The Investigation of Fracture Toughness Evaluation Methods of Oil Well Cements

Zonal isolation is critical to ensure safety of wellbore. Cements mechanical property tests are necessary to evaluate the zonal isolation. In the past, the mechanical properties of intact specimens had been used to predict the failure of cement sheath. However, low displacement efficiency, gas channeling, perforating and fracturing may lead to uneven surfaces or cracks of cement sheath after setting. In order to evaluate the long-term integrity of cement sheath with cracks, the fracture toughness evaluation methods of cements are introduced in this study. Four kinds of methods are adopted to evaluate the fracture toughness of cements, including CSTBD (cracked straight through Brazilian disc), CCNBD (cracked chevron notched Brazilian disc), SCBD (semicircular bending disc), and SENB (Single edge notched bending). The experiments are divided into size effect experiment and repeatability experiment to determine the influence of different specimen shapes on the results. The findings demonstrate a positive correlation between the specimen size and their fracture toughness, indicating an increase in fracture toughness as the size of cements increases. Among the four methods, CSTBD has the lowest size effect and the best repeatability. Thus the most fitting evaluation specimen for measuring the fracture toughness of the oil well cements is the CSTBD. Four types of cement formulations were examined to assess the suitability of fracture toughness assessment techniques, including basic cement slurry, elastic cement slurry, fiber cement slurry and low density cement slurry. The result shows that the fiber can increase the fracture toughness. The method recommended in this study could be used to evaluate the fracture toughness of cements and provide basic parameters for numerical simulation.