CO2 Fluid Research Articles

Diamond resorption and immiscibility of C-O-H fluid in kimberlites: Evidence from experiments in H2O – CO2 – SiO2 – MgO – CaO system at 1–3 GPa

Kimberlites are the deepest sourced magmas emplaced at the Earth's surface. They provide a “window” into the processes at the base of the subcratonic mantle. A better understanding of the origin, composition, and emplacement mechanisms of kimberlites is hampered by uncertainties in the contents of the two main volatiles, H2O and CO2. Diamond dissolution in H2O and in CO2 fluids produces distinct resorption features offering an opportunity to determine the composition of the magmatic fluid in kimberlites. Here we examined the relationship between H2O:CO2 ratio of the fluid and the style of diamond resorption by conducting experiments in C-O-H fluid saturated with silicates with variable H2O:CO2 ratios at the conditions of kimberlite ascent of 1–3 GPa and 1150–1350 °C. Our results showed that the geometry of etch pits on diamond and the resorption style evolve consistently as bulk CO2 content of the fluid changes from 0 to 50 to 50–90 and 90–100 mol%. The fluid composition at the run conditions was monitored by entrapment of synthetic fluid inclusions in olivine and quartz. The inclusions demonstrated the existence of a fluid miscibility gap at 1–3 GPa and 1250 °C with two fluid endmembers, an aqueous and a carbonic phase, which H2O:CO2 ratio at 1 GPa determined with confocal Raman microscopy is (H2O)0.62(CO2)0.38 and (H2O)0.12(CO2)0.88 respectively. Hence, diamond resorption morphology depends on the proportions of the end-member aqueous and carbonic fluids, which vary with the bulk composition of the fluid. The different density and ability of aqueous and carbonic fluids to dissolve silicates (olivine) would promote their separation in the rising magma column. Concentration of the lower density aqueous fluid towards the tip of the propagating dyke would facilitate more efficient fracturing of the country rocks and faster ascent of the kimberlite magma causing explosive eruption. We propose that preferential attachment of aqueous fluid bubbles would help to increase the buoyancy of olivine xenocrysts and possibly diamond in the kimberlite magma offering a mechanism for transporting the heavy mantle cargo.

Effects of pretreatments on physicochemical and structural properties of proteins isolated from canola seeds after oil extraction by supercritical-CO2 process

The present research was to investigate the influences of pretreatments on the structural and physicochemical properties of proteins isolated from canola seeds (CSP) after oil extraction by supercritical CO2 fluid. The results showed that the CSPs pretreated with microwave treatment (MV-CSP) and ultrasound treatment (UL-CSP) significantly differed in terms of molecular weight distribution, amino acid composition, and the primary conformation of CSP. The crystalline properties and secondary structural conformation as well as average particle sizes of CSPs were not significantly influenced by pretreatments, but the morphological characteristics were remarkably changed. The physicochemical properties including solubility, water/oil holding capacity, emulsifying capacity and stability, as well as foaming capacity and stability were substantially altered by the pretreatments. The MV-CSP and UL-CSP possessed better physicochemical characteristics. Therefore, microwave- and ultrasound-pretreatments could serve as potential technologies to improve the physicochemical properties of canola seeds proteins. Moreover, different oil extraction methods greatly influenced the quality and functional properties of isolated CSPs. The supercritical CO2 (SC–CO2) process protected intact functional properties and enhanced the overall quality of the isolated CSPs compared to solvent extraction.

Interface optimization of 4H-SiC (0001) MOS structures with supercritical CO2 fluid

Supercritical CO2 fluid is reported as an effective media in optimizing the SiO2/SiC interface in 4H-SiC(0001) MOS structures at a temperature as low as 150 °C. After SCCO2 treatment, the breakdown electric field is improved to 10.7 MV cm−1. The near-interfacial oxide traps is decreased from 1.62 × 1011 to 1.84 × 1010 cm−2. The interface state density at 0.2 eV below E C is reduced from 6 × 1012 to 2.5 × 1012 eV−1 cm−2. A dynamic reaction model is proposed to explain the defect passivation reaction. This technique can be effectively applied to interface semiconductor devices to enhance performance.

