INTERACTIONS BETWEEN ORGANIC-ACID ANIONS IN FORMATION WATERS AND RESERVOIR MINERAL PHASES

Formation waters from the Norwegian continental shelf have been analysed for short-chain organic acid anions. Acetic acid is the major component, and the observed levels are in the range 0.3-20 mM in uncontaminated formation water samples. Studies of individual reservoirs and formations over time and area show a quite uniform distribution of the acids within each unit for large volumes of fluid. The organic acid anion levels are generally low in reservoirs containing biodegraded oil. Statistical analysis has been used to find correlations between the organic and inorganic ion compositions and the reservoir conditions and shows a dominating negative correlation between the organic acid anion levels and the levels of inorganic ions, indicating that the main effect of the organic ions is to increase the stability of mineral phases. Laboratory experiments have also been performed and show similar effects for sandstone samples. Under oxidizing conditions the buffering capacity of the organic acid species is also important, as it prevents the development of extreme pH conditions caused by pyrite oxidation. In carbonate systems few systematic effects were observed. The results, both for the natural systems and the laboratory experiments, show that the presence of organic acids in the formation waters does not contribute to the development of secondary porosity in the reservoir rock. The observed relationships rather indicate decreasing concentration levels of inorganic ions in the formation waters with increasing levels of organic anions and thus an increase in mineral stability.