Failure modes in three-point bending tests of cement-steel, cement-cement and cement-sandstone bi-material beams

Abstract

Tensile strength of cement-steel and cement-rock interfaces is an important input parameter when predicting well integrity failure in petroleum industry as well as during underground CO2 storage. Laboratory tests of interface strength (e.g. the so-called pushout test) often provide estimates of shear rather than tensile strength. In this work, three-point bending test of bi-material beams was used to study tensile failure at cement-steel, cement-cement, and cement-sandstone interfaces. The tests revealed that cement-steel interfaces were the weakest ones, while cement-cement interfaces were the second weakest. Cement-sandstone interfaces were apparently quite strong: both tested cement-sandstone beams broke inside the cement, ca. 2–3cm off the interface. This surprising result, i.e. the interface being stronger than the hardened cement, was attributed to water suction from cement into the dry sandstone during setting, which was corroborated by the observed very uneven fracture surface. All bi-material beams had lower flexural strength than monolith cement beams.