Equation of state for CO and CO2 fluids and their application on decarbonation reactions at high pressure and temperature

Ab initio molecular dynamics simulations were performed at pressures and temperatures up to 160 GPa and 4000 K, in order to obtain equations of state (EOS) for CO and CO2 fluids. We found that polymerisation of CO and CO2 fluids starts at low pressures, and that including the effect of polymerisation is essential for accurate EOS. EOSs for CO and CO2 determined from methods using experimental data, or classical potentials that ignore the changes in speciation, should be treated with caution when extrapolated beyond the examined pressures and temperatures. The obtained data was fitted to a modified Lee and Kesler EOS for both CO and CO2 fluids. The thermodynamic calculations for the decarbonation reactions of both MgCO3 and CaCO3 using the derived CO2 EOS reproduced the experimental data and theoretical calculations at low pressures. Both MgCO3 and CaCO3 pure phases are found to be stable in the upper mantle compared to CO2. However, they both become destabilised when approaching lower mantle conditions.

CFD-DEM simulation of proppant transport by supercritical CO2 in a vertical planar fracture

The great potential of supercritical CO2 fracturing technique in the stimulation of unconventional reservoirs requires further insight into the mechanism of proppant transport in the fracture. In this paper, we adopted the computational fluid dynamics (CFD) coupled discrete element method (DEM) scheme to study the proppant transport in supercritical CO2 fluid, and verified this method via previous experiment data. Based on the visual analysis at the particle-level, transport mechanisms of proppant in the fracture were revealed. The difference in the proppant transport capacity between supercritical CO2 and commonly used fracturing fluids were demonstrated, and effects of slurry injection velocity, proppant concentration, injection temperature, and proppant diameter were discussed. Results indicate that the transport and placement process of the proppant can be divided into three stages, i.e., flat laying stage, proppant dune development stage, and proppant bank development stage. The supercritical CO2 slurry is characterized by obvious stratified flow in the fracture: the static bed at the bottom, the fluidization layer above it, and the pure fluid flow region at the top. Viscous drag transport, lifting transport, and fluidization transport are three proppant transport mechanisms for supercritical CO2 slurry flow, among which the fluidization transport plays a more dominant role. This work is expected to provide an in-depth understanding of the transport mechanism of supercritical CO2 slurry in a vertical planar fracture.

Experimental constraint on grain-scale fluid connectivity in subduction zones

The dihedral angle (θ) between olivine and aqueous fluid is a critical parameter in identifying the grain-scale fluid connectivity which controls the distribution and migration of aqueous fluids in subduction zones besides physical properties of mantle wedges. Recent magnetotelluric observations have suggested the occurrence of significant fluid circulation in deep fore-arc regions, which can be explained by infiltration of saline fluid with a low θ in the mantle wedge (Huang et al., 2019). Along with the salt component, non-polarized gas such as CO2, is a crucial constituent of subduction zone fluids. CO2 is known to increase the olivine–fluid θ under conditions in which the olivine does not react with CO2, which is in contrast to the effect of NaCl on θ. For a better understanding of the connectivity of multicomponent fluid in the mantle wedge, we experimentally constrained θ in olivine + H2O–CO2 fluid and olivine + H2O–CO2–NaCl (multicomponent) fluid systems at 1–4 GPa and 800–1100°C. For the H2O–CO2 system, we found that CO2 tends to increase θ at 1 GPa and 800–1100°C and at 2 GPa and 1100°C. In contrast, CO2 reduced θ even below 60° at relatively high-pressure (P) and low-temperature (T) conditions, in which the olivine partly reacts with CO2 to form magnesite and orthopyroxene (opx). The consumption of non-wettable CO2 components in aqueous fluid alone cannot explain θ lower than those in a pure H2O system. Additional experiments on olivine–magnesite + H2O and olivine–opx + H2O systems showed that the presence of magnesite or opx decreased the olivine–fluid θ, which implies that coexisting minerals affect the olivine–fluid interfacial energy by changing the fluid chemistry. The results of the multicomponent system showed that the effect of NaCl on θ is much more significant than that of CO2. Strikingly, θ was smaller than 60° in all the magnesite- and opx-bearing multicomponent systems. Our results suggest that slab-derived fluid can infiltrate into the deep fore-arc mantle wedge through an interconnected network even in a CO2-bearing multicomponent system at pressures above 2 GPa, which facilitates significant fore-arc fluid circulation. The contrasting effects of aqueous fluid and silicate melt on the seismic wave velocity in a wide condition may allow for the possibility of mapping partial melt in the mantle wedge.

