Casing collapse resistance is one of the key factors in well integrity design especially for production casings and the aim of the current study is investigating the physical phenomena that define the mechanism of failure in presence of two concentric cemented casing strings.

Collapse load mode, especially in specific conditions, becomes the governing load of the well.

Typical examples are salt or other creeping formations, which represent the most difficult load combinations to design and analyze. Salt creeping movements create three loading mechanisms that may result in a production casing failure: uniform loading, non-uniform loading and shear loading. The current oil industry approach, to deal with salt formations, is using two casing strings or either an individual "high collapse" casing column.

This paper describes results of a joint activity between Eni and Tenaris aimed at evaluating on full field scale the combined collapse resistance of two casing strings in three different cementing configurations.

The experiment consists of two different laboratory test set-ups. The first set is a collapse test, performed in a high-pressure vessel under hydrostatic conditions, to reproduce salt distributed Uniform Loading. Meanwhile the second test set, which is one of the first worldwide experiments of its kind, reproduces punctual Non-Uniform and Shear Loading. This second test is conducted with a mechanical press equipped with an ad-hoc designed indenting tool. The scope of the tool is to reproduce the effect of hard formation inclusions, in salt layers, acting on the casing's external surface. Test set-up and specimen's geometric configuration are optimized by specifically developed FEA analyses. To simulate real stress conditions, as the ones found in the well, tests are performed on different specimen configurations.

The base configuration consists of perfectly concentric casings with standard cement slurry in the annulus. Moreover, a configuration with 10% stand-off is also tested. At last, a configuration simulating the effect of contaminated cement or poor cement job is evaluated. The latter allows a better cement contribution assessment to final collapse resistance.

The results of the first set of experiments allows empirically defining a coefficient, "K-value", used to define collapse resistance of the assembly; the second set provides an indication about the entire system resistance to punctual loads and cement sheath integrity. Purpose of these tests is to extend the outcomes to several casing sizes thru the utilization of FEA model.

The obtained result of full-scale experiments is the way forward to better understand the collapse phenomena and approach the casing design for "pipe in pipe" cemented strings, with a significant impacts on well final cost and integrity.

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