The annular cement is often subject to temperature variations, for example heating during steam injection, or cooling during CO2 injection or hydraulic fracturing. Fractures caused by thermal stresses may create flow paths through the cement sheath thereby jeopardizing the well integrity. The objective of this work was to experimentally and numerically investigate the effect of thermal cycles and harsh cooling on downscaled wellbore sections with a focus on the cement sheath failure.
Downscaled wellbore sections were each prepared by cementing a casing section within a hollow cylinder of Berea sandstone. One specimen was subjected to a thermal cycling program that included cooling to −50 °C and heating to 80 °C. Two other specimens (dry and wet) were used in harsh cooling experiments with solid CO2 and liquid nitrogen. CT scanning was performed before the onset of, during and after the completion of the thermal cycling program or harsh cooling experiments.
No differences in the distribution of voids and fractures in the cement before and after the thermal cycling were observed. This indicates that the applied temperature range was not sufficient to cause damage in the cement sheath. On the other hand, the harsh cooling experiment using liquid nitrogen resulted in fracturing of both the cement and rock in the water-saturated specimen.
Finite-element simulations reproducing the thermal cycling program used in the laboratory experiment were performed. The simulations have shown that thermal radial stresses at cement/casing and cement/sandstone interfaces were relatively low, i.e., from 0.5 to 1.0 MPa, which might explain the lack of thermal-induced damage in CT images. The simulations also suggest that the radial fracture in the water-saturated specimen subject to harsh cooling was caused by a combined effect of tensile hoop stresses and stresses due to water freezing. The fracture was most likely initiated in the sandstone followed by further propagation into the cement. It is likely that well construction materials and the surrounding formation are water-saturated. Thus, cooling well below 0 °C, which may occur during a CO2 blowout, would increase chances for cement and formation failure.