Handling drilling and production waste associated with oily contaminated drill cuttings, NORM and produced water is an environmental and economic issue in offshore drilling and production operations. There are a number of waste management options and subsea injection into suitable geological formations can be a preferred one. This is because it can achieve zero discharge operations; eliminating transportation and spill risks. It can also be economically favorable. Although subsea drill cuttings and produced water re-injection operations have been conducted in an environmentally acceptable manner, system design and operational experience are limited. A few worldwide drill cuttings re-injection operations from mobile offshore drilling units are reviewed first in the paper. Lessons learned from these and other system design and operational experience include a detailed subsurface study and careful engineering, wellhead and injection system design and effective communication to handle emergency disconnect, quality control on well integrity and slurry injection procedure. Challenges and best practice guidelines for waste injection operations from subsea wellheads are given to minimize failure and maximize success.
Growing environmental regulations and constraints put pressure on drilling operations to find environmentally friendly and economically sound solutions for handling drilling waste. Drill cuttings re-injection (DCRI) operations into suitable geological formations have been successfully implemented all over the world.1–13 Figure 1 shows a schematic of DCRI process. Briefly, the drill cuttings from shale shaker are milled to small particles (300 microns or less), in the presence of water, usually seawater or other wastewater. The resulting waste slurry is pumped into sub-surface fractures created by injecting the slurry under high pressure into the disposal formation. Depending on drill cuttings generation rate, the waste slurry is injected either continuously or intermittently in batches. A continuous slurry injection scheme is expected to lead to an extended disposal fracture, while periodic re-injections would promote the development of multiple fractures and thus a compact domain of smaller multiple fractures from the wellbore. The batch process consists of intermittent injections of roughly the same volumes of slurry and shutting-in the well after each injection. This allows the disposal fracture to close onto the cuttings and to dissipate any build-up of pressure in the disposal formation. This may be impractical when re-injecting into low leak-off shale as the target disposal horizon when drilling rates and thus the slurry generation rates are relatively high. In practice, therefore, re-injections may shift from essentially continuous mode during drilling of large hole sections to periodic as the cuttings generation rate declines with the well approaching TD.
Although drill cuttings re-injection has been applied since early 1980s, these operations have mostly been restricted to fixed platforms; annular re-injection into subsea wells, where drilling operations were in progress, had not been feasible until early 1993. BP, through a Drilling Engineering Association (DEA) JIP, conducted the first field trial DCRI operation in March 1993 into the annulus of a subsea well from a mobile offshore drilling module unit (MODU).14, 15