Supercritical CO2 Saturation Research Articles

Zeta Potential of Supercritical CO2‐Water‐Sandstone Systems and Its Correlation With Wettability and Residual Subsurface Trapping of CO2

AbstractAlthough CO2 geological storage (CGS) is thought to be one of the most promising technologies to sequester the anthropogenic CO2 to mitigate the climate change, implementation of the method is still challenging due to lack of fundamental understanding of controls of wettability, which is responsible for residual trapping of the gas and its flow dynamics. One of the key parameters that controls the wetting state is the zeta potential, ζ, at rock‐water and CO2‐water interfaces. ζ in systems comprising rocks, carbonated aqueous solutions and immiscible supercritical CO2 have not been measured prior to this study, where we detail the experimental protocol that enables measuring ζ in such systems, and report novel experimental data on the multi‐phase ζ. We also demonstrate for the first time that ζ of supercritical CO2‐water interface is negative with a magnitude greater that 14 mV. Moreover, our experimental results suggest that presence of multi‐valent cations in tested solutions causes a shift of wettability toward intermediate‐wet state. We introduce a new parameter that combines multi‐phase ζ and relative permeability endpoints to characterize the wetting state and residual supercritical CO2 saturation. Based on these results, we demonstrate that ζ measurements could serve as a powerful experimental method for predicting CGS efficiency and/or for designing injection of aqueous solutions with bespoke composition prior to implementing CGS to improve the residual CO2 trapping in sandstone formations.

Effects of supercritical CO2 fluids on pore structure and fractal characteristics of bituminous coal

Enhanced coalbed methane recovery with CO2 coal seam storage (CO2-ECBM) technology is an important way to achieve China's strategic goals of carbon peak and carbon neutrality. Presently, to date there has been rarely research conducted on the effect of coal sample scale on pore structure under supercritical CO2 (ScCO2) fluids. In this study, a high-pressure geological environment simulation system was adopted to analyze coal samples of different scales for ScCO2 saturation. Subsequently, low-pressure nitrogen gas adsorption (LP-N2GA), mercury intrusion porosimetry (MIP), and low-field nuclear magnetic resonance (LF-NMR) were used to analyze the pore structure and fractal dimension changes in saturated coal samples at different scales. The experimental results show that the mesopore ratios of cylindrical and granular coal decrease by an average of 1.68% and 2.30%, respectively, after the saturation of ScCO2. The proportion of macropores in cylindrical coal increased by an average of 5.50% after ScCO2 saturation, while the proportion of macropores in granular coal changed by 176.86% compared to cylindrical coal. The fractal dimension of the ScCO2 saturated coal samples obtained with LP-N2GA, MIP, and LF-NMR all show a decreasing trend, again confirming the modification of the coal pore surface by ScCO2. Finally, a conceptual model is presented to analyze the mechanism of the effect of coal sample scale on the pore structure under ScCO2. The difference in the transport paths of ScCO2 molecules at different coal scales is the main reason for the difference in the evolution of the pore structure. In addition, the impact of the amount of adsorption obtained in the laboratory using coal samples of different scales on the assessment of the CO2 storage capacity was discussed. Therefore, the results of this study are expected to provide a reference for the CO2 storage capacity assessment of the CO2-ECBM project.

Impact of supercritical CO2 saturation temperature on anthracite microstructures: Implications for CO2 sequestration

