The importance of considering rock-water interaction when studying potential impacts of geological CO 2 sequestration on drinking water resources

Changbing Yang, Jean-Philippe Nicot, Rebecca C. Smyth, Katherine D. Romanak, and

Susan D. Hovorka

Summary It is of fundamental importance that water-rock reactions be taken into account in the interpretation of ground water chemistry. Not considering the rock contributions to the

rock-water-CO 2 reactive system can lead to incorrect extrapolations and overemphasize

the risk to water.

Conclusions of modeling exercises must also be seriously weighted by the soundness of the model assumptions. For example, we think that the role of sulfides in terms of risk to water as a consequence of CO 2 leakage in the report of Birkholzer et al. (2008) has been overemphasized, as these species are not the minerals commonly observed in fresh-water aquifers.

Introduction Researchers from the Bureau of Economic Geology (BEG) at the University of Texas at Austin are conducting experimental and modeling studies relevant to geologic sequestration of CO 2 (CO2GS) with specific regard to potential impacts to shallowgroundwater resources. In addition, BEG researchers have recently completed a fieldbased groundwater sampling study at the SACROC oilfield in west Texas and are continuing research on CO2GS at the Cranfield oilfield in Mississippi. Other researchers in the field of geologic carbon sequestration have recently published work on potential impacts to water resources as a result of CO 2 injection directly into deep brine formations, or by accidental release of injectate CO 2 into drinking water resources either through laboratory experiments or groundwater modeling studies.

Themes we want to emphasize as being critical to studies of potential impacts to drinking

water resources from CO 2 injection are:

• CO 2 occurs naturally in shallow aquifers. CO 2 concentrations in a single aquifer can vary temporally and spatially as a result of agricultural and industrial practices. For example, Scanlon et al. (2005, 2007) have documented enhanced recharge from land use changes. Enhanced recharge of groundwater through the vadose zone can potentially change CO 2 concentrations in the saturated zone.

• The capacity for carbonate minerals (carbonate rocks and carbonate cement in clastic rocks) to buffering changes in pH with influx of CO 2 is under represented in the literature (i.e. Carroll et al., 2009). An exception to this statement is Keating et al. (2009) who documented nicely the buffering capacity of carbonate cement in a high-CO 2 concentration natural analog system in New Mexico.

• Realistic rock-water reactions are critical to accurate prediction of impacts to groundwater as a result of CO 2 injection (numerous articles referenced herein). For example, Romanak et al. (in preparation) have also documented the importance of carbonates but in a more complex fashion through the incongruentdissolution of dolomite as a process dominating the hydrochemistry of the Dockum aquifer that overlies the SACROC oilfield in west Texas.

The following text is a review of recently published studies on potential impacts to groundwater from GS with some examples from BEG’s on-going and soon to be published work. We start by citing published work on the importance of rock-water interactions in both deep and shallow aquifers for interpreting results of groundwater modeling. We then focus on one specific study (Birkholzer et al., 2008) that has been completed by researchers at the Lawrence Berkeley National Laboratories (LBNL).

CO 2 -water-rock interactions discussed in literature Rock-water reactions are ubiquitous in groundwater systems and cannot be excluded from models used to predict impacts to groundwater from introduction of CO 2 . Work of Kaszuba et al. (2003) concludes that because of the importance of rock-water reactions in potential geologic sequestration systems, much more than simple aqueous dissolution of

CO 2 in water must be factored into any realistic model of potential impacts to deep formation fluids or freshwater aquifers from introduction of anthropogenic CO 2

.

Earlier work by Zhu and Burden (2001) states that the complexity of aquifer fluid chemistry is defined by the degree of interaction with aquifer mineralogy, which includes solid matrix, grain-coatings and colloids. They are critical of groundwater modeling efforts that only consider pore-fluid compositions and ignore or greatly simplify aquifer mineralogy. Conclusions from Zhu and Burden (2001) are that contaminant fate and transport model predictions depend upon mineral composition of the aquifer. In fact, they state that the amount of calcite in the aquifer matrix determines the distance of contaminant plume migration.

