Modeling North Sea Oil-Based Mud Cuttings Discharges To Assess Environmental Loading: An E&P Forum Study
Authors: Brandsma M.G. , McKelvie Scott 
Published: October 2008

Oil concentrations in the water column and benthic sediments caused by the discharge of oily cuttings were predicted at four North Sea sites using a computer model. The cuttings were drilled with oil based mud and were treated to reduce oil content prior to discharge. The predictions allowed the results of biological experiments to be related to field conditions in a multi-disciplinary research project. The goal of the project was to determine what amount of oil remaining on the cuttings is environmentally acceptable. Model results suggested that treatment reduced maximum and average oil concentrations in benthic sediments by more than 93% Frequently, the maximum concentration was reduced below the 1000 ppm identified in the biological experiments as the sediment oil concentration above which significant effects on biological communities may be observed. Reductions of oil concentrations in the cuttings were accompanied by reduced particle sizes and settling velocities. This resulted in increased dispersion at the seabed and caused water column oil concentrations to increase. The increased water column concentrations were still several orders of magnitude below toxic levels.

INTRODUCTION

Mineral oil based muds (OBM) have been used extensively throughout the North Sea Operating Area particularly when drilling highly deviated wells and water sensitive formations. When discharged, the drill cuttings are coated with a layer of the OBM. These cuttings can accumulate around drilling platforms and thereby affect the benthic community1.

Techniques have been developed to remove the oil from the cuttings prior to discharge and so minimize the environmental effects. Little information is available on the possible environmental effects of treated cuttings, although data are available on the effects of uncontaminated and untreated cuttings.

The Oil Industry International Exploration and Production Forum (E & P Forum) sponsored a research program to provide the necessary information to identify the amount of oil remaining on discharged cuttings for which the effects are environmentally acceptable. This research program comprised multidisciplinary studies (Table 1) covering the physical, chemical and biological aspects of cuttings discharges. These studies used a common set of drill cuttings and analytical methods to ensure comparability of results, a feature we think is unique to this research program. The studies are cited where appropriate.

The modeling study predicted the distribution of the discharged drill cuttings and the associated mineral oil through the water column and the resultant deposition of cuttings and oil on the seabed. The objectives of the modeling component were:to define the effect of removal of oil from discharged cuttings on the deposition behaviorto allow comparisons of the seabed area affected by drill cuttings at different locationsto define the areal extent of effects identified from the biological experiments.

This paper will describe the modeling component of the research and briefly relate the modeling results to the research program as a whole. Details of the work are available in the project report2.