Environmental Aspects of Produced Waters From Oil and Gas Extraction Operations in Offshore And Coastal Waters

Abstract

Environmental literature has been greatly concerned with possible detrimental effects of continued produced-water discharges in offshore and coastal areas. This review paper presents results from a number of studies that should relieve some of these concerns. Introduction This paper reviews the constituents of produced waters and their effects on both offshore and coastal marine environments. Considerable laboratory and field data were interpreted. This review paper was prepared to allay certain concerns expressed recently in environmental literature about possible detrimental effects on the marine environment of continued produced-water discharges in offshore and coastal areas. Petroleum Hydrocarbons in Petroleum Hydrocarbons in Produced Waters Produced Waters Inputs A recent National Academy of Sciences (NAS) report reviews the input of petroleum hydrocarbons to the marine environment from various sources. This study estimates that the worldwide input of oil to the oceans from all sources is about 6 million metric tons per year and from offshore drilling and production operations only about 0.08 million tons per year. Of this latter total, about 0.06 million tons comes from major spills. The remaining 0.02 million tons is attributed to minor spills (50 bbl or less) plus discharges of oilfield-produced waters during normal drilling and producing operations. Table 1 summarizes the input estimates from the different sources. Another particularly interesting comparison can be made between the produced-water input and the much higher continuous discharge of petroleum hydrocarbons from submarine seeps, estimated to total 0.6 million metric tons per year. Seeps have been discharging hydrocarbons into the marine environment for millions of years. Petroleum hydrocarbons also are being added continuously to the marine environment by erosion and discharge from uplifted sedimentary rocks containing hydrocarbons generated during burial. Thus, petroleum hydrocarbons are certainly not substances foreign to the marine environment. The relative amount of hydrocarbons contributed to the marine environment by oilfield-produced waters is so small that any over-all environmental effects from this input should be minimal. Fate Water coproduced with oil and gas and separated for discharge retains up to 50 ppm of separate-phase oil as small droplets. The water also may contain up to 35 ppm of dissolved hydrocarbons. These produced waters discharged from offshore operations are diluted with large volumes of moving water, The oil (dispersed and dissolved) immediately undergoes a number of changes that result in changes in concentration and composition. The mechanisms are dilution, evaporation, spreading, emulsification, air-sea interchange, biological degradation, and photo-oxidation. These natural processes quickly lower the actual measured concentrations of hydrocarbons. JPT P. 723