Supercritical CO2 uptake by nonswelling phyllosilicates

Geochemical evaluation of Washita-Fredericksburg formation as a carbon storage reservoir

Geological carbon sequestration is a promising technique to reduce atmospheric greenhouse gas emissions. The Washita-Fredericksburg formation in the southeastern United States is being considered as a prospective storage formation. This requires understanding the geochemical impact of CO2 injection on the formation, which is the focus of this work. Here, sandstone samples from the Washita-Fredericksburg formation are analyzed to understand their overall mineralogical composition and the potential geochemical processes that might occur following CO2 injection. Powder X-ray diffraction (XRD) analysis, Scanning Electron Microscopy (SEM) imaging, and image analysis were used to identify mineral phases. SEM images were processed to create a segmented mineral map, which was then used to calculate mineral volume fractions and porosity. Results show that the sample has a porosity of 20% and is mainly composed of quartz, K-feldspar, muscovite, and clays. Accessory minerals such as titanite were also found. Reactive transport models were constructed to assess potential CO2-brine-mineral interactions following CO2 injection. Simulation results suggest that the overall extent of mineral dissolution and precipitation reactions over 10,000 days is limited, with muscovite dissolution increasing porosity to 22%. Limited mineral reactions suggest more injected CO2 will exist in free and dissolved forms, which may require more extensive long-term monitoring.

Geochemical controls on CO2 interactions with deep subsurface shales: implications for geologic carbon sequestration

One of the primary drivers of global warming is the exponential increase in CO₂ emissions. According to IPCC, if the CO₂ emissions continue to increase at the current rate, global warming is likely to increase by 1.5 °C, above pre-industrial levels, between the years 2030 and 2052. Efficient and sustainable geologic CO₂ sequestration (GCS) offers one plausible solution for reducing CO₂ levels. The impermeable shale formations have traditionally served as good seals for reservoirs in which CO₂ has been injected for GCS. The rapid development of subsurface organic-rich shales for hydrocarbon recovery has opened up the possibility of utilizing these hydraulically fractured shale reservoirs as potential target reservoirs for GCS. However, to evaluate the GCS potential of different types of shales, we need to better understand the geochemical reactions at CO₂-fluid-shale interfaces and how they affect the flow and CO₂ storage permanence. In this review, we discuss the current state of knowledge on the interactions of CO₂ with shale fluids, minerals, and organic matter, and the impact of parameters such as pressure, temperature, and moisture content on these interactions. We also discuss the potential of using CO₂ as an alternate fracturing fluid, its role in enhanced shale gas recovery, and different geochemical tracers to identify whether CO₂ or brine migration occurred along a particular fluid transport pathway. Additionally, this review highlights the need for future studies to focus on determining (1) the contribution of CO₂ solubility and the impact of formation water chemistry on GCS, (2) the rates of dissolution/precipitation and sorption reactions, (3) the role of mineralogical and structural heterogeneities in shale, (4) differences in reaction mechanisms/rates between gaseous CO₂vs. brine mixed CO₂vs. supercritical CO₂, (5) the use of CO₂ as a fracturing fluid and its proppant carrying capacity and (6) the role of CO₂ in enhanced hydrocarbon recovery.

Enhanced Sorption of Supercritical CO2 and CH4 in the Hydrated Interlayer Pores of Smectite

Understanding the long-term confinement of supercritical fluids in the clay pores of subsurface rocks is important for many geo-energy technologies, including geological CO₂ storage. However, the adsorption properties of hydrated clay minerals remain largely uncertain because competitive adsorption experiments of supercritical fluids in the presence of water are difficult. Here, we report on the sorption properties of four source clay minerals-Ca-rich montmorillonite (STx-1b), Na-rich montmorillonite (SWy-2), illite-smectite mixed layer (ISCz-1), and illite (IMt-2)-for water at 20 °C up to relative humidity of 0.9. The measurements unveil the unsuitability of physisorption analysis by N₂ (at 77 K) and Ar (at 87 K) gases to quantify the textural properties of clays because of their inability to probe the interlayers. We further measure the sorption of CO₂ and CH₄ on swelling STx-1b and nonswelling IMt-2, both in the absence (dehydrated at 200 °C) and the presence of sub-1W preadsorbed water (following dehydration) up to 170 bar at 50 °C. We observe enhanced sorption of CO₂ and CH₄ in STx-1b (50 and 65% increase at 30 bar relative to dry STx-1b, respectively), while their adsorption on IMt-2 remains unchanged, indicating the absence of competition with water. By describing the supercritical adsorption isotherms on hydrated STx-1b with the lattice density functional theory model, we estimate that the pore volume has expanded by approximately 6% through the formation of sub-nanometer pore space. By presenting a systematic approach of quantifying the smectite clay mineral's hydrated state, this study provides an explanation for the conflicting literature observations of gas uptake capacities in the presence of water.