CO2 sequestration in coal seams is one of promising methods to achieve the goal of carbon neutrality, which has attracted the worldwide attention. Reservoir temperature plays a vital role in supercritical CO2 (ScCO2)-coal interactions and it even serves as a crucial factor in determining the CO2 trapping capacity and the feasibility of CO2 geological storage in these coal formations. The impact of saturation temperature on the interactions between coal and ScCO2 at geo-storage conditions is rarely reported in the literature, which directly affects the recovery of CO2-enhanced coalbed methane and the sealing integrity of structural trap provided by the caprocks. The microstructural changes of coal is also essential in prevention of upwards migration and CO2 leakages. In this paper, changes in coal microstructures including functional groups, mineral compositions and multiscale pore distribution at the saturation pressure of 10 MPa and various temperatures of 35–85 ℃ were comprehensively investigated using the combination of low-pressure CO2/N2 adsorption, X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The results show that micropores and mesopores are enhanced by 24.2–42.7% and 25.8–49.6%, respectively, while macropores are decreased by 4.9–19.5%. The carbonate minerals are most sensitive to ScCO2, and temperature rise accelerates the mineral dissolution to form more new pores within coal matrix. ScCO2 has strong extraction capacity for hydrocarbons, and approximately 80% of aliphatic structures are extracted from coal macromolecules. The higher saturation temperature, the more aliphatic groups are mobilized. Some isolated organic pores or closed pores are also extracted and damaged to become accessible for gas flow, which directly enhances the CO2 storage capacity in coal reservoirs. Once the temperature exceeds 65 ℃, the further temperature rise has less influence on coal microstructures. The results of this study may play a significant role in assessing the hydrocarbon recovery and the trapping of CO2 in coal reservoirs.

Coal microstructural and mechanical alterations induced by supercritical CO2 exposure: Role of water

Injection of CO2 into deep unminable coal reservoirs is regarded as an effective method to reduce greenhouse gas emissions. Coal seams with burial depth > 800 m are the potential reservoirs for CO2 sequestration, and CO2 is stored in the form of supercritical state. The interactions between supercritical CO2 (ScCO2) and coal are a complex process, which has influence on coal structures and strength. Recent studies are rarely reported regarding the impact of ScCO2-coal-water reactions on coal microstructures and mechanical strength. In this paper, comparative analysis was conducted on the microstructural changes and mechanical degradation of coal induced by ScCO2 treatment and ScCO2-water saturation. The results show that water has great influence on coal pore structure, functional groups, mineral compositions and mechanical properties during the reaction. ScCO2 saturation only triggers a small decline of 11.27–17.65% in aliphatic groups in coal, while approximately 62.75–85.92% of aliphatic groups are extracted by ScCO2-water. The largest hysteresis loops are found in samples saturated by ScCO2-water, followed by the specimens treated by ScCO2, illustrating that the complexity of coal pore structure is enhanced by ScCO2 interaction, and the presence of water can further increase the pore roughness. The UCS is reduced by < 13% after ScCO2 saturation on dry coal, but it is greatly decreased by 26.58–44.21% in the presence of water. In the process of interaction, water accelerates the hydrocarbon extraction and mineral dissolution to aggravate the degradation of coal strength. This work is conducive to understanding the process of CO2 geological sequestration.

Mechanical strength and porosity changes of bituminous coal induced by supercritical CO2 interactions: Influence of saturation pressure

Sequestration of CO2 in the geological formations has great potential to reduce greenhouse gas emissions, and the induced mechanical stability of coal reservoirs is deeply concerned with the long-term safety of CO2 geological storage. After injection, it is likely that CO2 is stored in the form of supercritical state in coal reservoirs. To date, our current knowledge is still lacking in the underlying interaction mechanisms between coal and supercritical CO2 (ScCO2) fluids under the influence of saturation pressure. In this work, ScCO2 saturation was performed on bituminous coal at the pressures from 7.5 to 25 MPa for saturation time up to 10 days. Mercury intrusion porosimetry (MIP), X-ray diffraction (XRD) and uniaxial compression measurements were conducted to evaluate the mechanical strength and microstructure changes of bituminous coal. The results indicate that coal pore structures are greatly changed by ScCO2 interactions, including the increase of pore volume (84.8–173.2%) and surface area (42.1–85.8%). The larger hysteresis effect is found in saturated specimens, illustrating that ScCO2 interactions enhance the pore roughness of coal. The carbonates in coal are sensitive to ScCO2 fluids, which are extensively dissolved during interactions. ScCO2 saturation pressure has significant impact on the mechanical properties of coal. Rapid reduction of mechanical strength (up to 35.6%) was identified when the pressure was increased from 7.5 to 15 MPa. Subsequently, limited further reduction (<9%) occurred as the saturation pressure is further increased up to 25 MPa. Mineral dissolution and porosity changes account for the mechanical degradation, which weakens the cementation degree and interaction force of coal matrix.

