Compaction Banding in High-Porosity Carbonate Rocks: 1. Experimental Observations

Identifying the mechanisms that control the formation of compaction bands is of high interest in reservoir mechanics since these structures may drastically affect the performance of geosystems operations. Considering the difficulty to identify compaction bands in carbonate samples tested in the laboratory, the Digital Volume Correlation technique is applied here and proves to be a relevant method. X-Ray Computed Tomography (XRCT) images of Saint-Maximin limestone centimetric samples are recorded before and after several triaxial loading stages and deformation maps are built. A new postprocessing method based on the analysis of the kinematics throughout the observed localization bands is proposed to identify their type. Compaction bands are identified at relatively high-confining pressures, while shear bands are observed at lower confinements. The brittle-ductile transitional regime reveals the formation of compactive shear bands, while a diffuse compaction is observed under hydrostatic loading. The effect of porosity heterogeneities on strain localization is explored by computing 3D porosity maps from calibrated XRCT images. Compaction bands are found to dominantly lay inside high-porosity zones, while shear bands can cross both high- and low-porosity zones. The band orientation is found to be controlled primarily by the confining pressure. Moreover, the porosity heterogeneity strongly affects the volumetric behavior inside the deformation bands, with a dilatant behavior identified in low-porosity zones in contrast to a compactive behavior observed in high-porosity zones. Finally, Scanning Electron Microscopy observations reveal that calcite grain crushing is dominant in high-porosity zones, while intergranular cement cracking occurs in denser zones.