Influence of Casing Shoe Depth on Sustained Casing Pressure

Abstract

In 2006 the Petroleum Safety Authority Norway (PSA) performed a well integrity survey. The survey indicated that about 20 % of wells on the Norwegian Continental shelf (NCS) may suffer from well integrity issues. Most of the problems were related to deficiency in annulus safety valve, tubing, cement and casing. Pressure build-up in annulus, i.e. sustained casing pressure, is one of the main indicators of a significant well integrity problem. Increased understanding on the field may help engineers to design wells with better integrity in the future. This thesis describes SCP and its most common causes with emphasis on the relation between casing shoe setting depth and the occurrence of SCP. Primary and secondary barrier is described together with the common practice of choosing casing shoe depth. Formation strength and its impact on setting depth is explained together with a brief introduction of the different formation integrity tests. Generic cases have been studied to determine the relation between unfavorable casing shoe setting depth and the occurrence of SCP. For each case there is a suggestion as how the well may be redesigned so that the risk of SCP is reduced. Information on the theme has been acquired through studying and comparing different papers, booklets, previous reports and reviews concerning the subject. The Norsok standard D-010 and 117 OLF recommended guidelines for well integrity have also been very informative during the study. To be able to avoid SCP and at the same time improve well design, it is important to properly understand how SCP arises. Changing the casing shoe setting depth to a more suited depth or formation cannot alone eliminate SCP. To eliminate SCP a good conversion between Top of Cement (TOC) and setting depth of the previous casing shoe is required. The best way of avoiding SCP because of casing shoe setting depth is to make a thorough investigation of the underground and carefully choose the setting depth.