Study on the constitutive model and brittleness variations of shale after imbibition in different fracturing fluids

Abstract

Water–rock reactions between shale and fracturing fluids can change the brittleness of shale, which affects the efficiency of hydraulic fracturing. In order to understand the effect of different fracturing fluids imbibition on shale brittleness, soaking experiments with different fracturing fluids (pH = 6, 7, 8) and different soaking times (15 days, 30 days, 90 days, 180 days) are conducted on shale samples. A new constitutive model combining a quadratic function and the Weibull distribution is applied to investigate the variations of shale brittleness. The results show that the model can well describe the stress–strain relationship of shale samples under uniaxial compression. The model parameter a is related to the initial elastic modulus E0 of shale samples. The significant decrease of a values indicates the decrease of initial elastic modulus after imbibition. The model parameter m is positively correlated with shale brittleness and can be used to compare shale brittleness preliminarily.Water–rock reactions and hydration expansion cause the deterioration of shale brittleness. According to the calculated values of the energy-based brittleness index (BI), the brittleness of shale shows decreasing–increasing–decreasing trends during the process of imbibition. The acidic fracturing fluid has the weakest effect on shale brittleness compared with the other two fracturing fluids used in this study.