Characteristics of Micro-fracturing in shales induced by dilute acid

Abstract The utilization of acid has been approved as a potential approach to enhance Stimulated Reservoir Volume (SRV) by creating secondary pores and microfractures in shale reservoirs. This paper proposed an acid soaking technique to quantify the generation of microfracturing behaviors. Two types of core samples from distinct shale depositional environments, namely Longmaxi carbonate-rich, and Yanchang clay-rich shale were emphasized. Hydrochloric acid (15%+ 3%KCL) was utilized to treat the shale samples for a period of 2 hours. Scanning Electron Microscope (SEM) and X-ray diffraction (XRD) experiments were performed to correlate the individual mineral phases. Inductively coupled plasma-optical emission spectrometry (ICP-OES) and X-ray fluorescence tests (XRF) were utilized, to quantify the elemental analysis of the distinct mineral phases. For post acidizing test conditions, three different types of microfractures, namely stress-induced micro-fractures, acid dissolution micro-fractures, and acid shrinkage micro-fractures were observed. For the clay-rich shale, the stress-induced microfractures were more prominent within the Kaolinite and the boundary edges of the Chlorite/Biotite regions. Whereas, shrinkage micro-fractures dominated the chlorite regions. The Longmaxi carbonate-rich shale, on the other hand, revealed more evidence of the creation of acid dissolution microfractures within Anhydrite, calcite, illite, and organic matter regions. According to the ICP-OES and XRF test results, the clay-rich shale sample underwent excessive ionic imbibition, as well as an ion dislodgment mechanism. However, the carbonate-rich shale experienced only the ion dislodgment mechanism from the shale interface. No ions reverted to the shale sample as the electric equilibrium was established. This paper would contribute immensely to the classification of different micro-fracturing features, and their characteristics for the distinct shale mineralogies.