Background and Experimental Setup
Since 1972 over 175 million metric tons of carbon dioxide (CO2) have been injected into the SACROC oil field to enhance oil recovery (EOR). About half of the CO2 has been co-produced with oil and recycled (separated from the oil and re-injected for further EOR). The remaining volume is assumed to be sequestered at 6,000 to 7,000 ft below surface. For this reason, and because SACROC oilfield has the longest history of CO2 injection of any oilfield in the Permian Basin, the site is an excellent natural laboratory for studying subsurface storage of carbon dioxide.
SACROC field experiments are being conducted on two scales. On a regional scale, research is focused on potential impacts to the shallow subsurface zones. GCCC and NMT researchers have sampled water wells as deep as 500 ft and measured CO2 soil zone concentrations within an ~800 mi2 area.
Deep subsurface experiments are centered on an experimental site that is roughly 0.4 square miles in area with CO2 injection wells located at each corner and an oil production well in the center. All of these EOR wells are 6,000 to 7,000 feet in depth. Three observation wells are located within several hundred feet of CO2 injection wells. Researchers from BEG, DOE NETL, Los Alamos National Laboratory, Schlumberger Carbon Services, and the University of Pittsburgh are attempting to monitor movement of CO2 in the deep subsurface using geophysical techniques. Geophysics data were collected before and after CO2 injection that began in October 2008.
The SWP project involves the four major areas of investigation listed below. GCCC is solely responsible for the groundwater studies. For more information on GCCC’s results, please click the links below.
The SWP project involves four major areas of investigation:
- Groundwater Studies Conducted by GCCC. Please click on the links below for information about each of the topics.
- Chemical Analysis Methods
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 field 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 below 4°C immediately after sampling and during shipping.
Laboratory analytes measured in BEG groundwater samples by LANL are: Al, Ag, As, B, Ba, Be, Br, Ca, Cd, Cl, CO3, Co, Cr, Cs, Cu, d13C, dD, d18O, F, Fe, HCO3, Hg, K, Li, Mg, Mn, Mo, Na, Ni, NO3, Pb, PCO2, PO4, Rb, Sb, Se, Si, Sn, SO4, Sr, TDS, Th, Ti, Tl, U, V, and Zn. Laboratory analytes measured by The University of Texas at Austin, Department of Geological Sciences (UT DGS), are: dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), methane (CH4), and CO2 from headspace gas in selected samples.
Analysis of the chemical controls on SACROC groundwater chemistry has been a complex and lengthy process during which we evaluated multiple processes prior to conducting extensive modeling. These evaluated processes include:
• Systematic changes in major element and isotopic chemistry along flow paths away from or across SACROC
• Systematic changes in groundwater chemistry with depth
• pH trends inside vs. outside of SACROC
• chemical trends related to stratigraphic unit
• variation of calcite and dolomite saturation indices with other geochemical parameters
• variations in all other analytes inside versus outside SACROC
• Chemical trends with Ca, Na, Cl, SO4, and
• Oxygen and deuterium trends.
BEG researchers conducted multiple phases of geochemical modeling that can be summarized as follows:
1. Modeling of major ions shows mixing (Permian, Dockum, Ogallala, and produced waters), cation exchange, and dedolomitization are the major geochemical processes.
a. Three samples “representative” of end members are used; however, the chemical variability of the samples precludes choosing discrete end members. This model only gives an idea of the basic carbonate geochemical processes.
2. The carbonate system is dominated by dedolomitization, not calcite dissolution, and is a consequence of mixing, not CO2 input.
a. Assume that more “evolved” samples have higher PCO2 due to either
i. degassing during dedolomitization in a closed system
ii. input of exogenous CO2
iii. input of microbial CO2
3. Carbon isotope variations result mostly from dedolomitization reactions which are slightly degassing.
a. Major assumptions are in the end member carbon isotope variability and the values used for modeling. Calcite and dolomite are not distinguished. The same 13C is used for calcite as for dolomite. Also, average values are used for injectate and microbial CO2. Variability in these values is not shown in the model.
- Surface Flux Studies Conducted by NMT and GCCC, with support from Applied Nanotech, Inc. and Bullock, Bennett and Associates.
- Geophysical Studies Geophysical surveys performed by Kinder Morgan, Dr. William Harbert of University of Pittsburgh and DOE/NETL, Dr. Bob Hardage of BEG, Los Alamos National Laboratory, and Schlumberger Carbon Services.
- Reservoir/Injection Zone Modeling Conducted by NMT and University of Utah.
Our field-based study of shallow (<500 ft) groundwater overlying and within an ~1,000 mi2 area of SACROC shows no impacts to drinking water quality as a result of over 35 years of deep subsurface (6,000-7,000 ft) CO2 injection. Modeling of stable carbon isotopes (d13C) of injectate CO2 gas, DIC in shallow and deep groundwater, carbonate mineral matrix, and soil zone CO2 suggests that no significant injectate CO2 has been introduced to the shallow groundwater.
Interpretation of groundwater flow regime, and concentrations of major ions and trace metals, indicate mixing of water types and water-rock interaction (i.e. dedolomitization) as major controls on groundwater geochemistry at SACROC. We think the popular assumption that carbonate parameters alone can be used as indicators of groundwater quality over a GS site is too simple, especially in complex hydrogeologic settings. We emphasize the importance of defining the regional groundwater system to (1) understand how it might react to introduction of CO2 and (2) identify the parameters best suited for monitoring over GS sites. Research is ongoing to define and group major geochemical aquifer systems and to assess the protocol that would be appropriate for each group.
Aquifer sampling and analysis of site-specific conditions may be needed to understand how an aquifer system will react to CO2 and the parameters best-suited for monitoring at sequestration sites. Research is ongoing to define and group major geochemical aquifer systems and to assess the protocol that would be appropriate for each group.
For a pdf of the complete report, please click here.
Numerous researchers from GCCC and BEG are involved with various aspects of the research including:
The following post-docs and students have contributed to the work and we acknowledge their important contributions:
- Federico Pozo
- Alejandra de la Rosa Illescas
- Kelly Mortensen
- Yihua Cai (current PhD student)
- Jiemin Lu (current Post Doc)