Stress-induced fractures in wellbore cement can form high-risk pathways for methane or carbon dioxide leakage yet little to no quantitative information on the impact of these fractures has been reported. To investigate this, scanning electron microscopy (SEM) and micro computed tomography (micro-CT) techniques were utilized to quantify the 2D and 3D geometrical parameters of cement fractures in mature thermal thixotropic cement samples that were subjected to pre- and post-peak compressive stress. A novel simulation method was also proposed to quantify the impact of the stress-induced realistic 3D fractures on the cement permeability.
Results show that: i-) For pre-peak samples, 90% of the 2D fractures have length and width smaller than 100 μm and 5 μm, respectively. Although higher compressive stress reshaped the 3D fractures and increased the fracture length and width, no well-propagated fractures were observed; ii-) For post-peak samples, distinctly visible (> 0.1 mm) well-propagated fractures were generated but failed to penetrate the entire sample, therefore the effect of stress-induced fractures (up to 1.0% strain) on cement sample's permeability is limited; and iii-) CT-based 3D visualization and simulation both show that inclusion of a correctly engineered fiber additive is able to blunt the fracture propagation in cement samples.
We conclude that up to the uniaxial compressive strength, the monotonic compressive stress is not likely to create leakage pathways in wellbore cement since the 2D fractures observed in SEM images are in limited dimensions and the large 3D fractures characterized in CT images have poor connectivity. Inclusion of a fiber additive is expected to enhance cement integrity by limiting the fracture propagation.