Steam Assisted Gravity Drainage (SAGD) development of thernAthabasca oil sands has led to a unique approach to horizontalrndrilling combining many existing and new technologies. Fromrna cementing perspective, preventing degradation of the cementrnsheath due to induced stress cracking and preventing gasrnmigration are the two main challenges that must be addressedrnin order to achieve the objective of long-term zonal isolationrnacross the length of the wellbore.rnOne of the main functions of cement is to prevent fluidrncommunication between formations or to surface throughoutrnthe life of the well, including after abandonment. Even whenrncementing operations have provided a good initial hydraulicrnseal, changes in downhole conditions can induce stresses torndestroy the integrity of the cement sheath resulting in a loss ofrnzonal isolation. Changes in downhole stress conditions thatrnoccur during the life of a SAGD well are particularly extreme.rnStresses in the cement sheath induced by extreme temperaturerncycling can result in severe mechanical damage and ultimaternfailure of the cement sheath, potentially leading to leakage ofrnsteam from the reservoir. Along with the requirement ofrnmaintaining cement sheath integrity, special cement designrnand placement considerations must be taken to prevent annularrngas influx and migration. Annular gas migration prior to thernsetting of the cement can lead to channels that would not onlyrnprovide a pathway for gas but also a potential conduit forrnsteam to escape into shallower zones or to surface.rnA new approach to cement design was undertaken to fulfill thernrequirements of cementing PanCanadian’s (now EnCanarnCorp.) SAGD wells in Christina Lake. A cement system withrnimproved flexibility to resist stress cracking was designed andrnimplemented. This system is engineered to have a lowrnYoung’s Modulus while maintaining high compressive andrntensile strengths as compared to conventional cement systems.rnThe permeability of the set cement is very low, yielding arnmaterial that is highly resistant to attack by corrosive fluids.rnFinally, the system is designed to be gas tight to prevent gasrnmigration. This paper presents data on the new cementingrntechniques and technologies used as well as case historyrnresults on the six wells cemented at Christina Lake.
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