As part of the continuous drive to improve environmental performance, drill cuttings discharges must be addressed and recommendations made for reducing the environmental impact of this waste stream.
The options for avoiding OBM discharges are to increase the use of water based drilling mud or implement total containment systems to prevent the discharge of synthetic or oil - based muds (synthetic mud/OBM) to seat.
The methods of total containment (cuttings re-injection - Cuttings Re-Injection), and transport of cuttings to shore for treatment and disposal) are operations involving mature technology.
Other technologies to consider are: high performance water based mud; equipment for sub-sea Cuttings Re-Injection; bulk transport of cuttings to shore; improved on-site cleaning technology and slurrification and discharge.
1. Introduction
In order to reduce the impact of cuttings discharged to sea, a considerable effort has been made over a number of years to develop drilling muds with an environmentally acceptable base fluid. Initially diesel base muds were replaced with low toxicity oil based muds (LTOBM). Although this represented an improvement, the slow rate of degradation of mineral oil based muds results in cuttings piles on the sea bed with a very long lifetime. As a result increasingly stringent legislation on the discharge of all OBM cuttings has been introduced, leading to the situations whereby no cuttings may be discharged if the level of OBM relative to the weight of dry cuttings exceeds 1%.
The principal methods for eliminating OBM discharges are by the containment of drilling waste for subsequent disposal, the use of water based muds (WBM) and the development of synthetic or pseudo oil based muds (synthetic mud / POBM) with similar performance characteristics but faster degrading than OBM.
The high cost of the synthetic base fluid makes pseudo muds considerably more expensive than mineral oil based muds. This has been justified by the fact that for many drilling operations the cost of a total containment system for drill cuttings disposal has been even higher and because the drilling performance of WBM is too low. However,the environmental performance of the synthetic mud has not matched expectations. In fact, laboratory tests indicate that the environmental behaviour of the majority of synthetic mud is very similar to that of OBM. With this evidence the continued use of costly drilling fluids is increasingly becoming insupportable both on economic and environmental grounds.
It is necessary to assess the environmental impact of all drilling muds in use and to design a practical solids controlhedule for phasing out the discharge of any found unacceptable. This must be done without compromising the need for drilling fluids able to drill demanding wells. This involves identifying the current forecast for the discharge activity in question, and analysis of the reduction drivers impacting that activity: environmental effects, public perception, assessynthetic mudent of legal trends, bench marking (nationally and internationally), best available technology, cost, etc.
The alternatives to disolids controlharging contaminated cuttings to the sea are: the use of genuinely low impact drilling fluids; employing an effective cuttings cleaning technique prior to dumping or containing the drill cuttings and either returning them to shore for treatment and disposal or re-injecting them in a disposal well offshore. These options each have technical limitations as well as economic implications which will differ between specific operations. In order to recognise this a method of comparing and ranking the possible solutions on a case specific basis should be found.
It should be recognised that a number of important factors in considering the handling of drilling waste are changing quite quickly. Such factors include the legislation in place, the level of understanding of the genuine impact of drilling fluids currently in use, the technologies available for cleaning or disposing of cuttings and the development of new drilling fluids.