In the North Sea, operators sample twice each day and prepare annual summaries that report the total produced water volumes, average oil content, and total amount of oil discharged to the sea. These reports are posted on web sites by the offshore operator associations for each country. The values in Table D-8 for the North Sea were derived directly from the available annual summaries, as described below.
The Netherlands Oil and Gas Exploration and Production Association reported annual oil discharges into Dutch waters in tonnes for 1987-1997 (NOGEPA, 1998). Table D-8 shows the average for the last three years, 1995-1997. NOGEPA (1998) also included a table listing oil discharges from produced water for Denmark for 1996.
The Norwegian Oil Industry Association published a summary of emissions to air and discharges to sea (NOIA, 1998) for the period 1990-1997. This report included total produced water volumes, amount reinjected, amount discharged to the sea, oil concentration, and total oil discharged in tonnes. The values for Norway in Table D-8 are means for the period 1996-1997, and include oil discharges from produced water and ballast and drainage water.
In the 1999 annual report by the United Kingdom Offshore Operators Association (UKOOA, 1999), produced water volumes, oil levels, and total oil quantity discharged in tonnes were provided for 1996-1998. These values are included in Table D-8.
For other international areas, where discharge summaries could not be obtained, a rough estimate was made, as follows. A “factor” was developed for the Gulf of Mexico offshore region, by dividing the oil discharge per year in tonnes by the oil production rate for this region. That is, the 1996-1998 maximum amount of 2,500 tonnes of oil from produced water discharges (representing an oil content of 29 mg/L in the produced waters, shown in Table D-8, was divided by the 1999 oil production rate of 1,354 kbbl/day, obtained from the U.S. Department of Energy (DOE, 1999), to get the minimum discharge amount. This approach avoids the need to convert from barrels to tonnes of oil. The U.S. Department of Energy provides data on the oil production rate for international regions. The percentage of production that is offshore is the same estimate used for estimating emissions for VOC. The best estimate was based on an oil and grease content of 60 mg/L, and the maximum estimate was calculated 100 mg/ L. Based on this analysis, other international oil production areas discharge 25,000 tonnes of oil and grease per year.
The total amount of oil discharged with produced water discharged for the late 1990s is estimated to be 36,000 tonnes. This volume cannot be compared with the estimate made in 1979 because of the different methods used to make the two estimates. The 1990s volume is based on detailed monitoring and should be considered relatively certain.
One issue that could affect the uncertainty of the amount of oil discharged with produced water is the use of the standard Environmental Protection Agency (EPA) gravimetric method for determining oil and grease in the United States (EPA Method 413.1). A study of three Gulf of Mexico platforms found that 2-17 percent of the oil and grease was hydrocarbon material; the nonhydrocarbon components in the oil and grease analysis are fatty acids, phenols, and related compounds (Brown et al., 1992). For three California platforms, petroleum hydrocarbons comprised 30-60 percent of the total hydrocarbons in produced water (Schiff et al., 1992). In the North Sea, total oil is measured by infrared spectroscopy, which also includes nonpetroleum hydrocarbons. Therefore, the total oil discharges in Table D-8 are likely to be high, by as much as a factor of two to five.
There have been some major changes in permitted discharges for the oil and gas production industry during the 1990s that are not included in Tables D-8 and D-9. In the United States, produced water discharges into coastal waters (into estuarine areas landward of the shoreline) in the Gulf of Mexico were prohibited by the late 1990s (40 CFR 435.43). Annual produced water discharges into coastal waters in Louisiana in the early 1990s were estimated to be 222,832,000 bbls and contained 1,170 tonnes of oil and grease (Boesch and Rabalais, 1989). By 1997, in the North Sea, discharge of oil-based drilling muds had been prohibited by all countries. In the United Kingdom, oil discharges with drilling cuttings were 3,965 tonnes and represented 40 percent of the total oil releases to the North Sea by the United Kingdom.