In the world of oilfield cementing, verification that the plug is in place and withstands the pressure is an important part of plugging operations. The consequences can be substantial under certain circumstances if the set plug does not hold as expected.
The famous words “I am confident it will work or has worked” can sometimes come back to haunt us. Planning and execution of an operation are not enough – it is also important to verify that execution and that the planned objectives have been achieved.
Following the standards
Typical government regulations require a leak test by application of a differential pressure to prove that the plug is indeed holding.
NORSOK Standard D-010 states that the pressure should be applied in the direction of the flow. However, if this is impractical, the pressure can be applied against the flow direction. It further states low-pressure leak test (1,5 MPa to 2 MPa for 5 min) should be performed before high-pressure leak testing. High-pressure testing should last for 10 min.
In the event, leak testing is not possible. Verification through assessment of job planning and actual job performance parameters are available options. These would include verification of the slurry sample under the pressure and temperature conditions of the well.
It is also noted. “For practical purposes acceptance criteria should be established to allow for volume, temperature effects, air entrapment and media compressibility. For situations where the leak-rate cannot be monitored or measured, the criteria for maximum allowable pressure leak (stable reading) shall be established.”
For open hole type plugs, tagging is essential. It's recommended to pressure-test plugs at 1000 psi above estimated formation strength. The top of the plug should be located/identified with wireline. A weight test can also be carried out to ensure depth and integrity of the plug.
Read more: Cement plugging: A nightmare waiting to happen?
other sound habits
There is no doubt that verification of barriers is necessary to make sure that plugs are holding as designed. For a good plug, bonding is a major factor. Therefore, hole preparation and placement are the first factors in achieving a successful verification.
Other sound habits include conducting use of an injectivity test to ensure the material can be placed as designed. Accurate placement and excellent bonding are the twin factors of plugging success.
Choice of material can also be a vital factor. Plugs for particular types of reservoirs can be improved by pumping a resin ahead of the cement plugs to ensure better sealing and reduce the chance for micro-annulus.
Read more: Dealing with micro annuli in casing cement
Challenges of interpretation
Common ways of conducting verification are tagging, pressure testing and long-term negative pressure testing. Under certain circumstances logging is also used. Interpretation of logging data can sometimes be more of an art than a science. Often due to the challenges of interpretation – which sometimes not very straightforward. Practical understanding is the key.
While there are several methods for barrier verification, pressure testing is the most effective. Negative and positive tests are in order. Industry literature points out that tests from the surface are best applied to reservoir/perforated zone area. Leaks in the secondary plugs may reflect a casing leak rather than plug failure.
where do we go from here?
Follow these simple steps to test your plug:
- Tag the plug (weight test where applicable)
- Inflow test
- Pressure test
- Static bubble observation – checking for gas migration
- Logging – for annular plugs
In the future, there will be technologies available that will make it easier to verify the integrity of plugs. Regardless of verification methods, the key to success is effective planning, proper material selection and accurate placement in the wellbore.
It's difficult and costly to remediate a leaking barrier.
Read more: Plugging in depleted reservoirs
