Experimental analysis of Wellbore cement-steel bond mechanics and characterization

Abstract

Portland cement is commonly used in everything from general construction to fluid barriers in environmental and petroleum structures. Wellbore integrity finite element models have shown that the bond strength between cement to steel is a weak point, but the failure mechanism is not well understood. Bond strength of cement to steel has been modeled using traction separation law which includes the bond strength, critical energy, and contact stiffness associated with the bond. However, experimental measurements only focus on the bond strength resulting in an incomplete analysis of the failure. A novel experimental methodology was developed to measure the tensile bond strength, critical energy, and contact stiffness of neat cement to steel bonds cured at atmospheric conditions for two days. Strain rates of 0.1 mm/min, 1.0 mm/min, and 10 mm/min were tested. The results showed that the 0.1 mm/min strain rate is too slow for bond testing since two of the bonding parameters experience relaxation during testing and result in low values. The 1.0 and 10 mm/min strain rates do not have statistical significance with respect to the bonding parameters. From the direct tensile tests of neat cement to steel, the bond strength is 0.61 MPa, the critical energy is 204.2 J/m2, and the contact stiffness is 1.00 E+09 N/m3.