A Cuttings Re-Injections Feasibility Study For A Minimum Facilities Platform
Authors: Ritchie G.M. , Zambonini M.J. , Binks S.L.M. 
Published: October 2008

In an effort to minimise potential liabilities associated with the generation of an oily cuttings pile, during development drilling from the Nelson platform, in the UK North Sea, a number of disposal / treatment options were investigated. This paper describes a study into one of the options considered, namely, re-injection of drilled cuttings.

The study was complicated by the fact that Nelson was designed as a minimum facilities platform with one of the major design bases of the drilling package being the use of a cuttings wash system, which would remain in place. Consequently space was very limited, Handling of cuttings from subsea satellite wells was also to be addressed.

Specifically the paper describes how a feasibility study and conceptual design was carried out concentrating on the design of surface equipment for cuttings re-injection and the downhole considerations.

The study indicated that it was feasible to inject drill cuttings from the platform wells. Two routes for m-injection were identified. It was however concluded that cuttings re-injection was not technically or economically feasible for the subsea satellite wells.

The E and P Forum "Guidelines for the Planning of Downhole Injection Programmed for Oil Based Mud Wastes and Associated Cuttings from Offshore Wells" were helpful as a guideline for carrying out the study1.

INTRODUCTION

Enterprise Oil (Enterprise) is developing the Nelson Field in block 22/1 1 and 22/6a in the UKCS on behalf of a consortium of partners including Shell, Esso and Elf. The platform has been designed by a Shell Project Management Team whilst Enterprise becomes operator after first oil. Enterprise pro-drilled eight wells from a template and two subsea satellite wells using low toxicity oil based mud.

Eighteen platform wells have been allocated slots to date. These wells are highly deviated and drilled through over pressured and highly reactive Tertiary shales before entering the normally pressured reservoir. Borehole stability analysis was carried out2 and Exxon Production Research have produced a borehole stability model which concluded that weighted oil based mud (directly related to hole angle) was requimi in order to drill directional wells successfully through the Tertiary shales. Hence, oil baaed mud is the preferred drilling fluid. A cuttings cleaning plant has been built on the platform to the same design as that used during the template drilling campaign. Levels of 4 to 5% w/w retained oil on cuttings were achieved which lies comfortably within the 10% WIW current legislation for existing production installations. This provides a clear way ahead until the end of 1996.

However, a study was initiated, by Enterprise, to examine other options for disposal / treatment of drilled cuttings for several reasons:The prognosed drilling programme has been lengthened such that up to four wells are planned to be drilled from 1997 onwards. Clearly, it is not feasible to drill the wells with existing oil based mud formulations and maintain the 1% w/w oil on cuttings discharge limit which comes into force after 1st January, 1997.