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This paper was prepared for presentation at the 1999 SPE/IADC Drilling Conference held in Amsterdam, Holland, 9-11 March 1999.

In order to assess the suitability of commonly used biodegradation techniques to simulate the environmental behaviour of pseudo oil-based fluids in the sea, a comprehensive study has been conducted. More than 40 biodegradation experiments, using different techniques, have been performed on one particular synthetic base fluid (an acetal). The results are compared to large scale simulation tests and a seabed survey in an attempt to identify the biodegradation method that most closely resembles the natural biodegradation process. Introduction Restrictions on the discharge of mineral oil based mud contaminated cuttings, coupled with the increasing environmental control required when using a drilling fluid, have led to the development of a range of pseudo oil based mud systems. The increasing use of such synthetic drilling fluids and allowable discharge of cuttings, has emphasised the need for a better understanding of the biodegradation process of water insoluble or poorly soluble materials. A range of different aerobic and anaerobic biodegradation methods have been used in the past to assess this particular environmental parameter with respect to drilling fluid components. Analyses of biodegradation data presented in this paper demonstrates the need for a standardised and commonly agreed method to be identified. The large variation obtained from the different biodegradation techniques indicates potential problem areas when selecting a relevant test method. In addition, variables such as test concentration, test medium (seawater vs. freshwater), bacterial count etc. seem to yield very different results within each individual test method used. Furthermore, the test results obtained from these biodegradation experiments on base fluids have, so far, been used to assess the environmental impact of potential, new pseudo based systems and to rule for or against existing fluids. ACETAL CHEMISTRY The base material is technically known as a dialkylether or acetal. Acetals are synthesised by condensation of aldehydes and alcohols eliminating water under acidic conditions: CnH2n + 1CHO+ 2HOCnH2n + 1 CnH2n + lCH(OCnH2n +1)2+ H2O Their known high stability in the alkaline range makes it possible to optimize the alkalinity in invert-emulsion drilling muds, for example with calcium hydroxide. The acetals can be converted by a simple acid treatment into aldehydes and alcohols which are used for the synthesis. For base fluids of invert-emulsion muds, combinations of aldehydes and alcohols must be used, which result in acetals with properties meeting the requirements of the base fluids, i.e. viscosity, flashpoint, pour point, toxicity and biodegradability. There is a broad range of combinations of aldehydes and alcohols which fulfill these requirements, but in practice only such acetals can be used, for which the raw materials are available in the required volume and at acceptable prices. The Acetal base fluid described in this paper is an isobutylaldehyddi-2-ethylhexyl-acetal. Both raw materials are based on oxosynthesis and are available in large volumes. Depending on the plant capacity of the acetal syntheses, this material can be supplied in unlimited volumes. The Acetal can also be used in mixtures with esters, olefins etc. Due to its high stability in the alkaline range it has very good cement contamination properties. TOXICITY The toxicity aspect of pseudo based drilling fluid systems is not an issue in this paper. Generally, most pseudo systems can be regarded as non-toxic. Of the most commonly used pseudo systems, the acetal based drilling fluid system is the least toxic. The test species used in the comparison, listed in Table 1, are the old SFT regulatory species. It is the Mytilus Edulis (blue mussel) which is the most sensitive specie to non-aqueous fluids. P. 387