Chemically induced compaction bands : 1 Triggering conditions and band thickness

7 8 Key points 9-Compaction band instabilities due to cataclasis and dissolution in rocks 10-Strong chemo-poro-mechanical coupling, grain breakage and chemical softening 11-Regions of instability, compaction band thickness and periodicity 12 13 Abstract 14 During compaction band formation various mechanisms can be involved at different scales. 15 Mechanical and chemical degradation of the solid skeleton and grain damage are important 16 factors that may trigger instabilities in the form of compaction bands. Here we explore the 17 conditions of compaction band formation in quartz-and carbonate-based geomaterials by 18 considering the effect of chemical dissolution and grain breakage. As the stresses/deformations 19 evolve, the grains of the material break leading to an increase of their specific surface. 20 Consequently, their dissolution is accelerated and chemical softening is triggered. By accounting 21 for (a) the mass diffusion of the system, (b) a macroscopic failure criterion with dissolution 22 softening and (c) the reaction kinetics at the micro level, a model is proposed and the conditions 23 for compaction instabilities are investigated. Distinguishing the micro-scale (grain level) from the 24 macro-level (Representative Elementary Volume) and considering the heterogeneous 25 microstructure of the REV it is possible to discuss the thickness and periodicity of compaction 26 bands. Two case studies are investigated. The first one concerns a sandstone rock reservoir which 27 is water flooded and the second one a carbonate rock in which CO 2 is injected for storage. It is 28 shown that compaction band instabilities are possible in both cases. (200 words) 29 30 31 32 Index terms 33 STRUCTURAL GEOLOGY: Mechanics, theory and modeling 34