Factors Affecting Methods for Biodegradation Testing of Drilling Fluids for Marine Discharge

Abstract

Although general agreement was reached between the OSPARCOM (Oslo and Paris Commission) countries in 1994 regarding testing of drilling chemicals for approval for use and discharge, there are still some practical variations concerning the implementation. In all standard environmental compliance areas, such as toxicity, bioaccumulation and biodegradability, there has been, or is, some concern about the reassessment of change. It is only recently that an agreeement has been reached on test species for toxicity assessment of drilling fluids and chemicals for North Sea discharge. Regarding approval for the use of synthetic based muds (SBMS), biodegradability has probably been the most controversial issue with respect to the test methods, test results, acceptable degradation rates and overall environmental impact. Drilling fluids used in SBMs have low water solubilities and are adsorbed onto drill cuttings. They are known to enter the marine sediments and will, in high concentrations, and when they are buried under a layer of cuttings or under sediments, accumulate in anoxic marine sediments. OSPAR has therefore required that both aerobic and anaerobic biodegradation test results be available for the base fluids of SBMs. Presently, a seawater biodegradation test protocol has been developed into an OECD Guideline for water soluble materials, but no standard test protocol has been agreed upon for poorly or non-water soluble materials. This is the case both for aerobic or anaerobic biodegradation tests. For the aerobic biodegradation test, several attempts have been made to come up with a standard seawater test, but presently different laboratories use different protocols. Small differences in existing test protocols for aerobic seawater tests have resulted in great variations in test results obtained between different test laboratories. For the anaerobic test, only a few attempts have been made to come up with a seawater test protocol and only a limited number of test results exist. It has been the general opinion among North Sea environmental authorities that rapid degradation will minimize the environmental impact, thus allowing fast recovery of the seabed. This argument was introduced at an early stage and lead to the development of a set of seabed simulation studies. The results of these studies generated alternative viewpoints. One argument was that aerobic degradation in a localized area will lead to anoxic conditions which immediately can have a lethal impact on the benthic fauna. Another argument was that although the base material may be relatively non-toxic, the by-products or any other constituents of the drilling mud, may be toxic. The ultimate issue concerning the use of any drilling fluid should therefore be environmental impact rather than fate. This paper presents the results from a three-year research program performed on behalf on Conoco Norway Inc. (CNI) on the standardization of biodegradation test protocols for poorly or non-water soluble SBMs and summarizes experiences from work conducted by chemical suppliers and oil operators, and evaluated on behalf of the Norwegian Oil Industry Association (OLF) in a project on acceptance criteria for drilling muds. Results from the CNI project include information on parameters of importance for standardization of the biodegradation test and their influence on deviations in test results.