Effect of Rock Physical and Mechanical Properties on Adhesion Strength at the Rock-Cement Paste Interface

This study investigates the influence of rock physical and mechanical properties on the integrity of the cement sheath barrier in oil and gas wells, with a focus on the rock-cement paste interface under geomechanical loading. Key rock characteristics, including structure, porosity, permeability, compressive strength, and tensile strength, were evaluated to determine their impact on adhesion strength and failure modes. Diametral compression tests were performed on disk-shaped specimens with a 2:1 diameter-to-thickness ratio, composed of semi-disks of rock and cement paste. The experimental program included Botucatu sandstone and Roman travertine, combined with both standard API and stabilized cement paste formulations. Variables such as cement curing time, sandstone stratification, and curing temperature were examined, while micro-CT, thin-section microscopy, and scanning electron microscopy (SEM) provided qualitative analysis of the interfaces. Results revealed contrasting behaviors between the two rock types: porosity did not influence adhesion strength in Roman travertine, whereas in Botucatu sandstone, high porosity and microroughness significantly enhanced interface bonding, often exceeding the cement paste strength. Curing time played a critical role at early ages, particularly at three days, but its influence diminished between seven and twenty-eight days. Curing temperature showed no effect on low-porosity sandstone interfaces, though higher temperatures (70 °C) improved the intrinsic strength of the cement paste. This work provides novel insights into the role of rock crystalline structure and porosity in CSB performance, contributing to a more comprehensive understanding of rock-cement interactions essential for ensuring long-term well integrity.