Shale damage simulation considering shale swelling during shale-liquid interaction

Abstract

Shale-liquid interactions can lead to shale swelling, shale damage and affect the mechanical and other properties of shale. The relationship between shale swelling and shale damage was not well understood. A better understanding of the relationship is essential for drilling and fracturing designs. In this work, we employ shale swelling tests, computed tomography scanning technology and numerical simulation to understand the relationship between shale swelling and shale damage. The experimental results show that the shale swelling is 0.23% after the experiment. When the shale swelling is small, the damage is weak. However, the shale damage growth rate is 495.05% when the shale swelling rate increases by 0.19%, and macrofractures are formed. Using COMSOL with MATLAB, we further explore the relationship between shale damage and shale swelling. The simulation results show that shale swelling has an important effect on shale damage, and this effect is verified by comparing the simulation and experimental results. Shale damage is weak or even absent when shale swelling is not considered, while the distinct shale damage can be observed when shale swelling is considered. The increase of shale swelling leads to the increase in shale damage, which leads to more obvious fractures expansion, resulting in an increase in permeability. When shale swelling is 0.4%, the permeability at the end of the simulation is 322% higher than the initial permeability. The variation in stress at 0.4% shale swelling is 21 times that without considering shale swelling, and the large variations in stress are an important factor in shale damage. The higher the in-situ stress, the lesser is the tendency for shale damage to occur. Shale damage occurs at high injection pressures, even when shale swelling is low. In addition, the more complex the fracture network, the higher is the tendency for shale damage to occur, and the permeability after the simulation was 6% higher than that of bedding model. Therefore, fracturing methods with high injection pressures should be used, and the hydraulic fracturing should be conducted at locations where natural fractures develop to achieve significant shale-liquid interactions and better hydraulic fracturing results.