A review on clay wettability: From experimental investigations to molecular dynamics simulations

Clay is one of the most important mineral components in geological formations, and it is widely used in many industrial applications. One clay property, which is of key importance in many areas, e.g. mineral processing, agriculture, fundamental geologic understanding, hydrology, oil/water separation and multi-phase fluid flow, is clay wettability. However, clay wettability is a complex parameter which is determined by clay surface chemistry, in-situ aqueous and non-aqueous fluid chemistries, and geo-thermal conditions. Thus, a systematic review of published results on the wettability of six different clay minerals (kaolinite, montmorillonite, illite, mica, talc and pyrophyllite) is provided here, spanning from experimental studies to molecular dynamics simulations. This is integrated with a critical discussion to elucidate the origin of significant inconsistencies in the reported data. Finally, a range of conclusions is clearly established and a future outlook is given. This review will thus advance the understanding of clay wettability and provide guidance for the various applications discussed.

A Review on the Influence of CO2/Shale Interaction on Shale Properties: Implications of CCS in Shales

Carbon capture and storage (CCS) is a developed technology to minimize CO2 emissions and reduce global climate change. Currently, shale gas formations are considered as a suitable target for CO2 sequestration projects predominantly due to their wide availability. Compared to conventional geological formations including saline aquifers and coal seams, depleted shale formations provide larger storage potential due to the high adsorption capacity of CO2 compared to methane in the shale formation. However, the injected CO2 causes possible geochemical interactions with the shale formation during storage applications and CO2 enhanced shale gas recovery (ESGR) processes. The CO2/shale interaction is a key factor for the efficiency of CO2 storage in shale formations, as it can significantly alter the shale properties. The formation of carbonic acid from CO2 dissolution is the main cause for the alterations in the physical, chemical and mechanical properties of the shale, which in return affects the storage capacity, pore properties, and fluid transport. Therefore, in this paper, the effect of CO2 exposure on shale properties is comprehensively reviewed, to gain an in-depth understanding of the impact of CO2/shale interaction on shale properties. This paper reviews the current knowledge of the CO2/shale interactions and describes the results achieved to date. The pore structure is one of the most affected properties by CO2/shale interactions; several scholars indicated that the differences in mineral composition for shales would result in wide variations in pore structure system. A noticeable reduction in specific surface area of shales was observed after CO2 treatment, which in the long-term could decrease CO2 adsorption capacity, affecting the CO2 storage efficiency. Other factors including shale sedimentary, pressure and temperature can also alter the pore system and decrease the shale “caprock” seal efficiency. Similarly, the alteration in shales’ surface chemistry and functional species after CO2 treatment may increase the adsorption capacity of CO2, impacting the overall storage potential in shales. Furthermore, the injection of CO2 into shales may also influence the wetting behavior. Surface wettability is mainly affected by the presented minerals in shale, and less affected by brine salinity, temperature, organic content, and thermal maturity. Mainly, shales have strong water-wetting behavior in the presence of hydrocarbons, however, the alteration in shale’s wettability towards CO2-wet will significantly minimize CO2 storage capacities, and affect the sealing efficiency of caprock. The CO2/shale interactions were also found to cause noticeable degradation in shales’ mechanical properties. CO2 injection can weaken shale, decrease its brittleness and increases its plasticity and toughness. Various reductions in tri-axial compressive strength, tensile strength, and the elastic modulus of shales were observed after CO2 injection, due to the dissolution effect and adsorption strain within the pores. Based on this review, we conclude that CO2/shale interaction is a significant factor for the efficiency of CCS. However, due to the heterogeneity of shales, further studies are needed to include various shale formations and identify how different shales’ mineralogy could affect the CO2 storage capacity in the long-term.

