Shell has experienced production casing deformation on several infill wells in an existing thermal development project in Canada. A full investigation identified trapped annular pressure (TAP) caused by unreacted water fraction in set cement between casing strings as a potential casing failure mode. A novel and innovative application of an existing technology (a cement blend incorporating hollow-glass microspheres) was developed, tested and implemented as an effective means to provide pressure relief in the cemented casing-casing annulus on steam injection wells.
Thermal cement blends from two suppliers were used in the testing program. The test slurries were cured in a test cylinder until compressive strength development tapered off, and then heated in increments to 320°C while pressure was recorded. Typical thermal blends containing a range of concentrations of hollow-glass microspheres were tested in this manner to establish a slurry design that would prevent the pressure build up from surpassing the temperature de-rated collapse resistance of the casing.
The tests using typical thermal cement blends with no glass bead additive resulted in rapid increase in pressure as the test cell was heated, and exceeded the temperature de-rated collapse value of the production casing string at temperatures much lower than typical steam injection temperatures. Tests performed on blends containing the glass spheres showed consistency and repeatability in results, and pressures were well below the de-rated collapse pressure of the production casing string at the maximum steam injection temperature.
Based on preliminary results, this investigation has concluded that the cement blend containing 10% BWOC hollow-glass microspheres is a viable alternative to conventional thermal blends, capable of providing hydraulic isolation to meet regulatory requirements and industry best practices, with the added benefit of providing a mechanism to mitigate TAP on steam injection wells.