E&P Notes (October 2020)

E&P Notes (October 2020)

The hydrothermal system of Bagni San Filippo (Italy): fluids circulation and CO2 degassing

Bagni San Filippo area is characterized by the discharge of thermal waters and deeply produced CO2-rich gases both from vents and soil diffuse degassing. The thermal waters are the results of the mixing between meteoric waters and hot fluids deriving from the condensation, at depth, of vapours uprising from a deep hydrothermal reservoir. This process gives rise to a relatively shallow thermal system at temperature close to 50°C, characterized by SO4-rich and Cl-poor waters and elevated PCO2 (~7 bar). Most of the incondensable gas of deep originated vapour is released as a free gas phase forming cold gas vents and localized spots of anomalous CO2 diffuse degassing. The location and the shape of these degassing zones are strongly controlled by the main tectonic structures of the area. Through detailed soil diffuse degassing surveys and hydrogeochemical modelling, we estimate at 226-326 t d-1 and at 965 t d-1 the deep CO2 emission and the amount of condensates discharged by the thermal springs, respectively. The thermal energy associated to the process results at ~ 29 MW, most of which (~ 25 MW) is associated with condensation occurring at depths greater than groundwater circulation.

A laboratory study on extracting active ingredients from scrophularia striata boiss using ultrasound-assisted supercritical fluid extraction technique

ABSTRACT The aim of this study was adopted survey a novel method with the frequencies of 37 and 80 kHz and the power of 50–310 W in the supercritical extraction system to extracting ingredients from Scrophularia striata Boiss plant which include analgesics, bacteria, depression and infectious agents, using ultrasound-assisted supercritical CO2 fluid extraction (USC CO2) or supercritical CO2 fluid extraction (SC CO2) techniques at frequencies of 37 and 80 kHz, temperature of 45, 55 and 65 °C, and pressures of 150, 180, 210 bar. The ultrasonic technique is underpinned by the formation of high-frequency ultrasonic waves that can generate cavities. Such cavities cause disturbances in the cell walls of solid matrix, which facilitate the solvent penetration and mass transfer and thus increase the extraction yield. According to the results indicated a 24-percent increase in yield for the USC CO2 extraction technique at a frequency of 37 kHz, optimal pressure of 180 bar, and temperature of 65 °C, in comparison to the SC CO2 technique. Such an increase is acceptable given that the USC CO2 technique is relatively inexpensive and pro-environmental.

Hydrosilylation of High Porosity Porous Silicon with 1-Hexene in Supercritical CO2 Fluid

