Abstract
We report experimental observations of permeation of CO2-rich aqueous fluids of varied acidic potential (pH) on three different shales to investigate mechanical, chemical, and mineralogical effects on fracture permeability evolution. Surface profilometry and SEM-EDS (scanning electron microscopy with energy-dispersive X-ray spectroscopy) methods are employed to quantify the evolution in both roughness on and chemical constituents within the fracture surface. Results indicate that, after 12 hours of fluid flow, fracture effective hydraulic apertures evolve distinctly under different combinations of shale mineralogy, effective stress, and fluid acidity. The evolution of roughness and transformation of chemical elements on the fracture surface are in accordance with the evolution of permeability. The experimental observations imply that (1) CO2-rich aqueous fluids have significant impact on the evolution of fracture permeability and may influence (and increase) shale gas production; (2) shale mineralogy, especially calcite mineral, decides the chemical reaction and permeability increasing when CO2-rich aqueous fluids flow through fractures by free-face dissolution effect; (3) clay mineral swelling reduces fracture aperture and additively calcite pressure solution removes the bridging asperities, which are the main reasons for fracture permeability decrease; (4) competition roles among clay mineral swelling, mineral pressure solution, and free-face dissolution determine how fracture permeability changes. Furthermore, a multiple parameter model is built to analyze effective hydraulic aperture evolution in considering above three mechanisms, which provide a reference to forecast fracture permeability evolution in shale formations.
Highlights
Researches verified that supercritical carbon dioxide (Sc-CO2) may work as fracturing fluid in shale stimulation works, which could create fractures surface with higher roughness and form complex fracture networks [1,2,3,4]
Permeability may increase due to free-face dissolution and chemical reaction by etching the fracture voids due to chemical dissolution [15] while permeability decrease could result from fracture asperity bridging by pressure solution [16, 17], stress corrosion cracking-induced diffusion [18], clay mineral swelling, and secondary mineral precipitation [19, 20]
We investigate the role of fluid acidity on fracture surface chemical element changes
Summary
Researches verified that supercritical carbon dioxide (Sc-CO2) may work as fracturing fluid in shale stimulation works, which could create fractures surface with higher roughness and form complex fracture networks [1,2,3,4]. Permeability may increase due to free-face dissolution and chemical reaction by etching the fracture voids due to chemical dissolution [15] while permeability decrease could result from fracture asperity bridging by pressure solution [16, 17], stress corrosion cracking-induced diffusion [18], clay mineral swelling, and secondary mineral precipitation [19, 20]. Those opposite phenomena, which always evolve concurrently, have been observed in experiments and in engineering works [21, 22]. The role of mechanical and nonequilibrium chemical effects on fracture permeability evolution is Geofluids (b)
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