Impact of Supercritical CO2 on Shale Reservoirs and Its Implication for CO2 Sequestration

Hydraulic fracturing has transformed the international energy landscape by becoming the go-to method for the exploitation of natural gas from unconventional shale reservoirs. However, in the recent years, the search for an alternative method of shale-gas exploration has intensified, because of various problems (e.g., contamination of ground and surface water, overexploitation of precious water resources, air pollution, etc.) associated with the usage of water-based fracturing techniques. The use of CO2 for shale gas exploitation has emerged as a better alternative to aqueous-based gas exploration techniques. CO2 when injected into deep shale reservoirs, transitions into supercritical CO2 (SC-CO2) when temperature and pressure condition exceeds the critical point, i.e., 31.1 °C and 7.38 MPa. In this paper, we comprehensively review the impact of SC-CO2 on shale gas reservoirs during the different stages of shale-gas exploration, i.e., (i) drilling, which involves the superiority of SC-CO2 over water-based drilling fluids, in terms of achieving under-balanced well condition, higher rates of penetration, and resistance to formation damage; (ii) fracturing, which involves factors affecting the tortuosity of fractures created by SC-CO2 fracturing, breakdown pressure, and proppant-carrying capacity; and (iii) injection, which involves the twin-headed benefit of enhanced recovery due to CO2/CH4 competitive adsorption and geological sequestration, CO2 vs CH4 excess sorption as a function of pressure, etc. Several research works have indicated discrepancies on how SC-CO2 impacts different shale properties. Some studies show low-pressure N2-gas-adsorption-derived surface area and total pore volume to be increasing with SC-CO2 imbibition, while others show a decreasing trend for the same. Similarly, for some shales, the quartz content, along with the clay mineral contents, decreased as the exposure to SC-CO2 increased, while in some other studies, with similar long-term exposure to SC-CO2, the quartz content was observed to increase along with the decrease in clay content and vice versa. Essentially, the increased exposure to SC-CO2 results in the dissolution of primary porous structures and fractures, and reformation of newer porous structure and conduits in shales. Nonetheless, these changes in the mineralogy weaken the microstructure of the rock bringing significant changes in the mechanical properties of the shales with implications on the wellbore stability and fracturing efficiency. The mechanical properties such as uniaxial compressive strength (UCS), Young’s modulus, and tensile strength decrease as the SC-CO2 saturation period increases. However, some studies have shown factors like bedding angle and phase-state of CO2 having varying effect on the strength behavior of the shales. Moreover, changes in the structure of shales caused by the creation of fractures and the reduction of their strength can also pose major risks, because of potential leakage of CO2 through these created pathways. How these processes would interact at field scale would control the sealing capacity, especially at field-scale for addressing long-term seepage of CO2.

Mechanical response and mineral dissolution of anthracite induced by supercritical CO2 saturation: Influence of saturation time

Injection of CO2 into coal seams is a critical path to achieve the goals of enhanced coalbed methane recovery and carbon sequestration. Under the in-situ reservoir conditions, CO2 is likely to be stored in the form of supercritical state. Up to date, little understanding is reported regarding the influence of supercritical CO2 (SC-CO2) saturation time on mechanical properties and structural changes of coal. In this work, anthracite specimens are prepared and saturated by SC-CO2 fluids at 40 ℃ and 11 MPa for saturation time up to 15 days. Uniaxial compression measurements, X-ray diffraction (XRD) tests and mercury intrusion porosimetry (MIP) were performed to explore the mechanical response and microstructural changes of coal. The results show that after SC-CO2 interaction, coal mechanical properties are remarkably altered, including the reductions of uniaxial compression strength (up to 65.42%), elasticity modulus (up to 59.07%) and tensile strength (up to 56.45%). Changes in coal mechanical behaviour induced by SC-CO2 interaction are time-dependent and mainly take place within 9 days, then followed by only a slight further reduction (approximately 4–6%). MIP results indicate that as the increase of interaction time, porosity and pore specific surface area are continuously increased from 12.12 to 18.36% and 3.56 to 6.75 m2/g, respectively. The reductions in coal mechanical strength are closely related to the increase of mineral consumption and coal porosity. Mineral dissolution, hydrocarbon extraction and differential swelling are mainly responsible for the softening effect of SC-CO2 on coal. This study provides clearer insights into the process of CO2 geological sequestration.