Wigand et al. (2008) go on to state that not only does introduction of supercritical CO 2 into geologic systems affect brine composition through aqueous dissolution, it also affects the intensity of rock-water interactions. The phenomenon of increased rock-water

interaction with increase in dissolved CO 2 in brine is becoming common knowledge through DOE/NETL- and EPA-sponsored research on geologic sequestration (i.e., Keating et al. 2009).

Review of LBNL shallow groundwater modeling LBNL (Birkholzer et al., 2008) evaluated mobilization of a set of trace elements (called hazardous constituents in the report 1 ), such as arsenic, barium, cadmium, mercury, lead, uranium and zinc from a set of mineral species that may potentially be mobilized in the

event of CO 2 leak from the deep storage reservoirs using a comprehensive computational model. The mineral assemblages in the aquifer rocks were selected indirectly upon evaluation of 38,000 groundwater samples from aquifers throughout the United States, retrieved from the Unites States National Water Information System (NWIS). The report concluded that some trace elements may be mobilized and exceed the maximum

1 We disagree with the term “hazardous constituents” used in the Birkholzer et al., (2008) report because the trace elements occur in groundwater naturally even if their concentrations may increase owing to CO 2 intrusion.contamination levels (MCL) resulting from enhanced dissolution of the mineral assemblages caused by CO 2 intrusion into shallow groundwater aquifers. However, the model results rely on a fundamental assumption with large uncertainties in model parameters. We disagree with the assumption for constructing the EQ3/6 models and the methodology they used for investigating mobilization of trace elements from the aquifer rocks. We believe that this report illustrates the trappings of solely using groundwater compositions to estimate impact of CO 2 on the system.

The fundamental assumption in the Birkholzer et al. (2008) report for constructing the EQ3/6 models is that mobilization of trace elements is mainly dominated by enhanced dissolution of a set of specific mineral assemblages (for instances, sulfides in Table 3.6, p37 in the report). This set of specific mineral assemblages was selected from mineralogical analysis of coal seam aquifers, which formed in very strong reducing environments. Based on such an assumption, the authors calculated saturation indices of the selected mineral assemblages over chemical analyses of 38000 groundwater samples and determined whether a specific mineral phase would be present in a rock in equilibrium with the water sample. In a second step, assuming this phase is present, the authors computed the amount of trace elements that could be released under addition of

CO 2 . This assumption is unreasonable and not well supported at all in the literature. There are many other factors that could affect mobilization of the tracer elements, such as aqueous complexation (by ligands), ad/desorption from metal oxide and clay surfaces, and solid solutions in common minerals (carbonates, feldspars, and clay minerals). For instance, behavior and contribution of arsenic in natural water are related to MeOx (either simple desorption in oxidizing conditions or reducing conditions) (for example, Smedley and Kinniburgh, 2002, Scanlon et al. 2009).

BEG researchers did a set of batch experiments by reacting rock samples taken from major aquifers in Texas with water and CO 2 gas at a pressure of 1 atm (Lu et al., 2009, Smith etal. 2009). They found that concentrations of some trace elements may be

elevated after CO 2 intrusion, such as arsenic (Figure 1). However, no evidences of XRD and SEM analysis indicated that arsenopyrite existed in the rock samples. Lead, which according to the Birkholzer (2008) could be the most problematic element after CO 2 intrusion, however, is non-detectable in the 10 batch experiments. Of the 114 analyses of potable groundwater in the vicinity of the SACROC oilfield in west Texas, 100 samples (88% of the total samples) had Pb concentrations below a detection limit of 0.0002 milligrams per liter (mg/L) (Smyth et al., in preparation). Our experiences indicate that Pb in drinking water resources commonly comes from corrosion of submersible water pumps and accompanying conduit pipes and not from the aquifer rocks.

