Measurement of mixed mode interfacial strengths with cementitious materials

In the oil and gas industry, a successful zonal isolation, which is the primary goal of cementing, is achieved by a strong bond at the cement-rock and the cement-casing interfaces. We introduce a universal test in conjunction with an analytical solution to measure the mixed mode interfacial strength of cementitious materials at the casing-cement or rock-cement interfaces. To this end, the most practical method is employed, which is the direct measurement of adhesion at the interface. This experimental setup is consisted of a rotational disk composed of two identical halves. Each half includes a cylindrical hole at its center, which can be filled by cement slurry, a rock cylinder or a steel bar. Therefore, different interfaces i.e. cement-cement, cement-steel, and cement-rock can be examined. After curing, the disk is subjected to two diametral point loads using a compressive frame. By adjusting the angle between the load direction and the overlapping interface, we can examine different combinations of modes I and II. On the other hand, an analytical elasticity solution is developed to calculate the critical shear strength that would initiate sliding between the interfaces. Analytical results show a non-uniform distribution of shear tractions along the sliding interface. Having the peak load obtained from experiment, derived formulations are used to evaluate the bonding strength of cementitious materials corresponding to different loading modes of pure shear, the combination of tension/compression and shear. The value of the current analytical derivation can be appreciated by comparison with the typical approach that assumes a uniform traction distribution along the sliding interface. We find that assuming uniform distribution will overestimate the shear strength at the cement-casing interface. The analytical solution in conjunction with the experimental results can also be used to calibrate cement interfacial friction properties.