Coupled multiphase flow and transport simulation to model CO2 dissolution and local capillary trapping in permeability and capillary heterogeneous reservoir

Deep CO2 storage reservoirs show a wide range of heterogeneous structures across different scales. Here, we show how high-resolution heterogeneity in both permeability and capillary entry pressure control dissolution and local capillary trapping of supercritical CO2 (ScCO2). Prior works mainly considered homogeneous reservoirs or simplified heterogeneity with horizontal and fixed capillary transition zones. Most of those studies ignored simultaneous free-phase ScCO2 spreading and solutal fingering. To advance understanding of heterogeneity effects, we have performed two-dimensional numerical simulations for ScCO2 injection into the middle of a storage reservoir. We have computed metrics including the spatial plume moments to compare the dissolution trapping and spreading of ScCO2 during the injection and post-injection period for different variances and correlation lengths of log-normal permeability and capillary entry pressure fields. The results show that the second-order spatial moment of the ScCO2 saturation field in the horizontal direction decreases by 10% as the permeability variance increases from 0.1 to 3.0. But it decreases by 50% as the capillary entry pressure variance becomes 1.1. For higher capillary entry pressure variance, the ratio of dissolved CO2 mass to free-phase ScCO2 mass decreases, showing higher local capillary trapping. The vertical distance between the centers of mass of the ScCO2 and dissolved CO2 plumes (representing the downward flow of dissolved CO2 through solutal fingering) is greater for heterogeneous reservoirs.

The influences of super-critical CO2 saturation on tensile characteristics and failure modes of shales

To investigate the mechanism of super-critical CO2 saturation on the tensile properties of shale, shale specimens were saturated with Sc-CO2, and inert gas (Ar) was employed as control group. Brazilian splitting experiment with different layering inclinations (0°, 30°, 60°, 90°) showed that after Sc-CO2 saturation, the Brazilian splitting strength (BSS) and Brazilian splitting modulus (E) of shale were decreased by 11.9% and 5.7%, respectively. Regardless of any direction of stress loading, the bedding planes of shale had been damaged. The failure mode of shale transformed from single tensile to tensile-shear mixed failure after Sc-CO2 saturation and less energy was required for shale failure. The weakening of carbonate cementation made the bedding planes more prone to damage, and the number of shear cracks after Sc-CO2 saturation was twice as much. The X-ray diffractometer, scanning electronic microscope and nuclear magnetic resonance results showed that the increase of porosity caused by calcite dissolution and organic matter extraction was the fundamental reason for the deterioration of shale tensile properties. Furthermore, gas pressure made shale more compact, and also induced new cracks formation in the meanwhile. Therefore, after Ar saturation, E and BSS of shale were increased and decreased, respectively.

