The cement/casing and cement/rock interface of a cemented annulus is a brittle material-composite. During well operations the bond is subjected to static and dynamic stress loads. The risk of bond failure depends on load frequency, its magnitude, and intrinsic properties of the bond, cement and casing. This work proposes to apply a certain low number of stress cycles on a cement/casing and cement/rock annular composite to evaluate the low cycle fatigue strength of the bond. The stress concept and the bonding mechanism are discussed in depth and innovative laboratory fatigue testing concepts are introduced.
In various applications like HPHT, deep-water, (steam) injection or geothermal wells the cement/casing bond is exposed to cyclic thermal loads and pressure variations. This could result in casing elongation/contraction/expansion and subsequently in cyclic radial and axial stresses at the cement/casing and cement/rock interface. This is due to different properties of the composite e.g. coefficients of thermal expansion, Young’s Moduli etc. An innovative testing apparatus is used to apply static and dynamic axial loads (compression/tension) on an annular cement/casing and cement/rock bond. The design of cell and the experiments clearly investigates when the bond could fail and when the cement matrix could fail.
In practice, the frequency of loads applied during the life of a well determines the failure of bond and the cement matrix. In order to evaluate the fatigue limits of the cement/casing bond, new test procedures are developed and an atmospheric test cell is designed and constructed. The cell consists of a piece of casing, a cemented annulus and an outer cell body. De-bonding is achieved by applying a cyclic tension and compression forces on the casing while keeping the cement and the outer cell body static. The advantage of this procedure is that the differential change in force reveal the exact cycle when the bonding failed. The test results are evaluated. The location of de-bonding is presented and discussed in detail. An outlook is given for to investigate the effect of casing surface roughness, surfactant contact time, casing coatings or special elastic cement properties on the life-time integrity or the cement/casing bond under cyclic stress loads in the next step.
Never before has the concept of classical laboratory fatigue strength testing been applied to evaluate the quality of a cement/casing/rock annular composite. A novel testing apparatus has been developed to investigate different effects during annular de-bonding.