Print this pageOrganic Species in Oil-Field Waters: Sources, Controls and diagenetic Water-Rock Interactions
KHARAKA, YOUSIF K
The nature, distribution, and interactions of dissolved organic species in formation waters from sedimentary basins have become reasonably well understood as a consequence of more than 25 years of intensive field observations and results from laboratory experiments and computer simulations. In most oil-field waters, acetate is the dominant organic ligand, with concentrations of up to 10,000 mg/L; it is followed by propionate and longer-chained aliphatic monocarboxylic acid anions. The total concentration of dicarboxylic acid anions is probably lower than previously reported, and likely is less than 500 mg/L, with succinate and glutarate being the dominant species. The highest concentrations of organic acid anions are present in formation waters from geologically young reservoir rocks at temperatures of 80-120 degrees C. Field and laboratory results indicate that the concentrations of organic species are controlled mainly by the rates of their generation from kerogen and liquid petroleum and by their destruction thermally or by bacteria; results do not support a control based on the attainment of redox equilibrium.
Geochemical models with an adequate thermochemical data base for organic species and complexes at temperatures to at least 200 degrees C, predict that organic species in subsurface waters play an important role in mineral diagenesis. Model simulations predict that these organic species (1)- are an important source and sink for protons ; (2)- buffer water pH, but increases in PCO[2] from organic or other sources are likely to result in calcite undersaturation; and (3) form strong complexes with dissolved cations, especially Ca, Mg, Fe and Al.