Co-pyrolysis of medium-low maturity shale and nano-NiO catalyst under supercritical CO2 atmosphere: Mechanisms and reaction kinetics

Abstract

On the basis of supercritical carbon dioxide (ScCO₂) thermal fluid, the introduction of a catalyst will significantly promote the development of oil resources from medium-low maturity shales. In this paper, after confirming that nano-NiO is the optimal catalyst among the three nickel-based catalysts (NiCl₂·6H₂O, nickel stearate, and nano-NiO), non-isothermal pyrolysis experiments on medium–low maturity shales both with and without nano-NiO catalyst loading were conducted under a ScCO₂ atmosphere. The conversion processes of hydrocarbon substances and the properties of the products were systematically analyzed to explore the synergistic effect mechanism of ScCO₂ and nano-NiO. The reaction kinetics models encompassing kerogen pyrolysis reaction and heavy hydrocarbon cracking reaction were established by coupling the nonlinear least squares method with a MultiStart optimization algorithm to describe the kinetics of organic matter thermal degradation and hydrocarbon generation. The results showed that compared to ScCO₂ conditions without nano-NiO catalysis, the additional introduction of nano-NiO catalyst effectively reduced the pyrolysis temperature of organic matter, enhanced the maximum weight loss rate of shale pyrolysis by 2.88 %, and boosted oil and gas yields by 32.3 % and 18.0 %, respectively. The synergistic effect of ScCO₂ and nano-NiO promoted the lightening and alkylation of hydrocarbon components and inhibited the high-temperature polymerization reaction, significantly improving the quality and stability of oil and gas fractions. The kerogen pyrolysis tends to convert into heavier hydrocarbon components, with proportion coefficients greater than 0.5, while the heavy hydrocarbon cracking primarily produces lighter oils and gases. The synergistic effect lowered the activation energy for kerogen pyrolysis reaction by 16.3 kJ/mol and increased the proportion coefficient of organic matter converted into oil, which is the desired outcome.