Seabed Surveys: the Best Means to Assess the Environmental Impact of Drilling Fluid Discharges?
Authors: Limia Jose M. 
Published: December 2007

With the ban in 1984 of the use and discharge of diesel based drilling fluids in the North Sea and their substitution with Low Toxicity Mineral Oils, the Oil and Gas Drilling Industry took an important step forward towards the reduction of the environmental impact of drilling operations.

This change to low toxicity OBM was based solely on the lower acute toxicity of these mineral oils to the shrimp Crangon crangon. Unfortunately this substitution did not render the expected results. Although the toxicity of the OBM was considerably lower than that of diesel, the same environmental impact was recorded years after the contaminated cuttings were discharged. It was observed that once the OBM cuttings were discharged, they create a permanent situation of organic enrichment in the sediment. This is mainly because these oils are biodegraded slowly under aerobic conditions and not biodegradable under anaerobic conditions. These findings prompted the regulatory authorities and environmental agencies, in North Sea countries, to reconsider the information required before granting permits for use and discharge of invert emulsion drilling fluids to the sea.

With the introduction in the UK of the revised Offshore Chemical Notification Scheme (OCNS), and more recently the Harmonised OCNF (HOCNF) for all North Sea countries, new additions to the required ecotoxicological information have been made: Toxicity to three species, Biodegradation and Bioaccumulation.

Toxicity

Skeletonema costatum

Acartia tonsa

Corophium volutator

Biodegradation

Aerobic

Anaerobic

Bioaccumulation

Log Pow Bioconoentration factor (BCF)

In Ref. 1 it is demonstrated that biodegradation continues to be the most controversial parameter. The variability found between the different approved protocols and testing laboratories is alarming. To illustrate this, the authors presented results obtained from biodegradation tests performed on an acetal following different test protocols. These tests produced biodegradation results ranging between 5 and 87%. This variability has created confusion in the Industry. Thus, based on information obtained with the currently approved biodegradation protocols, some fluids have been approved for use and discharge offshore and were subsequently discontinued when their biodegradation behaviour in the field did not verify the laboratory test results.

To better define the most appropriate method for measuring ready biodegradability, a ring test has been initiated. The need for this ring test in biodegradation was expressed at the SEBA (Sea Based Activities, PARCOM) meeting in 1995. The French delegation was asked to initiate and organise the project. The biodegradation results obtained on four products following four selected protocols will be compared. The protocols to be evaluated are as follows:Marine carbon dioxide evolution test (adaptation of the STURM test). The lead laboratory for this protocol will be INERIF (France).Marine BODIS test. The lead laboratory for this protocol will be Rogaland Research (Norway).Marine carbon dioxide head space. The lead laboratory for this protocol will be Shell Research (UK).OECD 306 Closed Bottle Test. The lead laboratory for this protocol will be VKI (Denmark). It is expected that preliminary results will be ready by the next SEBA meeting which will take place in February 1996. The project will be finalised during 1996.

P. 803