Re-injection of drilled cuttings into a subsurface formation has become a common practice for handling oil wet cuttings during drilling operations in the North Sea areas. Currently, re-injection is technically a straightforward process both on fixed installations and subsea developments. Furthermore, it is feasible to re-inject cuttings into the same well as being drilled.
Environmentally, cuttings re-injection is currently considered as an optimum solution. Emission of CO2 and further unfavourable environmental effects associated with the introduction of heavy and sophisticated equipment on the rigs compromises the priority. The paper outlines the emissions of CO2 from offshore re-injection operations and onshore treatment in the North Sea area. The rationale behind the priorities governing best handling option is critically addressed. It is shown that emissions to air are significantly reduced when cuttings is re-injected as opposed to treatment on land. This is even more expressed when 12 ¼"-sections are drilled since these operations require additional boats to handle increased volumes of drilled cuttings. If seabed deposition of oil wet cuttings was an accepted alternative, it is shown that the CO2 emissions would be significantly reduced.
Introduction
Residual materials from drilling and completion operations consist mainly of drill cuttings, drilling fluids and completion fluids. All processes involved in disposal or recycling of these residual materials will have an environmental impact. These impacts may in some cases be harmful for the environment and in other cases insignificant. Discharge of chemicals not benign to the in situ environment generate the highest impact. However, even in the enterprise of recycling there are environmental impacts; not necessarily from discharges to the sea or ground, but resulting from the emission of different gases like CO2 to the air.
During drilling operations most water based drilling fluid chemicals in use on Norwegian offshore operations are considered as not harmful to the environment. Therefore, discharge of drill cuttings and drilling fluid volumes to the sea are permitted. Discharge of cuttings contaminated with oil based drilling fluids or synthetic based drilling fluids are restricted. Discharge to the sea of cuttings containing less than one percent oil on cuttings can currently be permitted. Likewise, permits to discharge cuttings contaminated with synthetic drilling fluid can be given in some cases. Presently, there are no practical methods to remove a sufficient amount of the oil base fluid to reach the one percent limit. Therefore, cuttings contaminated with oil based fluids has to be treated as drilling waste and has to be handled by re-injection to the formation or land site handling.
Re-injection of cuttings into the formation is a widely used technique in Norway. The technique was first used on the Norwegian shelf by BP on the Gyda field in 19911. BP used a slurryfying unit on the platform and pumped the cuttings slurry down into the formation underneath the 13 3/8" shoe, through the annulus between the 13 3/8" and 9 5/8" casings. Since the first cutting re-injection operation was introduced, the technique has been applied on most platform based drilling operations where extensive use of oil based drilling fluid was planned. More recently, the technique was further developed to be applicable also for drilling operations from floating rigs2,3.
