RIASSUNTO
Abstract
This paper describes a new synthetic based drilling fluid (SBM) which is able to emulate oil based mud performance whilst complying with environmental legislation. Comparisons with oil based mud in the laboratory show that the system can provide similar performance at low synthetic:water ratio, high mud weight, high temperature and in the presence of contaminants. The laboratory results are supported by field data which demonstrate that the system can be managed like mineral oil based mud, giving good rates of penetration with minimum mud maintenance and low retention of the synthetic base fluid on cuttings discharged to the sea.
Marine toxicity, bioaccumulation and biodegradation data has been compiled for the SBM and the synthetic base fluid. Based on these results, the system has been accepted for use in UK and Norwegian waters, subject to sea bed surveys following its use in trial wells. In the UK, the system has been placed in provisional Group E of the revised Offshore Chemical Notification Scheme (OCNS) product categorisation. For operations in the United States, the mysid shrimp 96 hour LC50 toxicity test result significantly exceeds the limit required for use in the Western Gulf of Mexico outer continental shelf.
Introduction
Synthetic based drilling fluids have been developed to meet difficult drilling targets with reduced environmental impact. During the l980s, muds based on low toxicity mineral oils were heavily used, with unrestricted dumping of oily cuttings into the sea. It was originally thought that little environmental impact would result due to the low toxicity of these oils to marine species. However, it became clear that the petroleum derived mineral oils did not biodegrade adequately in the cuttings pile to allow degradation of the oil and recovery of the sea bed in an acceptable period.
The need, therefore, arose for a biodegradable drilling fluid which still possessed the high performance of mineral oil based muds. In spite of the strides made in the development of water based muds for controlling difficult shales, there are still many occasions when the use of oil based mud is necessary. For example, the viability of many offshore field developments depends on the ability to drill wells with high lateral displacement to obtain effective reservoir drainage. For many such wells, water based muds do not provide sufficient shale inhibition and lubrication, and oil based muds are viewed as the only effective mud types to allow the targets to be safely and economically reached.
The increasing legislative restrictions on the discharge of cuttings contaminated with mineral oil, together with the PARCOM3 directive on the discharge of ""oils of a petroleum origin"", led to the development of synthetic or naturally derived fluids which allowed synthetic based muds to be formulated with enhanced biodegradability and reduced toxicity. These muds have proved more acceptable to the environment, but have yet to achieve the properties of oil based mud.
The major difficulty lies in finding a balance between the environmental characteristics and the mud properties. For example, muds based on esters are known to have a low marine toxicity and biodegrade, but they suffer from high viscosity and can break down under alkaline conditions and at high temperature.
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