Challenging the 30 m Annulus Cement Requirement

Abstract The permanent plugging and abandonment of a well requires the establishment of at least two barriers according to the NORSOK D-010 standard. In common practice, a barrier is either a cement plug from formation to formation, which is all cement, or a combination of existing cement behind the annulus, the casing steel itself and a cement plug inside the casing, see Figure 1. The latter case is a cheaper and preferred solution as milling can be avoided and thus reducing cost, environmental footprint and a number of potential operational hazards. The cement between the casing and the formation is called the cement sheath and has typically been in place since the well was constructed. The evaluation of the status of this barrier element after years of production – called verification in many standards – is conducted using different types of logs. Leakage through the cement sheath can happen in three ways; along the interface between the cement sheath and the casing, leakage through the bulk cement itself as the cement has a certain permeability, leakage in cracks or channels present in the cement sheath and finally leakage between the cement sheath and the formation. Significant leakage through the cement sheath prior to permanent P&A typically causes an increased annulus pressure, which is monitored and dealt with if it happens prior to permanent P&A. In order to qualify the cement sheath when using the NORSOK D-010, 30 m of good bonding is required, i.e. the interface between casing surface and the cement is good in terms of that it most likely will not represent a leakage path along the interface, so called micro annuli. Logs are best at detecting interface phenomena and good logs means that we have strong indications that there are no micro annuli, but logs may also indicate weaknesses in the bulk cement between the casing and the formation.