Well Integrity Technical Section

There is an appraisal well which produces from gas-condensate reservoir. From the beginning of production there are pressures in A and B annuluses (the reason is not determined, there are only assumptions). During last month of production the pressures stayed more or less stable. But recently, suddenly pressure in A sharply dropped by about 60 atm in 2 hours and then returned to the previous value in 6 hours. During the same time period pressure in B dropped as well but by about 20 atm and then increased till its previous value. What are the possible explanations?

Hi Nijat,

Two trivial questions: was the pressure sustained, i.e. have you already bled it off and then it returned to the previous value? And second: was the pressure drop event caused by human action, e.g. bleeding off either A or B annulus, or changing production schedules? It would also be useful to know the absolute value of the pressures before and after the drop.

We can nonetheless attempt a few wild speculations, and suggest how you could test them:

• If there is pressure in the A and B annulus that may imply a shallow hydraulic connection - mind that an alternative hypothesis is that both are connected to a (deep) source, but the production casing and the wellhead are sealed.
• The annulus pressure may actually be due to production. Maybe your tubing packer never sealed and you production casing is leaking. Or maybe the SCP is due to a shallower source of pressurized fluids. You should be able to tell these two hypotheses apart based on pressure behavior.
• You seem to suggest that the sustained pressure values are different in both annuli. This could imply that the leak pathway can let through some gas to the top of each annulus, but doesn't allow annular fluids to escape. A related possibility is that the leak points through the two barrier elements (completion and production casing) are at different depths. The fact that it took long for pressure to drop and build up again seem to confirm that the leakage pathway is small.
• The first hypothesis that comes to mind is that the pressure drop was caused by a barrier element failure and that the flow out of both annuli slowly pressurized a secondary closed container. Since pressure dropped in 2 hours and then took longer to climb again, it is possible that this new failure is "larger" than the initial leakage pathway so pressure had to drop to an equilibrium value between all three containers (A+B annulus as well as the new unknown one). It should be fairly easy to infer sizes of containers and pathways from the pressure vs. time behavior. The secondary container must be closed (or at least leaking far less than the A+B annulus pathway) since pressure built up back to the original value.
• A possibility is that your intermediate casing (or your wellhead seal) failed and you recharged the C annulus. This, again, should be easy to test. It is less likely that you're leaking into a formation and very unlikely that you're leaking into an open system (surface aquifer or atmosphere).

Now another question: was the maximum pressure in the B annulus constant through the operation life or recently spiked? The reason for asking is that if the casing properties degraded with time, then the same is likely to happen with your newly pressurized container, and you may have a leak at surface within a few months.

Please remember that any assumption should be tested against all available evidence to come up with plausible explanations. And let me know if the above deductions are not clear and I need to reformulate them.

Best of luck,

Matteo

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Matteo Loizzo
Well integrity consultant
Berlin
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Excellent reply from Matteo here, with the limited information you have provided this will be have to be guesswork.

A sudden drop normally means something has broken by exceeded MAASP, and MAASP is NOT a constant. If you for instance have influx of fluid into a gas filled annulus or a temperature change occurs, the value can change significantly.

One possibility is that you have extended a fracture at the production casing shoe. The B-annulus drop may be caused by less ballooning effect of the reduced A-annulus pressure, but without absolute pressures it is not possible to conclude. Careful study of A&B pressures combined with volumes and compressibility of annulus contents should give the answers to this.
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Vidar Rygg
Project Manager
Sandaband Well Plugging
Tananger
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Original Message:
Sent: Dec 08,2014 04:28 AM
From: Matteo Loizzo
Subject: Sudden pressure drop in annuluses

Hi Nijat,

Two trivial questions: was the pressure sustained, i.e. have you already bled it off and then it returned to the previous value? And second: was the pressure drop event caused by human action, e.g. bleeding off either A or B annulus, or changing production schedules? It would also be useful to know the absolute value of the pressures before and after the drop.

We can nonetheless attempt a few wild speculations, and suggest how you could test them:

• If there is pressure in the A and B annulus that may imply a shallow hydraulic connection - mind that an alternative hypothesis is that both are connected to a (deep) source, but the production casing and the wellhead are sealed.
• The annulus pressure may actually be due to production. Maybe your tubing packer never sealed and you production casing is leaking. Or maybe the SCP is due to a shallower source of pressurized fluids. You should be able to tell these two hypotheses apart based on pressure behavior.
• You seem to suggest that the sustained pressure values are different in both annuli. This could imply that the leak pathway can let through some gas to the top of each annulus, but doesn't allow annular fluids to escape. A related possibility is that the leak points through the two barrier elements (completion and production casing) are at different depths. The fact that it took long for pressure to drop and build up again seem to confirm that the leakage pathway is small.
• The first hypothesis that comes to mind is that the pressure drop was caused by a barrier element failure and that the flow out of both annuli slowly pressurized a secondary closed container. Since pressure dropped in 2 hours and then took longer to climb again, it is possible that this new failure is "larger" than the initial leakage pathway so pressure had to drop to an equilibrium value between all three containers (A+B annulus as well as the new unknown one). It should be fairly easy to infer sizes of containers and pathways from the pressure vs. time behavior. The secondary container must be closed (or at least leaking far less than the A+B annulus pathway) since pressure built up back to the original value.
• A possibility is that your intermediate casing (or your wellhead seal) failed and you recharged the C annulus. This, again, should be easy to test. It is less likely that you're leaking into a formation and very unlikely that you're leaking into an open system (surface aquifer or atmosphere).

Now another question: was the maximum pressure in the B annulus constant through the operation life or recently spiked? The reason for asking is that if the casing properties degraded with time, then the same is likely to happen with your newly pressurized container, and you may have a leak at surface within a few months.

Please remember that any assumption should be tested against all available evidence to come up with plausible explanations. And let me know if the above deductions are not clear and I need to reformulate them.

Best of luck,

Matteo

------------------------------
Matteo Loizzo
Well integrity consultant
Berlin
------------------------------

Dear ninja 60 atm is huge pressure to drop. In gas well whenever you find sustained casing pressure you have to be very careful and diagonise the source. In most of the cases it is tubing thread or joint of accessories. Without the information it is not possible to advise. I will first doubt the joints of tubing and casings and also well head.and most probably gas has come from tubing to annulii.that should be relatively easy to fix.you need to calculate whether fracture presure at the shoe was attained at the outer casing shoe.it may so happened that pressure gas from tubing got communicated to A section through B section. Once gas got released through the casing shoe ofA pressure dropped in both the annuli .therafter started building again. Rate of build up is slow since leak point is very small.if it is so this cycle of drop of pressure will again happen. If the casings/annulling are getting pressurised from out side the casing you are in serious problem . Well may I repeat may heading for internal blow out. You need to critically examine the drilling history to ascertain that. I am only mentioning the possibilities. There may be more reason. Work on the well to eliminate cause one by one. P. K.borthakur ------------------------------ Pronip Borthakur cosultants Oil & Natural Gas Corp. Ltd. Mumbai ------------------------------
Original Message: Sent: Dec 07,2014 07:24 AM From: Nijat Hakimov Subject: Sudden pressure drop in annuluses There is an appraisal well which produces from gas-condensate reservoir. From the beginning of production there are pressures in A and B annuluses (the reason is not determined, there are only assumptions). During last month of production the pressures stayed more or less stable. But recently, suddenly pressure in A sharply dropped by about 60 atm in 2 hours and then returned to the previous value in 6 hours. During the same time period pressure in B dropped as well but by about 20 atm and then increased till its previous value. What are the possible explanations?