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Oil in the Sea III: Inputs, Fates, and Effects
the estimated annual tonnage of offshore oil produced for the year 1995 from API (2001). The year 1995 was the most recent year included in API (2001). Note that most of the rows are “zero” because there is no offshore production in those zones in 1995.
Estimates for VOC rates in the U. S. Gulf of Mexico come from the Minerals Management Service (MMS, 1994, OCS Study MMS 94-0046; Gulf of Mexico Air Quality Study, Vol. 1 Summary of Data Analysis and Modeling). The VOC rates are based on measurements in 1993 and projected here to 1995 using production rates derived from API (2001). It should be noted that MMS estimates for VOC rates are based on only a two-month summer sampling program involving two platforms offshore Texas and Louisiana within the land-sea breeze corridor. Clearly this is a small data set with considerable uncertainty. Nevertheless the number seem to be consistent with the other sites in Tables 2-2 through 2-6.
VOC for Southern California (zone K) were calculated from numbers provided by the California Air Resources Board (CARB) inventory for 1997 in the Santa Barbara Channel (available on request from www.arb.ca.gov). Actual VOC in Table D-7 were calculated using the California VOC rate times the 1995 production from API (2001). These VOC rates are by far the most accurate estimates since they were based on detailed component (valves, internal combustion engines, etc.) counts multiplied by an assumed VOC emission rate per component (Tier 3 level).
The best estimate for Alaskan offshore platforms were based on the GOM VOC rates times the relevant 1995 production from API (2001). The best estimate of total VOC released in North America in 1995 is 60 kilotonnes/year with a lower bound of 36 kilotonnes/year and upper bound of 226 kilotonnes/year.
Table D-7 summarizes the worldwide estimates. It shows a best estimate of 649 kilotonnes per year with lower and upper bounds of 191 and 1,322 kilotonnes per year, respectively.
VOC rates for the U.K. sector were provided by U. K. Offshore Operators Association (http://www.ukoaa.co.uk). Estimates for the Norwegian Sector were provided by Norwegian Petroleum Directorate (Einang Gunnar). The estimates were combined and labeled “W. Europe.”
No direct estimates of VOC data could be found for the remainder of the world so these were estimated by multiplying the VOC rate by the oil produced offshore in that region 1995 from API (2001a). One problem arose in doing this. The API (2001b) lumps Mexico with “Other Latin America.” In order to estimate VOC for North America, the production volumes from Pemex (2000) were subtracted from the API “Other Latin America” and the resultant added to the API “Venezuela” estimate to get the value for “S. America shown in Table D-7.
During oil production, water from the reservoir is also pumped to the surface. Under current industry practices, this “produced water” is treated to separate free oil and either injected back into the reservoir or discharged overboard. Produced water is the largest single wastewater stream in oil and gas production. The amount of produced water from a reservoir varies widely and increases over time as the reservoir is depleted. For example, in the North Sea, a maturing oil production area, the volume of produced water has increased at a rate of 10-25 percent per year over the period 1993-1997 in Norway (NOIA, 1998) and the United Kingdom (UKOOA, 1999). Norwegian oil fields produced about half as much water as oil (NOIA, 2000) in 1997. However, an increasing amount of the produced water is re-injected.
Produced water discharges are permitted as operational discharges. The oil and grease content is regulated by permit, and the allowable maximum concentrations vary by region and nation: For the U.S. Gulf of Mexico, the limit is 29 mg/L (USEPA, 1996); in the North Sea and Canada, it is 40 mg/L (PARCOM, 1986, PanCanada, 1999). Conventional treatment consists of oil/water separators, and there will have to be major technological advances before significant improvements in treatment efficiencies can be expected.
Tables D-8 and D-9 show the estimated volumes of water and oil discharges from offshore produced water discharges for North America and other major offshore producing regions, where available. Data from the 1985 Oil in the Sea report (based on 1979 offshore oil production volumes) are included for comparison. It should be noted that the 1979 estimates were calculated very indirectly; offshore oil production was multiplied by a water:oil ratio (which varied from 0.1 for the U.K. to 0.8 for the United States) and three concentrations for oil content (low, best estimate, and high) that varied by a factor of two. The 1979 estimate did not consider reinjection of produced waters.
PRODUCED WATER DISCHARGES IN NORTH AMERICA
The 1990s estimates were made using very different and more precise methods. In the United States and the North Sea, offshore operators are required to routinely monitor the volumes and oil content of produced water discharges and to submit reports to regulatory authorities to demonstrate compliance with discharge permits. Therefore, the 1990s estimates have a relatively high degree of certainty. For the United States, produced water discharge volumes and oil and grease content are reported in discharge monitoring reports (DMR) that are submitted monthly to the U.S. Environmental Protection Agency. Table D-8 includes detailed calculations for the different oil production areas in the United States and the North Sea.