Energy & Fuels, Vol.34, No.3, 3458-3466, 2020
Modification of Microscopic Properties of Shale by Carbonic Acid Treatment: Implications for CO2-Based Fracturing in Shale Formations
Recent research has demonstrated that CO2, working as a fracturing fluid, possesses unique advantages during shale reservoirs development. Since water may exist in formations or be introduced into formations during drilling, carbonic acid would form after CO2's dissolution into water. In this study, modification of microscopic surface properties of shale induced by carbonic acid treatment is comprehensively studied. Based on scanning electron microscope (SEM) imaging and energy dispersive spectrometer (EDS) mapping, the elemental evolution in the same position illustrates a certain correlation between the microstructure change and the dissolution of calcite and dolomite. Adhesion properties of the shale surface are revealed by atomic force microscopy (AFM). Adhesion force remains relatively stable for the quartz regions and it shows an irregular increase in the nonquartz regions after carbonic acid treatment. Stereo images obtained by a confocal microscope show that the surface becomes smoother after carbonic acid treatment despite the formation of dissolution cavities. According to nanoindentation tests, Young's modulus and hardness are significantly reduced and the shale sample becomes more ductile after acid treatment. These findings provide a deep insight into the microscale alterations in the shale surface induced by carbonic acid treatment, which contributes to the further assessment of CO2-based fracturing and CO2 sequestration.