Acid Fracturing in Shales: Effect of Dilute Acid on Properties and Pore Structure of Shale

Summary Many of the microfractures created during hydraulic fracturing are too small to be filled with proppants and are likely closed during production. However, for some shales that are rich in calcite (calcareous mudstones), such as the Bakken and Eagle Ford shale, dilute acids can be used while fracturing to maintain the conductivity of these microfractures under closure stress by nonuniformly etching the fracture surfaces. The mineralogy and pore structure of the shale and their evolution during acid fracturing are critical factors on the surface-etching profile and the fluid leakoff. Therefore, understanding how acid dissolution changes the microstructure, petrophysical properties, and pore structures of shale is essential in the design and application of acid fracturing in shales. In this paper, in-situ changes of microstructure and pore structure in shale before and after acid fracturing were demonstrated and visually compared for the first time, with a carefully designed microscopy work flow. Acidized sections of a shale core sample were meticulously isolated, and its microstructure, pore structure, and petrophysical properties were systematically studied and compared with nonacidized sections of the core. Evolution of mechanical properties of shale during acid treatment was also investigated. Microstructure changes were found to be strongly dependent on mineral distribution, and several patterns were identified: channels developed in carbonate-rich regions; cavities or grooves formed in carbonate-rich islands or carbonate rings; and surface roughness created in mixed zones of scattered carbonate and inert minerals. Inert minerals such as clay and organic matter (OM) stay relatively undisturbed in the structure, while some mineral grains can be dislodged from their original locations by dissolution of the surrounding carbonates. Many macropores with sizes up to 120 µm were created, and mesopores mostly associated with clay gained more accessibility. Significantly increased permeability and porosity were measured in an acidized shale matrix. Indentation hardness measurements show that, as expected, the hardness of the shale was reduced by acidizing. This means that for acidizing to work effectively, it is important not to etch the fracture surfaces uniformly. Doing so will result in a reduction in the fracture conductivity under stress. The microstructure changes introduced by acid fracturing demonstrated in this study will result in the formation of surface asperities, which is likely to improve the fracture conductivity of induced unpropped fractures. The acidized shale matrix close to the fracture surface with increased abundance of macropores and accessibility to mesopores may serve as a preferred pathway for fluid flow, as well.