(19)

(11)

EP 0 840 836 A1

(12)

(43) Date of publication: 13.05.1998 Bulletin 1998/20 (21) Application number: 96927604.0 (22) Date of filing: 23.07.1996

(51) International Patent Classification (IPC):  E21B 34/ 16( . ) E21B 43/ 12( . ) (86) International application number: PCT/EP1996/003285 (87) International publication number: WO 1997/004212 (06.02.1997 Gazette 1997/07)

(84) Designated Contracting States: GB IT NL (30) Priority: 24.07.1995 EP 19950202038 (71) Applicant: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V. 2596 HR Den Haag (NL)	(72) Inventors: DER KINDEREN, Wilhelmus Johannes Godefridus Josep NL-2288 GD Rijswijk (NL) KOORNNEEF, Pieter NL-2288 GD Rijswijk (NL)