The results from the well test are in and you just confirmed (or found out) that there is H2S in the production fluids and now everything must be “NACE compliant”, whatever that means! The tough questions will start: Does it really have to be CRA (Corrosion Resistant Alloys)? Can we get away with L80? How much is it going to cost? At this point you will have to involve that reclusive, quiet, boring guy, the one everyone avoids, your metallurgist!
He’s going to listen to you, then look at the data and he will pronounce the famous words “it depends…”. Yes we’ve all been there, you have heard those words, I had said them before, and well actually I use them all the time!!! And the reality is that it all does depend but with a bit of collaboration and communication you will get the answer.
You do need a metallurgist and this metallurgist will need to talk to a lot of people, but mainly needs to talk to your production technologist and your production chemist. Continuous communication will lead to an iterative process and the optimal solution, how does it work? That’s the purpose of this post.
Selection of tubing material is not a defined process by an international standard that can take you by hand in the same way that pipeline design or cathodic protection have a better define process. Tubing selection is dependent on a series of reservoir variables, production strategy and well integrity. There is not a single standard that dictates tubing selection although there are many standards that will required to be checked such as API 5CT, ISO 15156 / NACE MR0175; for well integrity Norsok has prepared a document Norsok D010, and OGP is working on a series of drafts for well integrity. There is available software for corrosion assessment and alloy suitability but none of them provide a definite answer. When it comes to software I personally prefer to use ECE or Socrates. But the software output is not the final answer. In my limited experience, the best way to tackle the problem is by an iterative process that may look like this.
- Corrosion assessment, your metallurgist needs to calculate the downhole conditions, if the corrosion rates grant the use of carbon steel, the need for inhibitor will be defined, if inhibitor is not enough then CRA must be consider, the question is which CRA? Your metallurgist will consider all variants. such as h2s, co2, chlorides, sulphur, organic acid, pH and using data from a model software will determine the suitability of a CRA, with a bit of luck that CRA will be qualified by NACE, if it isn’t then you might need to carry out experimental qualification, see part 2 and compare them to iso15156. Many tubing supplies have prepared their own materials selection diagrams, here’s one from Sumitomo and a table from V&M.Nippon Steel – Sumitomo OCTG CRA Selection Guide (Source: https://www.tubular.nssmc.com)Vallourec and Mannesmann CRA OCTG Selection Guide (Source: www.vallourec.fr/en/activities/our-markets/oil-and-gas/octg/)
- Once an alloy has been selected it is important to involve the production technology, he or she will review the feasibility of such selection, if carbon steel with corrosion inhibitor was selected then he will consider compatibility with the completion; the production technologist will also consider the selection against work over feasibility, design life events, and the overall well integrity strategy.
- The production chemist will help you select the corrosion inhibitor, and in collaboration with the production technologist the appropriate inhibition method, i.e. squeeze or continuous. The production chemist and production technologist will also inform the metallurgist on matters such as acid stimulations and other operations that may affect the materials selection
- Finally, the materials selection will also depend on the consider supply chain restrictions and the all mighty cost. At this stage most of the questions will be answered and the materials selection of the tubing will be an integral part of your well integrity management system (WIMS)
The point I want to stress is that materials selection for tubing (and completion and casing) is not an isolated process, its importance requires input from several disciplines and it is important that us metallurgists avoid working in isolation, only then we will be able to understand restrictions that are beyond H2S and chloride content, we need to see the big picture and then we can give you the best answer.
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