James J. Thordsen

Distribution and significance of dicarboxylic acid anions in oil field waters

The origin, distribution, and interactions of low-molecular-weight organic acid anions have become an intensively studied field in geochemistry since their widespread occurrence in formation waters of sedimentary basins was first documented by Carothers and Kharaka ( 1978 ). High concentrations (up to 10,000 mg 1~ ) of monocarboxylic (mainly acetate, propionate, and butyrate) and dicarboxylic (mainly oxalate, malonate, and succinate ) acid anions have been reported from many sedimentary basins, with the highest values present in relatively young (Cenozoic age) petroleum reservoir rocks at subsurface temperatures of 80-120 ° C [ see Lundegard and Kharaka (1990) for a recent review with references ]. Geochemical interest in these organic anions stems mainly from their important role in mineral diagenesis in sedimentary basins (Kharaka et al., 1986; MacGowan and Surdam, 1990). In particular, these species act as sources or sinks of protons, as a source of CO2 and as pH and Eh buffering agents (Lundegard and Kharaka, 1990). They also form complexes with cations and metals such as Ca, A1, Fe, Pb, and Zn (Kharaka et al., 1985; Harrison and Thyne, 1992). Data on the concentrations of monocarboxylic acid anions in subsurface waters from many sedimentary basins worldwide are available and are generally accepted as representing the aqueous concentrations at depth. Data on the concentrations of dicarboxylic acid anions are much more limited, some reported values are controversial, and the total values reported range widely from 0 to 2640 mg 1-~ (Surdam et al., 1984; Kharaka et al., 1986; Barth, 1987; MacGowan and Surdam, 1988, 1990). Because of the wide range in the reported concentrations and because dicarboxylic acid anions generally form stronger complexes with A1, Fe, and other cations than do monocarboxylic acid anions, we resampled four oil wells in the San Joaquin basin, California, in order to better assess the role of these acid anions in water-rock interactions in sedimentary basins.

Deep well injection of brine from Paradox Valley, Colorado: Potential major precipitation problems remediated by nanofiltration

Groundwater brine seepage into the Dolores River in Paradox Valley, Colorado, increases the dissolved solids load of the Colorado River annually by ∼2.0 × 108 kg. To abate this natural contamination, the Bureau of Reclamation plans to pump ∼3540 m3/d of brine from 12 shallow wells located along the Dolores River. The brine, with a salinity of 250,000 mg/L, will be piped to the deepest (4.9 km) disposal well in the world and injected mainly into the Mississippian Leadville Limestone. Geochemical modeling indicates, and water-rock experiments confirm, that a huge mass of anhydrite (∼1.0 × 104 kg/d) likely will precipitate from the injected brine at downhole conditions of 120°C and 500 bars. Anhydrite precipitation could increase by up to 3 times if the injected brine is allowed to mix with the highly incompatible formation water of the Leadville Limestone and if the Mg in this brine dolomitizes the calcite of the aquifer. Laboratory experiments demonstrate that nanofiltration membranes, which are selective to divalent anions, provide a new technology that remediates the precipitation problem by removing ∼98% of dissolved SO4 from the hypersaline brine. The fluid pressure used (50 bars) is much lower than would be required for traditional reverse osmosis membranes because nanofiltration membranes have a low rejection efficiency (5–10%) for monovalent anions. Our results indicate that the proportion of treatable brine increases from ∼60% to >85% with the addition of trace concentrations of a precipitation inhibitor and by blending the raw brine with the effluent stream.

Carbon isotope analysis of dissolved organic carbon in fresh and saline (NaCl) water via continuous flow cavity ring-down spectroscopy following wet chemical oxidation

This work examines the performance and limitations of a wet chemical oxidation carbon analyser interfaced with a cavity ring-down spectrometer (WCO-CRDS) in a continuous flow (CF) configuration for measuring δ13C of dissolved organic carbon (δ13C-DOC) in natural water samples. Low-chloride matrix (<5 g Cl/L) DOC solutions were analysed with as little as 2.5 mg C/L in a 9 mL aliquot with a precision of 0.5 ‰. In high-chloride matrix (10–100 g Cl/L) DOC solutions, bias towards lighter δ13C-DOC was observed because of incomplete oxidation despite using high-concentration oxidant, extended reaction time, or post-wet chemical oxidation gas-phase combustion. However, through a combination of dilution, chloride removal, and increasing the oxidant:sample ratio, high-salinity samples with sufficient DOC (>22.5 µg C/aliquot) may be analysed. The WCO-CRDS approach requires more total carbon (µg C/aliquot) than conventional CF-isotope ratio mass spectrometer, but is nonetheless applicable to a wide range of DOC concentration and water types, including brackish water, produced water, and basinal brines.