The Influence of Super-critical CO2 Saturation on the Mechanical and Failure Properties of a North Sea Reservoir Sandstone Analogue

Large-scale geological carbon storage in the North Sea will involve injecting CO2 in a super-critical state into deep saline aquifers. As CO2 is injected, stress changes due to fluid pressure and temperature changes may result in deformation and potentially may result in microseismicity if existing faults are reactivated. Microseismicity may provide additional information on in situ stress conditions and the progression of the CO2 plume. To assess failure criteria in the reservoir, we must know the mechanical response of reservoir sandstone due to injection of super-critical CO2 (scCO2) under reservoir stress conditions. This includes fracturing processes at the microstructural scale as well as at the macroscopic scale by reactivation of existing fractures. We conducted three triaxial tests on a sandstone analogue to deep North Sea reservoir rock: once pressurized, samples were saturated with either scCO2, brine, or a brine-scCO2 mixture. Each sample was then deformed axially to induce a through-going fracture, which was then reactivated—first by increasing the pore pressure under anisotropic stress conditions, then by axial reloading with a constant pore pressure. Throughout testing, we monitored acoustic emissions (AE) and measured ultrasonic P-wave velocities. AE were located within the sample, and their relative magnitudes were calculated, as well as their source mechanisms for a subset of high signal-noise ratio events. We observed: • Higher strength and stiffness for the sample saturated only with scCO2 than for samples containing brine, and a brine-scCO2 mixture. • No clear difference in fracture strength for samples with different fluid saturations. • Higher energy events released from fracturing of the stronger and stiffer scCO2-saturated rock. • For all samples, the AE response during fracture reactivation was more energetic and had a higher frequency content at higher effective stresses (by axial loading) than at lower effective stresses (by pore pressure increase). We found fluid saturation to have only a slight influence on the mechanical and failure properties. We observed no significant weakening with scCO2 saturation that might otherwise cause undesired inelastic deformation within the reservoir.

Experimental and numerical investigation of supercritical CO2 migration in sandstone with multiple clay interlayers

CO2 enhanced water recovery (CO2-EWR) is one of the most promising technologies for geologic carbon sequestration. The migration characteristics and displacement efficiency of supercritical CO2 (SC-CO2) after injection into a saline aquifer directly affect the feasibility and economy of a CO2-EWR project. Multiple clay interlayers often develop in the target sandstone reservoir and combine in different expansive ways, which could influence the SC-CO2 migration. In this study, core flooding experiments were conducted using a special sandstone with multiple thin clay interlayers and the fluid behavior was monitored by an X-ray computed tomography (X-CT) scanner. Based on the porosity matrix obtained from X-CT images, a two-dimension model based on Darcy flow was developed to simulate the fluid behavior under the experiment conditions. The experimental results show that SC-CO2 pathways were developed in the sand part of the sample, while the clay interlayers were fluid-proof. Comparing the experimental and simulation results, it is evident that the porosity distribution, clay interlayers directionality/extent and flooding direction were the main factors that affected the fluid behavior in the sample. There were positive correlations between porosity and SC-CO2 saturation after drainage processes. Flooding directions and clay interlayers were combined to affect the fluid migration. The clay interlayers had functions of barrier and collection when the flooding direction pointed towards the opening of a wedge-shaped semi-open area formed by clay interlayers. While this area was almost free of SC-CO2 when the injection direction reversed, resulting a smaller SC-CO2 saturation in sample during the backward drainage process.

Influence of supercritical CO2 saturation on the failure process of hot dry rock with acoustic emission monitoring

To investigate the effect of supercritical CO2 saturation on acoustic emission (AE) signal properties in hot dry rock failure process, AE tests were conducted on granite under uniaxial compression. The results revealed that the strength of granite was obviously decreased after supercritical CO2 saturation, and new frequency bands appeared in the high-frequency region of AE and dominant frequency dispersion was enhanced. Dominant frequency mode was evolved from single mode dominated by high frequency to complex mode distributed in multiple frequencies indicating the failure modes diversified. Furthermore, supercritical CO2 saturation resulted in more uniform distribution of energy events and wider distribution of magnitude during granite failure. The decrease of b value as the prediction basis for failure might not be applicable. In addition, the degradation of the cementation degree of granite mineral particles and particle size were considered to be the fundamental reasons for the variation of AE properties.

