Continuous or Stacked Cement Bond Logs – Does It Matter?

As part of the permanent plug & abandonment of a well, the annulus casing cement sheath may be considered a barrier element provided that its integrity can be verified. Operators use a combination of data to evaluate the integrity of the casing cement sheath; cement job parameters and displacement calculations, results from FITs and LOTs, bonding logs and sustained casing pressure being the most common ones. The cement length is a key quantity for qualifying annulus cement as a well barrier element. If using the standard NORSOK D-010 rev 5 as the decision basis and provided a correct positioning, a length of 30 m MD is required when using bonding logs as the means of verification. Since the annulus cement sheath usually is considerably longer that 30 m, 30 meters of good cement can be achieved either by presenting a bond log showing 30 m of continuous good cement or by presenting a bond log that in total has 30 m of good cement. Operators have different requirements and views when it comes to stacked versus continuous. To better understand the implications in terms of sealing ability of a continuous annulus cements sheath versus a stacked annulus cement sheath we have conducted studies of both quantitative and qualitative nature. For a quantitative comparison we have used Darcy’s law and capillary pressure, for a qualitative comparison we have looked at cracks, shear failure and the effect of zones with a poor bond log.