum requirements to protect underground sources of drinking wate r (USDW), including:

• geologic site characterization;

• monitoring of fluid movement;

• area of review (AoR) and corrective action;

• well construction, operation, mechanical integrity testing (MIT), monitoring, and plugging; and

• post injection site care and closure.

Definition of USDW in rule: An aquifer or portion of an aquifer that s upplies any public water system or that contains a sufficient quantit y of groundwater to supply a public water system, and currently supplies drinking water for human c onsumption, or that co ntains fewer than 10,000 mg/l total dissolved solids an d is not an exempted aquifer. The main concerns expressed in rule regard ing groundwater are (1) mitigating movement of fluids containing mobilized cont aminants from appropriate CO 2 receiving formations into USDWs and (2) the fact that injection ma y force naturally occurring brine from an appropriate CO 2 receiving formation into USDWs.28 SACROC groundwater study results are pertinent to the follo wing sections of the rule. Section II. What is EPA proposing? Subpart D. What are the risks associated with CO 2 GS? An improperly managed GS site has potential to endanger USDWs.

Risks can include:

(1) leaching and mobilization of contaminants (e.g., arsenic (As), lead (Pb), and organic compounds),

(2) changes in regional groundwater flow, and

(3) movement of saltier appropriate CO 2 receiving formation fluids into USDWs, causing degradation of water quality. Subpart J. Is injected CO 2 considered a hazardous waste under CERCLA?

CO 2 is not a hazardous waste if it is a pure stream, but impure streams may contain mercury (Hg). Also, CO 2 could react with groundwater (brine or freshwater unspecifi ed) to produce sulfuric acid (H 2 SO 4 ). Section III. Proposed Regulatory Alternative Subpart A. Proposed Alternative 6. Proposed plume and pressure fr ont monitoring requirements Monitoring temperature, pH, salin ity, heavy metals, and stable isotopes (carbon and oxygen) via designate monitoring wells are proposed. This section is ambiguous because although monitoring wells completed in the confining layer between appropriate CO 2 receiving formation (aka injection zone) and USDW are specifically mentioned, the freshwater zone is also

referenced. 8. Proposed well plugging, post-injec tion, site care, and site closure

requirements The only pertinent point here is the 50-year time frame for post injection monitoring, or time frame to be determined by

Director of EPA.Letter ID: 0379

Commenter: Rebecca C. Smyth

Organization: University of Texas at Austin, Bureau of Economic Geology, Jackson School of Geosciences,

Gulf Coast Carbon Center

Letter ID: 0379 Comment ID: 28184 Letter Lines: 17 to 35

Comment Code: 2400 2.4. Other Research Offered by Commenters (seeking comment)

Comment: Over 3 trillion standard cubic feet (TCF) or 150 million metric tons of

CO2 have been injected for enhanced oil recovery (EOR) at the SACROC oilfield in

west Texas since 1972. Roughly half of the injected CO2 has been produced and

reinjected as part of a recycling effort; an estimated 35 million tons is

permanently sequestered in the subsurface at SACROC. Clearly this is a large

scale CO2 injection site. As with the other four CO2 injection sites referenced

in Section II of this Notice, there has been no documented leakage of CO2 at

SACROC.

The attached document is a status report submitted to EPA Region 6 offices in

Dallas, TX in April 2009. This report summarizes DOE/NETL

‐ funded, through the

Southwest Regional Carbon Sequestration Partnership (SWP), research at SACROC.

Objectives of the combined research efforts of the Bureau of Economic Geology,

Los Alamos National Laboratory, New Mexico Tech, University of Pittsburgh, and

University of Utah are to (1) assess potential impacts to the shallow subsurface

over SACROC, (2) model the fate of CO2 injected into the subsurface at SACROC,

and (3) use geophysical methods to image the ~35 million tons of CO2 assumed to

be trapped in the subsurface.

Please refer to the PDF or see further down below for the text of the attachment.

Individual Response: EPA has reviewed the status report on the SACROC project. EPA acknowledges the

submittal and will continue to review studies related to the evaluation of actual field results and the

limitations of sampling and analysis, as well as the limitations of modeling. EPA adds that is has been

monitoring the progress at SACROC, among other GS projects. EPA intends to continue to collect data on

GS and to incorporate such data and information as it implements its adaptive approach to the GS

rulemaking to incorporate new research, data, and information about GS. For more information on the

adaptive approach, see the responses to comments on the proposed rule in the rulemaking docket.