Effect of supercritical CO2 saturation pressures and temperatures on the methane adsorption behaviours of Longmaxi shale

Abstract To investigate the effects of supercritical CO2 (SC–CO2) saturation pressures and temperatures on the methane adsorption capacity of shale, total organic carbon (TOC) analysis, X-ray diffraction (XRD) analysis, scanning electron microscope and energy dispersive spectrometer (SEM/EDS) analysis, low-pressure N2 adsorption (LP-NA), and high-pressure methane adsorption (HP-MA) were conducted on raw and SC–CO2–saturated (8, 12, 16 MPa; 40, 60, 80 °C) shale collected from the Sichuan Basin. Results indicate that the excess adsorption isotherm of shale to methane clearly exhibited excess adsorption characteristics with increasing adsorption pressures. This excess methane adsorption capacity decreased after SC-CO2 saturation and is mainly attributable to decreases in the TOC content, clay minerals and specific surface area (SSA) in the SC–CO2–saturated shale. The density of adsorbed phase methane was considered at different adsorption models, and the results suggest that the Langmuir-Freundlich (LF) and Dubinin-Astakhov (DA) models better describe methane adsorption behaviours than the Langmuir and Ono-kondo models. The methane adsorption capacity of shale is closely related to the solubility of SC-CO2, which decreased significantly with increasing SC-CO2 saturation time and pressure, while the influence of SC-CO2 saturation temperatures was not evident at low-pressure (8 MPa). This study provides a theoretical reference for CO2 enhanced shale gas recovery.

Effects of cyclic saturation of supercritical CO2 on the pore structures and mechanical properties of bituminous coal: An experimental study

The geological sequestration of CO2 in unmineable coal seams has gradually become one of the most effective means of responding to the global greenhouse effect. Presently, the cyclic injection of CO2 into coal seams is utilized in some enhanced CBM recovery projects with CO2 sequestration; therefore, a proper understanding of the effects of the cyclic saturation of supercritical CO2 (ScCO2) on coal is essential. Here, we present a series of uniaxial compressive strength (UCS) tests on bituminous coal subjected to both sustained and cyclic ScCO2 saturation. Nuclear magnetic resonance and acoustic emission (AE) are used to investigate changes in the pore structure distribution and porosity, as well as the fracture propagation. The results indicate that ScCO2 saturation enhances the continuity of the pore volume distribution, while cyclic saturation has a stronger influence on the pore structure. Furthermore, samples subjected to cyclic saturation exhibit significantly greater decreases in UCS and elastic modulus than the sustained-saturation samples, owing to mechanical fatigue caused by the cyclic saturation. The AE results show that cyclic saturation produces multiple signal releases in the form of unstable crack propagation, reducing crack closure and enhancing stable crack propagation. We also analyze the failure mechanism of coal samples under cyclic ScCO2 saturation in terms of the pore structure and mechanical changes experienced and discuss the influence of the cyclic injection of CO2 into coal seams. Hence, the results of this study are expected to provide a reference for the selection of appropriate CO2 injection methods and safety assessments for field projects involving coal-seam CO2 sequestration.

Experimental investigation on the influence of sub- and super-critical CO2 saturation time on the permeability of fractured shale

The permeability variation of shale due to the CO2 saturation induced deformation has significant influence on the overall CO2 injection capacity and shale gas production. In this study, the effect of subcritical (SubCO2) and supercritical CO2 (ScCO2) saturation and its action time on the permeability evolution of fractured shale sample was examined. Five saturation times (4, 10, 12, 18, 24 h), both for SubCO2 (6 MPa) and ScCO2 (8 MPa), were introduced to the shale sample. For each saturation cyclic, the permeability of shale sample was determined by helium injection with various injection pressures (2–6 MPa). Results indicated that the permeability of shale sample is significantly reduced after CO2 saturation, and is decreased with the increase of CO2 saturation time due to the time dependent adsorption-induced swelling. ScCO2 induced a higher permeability reduction and a longer permeability reduction process than SubCO2, due to its higher affinity and higher viscosity in shale. The permeability has a negative exponential relation with effective stress for SubCO2 and ScCO2 saturated shale samples. After CO2 saturation, the stress sensitivity of shale permeability increased with the CO2 saturation time, which is related to the time dependent mechanical weakening of shale induced by CO2-shale interaction. ScCO2 induced a larger increase in stress sensitivity of shale permeability than SubCO2, because it induced more remarkable weakening of mechanical properties in shale.

