After the multistage hydraulic fracturing operations, it is generally observed that the shale gas well is characterized by low flowback rate and high-salinity flowback water. It can be explained by the high water imbibition and ion diffusion capacity of gas shale that is significantly different from conventional sandstone reservoirs. The research on imbibition and ion diffusion is significant for the understanding of shale formation characterization. A new method to evaluate the mean pore size was developed using the experimental data of water imbibition and ion diffusion in gas-saturated shale. The mathematical model was derived theoretically depending on Darcy's law and continuum equation. The results show that the imbibition and diffusion rate can be calculated by imbibition-diffusion curves, and the ratio of imbibition rate to diffusion rate is related to mean pore size. The value of mean pore size is inferred, which is verified by the mercury injection test. This research is significant for volumetric analysis and chemical analysis after fracturing operations to understand the characteristics of gas shale.
Shale gas, one of the unconventional natural gas resources, becomes the focus of the world. The economical explorations have been taken in several basins of America, Canada and China. As shale gas formation has the characteristics of low-porosity and low-permeability, the multistage fracturing technology must be taken to realize economical exploration (Vera and Ehlig-Economides, 2014).
The field studies of network fracturing illustrate that the flowback efficiency of fracturing fluid is generally low. The flowback efficiency of shale gas wells in America is 20~40%, and that of several shale gas wells in Fuling of China is evenly lower than 5~10% (Zhong, 2011). Many researches explain that this is caused by spontaneous imbibition. High capillary pressure due to micro-nano pores and ultra-low initial water saturation ("super dry") leads to the strong imbibition capacity of shale formation that substantially exceeds conventional sandstone formation. (Lal, 1999). The clay mineral content of shale is high, which will result in strong chemical effects. When shale contacts with water, the water is imbibed into shale under the chemical effects. So, the imbibition mechanism in shale is more complicated than that in conventional sandstone. In particular, the existence of smectitie and illite can greatly enhance the water imbibition rate (Yang et al., 2016).