This paper presents case histories and discusses the laboratory validation of a sealant system for plug-and-abandon (P&A) operations in critical gas wells close to habitations. To ensure long-term isolation both inside and outside the original casing, the plan was to mill out the casing and cement and to place the plug from rock face to rock face. The milled-out section was planned so that in the worst-case scenario, the differential gas pressure would have to be supported by 25 m of cement plug, even though a greater height of cement would be placed inside the original casing above the milled-out section.
Two cement systems were designated as candidates for the plugging operation with slurry properties suitable for the application. The gas-sealing ability of the selected systems was validated with both small-scale and large-scale tests. One system, exhibiting both high flexibility and expansion, met the stringent specifications and did not leak until a differential pressure of more than four times the specification was applied. The paper will discuss the parameters of the selected system, thus providing guidelines for system selection for critical plugging operations. A detailed case history of one well (two plugs) is given along with the results from four other wells.