a b s t r a c tField observations demonstrate that shale gas wells feature a low flowback efficiency (<30%) and highsalinity flowback water (approximately 200kppm) after multistage hydraulic fracturing operations. The water recovery and salinity profile could be regarded as a critical method for volumetric and chemical analyses to characterize the reservoir properties and complexity of the fractured network. This paper aims to understand the relationship between fracturing imbibition and ion diffusion, which are responsible for inefficient water recovery and high-salinity flowback fluid, respectively. Comparative imbibition experiments are performed on different shale and sandstone samples, and an electrical conductivity meter is used to monitor the change in ion concentration change of the imbibition fluid. A mathematical model based on theoretical analysis is proposed to clarify the correlation between imbibition and ion diffusion. Both the experimental and analytical solution results show that the imbibition fluid conductivity resulting from ion diffusion is proportional to the square root of time, which is similar to the law of capillary-driven imbibition into porous media. Water imbibition into gas shale and ion diffusion into water proceed simultaneously in the opposite direction, and only the imbibition front contacting the pore wall with salt ions can cause the salt ions to dissolve and diffuse into water. The analytical solution results also indicate that the effects of the porosity, surface tension, contacting area and wetting angle on the water imbibition rate are in consistent with that of the ion diffusion rate. The permeability, however, shows a positive correlation with the imbibition rate and a negative correlation with the ion diffusion rate. The initial water saturation is negatively related to the imbibition rate, and positively related to the ion diffusion rate. In addition, smectite and I/S could enhance the imbibition and diffusion rates. It is observed that illite concentration has no relationship with the imbibition and diffusion rates, indicating that illite minerals do not significantly affect the imbibition/diffusion rate in these clay-rich shales. This research contributes to understanding the correlation between imbibition and ion diffusion, which is significant for flow-back analysis after fracturing operations.

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a b s t r a c tWellbore instability is a widespread problem when drilling in shale formations, particularly with waterbased drilling fluid. The main reason for the occurrence of wellbore instability is that stress concentration and pore pressure redistribution occur around the wellbore once a hole is drilled; current studies show that the drilling fluid temperature and solute mass fraction play important roles during the process. In this work, a non-linear fully coupled chemo-thermo-poroelastic finite element model is developed to quantitatively access thermal and chemical effects on time-dependent pore pressure and effective stresses; in addition, material constant sensitivity analysis of an inclined well drilled in a transversely isotropic formation is presented. The results reveal the following: fluid transfer is greatly affected by thermal and chemical osmosis, the lower temperature and higher solute mass fraction of the drilling fluid contribute to decreasing the pore pressure and are beneficial for wellbore stability, and thermal parameters (such as thermal osmosis coefficient and thermal diffusivity) and chemical parameters (such as reflection coefficient and solute diffusion coefficient) have high effects on the pore pressure and effective stresses. Anisotropy ratio analysis of the material constants indicates that the pore pressure and effective stresses are very sensitive to Young's modulus and the permeability ratio, but are not sensitive to Poisson's ratio. Therefore, the developed coupled chemo-thermo-poroelastic theory illustrates that optimization of the reduction of the drilling fluid temperature while maintaining a high solute mass fraction could enhance wellbore stability.

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