•Under the interaction of SC-CO2, the change of coal pore size is mainly attributed to dissolution, while the contribution of pressure is small.•Under the interaction of SC-CO2, micropores and ...mesopores are converted to macropores, increasing the connectivity of macropores.•Under the interaction of SC-CO2, the evolution of fracture networks is related to the nanopore characteristics in the matrix.
It is essential to understand the permeability evolution of coal reservoirs under the interaction of supercritical CO2 (SC-CO2) in order to be successful in both CO2 geological storage and extraction of coalbed methane. However, the influence of nanopore structure characteristics on the permeability evolution of coal exposed to SC-CO2 has not been elucidated. To this end, the quantitative analysis of the pores in high-rank coal was conducted through low-temperature N2 adsorption, low-temperature CO2 adsorption, and Nuclear Magnetic Resonance (NMR). The analysis revealed that micropores and mesopores are the major contributors to the specific surface area and pore volume of the nanopores, respectively. After SC-CO2 interaction, the pore size and pore number of the coal increased significantly, with small pores changing to large ones, mainly due to the dissolution of SC-CO2 with the contribution of gas pressure being negligible. The proportion of micropore and mesopore decreases while that of macropore increases after SC-CO2 treatment, especially SC-CO2 causes macropores to be continuous and increased the connectivity of fractures. The relationship between nanopore structure characteristics and the deformation of the fracture network under SC-CO2 interaction was studied. The results showed that the specific surface area (SSA) of the micropore and the volume fractal dimension (DLN2) of the mesopore were related to the change of fracture porosity, while the surface fractal dimension (DLN1) of the micropore, the specific surface area (SSA) and pore volume of mesopore were related to the change of fracture complexity. This indicated that the evolution of the fracture network under SC-CO2 interaction is related to the nanopore structure characteristics. A mechanical model was established based on the close correlation between nanopore structures characteristics in the matrix and the evolution of fracture networks under SC-CO2 interaction. This mechanical model elucidates the impact of nanopore alteration resulting from mineral dissolution induced by SC-CO2 on the evolution of fracture networks. This research provides a clearer understanding of the evolution in reservoir permeability during CO2 geological sequestration, offering valuable reference for safety assessments.
The wettability of shale, which determines the success of carbon dioxide-enhanced shale gas recovery (CO2-ESGR) and the safety of CO2 geo-storage, is influenced due to shale–CO2 chemical ...interactions. To date, the wettability alteration mechanism of shale by its surficial chemical groups is not well understood. In this study, the variations in the wettability, mineralogy, and infrared spectrum of shale after interaction with CO2 at different temperatures and pressures were studied using the sessile drop method, X-ray diffraction, and Fourier transform infrared spectroscopy analysis. The effect of the contents of minerals and chemical groups on shale wettability was investigated by correlation analysis. It is found that the pressure of CO2 has a larger influence on shale wettability than the temperature, whereas the temperature has a greater effect on the infrared spectra of shale. Quartz has the greatest effect on wettability followed by carbonate and clay minerals. Quartz is originally water-wet, and the negative correlation between the relative quartz content and the wettability of shale implies the deterioration of the quartz hydrophilicity after CO2 treatment, and the same applies to the oxygen-containing groups. These findings indicate the interactions among functional groups and their redistribution on the surface during shale–CO2 interactions. Si–O groups are derived more from the hydrophobic Si–O–Si groups than hydrophilic Si–OH groups after CO2 treatment, reducing the contribution of quartz to shale hydrophilicity. The weakened absorption ability of CO3 2– ions to water may be caused by the rotation and redistribution of the CO3 2– groups on the surface after the CO2 treatment. A longer aliphatic hydrocarbon length reduces the shale wettability. Among the hydrophilic −OH groups, free −OH has the largest influence on shale wettability followed by −OH–O, −OH–OH, and −OH−Π. This study has important theoretical significance for understanding the wettability alteration mechanism of shale.
