Mechanisms of tripartite permeability evolution for supercritical CO2 in propped shale fractures

Characterization of CO2 flow in propped fractures is important in defining the response to CO2 injection for reservoir stimulation and CO2 sequestration. We measure the evolution of permeability in propped fractures of shale to both adsorbing CO2 and non-adsorbing He under sub- and super-critical conditions. A tripartite permeability-pressure evolution curve is obtained when supercritical, consisting of a dual-U-shaped evolution first below and then exceeding critical pressure with a V-shaped fluctuation spanning the phase transition. The increasing adsorbed-phase-density and resultant swelling stress may control the permeability variation around the critical point. The inorganic adsorbent (mainly clay) may contribute to the secondary U-shaped evolution according to its sorption isotherm. The secondary adsorption may be generated by increasing sorption sites (competitive adsorption between CO2 and H2O) or through multi-layered sorption and stronger diffusion of supercritical CO2. Further constraint is applied through observations of permeability recovery between initial and repeat saturations to non-adsorptive He. An abnormal increment of permeability recovery ratio is obtained for secondary adsorption, which may be caused by the dehydration and shrinkage of the matrix and the dissolution of minerals. Mechanisms of permeability evolution for CO2 in shale are classified between organic and inorganic fractions. The contributions of adsorption to the permeability evolution are quantified by comparisons for permeation by CH4 and He. A flat X-shaped trend is apparent, in which the inorganic contribution to permeability increases with increasing pressure while the organic contribution to permeability decreases with increasing pressure. The ratio of inorganic contribution reaches 60-70% under supercritical conditions.