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Silurian marine shale in Sichuan Basin is the most significant target zone for shale gas resources in China. In this work, a combined experimental and molecular simulation study was conducted to characterize the thermodynamic and structural properties of the organic matter in Silurian shale. Organic geochemistry experiments and Fourier transform infrared (FTIR) spectroscopy were performed to provide structural parameters for the main skeleton of the organic matter. A realistic molecular model of the organic matter under typical reservoir conditions was generated by molecular dynamics simulations based on the experimental results and documented analytic data. The thermodynamic and structural properties of the organic matter model were discussed in detail. Clear correlations are found among geochemistry properties and structural parameters of organic matter, independent of organic matter type and maturity. Aromatic units in the organic matter are highly condensed, and the interunit linkages are mainly short methylene groups. Ether groups are the dominant oxygenated compounds, while aromatic sulfur is the main form of organic sulfur. Reasonable consistencies are found on results of compositions of the organic matter fractions and physical density between simulated and available experimental data. The isothermal compressibility and thermal expansion coefficient correspond to the general range of a liquid. In addition to micropores, the organic matter contains a large amount of ultramicropores, which contribute a lot to the high porosity and specific surface area. The porous network is highly connected with few dead pores. Interestingly, the introduction of bitumen fractions has little effect on the spacing of polyaromatic units, but it aggravates the relative slippage of polyaromatic units. Also, separation of lighter compounds is observed in the structure. The carbon dioxide molecules are closer to the oxygenated groups, while the nitrogen molecules and methane molecules are closer to the sulfur functional groups and nitrogen functional groups. This proposed organic matter model can serve as a starting point for further theoretical investigations on gas adsorption and transport mechanisms, representative of the organic matter in Silurian shale at molecular scale.

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Tight oil reservoirs are an unconventional hydrocarbon resource with great potential to help meet energy demands. Horizontal drilling and hydraulic fracturing has been extensively used for the exploitation of these unconventional resources, and fracturing fluids absorbed into formations by spontaneous imbibition (SI) is an important mechanism of oil production. In this paper, imbibition experiments combined with nuclear magnetic resonance were conducted to study the characteristics of fluid displacement in an oil/water system for tight sandstone. In addition, the relative contribution to oil recovery of different types of pores, effects of boundary conditions, and different surfactants on imbibition recovery was determined via the T 2 spectra of each sample. The results show that the tight sandstone features a multiscale pore structure, which is dominated by micropores and small mesopores. As the imbibition process begins, white oil is preferentially displaced from these relatively small pores by water and a large amount of oil production comes from the micropores. Boundary conditions are shown to have a significant impact on imbibition rate and ultimate recovery. Both are higher as the areas available for water imbibition increase. Deionized water with low concentrations of surfactants altered the wettability of the samples, from weakly water-wet to a strongly water-wet on the rock surfaces, while lowering interfacial tension (IFT) at the oil−water interface. Therefore, a higher oil recovery could be obtained to some extent, but enough IFT is still needed to ensure a large capillary force. Because conventional scaling equations do not account for the effect of wettability alteration, such as the addition of surfactants to a system, they cannot be employed to scale imbibition data well. This research demonstrates the imbibition characteristics of tight sandstone and several relevant affecting factors, providing crucial theory foundations for the development of tight oil formations.

High pressure sorption isotherms of pure CH4 and CO2 at 80 °C and CH4/CO2 mixtures at 30 and 80 °C on shale samples from Sichuan Basin, China, were measured by a modified volumetric method. The multicomponent sorption measurements were conducted on the mixtures with feed gas compositions of 79.50% and 48.62% CH4. The sorption isotherms of pure CH4 and CO2 were fitted by a modified Langmuir equation, and the sorption isotherms of the total and individual component of CH4/CO2 mixtures were fitted by an extended Langmuir (EL) equation. Qualitative and quantitative characterizations of selective sorption of CH4 and CO2 were discussed at different temperatures and gas compositions. The results indicate that the sorption capacity of pure CO2 is larger than that of pure CH4, with the Langmuir sorption capacity of pure CO2 being approximately 2.5 times of pure CH4. However, a higher sorption amount of CH4 than CO2 is obtained in competitive sorption of mixed gas with a feed gas composition of 79.50% CH4, suggesting that sorption behavior of individual component under competitive condition depends not only on the sorption affinity of the component, but also on the partial pressure of the component in the mixture. The separation factors calculated from multicomponent sorption data are in a range of 1.0–2.5, which are far less than that calculated from single gas sorption data, indicating that the presence of CH4 greatly weakens the preferential sorption of CO2. On the contrary, increasing temperature and CO2 content in the mixture will promote preferential sorption of CO2.