Abstract

In Nigeria today, the environmental acceptance of a non water-soluble drilling mud base fluid depends not only on its toxicity as measured from traditional bioassays, but also on its biodegradation under aerobic and anaerobic conditions. Presently, however, there is no widely accepted laboratory method for biodegradability assessment of drilling fluids.

Based on its relevance to environmental conditions affecting the biodegradation of drilling fluids, the recently developed Solid Phase Biodegradation Test Method was chosen for assessment of drilling fluids in Nigeria. The test was modified, essentially to ensure better simulation of Nigerian environmental conditions. Thus, the Nigerian version of the method is a 60 day "closed system" Test that entails the assessment of a given sample in both fresh and brackish/marine water simulated environmental conditions.

The biodegradation assessment of three drilling fluid types: an ester, an olefin and a mixture of both was carried out using the Nigerian version of the Solid Phase Test. The results obtained are presented in this paper. They compare favorably with and validate other Solid Phase Test results, which show olive oil to be a ready degradable substance and esters as having a generally higher percentage degradation than other synthetic base fluids. In this test, percentage biodegradation was observed to be higher in the brackish/marine water than in the freshwater simulated environment.

Introduction

All over the world, industrial activities such as drilling for oil and gas are increasingly being pursed hand in hand with environmental management. In line with this, stricter environmental regulations, standards and limitations are also developing.

For many years, drilling mud systems with oils as carrier fluids (Oil Based Muds (OBM) have generally been preferred to water based mud (WBM) due to their better drilling performance. Diesel and mineral oils containing aromatic hydrocarbons were the carrier (base) fluids in traditional OBM. Studies ^1,2 have shown that related to their toxicity and recalcitrance, OBM, when discharged into aquatic sedimentary environments, as has been the practice in many countries, are deleterious to seabed biological communities. The use of mud systems containing this class of base fluids was therefore banned years ago in most countries including Nigeria and biodegradability is presently widely acknowledged as an important parameter for environmental acceptance of drilling fluids.

A number of non water drilling mud base fluids known as synthetics have been developed in the last ten years to replace the traditional OBM carrier fluids. The synthetics, including esters, alpha olefins, acetals and n-alkanes posses' similar drilling performance to OBM but are more acceptable based on reaults of traditional toxicity bioassays which, suggest that they are generally non-toxic. In addition, they are widely believed to be readily biodegradable. However as has been shown ^3,4,5, with the exception of the esters, whether or not these fluids possess significantly improved biodegradation properties than mineral oils is not readily known.

The Department of Petroleum Resources (DPR) and the Federal Ministry of Environment (FMOE) regulate the oil and gas industry in Nigeria. In 1997, the DPR reviewed its regulatory requirements concerning the use of non-water drilling mud base fluids⁶. Discharge restrictions of 1% residual oil on cuttings (dry wt.) (10% in 1995 and 15% in 1991⁷) now apply to mineral oil based fluids or their mud systems while synthetic muds or fluids containing linear alpha olefins, isomerised olefins, poly alpha olefins have a 5% oil on cuttings discharge limitation. Additionally, all non-water drilling mud base fluids are required to undergo biodegradability testing under Nigerian environmental conditions as a permit requirement for their use in Nigeria ⁶.