Importance

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Abstract

The invention relates to a well casing 11 including at least two casing lengths 12, 13, 14, 15, 16 with a first casing section 12 having and outside diameter OD12 and an inside diameter ID12 being fitted and cemented in position extending downwardly from the top of the well. The remaining sections are installed subsequently each subsequent casing section 13, 14, 15, 16 having an outside diameter OD13, OD14, OD15, OD16 and an inside diameter ID13, ID14, ID15, ID16 and the diametrical difference D1, D2, D3, D4, D5 between the outside diameter OD13, OD14, OD15, OD16 of the sections 13, 14, 15, 16 being less that the internal diameter ID12, ID13, ID14, ID15 of the corresponding fitted sections 12, 13, 14, 15 being an amount which is just sufficient for the subsequent sections 13, 14, 15, 16 to pass down through the internal bore of the corresponding fitted section 12, 13, 14, 15. Preferably the average diametrical difference is less than W mm such that the outside diameter OD12 of the first section 12 can be as small as possible and is at most equivalent to: OD12=(W.times.(n-1)+2.times.T.times.n)+ID16, where T is the average wall thickness of the casing sections 13, 14, 15, 16, ID16 is the internal diameter of the last section, n is the number of casing sections and W is the average of the diametrical differences. The well casing is made of continuous coiled tubing and W is less than 15 mm and greater than 0.1 mm. The well casing is made of joined tubing and W is less than 25 mm and greater than 1 mm.

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First Claim