A common operation performed in oilfields is that of plug and abandonment (P&A) whereby a well is permanently closed at the end of its economic life. The regulatory requirements for this operation may vary between countries, however, it is imperative to establish that there is no potential for uncontrolled fluid flow to surface and freshwater aquifers are hydraulically isolated in the wellbore. This entails an accurate assessment of casing and cement integrity including an unambiguous determination of the top of cement (TOC). Towards the end of the plugging operation, a cut is placed above TOC, and the casing is retrieved from the well.
There are several challenges associated with cement evaluation in old wells. The well may have gas or gasified fluid in the wellbore, depleted formations making it difficult or impossible to fill the wellbore with liquid prior to cement evaluation logging, casing damage or open perforations may preclude application of pressure to differentiate free pipe from micro-annulus conditions and records on cement and operations history may be sparse. Drilling muds are often weighted using barite or other weighting materials. With an incomplete removal of mud, during well completion, these heavy materials sag, settle and solidify, behind casing, over the life of the well. An often-encountered consequence of solidified mud is to shift casing free-point to a depth well above TOC. Traditional cement evaluation tools find it difficult to identify this condition leading to unsuccessful casing cut and retrieval attempts, and associated time and cost overruns.
A new cement logging technique uses electro-magnetic transducers and multiple wave modes to overcome operational and evaluation challenges met in P&A operations. We will present a case study, from Southeast Asia, where the operator was concerned about risks associated with solids sag, above cement, in a 60 years old well. The new service provided accurate cement evaluation across depleted reservoir sections and over intervals with micro-annulus condition. Top of cement was clearly differentiated from the top of solid mud behind casing. A free-point depth was selected and casing was cut and pulled in first attempt. Operator estimated an overall saving of two days in rig time.