Study on the anisotropic permeability in different rank coals under influences of supercritical CO2 adsorption and effective stress and its enlightenment for CO2 enhance coalbed methane recovery

The permeability of coal reservoir is dynamically changed during CO2 enhance coalbed methane recovery projects (CO2-ECBM) and is controlled by the two main influencing factors: effective stress and CO2 adsorption. Six coal core samples parallel and vertical bedding plane directions were drilled, which were then adopted to conduct the permeability tests under conditions of cyclic loading/unloading and supercritical CO2 saturation. Results show that the permeability is depended on the coal rank, the middle rank coal possesses the highest initial permeability because of its moderate compaction and largely generated cleats during the coalification. The permeability is anisotropic, manifesting in the permeability in parallel bedding plane direction is larger than that in the vertical bedding plane direction. The stress sensitivity coefficient and IPLRi in parallel bedding plane direction exceed that in vertical bedding plane direction, however, PALR exhibits the opposite law. Hereat, the fractures parallel to bedding plane are susceptible to the stress change and the cleats vertical to bedding plane are sensitive to the ScCO2 adsorption. In view of the irreversible permeability damage by the stress change and CO2 adsorption, experiments in this paper confirms that prefracturing can dramatically decrease the IPLRi, stress sensitivity coefficient and PALR of coal seams, which may be the potential and effective measure to enhence the injectivity of CO2 in coal seams.

Experimental investigation on the mechanical properties of a low-clay shale with different adsorption times in sub-/super-critical CO2

Knowledge of the effect of carbon dioxide (CO2) on the mechanical properties of low-clay shales is essential to shale gas production and CO2 sequestration. In this paper, a series of uniaxial compressive strength (UCS) variable-time experiments were performed on low-clay shale samples saturated in sub-/super-critical CO2. The crack propagation process and micro scale variations were recorded by acoustic emission (AE) sensors with 3D ARAMIS technology and SEM tests together with EDS analysis. According to the experimental results, sub-/super-critical CO2 adsorption weakens the strength and increases the ductility of the shale. The UCS and Young's modulus decrease with the increase of saturation time. Compared to samples saturated in sub-critical CO2, samples saturated in super-critical CO2 present lower strength and Young's modulus. AE results show that samples saturated at a longer time in sub-/super-critical CO2 present a higher number of peak cumulative AE energy. Super-critical CO2 saturation creates more AE energy than sub-critical CO2 saturation. Based on the SEM results, sub-/super-critical CO2 adsorption creates some new pores in shale samples which lead to the strength decreasing. EDS analysis presents that CO2 adsorption increase the C content of the shale which demonstrates the occurrence of chemical reactions in the shale.

Experimental study of the effects of sub- and super-critical CO2 saturation on the mechanical characteristics of organic-rich shales

