Experimental Study on the Mechanical Properties of Shale After Long-term of Immersion in Fracturing Fluids with Different pH

Due to the need of breaking the formation during hydraulic fracturing, the fracturing fluid is usually weakly acidic or weakly basic, and the contact of the fracturing fluid with shale leads to significant alterations in its mechanical properties. However, the mechanical characteristics of shale using fracturing fluids of different pH still require further investigations. In this study, fracturing fluids with pH values of 6, 7 and 8 are considered to perform shale soaking experiments. The mechanical properties of shale after soaking for 15 days to 12 months, are tested using uniaxial compressive strength tests. The microscale variations of shale during imbibition are recorded by X-ray diffraction (XRD) tests and electron microscopy (SEM) tests, together with EDS analysis. The experimental results demonstrate that after 12 months of imbibition in acid, neutral and alkaline fluids, the strength and Young’s modulus of shales decrease by almost 28.9%, 24.3%, 32.2% and 32.2%, 36.1%, 46.8%, respectively. During the imbibition, these values show decreasing–increasing–decreasing trends. The SEM and EDS results show that a large amount of pyrite and clay minerals are dissolved in the reaction process. In the soaking process, the pH of fracturing fluid highly decreases in the first 15 days of immersion, and then gradually increases. The water–rock reaction and hydration expansion cause the changes of the microstructure of shale and the pH of fracturing fluids. They finally result in macroscopic mechanical properties changes. It can be deduced from the long-term test results that the initial pH value of the fracturing fluid has a small impact on the mechanical properties of shale. The obtained results are helpful to better understand the influence of hydraulic fracturing on the mechanical characteristics of shale, and to optimize the shale gas recovery.