A solid-phase biodegradation test designed to assess the fate in marine sediment of base fluids used in synthetic drilling muds in the offshore drilling industry was evaluated. The test was intended to provide conditions for degradation which more closely parallel those encountered in the marine sedimentary environment, with respect to temperature, microbial flora, anaerobic characteristics and other physico chemical conditions, than existing biodegradation tests.
The basic experimental design consisted of sets of glass jars filled with marine sediment which had been homogeneously mixed with synthetic mud base fluid. The jars were maintained in fibre-glass troughs through which a continuous laminar flow of seawater was passed. At set times, triplicate jars were removed, the entire contents of each jar mixed thoroughly and, following solvent extraction, the concentration of base fluid remaining in the sediment was determined by gas chromatography with flame ionization detection.
The biodegradation rates of two synthetic mud base fluids, an ester and a blend of n-alkanes, poly-alpha olefin and linear-alpha olefin, were compared. A naturally occurring triester, olive oil, was used as a positive biodegradable control. A mud, a sand and a 1:1 (wt/wt) mixture of mud and sand were used to examine the effect of particle size on biodegradability.
The test was able to reproducibly distinguish between easily biodegradable and poorly biodegradable synthetic mud base fluids. In this system, biodegradation rates for the ester were faster than for the blended fluid, The rapid metabolism of the ester resulted in a depletion of oxygen, a drop in the oxidation-reduction potential and the occurrence of sulphide events in the sediments. Biodegradation of the ester was significantly slower in sand compared to mud, while no measurable biodegradation of the blended fluid took place in the sand. The results indicate that some synthetic drilling fluids may be poorly degraded. The implications for the perceived environmental friendliness of these fluids are discussed.
The deposition of drill cuttings on the seabed is probably the single most important factor in the environmental impact of offshore drilling operations in the North Sea Since the late 1970s, the use of oil-based drilling muds in the North Sea has increased due to their improved drilling efficiency over water-based muds in deep and directionally drilled wells. The biological impact of these oil-based muds is greater than that of water-based muds and the impact zone of the cuttings pile may extend to 3,000 m from the platform. The first oil-based muds contained diesel oil which can consist of a high proportion of highly toxic aromatic hydrocarbons which have the tendency to persist in the environment. Concern over the possible long-term effects of diesel oil resulted in its replacement with low toxicity oils such as mineral oil which contains lower levels of aromatic compounds. In 1992 the Paris Commission for the prevention of oil pollution set down maximum discharge levels of 10% mineral-oil-on-cuttings with a further stipulation of a 1% threshold for all North Sea drilling operations from January 1997. Subsequently, the number of wells drilled using oil-based mud has declined. In 1991 75% of a total of 333 wells drilled were drilled with oil-based mud resulting in an estimated 11,000 tonnes of oil being discharged. In 1995 less than 30% of the 342 wells drilled were drilled with oil-based mud resulting in the discharge of about 3,000 tonnes of oil. Measured levels of oil-on-cuttings however, have not improved below 10%.
In the absence of alternative technology, such as improved cuttings cleaning, and because of the perceived difficulty and cost of bringing cuttings to shore at most central and northern North Sea sites, Industry currently views synthetic drilling muds (SMs) as the preferred means of compliance with the 1 % oil-on-cuttings legislation. SM-on-cuttings discharges are not currently regulated to a specific target, although Industry is requested, where possible, to meet the previous 10% threshold operative on mineral-oil-based muds. The number of wells drilled using SMs has increased from less than 25 in 1992 to more than 100 in 1995 and an increasingly diverse range of base fluids including esters, linear-alpha olefins (LAO), poly-alpha olefins (PAO), linearalkyl benzenes (LAB), n-alkanes (n-paraffin) and ether derivatives are used in SMs (Fig. 1).