Test of Analytical Approaches for Assessing Dispersal of Barium Free Drilling Discharges on Seabed

Abstract

Abstract During planning of a drilling operation in the North Sea with planned discharges from the top hole section, it was revealed that the drilling location was bordering to a predefined area for sand-eel (Ammodytes marinus). As sand-eel spawning is strongly related to limited habitat, spawning grounds are given high value on the uniqueness, life-historical importance and productivity. Due to the presence of sand-eel, mitigating measures were implemented to minimize the risk of smothering sand-eel areas by: 1)Use of cuttings Transport System (CTS) in order to lead the discharges away from the defined sand-eel area.2)Use of particle free drilling fluid for minimizing spreading of particles. As a consequence barite is not included in the drilling mud composition. Barite contains high amounts of Ba and may generate particles with a potential of being transported over long distances. Environmental authorities required documentation of drilling discharges not spreading to the sand-eel habitat. The question is then how one can monitor and document the spreading of barium free drilling discharges on seabed? Application/Development Normally monitoring of drilling operations close to vulnerable habitats in Norway is done with an array of measuring techniques, covering turbidity measurements, sediment traps, current measurements and sediment analysis using barium as a tracer. In this case barium was not present in the mud and could therefore not be used as a tracer. A new approach was therefore chosen. This approach included use of Scanning Electron Microscopy (SEM), picture analysis, grain size and organic matter analysis from a total of 36 sampling stations. Results, Observations, and Conclusions The monitoring approach gave relatively clear results of the dispersion pattern of the drilling discharges. Results were comparable with observations from monitoring of other drilling operations. Especially the SEM approach gave rather clear results, and these findings were supported by the analysis of grain size distribution. The approach used is considered to be relatively cost effective compared to standard monitoring approaches used in Norway. Significance of Subject Matter A combination of picture analysis and grain size analysis is an alternative approach for documenting spreading of drill cuttings discharges. It may be a cost efficient alternative in cases where barite is not present in the mud.