Mechanical performance of cement significantly affects cement sheath integrity and long-term zonal isolation, especially in complex high-temperature, high-pressure (HTHP) wells drilled under challenging operational conditions. A majority of cement sheath mechanical failures are believed to occur in tension rather than compression, which has led to increased focus on cement tensile strength. However, most technical literature related to cement tensile strength refers to testing at ambient temperature and pressure, which does not represent actual downhole conditions. In this study, an HTHP tensile strength testing device is used to measure cement tensile strength in situ at simulated downhole conditions, and these data are compared with measurements gathered at ambient conditions.
In this work, class H cement was mixed with mechanical enhancers, an anti-settling agent, and an anti-foaming agent at 16 to 16.4 ppg. Several mechanical enhancers, including polymeric and fiber additives, were investigated. The cement samples were cured inside the HTHP tensiometer at downhole conditions, and the tensile strength was measured in situ at the same curing conditions.
The in-situ tensile strength results were compared with results from the traditionally used uniaxial method at ambient conditions. The difference between the tensile strength tested in situ and at ambient conditions varied depending on the type of mechanical enhancer. For example, a significant increase in in-situ tensile strength was observed for cement with polymeric additives.
This study demonstrates that testing environment significantly affects the measured mechanical properties of cement, and conventional ambient measurements may not accurately reflect downhole performance. It draws attention to industrywide concerns about developing cement systems with mechanical properties optimized for HTHP and other challenging downhole environments.