GENERATION OF SHORT-CHAIN ORGANIC-ACIDS FROM CRUDE-OIL BY HYDROUS PYROLYSIS

The short-chain, water soluble carboxylic acids that are found in all oil-field waters are generally assumed to have been generated in the source rock together with the petroleum. This requires a migration mechanism that is efficient, even for very water soluble compounds, which has been shown to be problematical. An in situ source would explain the lack of migration-related effects. To evaluate whether the emplaced oil is a possible source for the short-chain acids (SCA), hydrous pyrolysis of a variety of crude oils, and light and heavy oil fractions, was performed. Mineral phases considered representative for North Sea reservoirs were used to give a realistic chemical environment during the pyrolysis. The experiments showed generation of organic acids in the range 1.1 x 10(-7)-8.58 x 10(-6) mol/ml crude oil, which is sufficient to explain the concentrations observed in North Sea oil-field waters. The influence of the experimental conditions was examined and showed increasing yields of SCA with increasing temperature, presence of mineral phase and the addition of oxidation agents. The type of mineral phase, and especially oxidation agent, was important both for the yields and the product distributions. Oxygen in air, and hydrogen peroxide in water, gave a 10-fold increase in yields, while iodine had little effect. Acid generation from redox processes is thus considered to be the quantitatively important process compared to organic cracking reactions.