Abstract

During the past few years more and more attention has been paid to the estimation of the environmental impact of the discharge of produced water from oil and gas production platforms. In these studies most often a traditional PEC:PNEC approach is used on the basis of dilution of produced water in sea water (PEC), in comparison to toxicity data for a dilution series of the produced water (PNEC). Spatial and temporal variation in the concentration of chemicals is summarised in frequency distributions. Although an indication of the ecological risk of the discharge of produced water can be obtained, it gives no information about what chemical components are causing the estimated environmental risk.

It is obvious that some components (either deliberately added production chemicals or components naturally occurring in formation water) may contribute more to the environmental risk than others. A newly developed method, based upon the concept of combination toxicology, makes it possible to calculate the overall ecological risk of produced water from the ecological risk of individual components. By focusing mitigating measures on components with the highest contribution to the overall risk, reduction of the ecological risk can be achieved more efficiently and probably less expensive. In this paper the theoretical background of the method will be presented and demonstrated with some case studies.

Introduction

With the production of oil and gas relatively large volumes of produced water are generated, which can be re-injected into the well or discharged into the ambient environment. As the requirements for re-injection are rather high and the process is very costly, the produced water is usually discharged from the platform. Before the sea water is pumped into the well, production chemicals may be added to facilitate the production process (e.g. emulsifiers) or to protect the installation (e.g. corrosion inhibitors). Although the dosage is specified to ‘use’ all chemicals in the well, any surplus may still be present in the produced water. Usually the produced water also contains chemical compounds, such as metals and various hydrocarbon compounds, which originate from the formation from which the oil or gas is produced.

Because of the complicated composition of produced water, which may change over time and between platforms, it is not possible to generate a quick estimation of the environmental impact of the discharge of produced water. Although it is possible to perform toxicity tests with the produced water, it only provides information about a particular platform at a specific moment. In general it is too expensive to perform such a series of tests for several platforms.

In this study the environmental impact of produced water discharges from oil (or gas) production platforms is studied, using a method to estimate the environmental impact of complete produced water discharges with toxicity information of the individual constituents. The input parameters for this study were derived from chemical analyses of the produced water, as well as toxicity information for the constituents. As is the case with all chemical mixtures, the toxicity of produced water is determined by the chemical composition of the produced water. The main groups of chemical components in produced water are aliphatics, aromatics, PAH's, organic acids, metals and production chemicals. For each of these groups toxicity data can be obtained (either from literature or by performing toxicity test), and it is therefore possible to estimate the environmental impact of produced water using the toxicity of the individual components.

The advantage of the approach used in this study is that the total risk is based upon the relative contribution of each component of the produced water. This gives the managers the opportunity to define effective risk reducing measures, focused on the components with the highest environmental impact.

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