Effects of Compositions and Fractal Pores on CO2 Adsorption in Lacustrine Shale

Lacustrine shale reservoirs hold promise for CO2 geological sequestration and enhanced shale gas/oil recovery, while the CO2 adsorption capacity and its controlling factors are still unclear in lacustrine shales. Using a volumetric-based adsorption apparatus, CO2 adsorption experiments were performed at 50 °C on the Ch7 lacustrine shale samples from the Yanchang Formation in Ordos Basin, China. Basic petro-physical experiments, low-temperature N2 adsorption, and field emission scanning electron microscopy were used to characterize shale properties and fractal pores in the lacustrine shales. Further, the effects of shale compositions and fractal pores on CO2 adsorption capacities were serially investigated. The results show that Ch7 lacustrine shales are characterized by being rich in their TOC (total organic carbon) content, high in their clay content, but low in their quartz content, which is distinguished from the mineral compositions in marine shales. The pore size distributions are multi-modal with a main peak and two secondary peaks. Meanwhile, two-regime pore fractal characteristics were identified in the Ch7 lacustrine shales, and the fractal dimensions of the pore surface and spatial structure were calculated based on the FHH (Frenkel–Halsey–Hill) model with D1 and D2 ranging from 2.586–2.690 and 2.756–2.855, respectively. CO2 adsorption isotherms present an initial phase of rapid adsorption followed by a slow saturation and were fitted using the Langmuir model with Langmuir volumes in the range of 2.16–6.89 cm3/g for Ch7 lacustrine shales. TOC is crucial for enhancing the CO2 adsorption capacity, whereas the effect of clays on CO2 adsorption is complex because of the reverse effects of clay-related pores and other pores filled by clays. Micropores (<2 nm) dominate the CO2 adsorption capacity because they offer a larger unit-specific surface area and possess a higher adsorption potential compared to meso- (2–50 nm) and macro- (>50 nm) pores. Moreover, the D1 is positively related to the CO2 adsorption capacity as a larger D1 coincides with more heterogeneous fractal pore surfaces and more available locations for CO2 adsorption. This work provides useful knowledge and important data for estimating the CO2 geological storage potential in lacustrine shale reservoirs.