The surface of as-formed porous silicon (PSi) is terminated by Si-H bonds. Hydrogen-terminated PSi is generally unstable due to the high reactivity of these Si-H bonds. Thermally activated hydrosilylation of organic groups on hydride-terminated PSi enhances PSi stability because Si-C bonds and much more stable than Si-H bonds and also because of steric hindrance provided by the long organic molecules. In our previous work, gas-phase hydrosilylation for PSi with porosity of 68% was achieved by using 1-hexene, and highly stable luminescence was obtained by organic group termination [1].The higher the porosity, the higher the luminescent efficiency of PSi. When the porosity is higher than ~75%, however, it is extremely difficult for the as-formed PSi to preserve its fine structure due to the surface tension induced during air-drying. The supercritical fluid is characterized for its zero surface tension [2]. In this work, the process of both drying and hydrosilylation in the supercritical fluid for high porosity PSi was developed. The effect of this process was investigated in terms of FTIR and photoluminescence (PL) measurement.PSi layers were formed by anodization of p-type, (100)-oriented, and 5-10 Ω・cm silicon substrates with an Al back contact. Layers with porosity of 80% and thickness of approximately 1, 2, 5, and 10 mm were obtained by etching 200, 400, 1000, and 2000 s in the solution of HF (55 wt. %): ethanol = 1:1, at a constant current density of 10 mA/cm2. Following formation, PSi was rinsed in ethanol, then, without drying, placed inside a high-pressure reactor (volume: 5.5 ml) filled with ethanol. The drying and hydrosilylation were accomplished through the following procedures. First, liquid CO2 was flowed through the reactor to expel the ethanol inside and continuously purged until the reactor pressure was up to 10 MPa. Then, the reactor was heated at 80°C to yield the supercritical fluid CO2. The supercritical condition of 10 MPa and 80°C was maintained for 30 min to dry the PSi sample. After drying, the CO2 was continuously pumped into until the reactor pressure up to 15 MPa, and the reactor was heated at 150 °C. At this supercritical condition of 15 MPa and 150°C, CO2 - 1-hexene solution was supplied for 4 hours to achieve hydrosilylation. Finally, the reactor was cooled to the room temperature, and the supercritical CO2 was released.FTIR spectra of PSi samples before and after the hydrosilylation process were investigated. The FTIR results showed substitution of hydrogen on the PSi surface by organic groups via hydrosilylation.Photoluminescence of these samples were measured by exciting with a laser diode (wavelength is 405 nm) for 4 hours. Figure 1 illustrates the PL vs. time results of two samples (porosity 80%, thickness 2mm). Compared with the PL of the sample dried in the supercritical fluid, the sample after drying and hydrosilylation process shows good stability. The result indicates the availability of supercritical process for high porosity PSi. Gelloz, T. Iriyama, N. Takura, and E. Kondoh, and L. Jin, ECS Journal of Sol. Stat. Sci. and Tech. 8 (9), R109-R113 (2019).Kondoh and H. Kato, Microeletron, Eng. 64, 495-499 (2002). Figure 1

Sound velocity and refractive index of pure N2 fluid and of equimolar N2-CO2 fluid mixture up to 15 GPa.

The sound velocity and refractive index of pure N2 and of the equimolar N2-CO2 mixture are measured up to 15 GPa and 700 K in a resistive heating diamond anvil cell. The refractive index vs pressure is obtained by an interferometric method. The adiabatic sound velocity is then determined from the measurement of the Brillouin frequency shift in the backscattering geometry and the refractive index data. No phase separation of the N2-CO2 fluid mixture is observed. The fluid mixture properties are discussed in terms of ideal mixing.

Hydrosilylation of High Porosity Porous Silicon with 1-Hexene in Supercritical CO2 Fluid

Hydrosilylation of organic groups on hydrogen-terminated porous silicon enhances its stability and increasing the porosity of the porous silicon improves its luminescence efficiency. For as-formed thick porous silicon layers with high porosity, however, it is very difficult to preserve its fine structure during air-drying. Here, a process of both drying and hydrosilylation of highly porous silicon in ambient supercritical fluid was developed. 1-hexene was used for the hydrosilylation. The parameters of ambient temperature and processing time have been investigated for optimal process. The effects of the process were evaluated by means of Fourier transform infrared spectroscopy. Supercritical fluid hydrosilylation of porous silicon with thickness of 10 μm and porosity of 80% was achieved. Stable photoluminescence of processed porous silicon was also obtained.

Fluxing of mantle carbon as a physical agent for metallogenic fertilization of the crust

Magmatic systems play a crucial role in enriching the crust with volatiles and elements that reside primarily within the Earth’s mantle, including economically important metals like nickel, copper and platinum-group elements. However, transport of these metals within silicate magmas primarily occurs within dense sulfide liquids, which tend to coalesce, settle and not be efficiently transported in ascending magmas. Here we show textural observations, backed up with carbon and oxygen isotope data, which indicate an intimate association between mantle-derived carbonates and sulfides in some mafic-ultramafic magmatic systems emplaced at the base of the continental crust. We propose that carbon, as a buoyant supercritical CO2 fluid, might be a covert agent aiding and promoting the physical transport of sulfides across the mantle-crust transition. This may be a common but cryptic mechanism that facilitates cycling of volatiles and metals from the mantle to the lower-to-mid continental crust, which leaves little footprint behind by the time magmas reach the Earth’s surface.

