The permeability of tight shale formations varies from micro-Darcy to nano-Darcy. Recently, hydrogen peroxide (H2O2) was tested as an oxidizer to remove the organic matter in the rock in order to increase shale permeability. In this study, shale particles were reacted with hydrogen peroxide solutions under different temperature and pressure conditions in order to “mimic” underground geology conditions. Then, low-temperature nitrogen adsorption and desorption experiments were conducted to measure the pore diameters and porosity of raw and treated shale samples. Moreover, scanning electron microscopy (SEM) images of the samples were analyzed to observe pore structure changes on the surface of shale samples. From the experiments, it was found that the organic matter, including extractable and solid organic matter, could react with H2O2 under high temperature and pressure conditions. The original blocked pores and pore throats were reopened after removing organic matter. With the increase of reaction temperature and pressure, the mean pore diameters of the shale samples decreased first and then increased afterwards. However, the volume and Brunauer–Emmett–Teller (BET) surface areas of the shale particles kept increasing with increasing reaction temperature and pressure. In addition to the effect of reaction temperature and pressure, the pore diameter increased significantly with the increasing reaction duration. As a result, H2O2 could be used to improve the shale permeability.

The existence of nanosized pore systems differentiates isotopic gas transport inside a shale matrix from conventional continuum flow. In this study, a novel self-similarity mathematical model was developed to investigate the effects of gas flow transport (both slip flow and free molecular diffusion flow) on isotopic gas fractionation for four different shale samples (S1 and S2 from north Germany and S3 and S4 from Xiashiwan Field, Ordos Basin, China). In this model, the nonlinear permeability and diffusion coefficients were developed for the isotopologues (12CH4 and 13CH4), respectively. By selecting appropriate exponents of the pressure gradient for 12CH4 and 13CH4, respectively, the estimated isotopic methane concentration and production rate showed a good agreement with experimental data. The developed model shows that the gas concentration of the isotopologues in samples S1 and S2 increases with time following a power law. Similarly, the gas production rates of the isotopologues in samples S3 and S4 decay with time following a power law. Moreover, the exponents of the pressure gradient for the isotopologues are close to 4 for samples S1 and S2, indicating that the effect of slip flow on isotopic gas fractionation cannot be ignored. For samples S3 and S4, the exponents of the pressure gradient for the isotopologues increase with temperature rising, which shows the promotion of isotopic gas fractionation under higher heating temperatures. The slight difference between the exponents of the pressure gradient for the isotopologues for the same shale sample reveals that the isotopic gas fractionation of carbon is a slow process.

In this study, two shale samples with different maturities, from Geniai, Lithuania (Ro = 0.7%), and Wenjiaba, China (Ro = 2.7%), were selected for open-system pyrolysis experiments at 400 °C and 500 °C, respectively. The generation of isotopic gases from the shales with different maturities was investigated, and the effects of pyrite catalysis on the carbon isotopic compositions were also studied. It was found that CO2, CH4 and their isotopic gases were the main gaseous products of the pyrolysis of both shales, and more hydrocarbon gases were generated from the low-maturity Geniai shale. The δ13C1 values fluctuated from −40‰ to −38‰, and δ13C2 showed higher values (−38‰~−34‰) for the Geniai shale. In addition, its δ13CCO2 values ranged from −28‰ to −26‰. Compared with the Geniai shale, lower δ13C1 values (−43‰~−42‰) and higher δ13CCO2 values (−19‰~−14‰) were detected for the Wenjiaba shale. As temperature increased, CH4 became isotopically lighter and C2H6 became isotopically heavier, which changes were due to the mass-induced different reaction rates of 12C and 13C radicals. Furthermore, the pyrite made the kinetic isotope effect stronger and thus made the CH4 isotopically lighter for both shales, especially at the lower temperature of 400 °C.