ABSTRACT:
In wellbores, cement plays an important role in wellbore integrity. As wells age and are stressed during their life cycle, the cement sheath may deform, altering its permeability and, perhaps compromising its integrity. In this study, we use flow measurements (calculated permeability) to provide real-time insight into damage incurred during triaxial deformation of neat cement. Cracks may be induced during deformation and their linkage may be sensed in the flow measurements. Conversely, cracks and pores may be closed during deformation, arresting fluid flow.
We subjected room temperature specimens of neat Portland cement to confining pressures (0.7, 2.1, 13.8 MPa) and measured helium flow continuously during triaxial deformation. Axial displacement across a specimen was periodically halted to perhaps assure steady flow rate throughout the sample. We observed the apparent permeability to decrease from 0.8 to 0.7 to 0.2 µD with the imposed confining pressure increase. Each specimen, when subjected to differential stress, exhibited a slight decrease in apparent permeability, implying disconnects of flow paths. For the two lower confining pressures, apparent permeability began to increase just prior to macroscopic failure, suggesting microcrack linkage. For the 2.1 MPa confining pressure test, apparent permeability increased by a factor of three at macrofracture, and for the 0.7 MPa confining pressure test, apparent permeability increased by a factor of thirty at macrofracture. At 13.8 MPa confining pressure, apparent permeability only decreases during triaxial loading, implying that poroelastic compaction restricts flow pathways and connectivity of appropriately oriented cracks for axial flow decreases during deformation. Failure by macrofracture did not occur in this sample.
Optical and scanning electron microscopy of deformed specimens indicate that pores and microcracks interact in complex manners, similar microcrack densities are observed in both 0.7 and 13.8 MPa test specimens, and pores represent both microcrack origination and localization sites. Larger pores (entrapped air voids) are sheared, flattened, and sites of crack opening. Micron-scale capillary porosity, determined using SEM image processing, is similar for all specimens.
The results from these few experiments indicate that microfracturing of cement during triaxial deformation results in permeability increases at low confining pressures. At the greater pressure, although microfracturing is observed, compaction and lack of microfracture interconnectivity have a greater effect on flow pathways, resulting in a permeability decrease during deformation.