Decarbonation Reactions Involving Ankerite and Dolomite under upper Mantle P,T-Parameters: Experimental Modeling

An experimental study aimed at the modeling of dolomite- and ankerite-involving decarbonation reactions, resulting in the CO2 fluid release and crystallization of Ca, Mg, Fe garnets, was carried out at a wide range of pressures and temperatures of the upper mantle. Experiments were performed using a multi-anvil high-pressure apparatus of a “split-sphere” type, in CaMg(CO3)2-Al2O3-SiO2 and Ca(Mg,Fe)(CO3)2-Al2O3-SiO2 systems (pressures of 3.0, 6.3 and 7.5 GPa, temperature range of 950–1550 °C, hematite buffered high-pressure cell). It was experimentally shown that decarbonation in the dolomite-bearing system occurred at 1100 ± 20 °C (3.0 GPa), 1320 ± 20 °C (6.3 GPa), and 1450 ± 20 °C (7.5 GPa). As demonstrated by mass spectrometry, the fluid composition was pure CO2. Composition of synthesized garnet was Prp83Grs17, with main Raman spectroscopic modes at 368–369, 559–562, and 912–920 cm−1. Decarbonation reactions in the ankerite-bearing system were realized at 1000 ± 20 °C (3.0 GPa), 1250 ± 20 °C (6.3 GPa), and 1400 ± 20 °C (7.5 GPa). As a result, the garnet of Grs25Alm40Prp35 composition with main Raman peaks at 349–350, 552, and 906–907 cm−1 was crystallized. It has been experimentally shown that, in the Earth’s mantle, dolomite and ankerite enter decarbonation reactions to form Ca, Mg, Fe garnet + CO2 assemblage at temperatures ~175–500 °C lower than CaCO3 does at constant pressures.

Genesis of the Maoping carbonate-hosted Pb–Zn deposit, northeastern Yunnan Province, China: evidences from geology and C–O–S–Pb isotopes

The Maoping Pb–Zn deposit (~ 3 Mt Pb + Zn reserves with grades of 12–30 wt%) is one of the largest Pb–Zn deposits in the Sichuan–Yunnan–Guizhou (SYG) metallogenic province, which has contributed a tremendous amount of lead and zinc resources for China. To obtain a further understanding of the sources of ore-forming materials and ore genesis of the deposit, S–Pb isotopes of sulfides and C–O isotopes of ore-stage calcites were systematically collected from representative orebodies at different elevations with a Finnigan MAT-253 mass spectrometer. The calcites separated from the sulfides of the No I and No II orebodies shared identical δ13CPDB values (− 5.3 to − 0.8 ‰) and δ18OSMOW values (+ 14.5 to + 21.8 ‰) with those of the calcites in the SYG region, suggesting that CO2 in regional ore-forming fluids possibly had a homologous C–O source that originated from a ternary mixture of the dissolution of marine carbonate rocks, degassing process of the Emeishan mantle plume, and dehydroxylation of sedimentary organic matter. The No. I-1 and No. I-2 orebody was hosted in the same strata, but the sulfur source of No. I-1 orebody (+ 13.1 to + 19.0 ‰) with equilibrated sulfur fractionation (δ34Ssphalerite δ34Sgalena) were different. They were derived from the allopatry thermochemical sulfate reduction (TSR) of overlying Carboniferous sulfates in the ore-hosting strata and local TSR of sulfates in the ore-bearing Upper Devonian Zaige Formation, respectively. The narrow and uniform Pb isotopic ratios of single galena grains collected from sulfides with 206Pb/204Pb of 18.713–18.759, 207Pb/204Pb of 15.772–15.776 and 208Pb/204Pb of 39.383–39.467 indicate a well-mixed metal source(s) that consist of Proterozoic Kunyang and Huili Group basement rocks and Devonian to Middle Permian ore-hosting sedimentary rocks. Besides, the late Permian Emeishan basalts are difficult to contribute metals for regional Pb–Zn mineralization despite a closely spatial relationship with the distribution of the Pb–Zn deposit. This is supported by Pb isotopic ratios plotting above the average upper crustal Pb evolution curves and staying far away from that of the age-corrected Emeishan basalts. Hence, taking into account of the similarities in tectonic setting, ore-hosting rock, ore assemblage, wall rock alteration, ore-controlling structure, and ore-forming materials and the differences in relationship with regional magmatism, fluid inclusion characteristic and ore grade between the Maoping deposit and typical MVT Pb–Zn deposit, the ore genesis of the Maoping deposit should be an MVT like Pb–Zn deposit.