The interaction between carbon dioxide (CO2) and shale during the process of CO2 sequestration and shale gas recovery could significantly affect mechanical properties of the shale. In the current study, we performed experiments at 38 °C on shale samples from the Sichuan Basin aiming at investigating the effects of sub-critical CO2 (SubCO2) and super-critical CO2 (ScCO2) saturation on shale mechanics. Uniaxial compressive strength (UCS) test, X-ray diffraction (XRD) analysis, energy dispersive X-ray spectroscopy (EDX) analysis and acoustic emission (AE) analysis were conducted on the raw and CO2-saturated (4, 6, 8, 12 and 16 MPa) shale samples. Results indicate that SubCO2 saturation (4 and 6 MPa) causes reduction in UCS and elastic modulus (E) of the shale up to 22.9% and 23.1%, respectively. More significantly, ScCO2 saturation (8, 12 and 16 MPa) causes up to 33.9% reductions of UCS and 34.0% reduction of E. This phenomenon can be attributed to a higher adsorptive potential and dissolution capacity of the ScCO2 fluid. In addition, these two parameters gradually vary with increasing saturation pressure: They slowly decrease and reach minimum values at 12 MPa, and then slightly increase as the saturation pressure increases from 12 to 16 MPa because of the compression effect resulted from higher fluid pressure. Results of AE analysis show that, compared with the raw shale samples, cracks of the CO2-saturated samples have a longer closure stage while a shorter stable and unstable crack propagation stage. It reveals that mechanical weakening of the shale is controlled by microscopic damages that resulted from CO2 saturation. Overall, our study confirms that the influence of CO2 saturation on the mechanical characteristics of organic-rich shale samples is closely related to gas pressure and phase state of CO2.

Effects of CO2 on P-wave attenuation in porous media with micro-cracks: A synthetic modelling study

The presence of CO2 in hydrocarbon reservoirs can cause significant changes in seismic wave properties. In turn these properties can be used to map CO2 saturation in hydrocarbon reservoirs or aquifers — either from natural sources or by injection from the surface. We present the results of a synthetic modelling study of the effects of supercritical CO2 saturation on P-wave attenuation in a medium consisting of four horizontal layers, including a target aquifer. The target aquifer is modelled fully by an effective medium containing pores saturated with brine and/or CO2 and randomly-aligned microcracks at different densities. The other layers are modelled solely by their bulk seismic velocities and densities. We first compute synthetic seismograms for a reference case where the third layer is completely isotropic with no cracks, no pores and no fluid saturation. We then calculate synthetic seismograms for finite crack densities of 0.01, 0.02 and 0.03 at varying degrees of CO2 saturation in the third layer. The results of our analysis indicate that attenuation is sensitive both to CO2 saturation and the crack density. For a given crack density, attenuation increases gradually with decreasing percentage of CO2 saturation and reaches a maximum at around 10% saturation. The induced attenuation increases with crack density and with offset. These observations hold out the potential of using seismic attenuation as an additional diagnostic in the characterisation of rock formations for a variety of applications, including hydrocarbon exploration and production, subsurface storage of CO2 or geothermal energy extraction.

Influence of CO2 adsorption on the strength and elastic modulus of low rank Australian coal under confining pressure

Precise knowledge of changes in the CO2 adsorption-induced mechanical properties of deep coal seams is necessary for the safe and successful implementation of carbon dioxide-enhanced coal bed methane (CO2-ECBM) recovery. To date, little attention has been paid to the geo-mechanical property alterations in coal seams during CO2 adsorption under in-situ stress conditions. The aim of this study is therefore to discover how coal seam integrity varies with the introduction of CO2 under in-situ stress conditions, by conducting a series of tri-axial strength tests of Australian brown coal samples. The effect of CO2 exposure time on coal's mechanical properties was also investigated under super-critical CO2 saturation (10MPa) conditions, in order to determine the effect of long-term CO2 injection on coal seams' mechanical properties, as CO2-ECBM is a long-term process.According to the test results, irrespective of coal rank, the strength reduction in coal with CO2 injection under field conditions is significantly less than would be expected based on simple laboratory testing such as uniaxial tests. However, in any stress environment, high rank coals are subjected to greater strength and stiffness reductions with CO2 adsorption than low rank coals, due to their well-developed cleat system, and the injection of super-critical CO2 induces greater mechanical property alterations in coal than sub-critical CO2 injection. These strength and elastic modulus changes in coal with CO2 adsorption can be presented using a simple Langmuir-type equation, regardless of rank. Furthermore, the observation of the effect of long-term CO2 saturation on coal's mechanical properties revealed that, although CO2 adsorption-induced mechanical property alterations in coal are mostly completed with the first interaction with CO2, further structural re-arrangement may occur at a slower rate over time.