An oil well requires a cement sheath between the casing and the geological formations to isolate different zones of the formations and prevent the migration of hydrocarbons or water from one layer to another. This is the so-called zonal isolation. Since wellbores are being drilled to depths of 30,000 ft or more, oil well cements are subject to wide ranges of pressure and temperature. It’s quite a challenge to study the behavior of cement sheaths under such conditions. No experimental data are currently available with regard to mechanical properties of oil well cements hydrated under in situ conditions, i.e. high pressure and high temperature.

We have been collaborating with Halliburton Energy Services in the past few years to develop a unique set of pressure test cells that permit the hydration of cement under in situ conditions and allow determining various mechanical properties without exposing the specimen to ambient conditions. The most recently developed cell (Fig. 1) can be used to measure chemical shrinkage, compressive strength, and tensile strength of cement hydrated under high pressure. The pressure is applied by syringe pumps with water as medium. Chemical shrinkage is measured by recording the total amount of water entering the cell during hydration. Compressive strength is measured by increasing the axial pressure on the sample inside the cell. The compression test within the cell has the advantage over traditional test methods in that it eliminates the sample end effect problem (radial confinement exerted by loading platens).

Conducting a direct tensile test inside the cell is extremely difficult. As a matter of fact, it is even difficult to determine the direct tensile strength of cementitious materials under ambient conditions due to the eccentricity of the applied load. There exists no ASTM standard test method to determine the strength of concrete in direct tension. Indirect methods for testing tensile strength, such as the Brazilian splitting tensile test, are normally used for concrete under ambient conditions. We are currently studying the feasibility of another indirect tensile test method initially conceived by Bridgeman around 100 years ago and further validated by other researchers such as Clayton and Mindess later on. The concept is based on the fact that a cylindrical sample will have a tensile type of failure when pressure applied to its curved surface is increased by the amount approximately equal to its direct tensile strength. This allows the determination of tensile strength without releasing the hydration pressure. However, neither the splitting tensile strength nor Bridgeman tensile strength is the same as the direct tensile strength. Our goal is to first correlate the direct tensile strength with the two indirect tensile strengths under ambient conditions and then to extrapolate the correlations to specimens hydrated under in situ conditions.