Nasir,
The allowable leak rate is a function of the criticality of the test. The more critical the test, and the higher the consequences as a result of a bad test, determines how much confidence you need to have in your barriers. This makes the test criteria a variable that depends on the purpose of the test itself.
Due to large volume, compressibility of fluids, entrapped air, wellbore fluid heating/cooling, etc a perfect straight line test is rarely accomplished in a well. For a tree, a small volume test, a straight line test proves that you have good isolation and is expected.
If the test is just to confirm that a bottom plug has been set before cement is placed on top of it to isolate a zone, then a short test with a high leak rate may be OK. A test with a 2-5% drop after 10 minutes may be OK. Some regulations allow a deep set cement liner to have a 10% pressure loss in 30 minutes.
If the test is to confirm the bottom plug is providing complete isolation, before removing a tree for example, then a very good test, with a small leak rate over a long time, would be expected. In this case, there will be people exposed to any sudden release and a chance wellbore fluid could be released to the environment...e.g. high consequences. In this case, a 1% drop every 10 min over a 30 min test, might be OK. And if the plugs are leaking then rerun the test with new plugs rather than accept a poor quality test.
For a new completion, with clean brine in the tubing, this criteria should not be a problem. I would certainly use this type of criteria as the integrity test for a Sour Gas well before I brought the well on production. But, this might be too low a leak rate for a well with complicated fluid system (water, oil, mud, etc) and a 2-3% drop per 10 min over 30 min a more suitable criteria.
For a production annulus and packer, with typically a clean water based packer fluid, I would expect a 1% every 10 min test, over 30 minutes, to be passed with ease. If the packer does not pass that test I would be concerned about the integrity of my barrier system and would retest.
In my examples, the test criteria of 1% per 10 min for a 30 min test is just an example...not a recommendation. You need to look at the critical tests your team is performing and establish an acceptable leak to minimize the risk to your operation. A shallow plug to allow tree removal may be different from a deep plug just required to set a packer.
I hope this helps. Please let me know if you have any questions.
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Dan Gibson
TheWellDoctor
Completion / Well Integrity Advisor
Gibson Petroleum Consultants
FulshearTX
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