Digital quantification of fracture in full-scale rock using micro-CT images: A fracturing experiment with N2 and CO2

How to effectively and cleanly exploit unconventional resources is currently being widely concerned. Due to the unavoidable limitations of water-based fracturing, CO2 and N2 etc., are regarded as alternative fluids for some specific reservoirs such as the arid area or high clay swelling formation. To investigate the characteristics of fracture induced by nonaqueous fluid in unconventional tight formation, fracturing experiments on tight sandstone using different fluid of water, N2, liquid CO2(L-CO2) and supercritical CO2(SC-CO2) were conducted. A novel fracture analysis approach based on CT scanning is also proposed to quantitatively analyze the micromorphology of fractures including fracture network pattern, fractal dimension (FD), aperture, area ratio (AR) and fracture volume etc. Results show that water and N2 can only induce a relatively simple fracture pattern. Whereas, fractures induced by CO2-based fluid have more complex fracture geometry with many fracture branches and various propagation orientations. Quantitative analysis based on image processing and fracture reconstruction algorithm indicates that the CO2-based fluid has up to 4.4 times larger fracture-induced capacity and can create fracture with up to 2.6 times higher complexity and 23.4% higher roughness than that generated by water-based fracturing. Comparing to SC-CO2, L-CO2 and water, N2 has the least fracture-induced capacity (83% of water) and produces more smooth and uniform fractures. On the other hand, the distributions of deviation angle suggest that fracture propagation is not only influenced by stress state, but also significantly affected by the original bedding plane. This experimental study proposes a precise and effective approach for fracture characteristic quantification, which can be further implemented into fracture simulation model. It also provides theoretical reference and guidance for field operation of waterless fracturing in development of unconventional oil and gas.

Adsorption behavior of CO2 in magnesite micro-pores at high temperature and pressure

The fluid inclusions in mantle rocks and melt indicated that a large amount of CO2 fluid exists in the deep earth, which is of great significance for understanding the deep carbon cycle and the composition of mantle. However, it was also suggested that carbonate minerals were likely to be the main host of mantle carbon. At the same time, the distribution and behavior of carbon in the mantle still remain a puzzle. In this paper, the adsorption behavior and occurrence characteristics of supercritical CO2 in magnesite (MgCO3) pores were studied by the Grand Canonical Monte Carlo method (GCMC) under the different conditions of CO2 pressures (0–100 ​MPa), temperatures (350–1500 ​K) and the pore sizes (7.5–30 ​Å). The simulated results showed that the adsorption of CO2 in magnesite was a physical adsorption, which was mainly controlled by the intermolecular force. The gas adsorption became more stable when the adsorption site shifted from the high energy site to the low energy site with increasing pressure (P) and decreasing temperature (T) and pore size. At the same time, the variations of excess adsorption amounts of CO2 in the pores of magnesite (Nexcess) under the different conditions were quantitatively calculated. It was found that the Nexcess decreased with increasing T, but increased with increasing P and pore size. The results favor understanding the CO2 migration, seismic precursor observations, and heat transfer process in the deep earth.

Identification of lipophilic components in Citri Reticulatae Pericarpium cultivars by supercritical CO2 fluid extraction with ultra-high-performance liquid chromatography-Q Exactive Orbitrap tandem mass spectrometry.

To systematically identify the lipophilic constituents of Citri Reticulatae Pericarpium from different cultivars, supercritical CO2 fluid extraction and ultra-high-performance liquid chromatography-Q Exactive Orbitrap tandem mass spectrometry were integrated for the component analysis of 18 batches of Citri Reticulatae Pericarpium from 12 cultivars for the first time. A total of 57 components from the supercritical CO2 fluid extracts were demonstrably or tentatively identified by the obtained parent peaks, fragment peaks, and retention times. In total, two flavonoids, six organic acids, nine coumarins, three aldehydes, seven esters, three terpenes, one limonoid, and five other compounds were detected for the first time; notably, coumarin components have not yet been reported in Citri Reticulatae Pericarpium. Furthermore, the extract constituents differed between cultivars. In particular, organic acids were more abundant in Citrus reticulata "Chachi" than in other cultivars, and pterostilbene was exclusively found in Citrus reticulata "Yichangju". The results showed that a greater variety of compounds in Citri Reticulatae Pericarpium could be extracted by supercritical CO2 fluid extraction and detected by ultra-high-performance liquid chromatography-Q Exactive Orbitrap tandem mass spectrometry. This study provides a more scientific basis for further analysis of the pharmacological activity and quality of Citri Reticulatae Pericarpium components from different cultivars.