In conclusion, the methodology employed in the Birkholzer (2008) report on potential impacts of CO 2 addition to shallow groundwater resources by using only numerical modeling techniques is questionable and may be misleading if the controlling minerals/or controlling phases assumed in the numerical models are not the right ones. In addition, large uncertainties in the model parameters, such as Eh values and solubility constants may lead to wrong conclusions. Regulations of CCS based on such conclusions drawnfrom the model results presented in the Birkholzer (2008) report could be biased and may be impractical when applied to actual sites.

1.E-01 1.E+00 1.E+01 1.E+02

-12 -8 -4 0 4 8 12 16

Time (days)As ppb Batch 1

Batch 2

Batch 3

Batch 4

Figure 1. Evolution of arsenic concentration in four batch experiments (time 0 represents introduction of CO 2 into the batch reactor)

References

Birkholzer, J., 2008, Research project on CO2 geological storage and groundwater resources: water quality effects caused by CO2 intrusion into shallow groundwater: Technical Report LBNL-1251E, 352p.

Carroll, S., Hao, Y., and Aines, R., 2009, Geochemical detection of carbon dioxide in dilute aquifers: Geochemical Transactions 10:4; doi: 10.1186/1467-4866-10-4.

Kaszuba, J.P., Janecky, D. R., Snow, M.G., 2003, Carbon dioxide reaction processes in a model brine aquifer at 200 C and 200 bars: implications for geologic sequestration of carbon: Applied Geochemistry, v. 18, p. 1065-1080.

Keating, HE, Julianna Fessenden, Nancy Kanjorski, Daniel J. Koning, and Rajesh Pawar, 2009, The impact of CO2 on shallow groundwater chemistry: observations at a natural analog site and implications for carbon sequestration, under review.Lu, J., Partin JW, Hovorka, SD, Wong, Corinne Wong (2009) Potential risks to freshwater resources as a result of leakage from CO 2 geological storage—a batch-reaction experiment, Environmental Geology, under review.

Scanlon BR, Reedy RC, Stonestrom DA, Prudic DE, Dennehy KF, 2005, Impact of land use and land cover change on groundwater recharge and quality in the southwestern USA. Global Change Biol 11: 1577-1593

Scanlon BR, Reedy RC, Tachovsky JA, 2007, Semiarid unsaturated zone chloride profiles: Archives of past land use change impacts on water resources in the southern High Plains, United States. Water Resour Res 43: Art. No. W06423

Scanlon, B. R., Nicot, J. -P., Reedy, R. C., Kurtzman, D., Mukherjee, A., and Nordstrom,

D. K., 2009, Elevated naturally occurring arsenic in a semiarid oxidizing system: Southern High Plains aquifer, Texas, USA: Applied Geochemistry, doi:10.1016/j.apgeochem.2009.08.0004.

Shiraki, M., and Dunn, T.L., 2000, Experimental study on water-rock interactions during CO2 flooding in the Tensleep Formation, Wyoming, U.S.A.:Applied Geochemistry, v. 15, p. 265-279.

Smedley, P.L., Kinniburgh, D.G., 2002. A review of the source, behaviour and distribution of arsenic in natural waters. Appl. Geochem. 17, 517–568.

Smyth, R., S. Hovorka, J. Lu, K. Romanak, J. Partin, C. Wong, C. Yang, 2009, Assessing risk to fresh water resources from long term CO2 injection – laboratory and field studies, Energy Procedia, p1957-1964.

Smyth, R., S. Hovorka, J. Lu, K. Romanak, J. Partin, C. Wong, and C. Yang, 2009, Hydrogeology of Dockum groundwater near the SACROC CO2 EOR oilfield, Scurry County, Texas, in preparation.

Wigand, M., Carey, J.W., Schutt, H., Spangenberg, E., and Erzinger, J., 2008, Geochemical effects of CO2 sequestration in sandstones under simulated in situ conditions of deep saline aquifers: Applied Geochemistry, v. 23, p. 2735-2745.

Zhu, C. and Burden, D. S., 2001, Mineralogical compositions of aquifer matrix as necessary initial conditions in reactive contaminant transport models: Contaminant Hydrology, v. 51, p. 145-161.