Numerical study on micro-cracks and permeability changes linked to clay swelling after fracturing in shale rock

The shale rock, with relatively low permeability and porosity, is usually challenging to be developed economically. Hydraulic fracturing is widely used as a stimulation technique for shale gas exploitation. Unlike tight gas reservoirs, shale gas reservoirs appear to show different characteristics under hydraulic fracturing, with a low flow-back rate leading to high productivity. This might be linked to micro-cracks generated by the clay swelling when fluid penetrates the rock, as shown in previous fracturing experiments. However, detailed failure patterns and associated permeability changes under different conditions (e.g., in-situ stress, pore pressure, and clay content) remain ambiguous. In this study, numerical simulations were adopted to address these issues. A solid model with cohesive elements based on the Finite element method was used to predict the generation of micro-cracks. Further, a permeability evaluation model considering the micro-cracks was applied to estimate the permeability changes. Our simulation results indicate that clay swelling can produce shear cracks. The number of shear cracks is substantially influenced by the swelling stress and clay contents. Tensile cracks, together with a complex fracture network, can be observed when the pore pressure or stress difference increases. The permeability increases significantly when a complex fracture network is formed. This study reveals that micro-cracks can be induced by the interaction between fracturing fluid and shale rock, which may help explain how low flow-back rate results in high productivity in shale gas reservoirs.