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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.

Cemented completion provides an excellent opportunity for low cost drilling techniques and simple well designs to reduce well delivery costs without impact on safety or well integrity. In this concept, the completion is run straight into the open hole and cemented in place. There is no need to run a liner and perform a traditional clean-up. The cement effectively replaces the production packer. Achieving a good cement job is essential to a successful operation. A failed cement job would require cutting and pulling of the completion before running a conventional completion and packer, unless there was a high confidence in the outcome of a remedial cement job. Therefore good conditions for cementing are a definite requirement. Gas production and gas storage wells are particularly demanding on the cement sheaths. A failure of the sheath through de-bonding, cracking or plastic deformation results in formation fluids to enter the annulus, which pressurises the well and makes it unsafe to operate, the consequences are either losing the well altogether or, at best, an expensive intervention. Advanced cement technology combined with state of the art software are used today to ensure a complete long-term zonal isolation of the cemented completion. The software can model the stresses in a particular well over its entire life, including changes to the formation properties, well completion parameters, formation in-situ stresses, and operating envelope and can compare the properties of different available cements to enable the optimum cement to be selected (hole size, cement sheath thickness). The following paper will describe the process and the engineered cement used to ensure integrity of the cemented completion and lists case histories of the 3 cemented completions successfully cemented Onshore Netherlands. Introduction The cemented completion concept is a fundamentally simple one whereby the reservoir open hole section is drilled to the required depth. Following electric logging (if required), the completion tubing is run into the open hole and cemented back inside the previous casing shoe or window. The Christmas tree is then installed and the well perforated in the normal manner. Cemented completions have already been used in low cost operating areas around the world. Well CA-28S2 drilled from the Cormorant Alpha platform in June 1999 marked the first cemented completion to be performed in the North Sea by Shell. CA-28S2 was selected as the ideal candidate for evaluating the method due to it's short 6" open hole length of 708 ft [215.79m] and near vertical inclination. Since then open hole lengths have been progressively increased, culminating in CN-28S1 which was successfully completed with 6500ft of 8 1/2" open hole at 67 degrees. In 2003, 79 cemented completions were installed by Shell globally yielding a considerable saving. In the Netherlands, the land asset have after an initial trial with 5" completion 2003, adopted 3 ½" cemented completions as a base case to the slim well low cost land drilling campaign that commenced in 2005 Fig.1. By end 2006 this has been applied successfully to eight gas wells. Cemented Completion Benefits The advantage of cemented over conventional completions is commercial in that it saves 5 to 6 days rig time during installation by eliminating such operations as running and cementing liner, wellbore clean up and packer setting procedures. In addition, the amount of liner and completion accessories required such as packers, liner hangers, Polished Bore Receptacles (PBR), nipples are greatly reduced. Savings depend on the completion and well design.

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