Modification of Microscopic Properties of Shale by Carbonic Acid Treatment: Implications for CO₂-Based Fracturing in Shale Formations

Abstract

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.

Comprehensive Review of Property Alterations Induced by CO2–Shale Interaction: Implications for CO2 Sequestration in Shale

“…The contents of carbonate, clay, and feldspar minerals in shale generally show decreasing trends after CO 2 –shale interaction. However, the content of quartz generally shows an increasing trend after CO 2 exposure due to the weak dissolution of quartz in shale and the decrease in the relative content of other more easily dissolved minerals. − In some specific cases such as at a higher temperature condition of 80 °C, the content of quartz in shale decreased after reaction with CO 2 , which may be related to the fact that the reaction rates of silicate–CO 2 could be speeded up at a higher temperature condition. , In addition to the element mobilization caused by the mineral dissolution, precipitation of dissolved minerals such as kaolinite and quartz can simultaneously occur, which has been observed experimentally. − The dissolution of the minerals consumed protons in acid solution, increasing the pH in it. As the pH value rises to a certain extent and the mineral ion concentration achieve a certain level, mineral precipitation occurs.…”

Nanoscale Analysis of Shale Matrix Alteration after Supercritical CO2Treatment: Implications for scCO2Fracturing in Shales

“…Nanoindentation is a technique capable of measuring the nano- to microscale mechanical properties of a sample. Some researchers have used it in conjunction with SEM–EDS to investigate the micromechanical behavior of Longmaxi shale exposed to the scCO 2 –water environment. ,, …”

CO2 Mass Transfer and Oil Replacement Capacity in Fractured Shale Oil Reservoirs: From Laboratory to Field

“…Extensive experimental studies (Zhou et al, 2016;Zou et al, 2018;Li et al, 2020a;Li et al, 2020b) and numerical simulation studies (Li et al, 2018b;Meng et al, 2019;He et al, 2020) have shown that CO 2 fracturing fluid is beneficial to fracture morphology and fracture scale. Moreover, CO 2 -water-rock reaction also plays a positive role in the realization of large-scale hydraulic fracturing (Fischer et al, 2010;Zhang et al, 2018;Tian et al, 2020;Zhou et al, 2020a). In addition to the benefits of CO 2 for fracture propagation and fracture morphology, another huge advantage of CO 2 fracturing is the CO 2 -oil interactions.…”