Well Integrity Technical Section

Hi dear,

How Maximum allowable annulus surface pressure can be calculated for a,b & C annuli if Top of cement is different for all casings.  Production casing is pressure tested at 1000 psig and we r observing more than 1000 psig at production casing annulus. how could MAASP be calculated to operate well in safe well operating envelope. Is there any excel calculation sheet,who would share or if is there any calculation procedure.

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Lajpat Rai
Petroleum Engineer
United Energy Pakistan Ltd
Karachi City
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Lajpat,

you need to understand what MAASP is about.

In most cases it relates to and you need to know where gas (or the gas bubble) is.

During production phase the gas, assuming it's a gas influx, almost certainly would have migrated to surface. The key issue in determining MAASP here is 

1. Where the gas- liquid interface is in each annulus (and the gradient of the fluid above and below each interface).

2. Where each relevant shoe is. (And its leak off or Frac pressure).

Generally, but not always, the tubulars are rated higher than the pressure generated by the reservoir / bubble and can be ignored. However on occasions you need to consider for each annulus the tubular rating, ie 

a the collapse of the inner string

b the burst of the outer string

The calculation depends on well configuration (partly whether cement top is above or below the shoe). This defines the weak point in the well, ie is the formation or one of the tubulars the weak point?

You need to consider the length of gas and the length of mud/ fluid above the shoe / TOC and the gas and fluid gradients. 

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During the drilling phase the gas initially will be deep then migrate to surface (quite quickly in many cases).

MAASP during drilling is generally considered when the bubble is below the casing shoe. So iI is based on the shoe leak off / Frac pressure and the gradient of mud above the shoe. Once the bubble is above the shoe MAASP generally can be ignored. Again not always as said above.

Hope this helps.

TG
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Terry Greaney

Consultant

Brisbane

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To continue:

MAASP is generally thought of as the pressure when the casing shoe fractures during drilling, resulting in losses at the shoe and in worst case an underground blowout.

This is less likely to happen with production casing (shoe is deep and cemented off).

This is when the tubular ratings might need to be considered, ie MAASP is where the inner or outer tubular fails.

Cement quality also needs to be considered. If cement is poor (or casing leaks ) pressure may reach a weak formation despite the TOC seeming to be above the shoe.

Most well control manuals have the formula for the normal drilling scenario.

1000 psi doesn't seem a particularly high rating for a production annulus. You need to reassess your casing rating and safety factor.  In future maybe test higher.

Pressure in the annulus may be reservoir generated (ie tubing / casing leak), may be thermal (trapped liquid heated by production) or could be gas coming through poor cement.

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Terry Greaney
Consultant

Brisbane

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Hi Laipat,

If you have access to ISO standards, full MAASP calculations for the A, B and C annuli  can be found in Annex K of ISO/TS 16530-2 Well Integrity - Part 2: Well Integrity for the Operational Phase.

Regards,

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Mark Bourne
Well Integrity Specialist
NOPSEMA
PerthWA
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Dear lajpat,

I think Terry has already explain clearly.

The consideration of MAWOP is based on your formation fract pressure or burst pressure of casing.

Then Surface Annulus pressure to be limit below the limit after consider hydrostatic pressure of annulus (cement and fluid)

Ideally Annulus pressure is zero, but some times it has due to

- Thermal effect

- Sustain Casing Pressure

- Injected to prevent Vacuum

From your case, just check the hand over data from DRL and evolution pressure during this well start up till now.

And the most important think is your well parameter (WHFP, WHFT, Qg, WGR, CGR).

If your well is HP well, is might be small communication at tubing or leak at the packer (if multi zone), but thermal effect also may occur if your WHFT is high and a lot of liquid is still on annulus.

In my experience, i bleed off into 20% then observe in 24 hour. Do leak rate measurement. If less than 15 scfm, the barrier still consider work properly.

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Ray Hutahaean
Well Performance Engineer
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Ray,

Just a small doubt, the acceptable leak off rate that you have mentioned is 15scfm (what I understand from this is 15 scf/min), which I feel is too high for a leak off rate, or have i interpreted it wrong?

Also, can you help me with any such guideline covering acceptable industry wide leak off ranges during Pressure Tests of:

1. Cement Plugs

2. Sand Plugs

3. Well Intervention surface rig up like Coiled Tubing, Frac set up etc.

As mostly I find opinion on them varying from person to person. Are there any API/Industry recommended guidelines for these?

Thanks,
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Sharad Goenka
Petroleum Engineer
Cairn Energy India Pty. Ltd.
Gurgaon
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Sharad,

API 6A recommends the leak rate for valves, gate & plug, chokes, actuators and hangers, THA.

Regards,

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Lajpat Rai
Petroleum Engineer
United Energy Pakistan Ltd
Karachi City
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