CO2-water-shale interaction induced shale microstructural alteration

Abstract

To better understand the CO2 enhanced shale gas recovery process, it is of great significance to investigate the CO2-water-shale interaction and its effect on the microstructural alteration in shale. To clarify the microstructural alteration of shale exposure to CO2-water at different CO2 pressures, X-ray diffraction analysis, low-pressure nitrogen gas adsorption (N2GA), nuclear magnetic resonance (NMR) and fractal analysis were carried on shale samples before and after pure water and CO2-water (PCO2 = 6, 10, 14 MPa; T = 45 °C) exposure. After CO2-water exposure, the contents of quartz in shale were increased, while the content of carbonate and clay minerals in shale were decreased with the increase of CO2 pressure. With the increase of CO2 pressure, the total specific surface area (TSSA) in shale were decreased with a maximum reduction of 29.40%, while the porosity was increased from 2.88% to a range of 2.93%-3.16%. The average pore size (Rave) in shale was also increased and the pore connectivity in shale was improved, micropores and mesopores in shale are gradually transferred to macropores. Both the pore surface fractal dimensions obtained from N2GA (DL1) and NMR (DN1), and the pore volume fractal dimensions obtained from N2GA (DL2) and NMR (DN2), were shown a reduction tendency with the increase of CO2 pressure, which reflected that the roughness and complexity of pore structure and morphology in shale decreased. The results indicated that the effect of super-critical CO2 (10 MPa, 14 MPa) on the pore structure alteration was more significant than that of sub-critical CO2 (6 MPa). The fractal dimensions calculated by N2GA and NMR tests had a good consistency, DL1, DN1, and DL2, DN2 shown positive correlations with TSSA and TPV (total pore volume), while shown negative correlations with Rave, which can be used to evaluate the gas adsorption and flow behaviors in shale, respectively.