Microbially-induced calcium carbonate precipitation (MICP) is a promising grouting material for subsurface remediation due to its water-like viscosity and excellent penetration. Current studies of ...MICP-grouting for subsurface remediation of both rock fractures and highly-permeable rock matrix focus on the spatio-temporal distribution of precipitated bio-CaCO3 and the resulting reduction in permeability. Conversely, we focus on the improvement of mechanical response following MICP-grouting. We contrast the improved mechanical response of MICP-treated Berea sandstones with distinctly contrasting initial mechanical properties - contrasting associated pre- and post-treatment microstructures with various durations of MICP-grouting. Results indicate that although the precipitated CaCO3 mass with time within these two rock types is similar, significant differences exist in the evolution of mechanical properties (UCS, Young's modulus and brittleness). The evolution of mechanical properties for the low-strength sandstone (initial UCS 25.7 MPa) exhibits three contrasting phases: an initial slow increase, followed by a rapid-increase and then saturation and asympotic response. After ten cycles of MICP-grouting, UCS, elastic modulus and brittleness index for low-strength sandstone increase by 229%, 179% and 177% compared with before grouting. In contrast, the mechanical properties for the high-strength sandstone (initial UCS 65.1 MPa) are not significantly enhanced, increasing UCS by only 22%, 14% and 12%. Imaging by scanning electron microscopy (SEM) indicates that the cementing minerals fill the quartz framework for the high-strength sandstone but are sparse for the low-strength sandstone. Sandstone is a clastic sedimentary rock consisting of a framework of quartz grains bonded by cementing minerals. For the high-strength sandstone infused with a large mass of cementing minerals, the calcium carbonate crystals only precipitate in the gaps between the cementing minerals or adhere to the cementing minerals. This is only capable of relatively limited enhancement in the bio-bonding strength and volume of the quartz framework. For the low-strength sandstone with fewer cementing minerals, the precipitated calcium carbonate is evenly distributed on the surfaces of the quartz gains. The bulk strength is progressively increased with the ongoing bio-cementation between quartz gains. Cementing mineral contents not only exert a considerable control on the integral mechanical properties and penetration for the sandstone, but also have a direct influence on the microscopic distribution of bio-accumulated CaCO3, controlling the effectiveness of bio-cementation by incrementing the mechanical properties.
•Distribution of bio-accumulated CaCO3 is strongly controlled by sandstone porosity and its structure.•Open-pore sandstones favor precipitation over grains and tight sandstones within pores.•Thus, low-strength and high relative porosity samples return the greatest gains in strength and modulus.
•Mode I fracture toughness, absorbed energy and energy-release rate of shale decreased after Sc-CO2 saturation.•Sc-CO2 saturation more rapidly cause the initiation and propagation of shale mode I ...cracks with main fracture mode of transgranular cracks.•The influence mechanism of Sc-CO2 on mode I fracture toughness and crack propagation of shale was revealed.
The mode I fracture toughness is a critical parameter which defines the rock’s resistance to crack propagation, especially in hydraulic fracturing. Recently, Supercritical carbon dioxide (Sc-CO2) has been proposed as a fracturing fluid candidate in hydraulic fracturing stimulations of shale reservoirs. However, its effects on fracture toughness transition and crack propagation behaviors have not been understood well. In this study, we performed a series of semicircular bend specimens (SCB) before the Longmaxi shale specimens’ saturation. Three-point bending tests along divider orientation showed that after Sc-CO2 saturation, mode I fracture toughness (KIC), elastic modulus (E) and absorbed energy (Ue) of shale were decreased by 22.1%, 24.5% and 44.3%, respectively. High speed camera images indicated that after Sc-CO2 saturation, mode I crack directly propagated straight along artificial pre-crack direction, decreasing the degree of crack deviation as in KIC. The results of Cronos high-precision 3D scanning system and scanning electron microscopy (SEM) revealed complicated fracture mechanisms (transgranular, intergranular and mutual coupling crack mechanisms) of shale after Sc-CO2 saturation, which reduced the roughness and area of fracture crack surface. The generation of pore and cracks was the main reason for the decrease of shale resistance to fracture. Furthermore, Sc-CO2 saturated shale specimens only needed to absorb less energy to more rapidly cause the initiation and propagation of mode I cracks with the main fracture mode being transgranular cracks.
