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The concentrations of organic acids in produced water are highly variable. Because of the information shown in others chapters of this book about the toxic effects of organic acids, it is of major interest to understand the mechanisms controlling the occurrence of this compound group in produced water. This chapter focuses on in-reservoir biogeochemical processes which may produce organic acids as products or by-products. The biodegradation processes inside reservoirs decrease the hydrocarbon content of petroleum. Additionally, an increase in oil acidity as measured by total acid number (TAN) is frequently observed. Since the formation waters of reservoirs are in close contact with these processes, the production of smaller, more polar petroleum constituents will also have an effect on the composition of organic molecules in the produced water. This manuscript reviews the literature with respect to acids production of petroleum biodegradation and the effects of water re-injection on such processes.

This paper demonstrates how stable carbon isotopes can be utilized to identify the source of oil-based drill cuttings discharges and estimate the time of their deposition. The base-oil used in drilling mud systems may be biodegraded by seabed microflora. Effects of such processes make it difficult to identify sources of discharges based on the oil's molecular structure. However, the stable isotope fingerprint of the base oil may vary for each batch of base-oil. Because carbon isotopic ratios in compounds with more than ten carbons are resistant to fractionation during biodegradation, this may be used as a tool to identify the source of drill cuttings discharges.