Changes in the microstructure of low-rank coal after supercritical CO2 and water treatment

With a focus on different CO2 pressures and H2O, the influences of the ScCO2–H2O coupling effect on the microstructures of low-rank coal samples were compared and analyzed, offering further analysis of the CO2 sequestration capacity in coal seams rendered unworkable owing to the effect of water. By using nuclear magnetic resonance (NMR) and X-ray powder diffraction (XRD), the changes in porosity, pore size, pore size distribution (PSD), fractal dimension, and minerals in coal samples of the two states were compared and analyzed. XRD analysis revealed that a large number of carbonate rocks (calcite) and aluminosilicate minerals (clay minerals) were found in coal. ScCO2 presented the optimal dissolution effect in the water-saturated samples. NMR analysis showed that compared with a single CO2 fluid, the pore structures of the coal samples varied more remarkably under the coupling effect of CO2 and H2O. Moreover, under the supercritical state of CO2, the effect on the coal sample was the greatest. With the increase in pressure during the CO2 treatment, the porosity of the coal samples increased, and the proportion of macropores grew, indicating the transformation of small pores into large pores. The roughness of adsorption pores increased, whereas the complexity and heterogeneity of seepage pores in the coal samples after ScCO2 treatment declined. Generally, the ScCO2–H2O coupling effect on coal samples caused further changes in the pore structures, promoting mineral dissolution in the coal samples.

New Closed Blender Reduces Footprint, Gasification of CO2in Waterless Fracturing

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 19770, “Design, Optimization, and Application of a Closed Blender for CO2 Waterless Fracturing,” by Qinghai Yang, Siwei Meng, and Chuan Yu, PetroChina, et al., prepared for the 2020 International Petroleum Technology Conference, Dhahran, Saudi Arabia, 13-15 January. The paper has not been peer reviewed. Copyright 2020 International Petroleum Technology Conference. Reproduced by permission. Carbon dioxide (CO2) waterless fracturing uses liquid CO2 to replace water as the fracturing fluid in reservoir stimulation. The continuity and reliability of the blender are key factors determining performance of the operation. The complete paper proposes a novel closed blender that uses a vertical, rather than horizontal, tanker. This modification can reduce the footprint and effectively suppress CO2 gasification. The field practice suggests that the developed closed blender combines the advantages of both vertical and horizontal blenders and ensures successful implementation of CO2 waterless fracturing operations. Introduction The CO2 waterless fracturing process, as an alternative technology for developing unconventional reservoirs, greatly reduces consumption of water resources in the development of unconventional resources. In addition, compared with traditional fracturing measures, this technology has the advantages of low reservoir damage, complex artificial fractures, good energy-storage effect, and a high degree of recovery because of the unique physical and chemical properties of CO2. The CO2 waterless fracturing process requires the following equipment: CO2 tank trucks, booster pump trucks, sand blenders, high-pressure pump trucks, and low-and high-pressure manifolds. The blender acts as the core equipment for dry fracturing of liquid CO2, its main function to mix CO2 with proppant. It then feeds the mixed sand fluid to the fracturing pump truck for CO2 water-less fracturing. In the fracturing process, CO2 on the ground is required to be under low temperature and high pressure continuously to maintain its liquefaction. In addition, CO2 fluid has characteristics of low viscosity, high friction resistance, corrosiveness, and poor lubrication. Compared with traditional hydraulic fracturing processes, the sand blender for CO2 waterless fracturing has specific requirements. Components of the Closed Blender Drawing lessons from design of internationally available closed blenders for CO2 waterless fracturing, the authors have developed independently a new generation of closed blender, mainly composed of a sand tanker, a sand-mixing system, a blender manifold (a discharge manifold and a suction manifold), a chemical-addition system, a hydraulic system, an electronic control system, and a data-acquisition system (Fig. 1).