The surface wettability and morphology of shale may both be modified by SiO2 nanofluid (SNF), directly influencing the capillary force and CO2 geo-storage. However, the literature requires more ...information and studies with respect to nanoparticles’ effect on shale surface at reservoir conditions of high pressure and temperature. Here, we investigate the stability of nanoparticle layers formed by SNF on Longmaxi (marine) and Yanchang (continental) shale in ScCO2 and nanoparticles’ efficiency of wettability reversal at different nanofluid aging times and concentrations. Besides, low-pressure nitrogen gas adsorption (LP-NA) was performed to evaluate the effect of nanoparticles on shale pore structures after ScCO2 exposure. Results indicate that the nanoparticle structure could be stable in ScCO2 and render the wettability of shale samples to be more hydrophilic, while the quartz-rich Longmaxi shale surface shows a denser nanostructure than the clay-rich Yanchang shale surface does and presents a stronger water wettability. More micropores are transformed into mesopores or macropores for ScCO2- and SNF-treated Longmaxi samples than solely ScCO2 samples, but the pore structure parameters show trivial alteration in Yanchang samples after SNF treatment. The difference in the capillary force recovery between Longmaxi (recovered by 660.5%) and Yanchang (recovered by 500.6%) shale implies that silica nanofluid may be more suitable for Longmaxi formation in the CGS application.
•The ability of SiO2 and Al2O3 nanoparticles to retain wettability reversal of CO2-water-shale system is studied.•The wettability-enhancing effect of SiO2 and Al2O3 nanofluid with different nanofluid ...concentrations and aging time on contact angles is obtained.•The effect of SiO2 and Al2O3 nanoparticles on water wettability and functional groups of shale after a period of ScCO2 treatment is elaborated.•SiO2 nanofluid showed better performance compared to Al2O3 nanofluid in wettability reversal after ScCO2 treatment.•SiO2 and Al2O3 nanoparticles can recover CO2 structural trapping and increase mineral trapping potential in shale formation.
The wettability of rocks is closely related to the CO2 geo-storage (CGS) security after CO2 injection into the reservoir. Nanofluids can be used to efficaciously alter the wettability of the CO2-water-rock system toward an advantageous state (water-wet) for CGS. In this study, we investigated the potential of SiO2 and Al2O3 nanofluids at different aging time and concentrations to retain the wettability reversal of CO2-exposed shale from the Sichuan Basin after supercritical carbon dioxide (ScCO2) exposure. Fourier Transform Infrared Spectrometer and field emission scanning electron microscopy tests were performed to evaluate the variations of functional groups and surface topography of shale. Results indicated that both nanoparticles efficiently retain the wettability reversal of shale after a period of ScCO2 treatment, which is conducive to structural trapping in the CGS project. The efficiency was improved by increasing the nanofluid aging time and concentrations, while beyond a certain value, the contact angle value changed trivially. Quasi-homogeneously distributed on the shale surface in ScCO2 may contribute to the wettability alteration of shale in ScCO2, whereas there are still removals of nanoparticles in some sites. The variations of functional groups suggest that the existence of nanoparticles may mildly expedite the CO2 mineralization, signifying the change of wetting behavior of shale in ScCO2 depends on the wettability of nanoparticles, and the nanofluids treatment may exert a positive influence on mineral trapping in the CGS project. The findings in this research provide an important basis for raising the CO2 geological storage security and potential via nanofluids.
