Well integrity is a key requirement for continuing heavy oil development and production. Cement sheath integrity is crucial to maintaining zonal isolation, which prevents unwanted fluid migration, especially oil, gas and steam, outside of the wellbore. Current commercial production techniques in heavy oil use steam to provide heat to mobilize oil and allow production. These production techniques apply significant stress to the cement sheath which could lead to cement sheath failure. A computer simulation assessing the stress in the cement sheath is presented. Flexible cement systems can be designed so that the cement sheath can withstand the expected stresses during the life of the well.
Preventing cement breakdown can be achieved by improving the installation practices and by assessing and reducing the stresses imposed on the cement during the life of the well.
A case history detailing the installation of a flexible cement system in a SAGD production well is presented. Once a flexible cement system has been selected, careful planning and process control is required to deliver the system with the desired properties. Best practices must be used for placement of the system in the annulus. Industry best practices, especially the use of centralizers, viscous fluid spacers and pipe movement were employed. Excellent zonal isolation is achieved with the flexible cement system.
Several challenges also present themselves for the installation and also the evaluation of the cement sheath. Centralization and hole size play an important role in the placement of the cement system in the annulus. Assessing the well zonal isolation integrity is the next step once the cement is in place. Operating parameters for a particular well can be determined using numerical modelling to simulate the stress that the cement sheath will be exposed to.
The Athabasca oil sands of north eastern Alberta contain significant oil resources. The oilsands deposits which the Alberta Department of Energy (2007) indicates covers 140 200 square kilometers and contains 1.7 trillion barrels of bitumen in place is mapped in Fig. 1. The bitumen near the surface can be recovered using strip mining techniques. The Department of Energy also reports that it is estimated that 82% of the total bitumen ultimately recoverable will be with in-situ techniques. One method of exploiting the remaining resources is by means of steam assisted production, specifically steam assisted gravity drainage (SAGD). SAGD involves drilling two shallow wells five metres apart vertically and extending both wells horizontally through the reservoir, with one above the other. This well configuration is shown in Fig. 2. The top well is used for steam injection to help lower the viscosity of the bitumen and allow production from the lower well. The injection of steam provides specific challenges to the design of cement systems.