Abstract: Shale gas is an important unconventional source of natural gas and has become a research hot topic in recent years. However, current studies have been mainly focused on the extraction technology improvements and environmental monitoring while waterrock interaction during shale gas exploration has seldom been reported. In this work, simulated experiments were conducted on the CO₂fracturing fluidshale interaction at 90 ℃ temperature and 10 MPa pressure, using selected shale rock samples. The main goal was to investigate the changes in rock mineral and flowback fluid compositions following sequential injections of hydraulic fracturing fluid and supercritical CO₂. The results showed that the fracturing fluid could cause shale to erode, and the presence of supercritical CO₂ could further enhance dissolution of minerals, which resulted in increase of pore size and micropore formation, making more channels available for shale gas and oil to migrate. This migration enhancement, however, may pose environmental threats and should not be ignored. As our study showed, many elements, including large amounts of Ca, Mg, Si and small amounts of Fe and Mn, were dissolved from the shale. Furthermore, under acidic environments of high temperature and pressure, volatile organic compounds (VOCs) were easily produced by the reaction system made of supercritical CO₂, organic matters (in shale) and surfactants (in fracturing fluid). Moreover, VOCs remained in the stratum, and very likely leaked through the pores or faults and polluted the aquifer. Also, flowback fluid was more difficult to treat since various VOCs and elements were mixed with the high salinity solution. This study should help to understand the waterrock interactions as well as potential environmental risks during shale gas exploration.

Key words:  shale gas, fracturing fluid, supercritical CO₂, waterrock interaction, environmental risk