Geochemical Models of Rock-Water Interactions in the Presence of Organic Acids

Summary

Theoretical analysis of the role of organic acids (OA) and acid anions (OAA) in sedimentary environments is an effective way of evaluating the importance of inorganic-organic interactions in rock-water systems. Geo-chemical modeling provides insight into the role of these organic constituents in aqueous sedimentary environments by defining the boundary conditions under which such interactions may be significant. In many cases, carefully designed geochemical models allow conflicts to be resolved among differing experimental results and among various working hypotheses. The approach can be limited by lack of understanding of the processes being modeled, lack of testable working hypotheses, or lack of either experimental or field observations against which to calibrate model results. Interpretation of results from chemical models always needs to accommodate uncertainties in the supporting thermodynamic data.

Geochemical models of rock-water interactions in the presence of organic acids, using a combination of experimental and estimated thermodynamic data, result in several important predictions: first, trivalent cation-difunctio-nal acid anion complexes are the most stable, whereas monovalent cation-monofunctional acid anion complexes are the least stable. Second, the stability of all complexes is pH-dependent and, whereas aluminum oxalate dominates the species distribution of aluminum under acidic conditions, in alkaline waters, inorganic aluminum species are predominant. Calculations using reaction path models reveal additional constraints on the role of OAA in geologic processes. The roles that OA and OAA play in modifying mineral solubility equilibria depend, among others, on pH, acid composition, competing inorganic equilibria with CO₂ and temperature. Considerations of simple system geochemistry thus cannot be readily used to predict the effect the OA/OAA may have in modifying rock-water interactions. Indeed, in some circumstances OAA-bearing waters are less effective at producing porosity in an arkosic sandstone than are OAA-free waters.

An overview of the role of OA in sedimentary processes is that they contribute to overall patterns of fluid-rock interaction, but appear unlikely to dominate such reactions except in restricted geochemical environments where concentrations are in excess of typical values. Such environments might include wetlands, gasoline-contaminated groundwaters, within organic-rich shales, and within sandstones immediately adjacent to such rocks.