Swelling Phenomena of the Nonswelling Clay Induced by CO2 and Water Cooperative Adsorption in Janus-Surface Micropores

With the development of microscopy and sensor techniques, it becomes evident that nonswelling clays show swelling behavior under CO₂-water mixture environments at high pressures and temperatures. The examples include Illite, muscovite, and kaolinite-rich rock samples. Here, we investigated the underlying mechanisms of kaolinite swelling induced by CO₂ and water using molecular simulations and low-pressure gas adsorption experiments. The results suggest the cooperative adsorption behavior of CO₂ and water on contact with kaolinite micropores, which have distinct wettabilities on the two adjoining interlayer surfaces. Even if clay-bound water exists, CO₂ can enter the micropores to induce swelling. The measured micropore volume, simulated equilibrium stable interlayer distance with pure water, and that with CO₂-water mixture were used in the swelling estimation, which shows good agreement with our experiments. The CO₂ and water molecule distributions inside the interlayer micropores verify the importance of the wettabilities of the kaolinite surfaces in this cooperative adsorption behavior. The result extends the traditional understanding of the swelling mechanism, i.e., cation hydration and subsequent osmotic processes. In addition to earlier observations of kaolinite swelling behavior with potassium acetate, our study indicates the significance of the subtle balance of the noncovalent interactions between CO₂, water, and the kaolinite Janus surfaces.

Pore-Scale Experimental Investigation of the Effect of Supercritical CO2 Injection Rate and Surface Wettability on Salt Precipitation

Injectivity is one of the most important factors to evaluate the feasibility of CO₂ geological storage. Salt precipitation due to the mass of dry CO₂ injected into a saline reservoir may cause a significant decrease in injectivity. However, the coupling effect of injection parameters and reservoir conditions on salt precipitation is not clear. Here, we conducted pore-scale visualization experiments to study the morphology and distribution of salt precipitation in porous structures and their effects on permeability reduction. The experimental results are achieved by controlling the supercritical CO₂ injection rate and the surface wettability at the reservoir temperature and pressure. It is found that for hydrophilic and neutral porous surfaces, ex situ precipitation occurs and blocks the entirety of pore throats and bodies, which results in a significant reduction in permeability. Increasing the CO₂ injection rate can suppress the capillary reflow and prevent the permeability reduction. For a hydrophobic porous surface, in situ precipitation occurs and occupies much smaller pore volume, which has a slight effect on permeability reduction compared to the hydrophilic samples at the same injection rate. Increasing the CO₂ injection rate and dewetting the injection well and formation nearby reduce the possibility of salt accumulation, which has less effect on CO₂ injectivity.

Supercritical CO2 and CH4 Uptake by Illite-Smectite Clay Minerals

Clay minerals abound in sedimentary formations and the interaction of reservoir gases with their submicron features have direct relevance to many geoenergy applications. The quantification of gas uptake over a broad range of pressures is key toward assessing the significance of these physical interactions on enhancing storage capacity and gas recovery. We report a systematic investigation of the sorption properties of three source clay minerals-Na-rich montmorillonite (SWy-2), illite-smectite mixed layer (ISCz-1), and illite (IMt-2)-using CO₂ and CH₄ up to 30 MPa at 25-115 °C. The textural characterization of the clays by gas physisorption indicates that micropores are only partly accessible to N₂ (77 K) and Ar (87 K), while larger uptakes are measured with CO₂ (273 K) in the presence of illite. The supercritical excess sorption experiments confirm these findings while revealing differences in uptake capacities that originate from the clay-specific pore size distribution. The lattice density functional theory model describes accurately the measured sorption isotherms by using a distribution of properly weighted slit pores and clay-specific solid-fluid interaction energies, which agree with isosteric heats of adsorption obtained experimentally. The model indicates that the maximum degree of pore occupancy is universal to the three clays and the two gases, and it depends solely on temperature, reaching values near unity at the critical temperature. These observations greatly support the model's predictive capability for estimating gas adsorption on clay-bearing rocks and sediments.