Abstract

Environmental studies of offshore drilling activities, conducted over a period of more than two decades, have shown that the mercury present at trace levels in drilling fluid barite has very limited bioavailability to marine organisms. However, media reports during 2002 about the presence of mercury in fish raised the level of interest in the potential for drilling discharges to contribute to mercury levels in fisheries resources.

The exploration and production industry responded to this interest by preparing a comprehensive review of the scientific literature on the fate and effects of mercury in the marine environment, updating information on the quantities of barite (the main source of the low levels of mercury in drilling wastes) discharged during drilling, and collecting new data on the potential for inorganic mercury in drilling discharges to be converted to methylmercury once discharged into a marine environment. The literature review indicated that the presence of low levels of inorganic mercury in drilling discharges poses minimal environmental risk. A review of data on discharge volumes showed that modern drilling technology has reduced the amount of barite used per well by 50 – 80%. New field data showed that marine sediments close to drill sites did not have higher methylmercury concentrations than did sediments from more distant sites. This indicated that there is low potential for the conversion of inorganic mercury in drilling discharges to the more readily bioavailable organic methylmercury form. The overall results of this effort support the conclusion that the low level of inorganic mercury in drilling discharges does not contribute to organic mercury levels in fish.

This paper provides a general review of mercury in the environment, its fate and effects, and identifies the most significant sources. Against this background, the relative contribution and behavior of inorganic mercury from drilling discharges is presented. Industry research efforts over the years evaluating the fate and effects of mercury in the environment are reviewed with an emphasis on studies of mercury concentrations in drill site sediments, the chemical forms of inorganic mercury in drilling fluid discharges, and recent studies of the potential for conversion of inorganic mercury in barite to the more bioavailable organic methylmercury form.

Introduction

During 2002, media reports raised interest levels in the potential for inorganic mercury in drilling discharges to contribute to mercury levels in fisheries resources. Media reports cited measurements of elevated levels of total mercury in sediments around Gulf of Mexico offshore platforms as the basis for the concern that inorganic mercury in drilling discharges might be contributing to organic mercury levels in fish.

The potential environmental impact from heavy metals, including mercury in drilling discharges, were raised and evaluated (1) early in the development of the offshore drilling industry. Studies have been conducted by agencies, academia and industry to examine discharge amounts, chemical forms and concentrations, bioavailability, bioaccumulation, fates and effects in the environment, and human health risks associated with drilling fluid discharges.

To respond to concerns raised by the media, E&P operators, drilling contractors, and drilling fluid companies embarked on a joint effort to collect information to place inorganic mercury in drilling charges in an appropriate perspective. To do this, discharges were evaluated relative to other inputs of mercury to the environment, the results of previous research on the effects of mercury in drilling discharges were summarized, and critical new data concerning the fate of mercury in drilling discharges were collected. This paper summarizes the results of that effort.

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