SACROC Research Project: Chemical Analysis Methods

Overview

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.
   • 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.
   • Assume that more "evolved" samples have higher PCO₂ due to either
     • degassing during dedolomitization in a closed system
     • input of exogenous CO₂
     • input of microbial CO₂
3. Carbon isotope variations result mostly from dedolomitization reactions which are slightly degassing.
   • 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.