Quantifying the Impact of 2D and 3D Fractures on Permeability in Wellbore Cement after Uniaxial Compressive Loading

Abstract

Summary 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 microcomputed tomography (micro-CT) techniques were used 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, for prepeak samples, 90% of the 2D fractures have length and width smaller than 100 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. 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. CT-based 3D visualization and simulation both show that inclusion of a correctly engineered fiber additive can blunt the fracture propagation in cement samples. We conclude that the fractures in cement matrix created by the monotonic compressive stress (up to the limit of uniaxial compressive strength) are not likely to form continuous leakage pathways. This is because the 2D fractures in cement matrix as shown by SEM images are in limited dimensions, whereas the 3D fractures in cement matrix observed from CT-based 3D models have poor connectivity, generally indicating that leakage pathways of significant permeability would not form as a result of compressing the cement samples up to their uniaxial compressive strength limit. Inclusion of a fiber additive is expected to enhance cement integrity by limiting the fracture propagation.