(19)

(11)

EP 0 491 841 A1

(12)

(43) Date of publication: 01.07.1992 Bulletin 1992/27 (21) Application number: 90914581.0 (22) Date of filing: 20.09.1990

(51) International Patent Classification (IPC):  E21B 21/ 08( . ) E21B 49/ 00( . ) E21B 47/ 06( . ) (86) International application number: PCT/US1990/005263 (87) International publication number: WO 1991/004500 (04.04.1991 Gazette 1991/08)

(84) Designated Contracting States: AT DE DK ES FR GB IT NL (30) Priority: 20.09.1989 US 19890408650 (71) Applicant: CHEVRON RESEARCH AND TECHNOLOGY COMPANY San Francisco, CA 94120-7141 (US)	(72) Inventor: WEAKLEY, Robert, R. The Woodlands, TX 77381 (US) (74) Representative: Nash, David Allan  Haseltine Lake & Co. Hazlitt House 28 Southampton Buildings Chancery Lane London WC2A 1AT London WC2A 1AT (GB)