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.
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. Introduction Sustained casing pressure (SCP) is recognized as a major issue in many parts of the world with significant numbers of wells reported as having SCP.[1]In many cases, depending on the severity of the problem, the SCP can be monitored and controlled during the producing life of the well without major intervention.However, any SCP issues must be resolved prior to abandonment as regular monitoring and control will not be possible once the well is abandoned. This paper discusses the procedure used to determine a solution to abandon a well with SCP after initial remediation attempts were only partially successful.Meeting standards for long-term abandonment was a critical requirement because the well was in proximity to habitations.The process used will be discussed in detail in the case history section, but the general concept followed those that have been discussed earlier.[2]However, the main issue in this case was to validate the performance of an optimum plugging material for the wells to ensure long-term isolation of all the subterranean zones. Generally, conventional cement systems have been used in the majority of P&A applications.However, it is known that these systems show internal shrinkage during the hydration process that can lead to bulk shrinkage, if there is no access to an external water source,[3] as is the case when cement is placed inside tubing or next to an impermeable formation.Bosma et al. [4,5] analysed the performance of several potential sealant systems and have shown that most systems shrink to some extent during setting.The authors recommended that appropriate sealant systems for P&A applications should have a low Young's modulus, a Poisson's ratio close to 0.5, and be placed so that they will become thermally stressed, ensuring a tight seal.
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