Executive Summary: Recent results of the SACROC
 

Groundwater Geochemistry Study
 

Romanak, K.D., Smyth, R.C., Yang, C., Hovorka, S.D., Nicot, J.P., Lu, J.
 

Gulf Coast Carbon Center, Bureau of Economic Geology, The University of Texas at Austin
 

Executive Summary

The purpose of this document is to report a summary of recent results and research of the

SACROC groundwater quality study and their relation to geologic sequestration (GS) of CO2. The

research was conducted by scientists at the Gulf Coast Carbon Center (GCCC) as part of the

NETL Southwest Carbon Partnership Phase II (SWP) research. The results of this study were not

available before the publication of the July 25, 2008, proposed rule, “Federal Requirements

Under the Underground Injection Control (UIC) Program for Carbon Dioxide (CO2) Geologic

Sequestration (GS) Wells'' (73 FR 43492). The work is currently being written for peer review

publication in Applied Geochemistry to be submitted by the end of 2009, and is useful for

consideration of amendments to the Safe Water Drinking Act. Additional details can be found in

Smyth et. al., 2009; Romanak et. al., 2008 and 2009; and in the document, Modeling shallow

groundwater geochemistry and carbon isotopes: test of methodology for CO2 storage evaluation

at an EOR site, west Texas, USA, published at http://www.beg.utexas.edu/gccc/.

Our examination of groundwater geochemistry at a site where CO2 has been injected for

decades shows that monitoring of natural parameters (pH, alkalinity, etc.) in shallow aquifers

may be impractical and/or ineffective for evaluation of CO2 storage permanence. At the very

least, detailed analysis of background variability and processes controlling groundwater

geochemistry would be required on a case by case basis to confirm correct interpretations. Our

analysis shows that to detect possible CO2 leakage into an underground source of drinking

water (USDW) it is important to characterize the hydrogeologic context. Sampling at a few

wells without understanding the hydrogeologic context could lead to erroneous

interpretations. Errors could be either at sites where CO2 leakage has occurred but is not

detectable or at sites where non‐injection induced changes are mistakenly assumed to indicate

leakage. We have found that background variability in aquifers may mask the effects of even

moderate anthropogenic CO2 input at GS sites rendering it impossible to discriminate changes

in natural biogenic CO2 from anthropogenic inputs.

Introduction

Monitoring groundwater resources above geologic carbon sequestration reservoirs is proposed

for ensuring that potable water supplies remain protected and also for establishing that CO2 is

adequately sequestered with respect to the biosphere. Research regarding the impact of CO2

leaks on potable aquifers is focused on, 1) assessing geochemical outcomes of increased CO2

and their effects on drinking water quality, 2) determining the potential for widespread usage

of shallow aquifer geochemistry for CO2 storage evaluation and, 3) identifying the parameters

best suited for monitoring groundwater resources.
                         

                           

                         

                             

                             

                             

                   

        

                               

                                 

                           

                               

                        

                      

                               

                       

                               

                             

                           

                    

                                   

           

       

                           

                       

                                

                             

                           

                                   

                            

                   

                             

                           

                         

                             

                      

                   

                     

               

The success of using shallow groundwater geochemistry as a tool for CO2 storage evaluation

depends upon the ability to identify one or more geochemical parameters sensitive enough to

provide a recognizable geochemical shift upon addition of exogenous CO2 to the aquifer. This

requires a shift that is larger than normal background variability and is therefore detectable as

an indicator of change. Widespread application of these methods in a variety of GS locations

also necessitates that the type and magnitude of geochemical shift is similar or at least

predictable in any geochemical environment even though processes and mineralogies differ

widely among aquifers.

