Abstract

The implementation of a well integrity management system in a mature liquid hydrocarbon storage field brought to light essential questions about the present condition and likely evolution of the barrier elements in the years to come. Are there defects in the cement sheaths that were not detected at the time of site construction more than 50 years ago? Are there aging processes, such as casing corrosion or cement degradation, that could limit the field's useful life or even present an immediate risk? And, finally, what were the exact properties of cement and casing, given that the information available in drilling reports is very limited?

At the start of the campaign, the perceived need and available technology meant using a logging tool requiring pressure, and therefore the inspection would typically involve 2 workovers and a period of 3 weeks. However, tool selection focused on the actual aging risks, together with the use of new transmitters and relentless operational optimization improved operational efficiencies allowing two wells to be logged in two consecutive days, with important savings in direct and indirect cost as well as minimization of operational risk. The cost reduction did not come at the expense of actionable information: the wireline tools selected, together with detailed preparation and carefully supervised execution led to outstanding data quality being collected.

The implementation of a consistent method of absolute log calibration and quantitative evaluation allowed us to characterize cement properties and defects, as well as their evolution with time. A salient signature, common to most storage sites in salt, is high cement quality across the salt, with a sudden unexpected, partial loss of bond across the anhydrite that overlies the rock salt formation. This was recognized as a benign form of "sulfate attack": The precipitation of secondary ettringite within the cement matrix results in expansion and thus debonding across the stiff anhydrite, whereas creeping salt pushes cement back on the casing. Lead cement densification was also observed near shallower aquifers, also rich in sulfates, with clamping provided by marls in this case. After optimizing the logging suite, it was concluded that there was no risk of erosion, wear or corrosion, external as well as internal. The analysis also improved understanding of cement behavior across salt and the role of creeping formations.

A streamlined approach at integrity assurance, whereby the right questions are asked by the management system, optimum inspection protocols are selected and carefully carried out, and acquired data is processed using advanced quantitative techniques, allowed us to understand characteristics and dynamics of barrier elements and to conclude that safe operation of the site is possible and that there is no fixed life span.

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