Predicted Impacts From Offshore Produced Water Discharges on Hypoxia in the Gulf of Mexico
- Victor J. Bierman (Limno-Tech Inc) | Scott C. Hinz (Limno-Tech Inc) | Dubravko Justic (Louisiana State University) | Don Scavia (U. of Michigan) | John A. Veil (Argonne National Lab) | Kent Satterlee (Shell E&P Co.) | Michael E. Parker (ExxonMobil Production Co.) | J. Scott Wilson (U.S. Envir. Protection Agency)
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- Society of Petroleum Engineers
- SPE Projects, Facilities & Construction
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- June 2008
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- Journal Paper
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- 2008. Society of Petroleum Engineers
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Summer hypoxia (dissolved oxygen < 2 mg/L) in the bottom waters of the northern Gulf of Mexico has received considerable scientific and policy attention because of potential ecological and economic impacts. This hypoxic zone forms off the Louisiana coast each summer and has increased from an average of 8,300 km2 in 1985-1992 to over 16,000 km2 in 1993-2001, reaching a record 22,000 km2 in 2002. The almost threefold increase in nitrogen load from the Mississippi River Basin (MRB) to the Gulf since the middle of the last century is the primary external driver for hypoxia.
A goal of the 2001 Federal Action Plan is to reduce the 5-year running average size of the hypoxic zone to below 5,000 km2 by 2015. After the Action Plan was developed, a new question arose as to whether sources other than the MRB may also contribute significant quantities of oxygen-demanding substances. One very visible potential source is the hundreds of offshore oil and gas platforms located within or near the hypoxic zone, many of which discharge varying volumes of produced water.
The objectives of this study were to assess the incremental impacts of produced water discharges on dissolved oxygen in the northern Gulf of Mexico, and to evaluate the significance of these discharges relative to loadings from the MRB. Predictive simulations were conducted with three existing models of Gulf hypoxia using produced water loads from an industry study. Scenarios were designed that addressed loading uncertainties, settleability of suspended constituents, and different assumptions on delivery locations for the produced water loads. Model results correspond to the incremental impacts of produced water loads, relative to the original model results, which included only loads from the MRB.
The predicted incremental impacts of produced water loads on dissolved oxygen in the northern Gulf of Mexico from all three models were small. Even considering the predicted ranges between lower- and upper-bound results, these impacts are likely to be within the errors of measurement for bottomwater dissolved oxygen and hypoxic area at the spatial scale of the entire hypoxic zone.
Summer hypoxia in the bottom waters of the northern Gulf of Mexico has received considerable scientific and policy attention because of potential ecological and economic impacts from this very large zone of low oxygen, and because of the implications for management within its massive watershed (CENR 2000; EPA 2001). These regions of oxygen concentrations below 2 mg/L that form off the Louisiana coast each spring and summer increased from an average of 8,300 km2 in 1985-1992 to over 16,000 km2 in 1993-2001 (Rabalais et al. 2002), and reached a record 22,000 km2 in 2002. There is significant interannual variability and no comprehensive records of areal extent exist prior to 1985. Fig. 1 is a composite plot that shows the frequency of occurrence of midsummer hypoxia in the Gulf of Mexico from 1985 to 1999.
An assessment of hypoxia causes and consequences (CENR 2000; Rabalais et al. 2002) concluded that the almost threefold increase in nitrogen load to the Gulf (Goolsby et al. 2001) is the primary external driver that stimulated the increase in hypoxia since the middle of the last century. This riverine nitrogen input stimulates coastal algal production and the subsequent settling of organic matter below the pycnocline. Because the pycnocline inhibits vertical oxygen flux, decomposition of organic matter below the pycnocline consumes oxygen faster than it is replenished, resulting in declining oxygen concentrations during the period of stratification.
The Federal-State-Tribal Action Plan for reducing, mitigating, and controlling hypoxia in the northern Gulf of Mexico (EPA 2001) included a goal of reducing the 5-year running average size of the hypoxic zone to below 5,000 km2 by 2015. After the Action Plan was developed, a new question arose as to whether sources other than the Mississippi River Basin (MRB) may also contribute significant quantities of oxygen-demanding substances. One very visible potential source is the hundreds of offshore oil and gas platforms located within or near the hypoxic zone. Many of these platforms discharge varying volumes of produced water. Produced water is trapped in underground formations and is brought to the surface along with oil or gas.
Fig. 2 (J.P. Smith, ExxonMobil Upstream Research Company, personal communication) shows the frequency of occurrence of midsummer hypoxia superimposed on a lease block map of the Gulf of Mexico. The blue dots on the map show platforms in the region. There are an estimated 287 platforms in the hypoxia zone (Veil et al. 2005); however, not every platform is a produced water discharge point. The red boxes indicate the lease block areas that are in the hypoxic zone. Each box is marked with the number of lease blocks in the area that have produced water discharges and that are in the hypoxic zone.
Until recently, only limited data characterizing oxygen demand, nutrient concentrations, and loadings from offshore produced water discharges had been collected. These discharges are authorized by a general permit issued by EPA under the National Pollutant Discharge Elimination System (NPDES). As part of the reissuance of this permit in 2004, EPA required that the industry provide information on the amount of oxygen-demanding substances contained in the produced water discharges.
The objectives of this study were to assess the incremental impacts of produced water discharges on dissolved oxygen conditions in the northern Gulf of Mexico, and to evaluate the significance of these discharges relative to loadings from the MRB. This study was conducted using the three existing models of Gulf hypoxia described in Scavia et al. (2004). Results from this study will provide EPA with an initial assessment of the appropriate forward path for how to incorporate produced water discharges within the overall framework for controlling nutrient loadings to the Gulf of Mexico as a management tool for reducing the occurrence and extent of hypoxia.
This paper is based on a report submitted to EPA in July 2006 (Limno-Tech 2006) and it contains the principal study results and conclusions. The complete study, including detailed descriptions of the models and all tabular and graphical results, is documented in the report to EPA.
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