Indeed the position of many researchers is that a predictable consequence of input of CO2 into

a dilute aquifer is an increase in dissolved inorganic carbon (DIC) as alkalinity, H2CO3 or HCO3

‐

, a

decrease in pH, and mobilization of cations including toxic metals from mineral dissolution and

desorption reactions (Assayag et. al., 2009; Apps, et. al., 2009; Carroll et. al., 2009; Wang and

Jaffe, 2004). A popular suggestion is that these parameters are therefore suitable for

monitoring groundwater resources. The results of the SACROC ground water study, which is

the only field study to date of potable water at a large‐scale industrial CO2 storage analogue,

give strong evidence against such assumptions. At SACROC, natural and man‐induced processes

that are common in many shallow aquifers have created; 1) a geochemical system that is more

complex and reacts differently to CO2 than the simple system put forth by previous researchers,

and 2) a background variability that would mask even a relatively large‐scale exogenous CO2

input. The implications are that GS monitoring strategies using natural groundwater

constituents must, at the very least, be site‐specific and may not be useful at all owing to the

variability and complexity of groundwater systems.

Research Objectives and Approach

To test methodologies for CO2 storage evaluation using shallow aquifers, we targeted a potable

water source (Dockum aquifer) above a long‐duration (>35 years) large‐scale (~250 km2

)

engineered CO2 injection site in west Texas. The SACROC oil field in Scurry County, TX, currently

operated by Kinder Morgan has injected CO2 for enhanced oil recovery (EOR) since 1972 with

approximately 50% recovery and recycling of injected CO2. This site provides the opportunity to

investigate (1) if CO2 from over 35 years of injection can be identified in the shallow aquifer and

(2) if groundwater chemistry can be effectively used as a monitoring tool at sequestration sites.

To accomplish our objectives, we compared the groundwater geochemistry of samples

collected in the area of CO2 injection (inside SACROC) to that of groundwater samples collected

from areas that have not experienced CO2 injection (outside SACROC). In addition, we utilize

regional historical data from the Texas Water Development Board (TWDB) collected either prior

to or spatially removed from any influence of engineered CO2 injection from the SACROC or

nearby Sharon Ridge Field to establish regional background variability in geochemistry.

Geochemical perturbations lying outside the normal range of groundwater variability could

signal anthropogenic CO2 input. In addition, we use stable carbon isotopes to source‐identify

dissolved CO2 to distinguish natural microbial inputs from anthropogenic ones.
                       

                         

                       

                         

                           

                         

                   

                       

                         

                         

                                  

                         

                           

                               

                   

     

                         

                         

                   

                     

                         

                   

                     

                     

                           

                           

        

                         

                         

               

                     

     

               

                             

                               

                 

                     

                         

                           

                         

Methods

Approximately 113 fresh to slightly saline groundwater samples within Triassic‐age Dockum and

8 samples from Permian‐age evaporite deposits were collected from 60 private water wells (50‐

to 500‐ft‐deep) and 1 spring during five water sampling trips, June 2006 ‐ November 2008.

Samples were analyzed in the field for alkalinity, dissolved oxygen, conductivity, pH, and

temperature and in the laboratory for major cations and anions, trace elements, δ13C, 18O, and

2

H isotopes, dissolved organic and inorganic carbon, and headspace gases. Ten samples of

brines co‐produced by Kinder Morgan from the 6,000‐ to 7,000‐ft injection/production zone

were also analyzed for these parameters. Additional geochemical data on co‐produced brines

were provided by Kinder Morgan and historical records for Dockum and Permian water

geochemistry collected by the Texas Water Development Board between 1936 and 2008 from

eight Texas counties were also added to the data set. Carbon isotopes of CO2 being injected for

enhanced oil recovery, soil organic matter, and soil carbonate from Dockum cores were

collected and analyzed for δ13C. Two samples of Dockum core from depth intervals of 450‐460

ft and 370‐380 ft were examined for mineralogy using SEM with aid of an energy dispersive X‐

ray system which can identify chemical composition of point area.

