Continued access to invert drilling fluids is a pre-requisite for efficient drilling operations across much of the North Sea. However, legislation now specifies that the oily cuttings, generated in the drilling process, have to be cleaned to a target level of 1% residual oil, on a weight by dry weight basis, prior to discharge overboard or they have to be disposed of other than to the seabed.
A number of options for disposal are being developed (ref 1) but one that appears particularly attractive is the grinding and slurrying of oily cuttings. The resultant fluid is used to induce fractures in the sub- surface formation within which its permanent disposal is achieved. This process has been utilised successfully onshore in Alaska and from fixed platforms offshore in the Gulf of Mexico, the Norwegian North Sea and on the UKCS (ref. 2 and 3).
In each case the wellheads have been easily accessible, providing simple access to the well or annulus for injection. For subsea wells drilled from a floating exploration drilling vessel, access to the annulus is much more complex, requiring modification to both the permanent guide base (PGB) and the subsea wellhead.
This paper is a report of a successful field trial of one such subsea system where the permanent guide base (PGB) and subsea wellhead have been modified to allow access to an appropriate annulus for slurry injection.
A Drilling Engineering Association (DEA) project was established in 1990 to investigate the feasibility of injecting a cuttings slurry into the annulus of a subsea well. This project was supported by 12 participating Operators (Agip, British Gas, British Petroleum, Conoco, Elf, Mobil, Norsk Hydro, Ranger Oil, Shell, Statoil, Texaco and Total) with Thule Rigtech as project managers. BP Exploration, at that time, held a patent for a design of a subsea cuttings injection wellhead (ref. 4), an adaptation of the Universal Subsea Wellhead which was detailed in SPE/IADC paper 18699 (ref 5). Dril- Quip were contracted by the project to develop and design a subsea wellhead system from the patent, with support from BP Exploration's Drilling Technology Division.
The resultant design, manufacture and stack-up testing of the PGB/wellhead assembly will be presented in this paper, including the general safety design philosophy. The deployment of the system will be outlined and key elements of its operability will be detailed. The injection trial itself will then be presented, as it relates to the functionality of the PGB and wellhead assembly. Finally, the applicability of this approach to waste disposal will be discussed within the overall framework of continued invert emulsion drilling fluid use in the North Sea operating area.
Figure 1 presents a schematic of the slurrying process, depicting, in this case, annular injection.
Oily cuttings, separated from the circulating fluid by the solids control equipment, pass to a grinding/mixing/dispersing process plant where the slurry of finely divided particles in water is generated.