Results and Discussion

Consideration of regional geology and stratigraphy of the area, geochemical trends of collected

data, and past and present farming and pre‐CO2‐EOR practices, was used to identify

background geochemical influences on Dockum geochemistry. The major influences are; 1)

mixing of Dockum groundwater with underlying Permian evaporite waters along a well‐

documented Permian dissolution zone (i.e., Dutton, 1989), 2) leaching into the Dockum of

historically‐produced brine formerly stored in unlined surface pits, 3) temporal geochemical

variations related to Dockum aquifer pumping and local irrigation practices, 4) groundwater

interaction with spatially and compositionally variable stratigraphic units, 5) cation exchange,

and 6) dedolomitization. Potential sources of CO2 in Dockum groundwater were identified as; 1)

CO2 injected into the production zone, 2) CO2 produced from microbial activities, and 3)

degassing from rock‐water reactions.

The findings at SACROC are pertinent to a wide variety of hydrogeologic environments where

GS sites may be implemented and therefore must be considered when developing a near‐

surface monitoring program. They illustrate the importance of understanding hydrochemical

context and background variability to implementing monitoring protocol. These points are

further illustrated below:

1.	 Geochemical Context ‐ The geochemical driving forces of the hydrochemical system must

be defined before the consequences of CO2 input can be predicted. In the Dockum aquifer,

as well as in many other aquifers, mixing of concentrated waters (in this case Na‐Cl brines

with cation exchange and Ca‐SO4 Permian evaporite waters) drives dedolomitization.

Dissolution of dolomite and precipitation of calcite (dedolomitization) fueled by calcium

ions is widely documented and common in many hydrologic systems (Pacheco and Szocs,

2006; Back and Baedecker, 1989; Back et. al., 1979; Plummer and Back, 1980; Hanshaw

and Back, 1985) and creates a significantly different carbonate environment than that of
                     

                         

                           

                           

                         

                   

                     

                     

                 

               

                   

                         

                             

                         

                         

                             

                   

                     

                   

                       

                           

                                 

             

                             

                   

                         

                       

                       

                       

                 

        

                           

                         

                       

                           

                         

       

simple calcite dissolution. Whereby calcite dissolution is driven by CO2, dedolomitization is

driven by calcium ions which create undersaturation of the system with respect to

dolomite and oversaturation with respect to calcite. Input of CO2 into these very different

systems has been modeled to produce very different outcomes. In the simple system, DIC

increases with increases in the partial pressures of CO2 (PCO2) as previous researchers

have predicted. However, in the case of dedolomitization, evolution of groundwater

proceeds towards lower HCO3‐ concentrations. Increases in PCO2 simply shift this overall

trend towards higher HCO3‐ but retain the same overall evolutionary trajectories. This

complex geochemical reaction would yield inaccurate conclusions if dedolomitization had

not been identified as a dominant process.

2.	 Background variability ‐ Because CO2 exists naturally in an aquifer, stable carbon isotopes

were used to determine if exogenous CO2 in the Dockum aquifer could be distinguished

from natural CO2. Stable carbon isotope ratios of various carbon sources in the system, the

PHREEQC numerical code, and isotope mass balance equations were used to; 1) model

isotopic trends during dedolomitization under open and closed systems and, 2) identify the

most likely source of CO2 in the Dockum aquifer. We modeled carbon isotopic and DIC

trends during dedolomitization in open‐ and closed‐systems with and without injectate

and microbial CO2 input. Whereas isotopic trends for calcite dissolution are

straightforward, isotopic trends during dedolomitization are complex. When coupled with

high background geochemical variability such as in the Dockum aquifer system, stable

carbon isotopes do not yield definitive results. Hence stable carbon isotopes do not appear

to be the best tool for positive identification of injectate CO2 in the Dockum aquifer – and

possibly in other complex aquifer systems as well.

Conclusions

Based on a field study and modeling of a long‐running, large‐scale engineered CO2‐EOR site, we

conclude that shallow aquifer geochemistry has limited applicability for monitoring

permanence of CO2 storage at GS sites. Many natural and man‐induced factors create

hydrochemical variability that may mask the effects of CO2 input in common hydrogeologic

environments. Detailed costly studies of each geochemical system would be necessary to

assure that interpretations are correct. Assumptions about CO2 leakage formed by observation

of geochemical parameters without knowledge of controlling geochemical processes would

lead to incorrect conclusions.

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