High Pressure Temperature Conditions Research Articles

Constraining composition and temperature variations in the mantle transition zone

The mantle transition zone connects two major layers of Earth’s interior that may be compositionally distinct: the upper mantle and the lower mantle. Wadsleyite is a major mineral in the upper mantle transition zone. Here, we measure the single-crystal elastic properties of hydrous Fe-bearing wadsleyite at high pressure-temperature conditions by Brillouin spectroscopy. Our results are then used to model the global distribution of wadsleyite proportion, temperature, and water content in the upper mantle transition zone by integrating mineral physics data with global seismic observations. Our models show that the upper mantle transition zone near subducted slabs is relatively cold, enriched in wadsleyite, and slightly more hydrated compared to regions where plumes are expected. This study provides direct evidence for the thermochemical heterogeneities in the upper mantle transition zone which is important for understanding the material exchange processes between the upper and lower mantle.

High pressure-temperature single-crystal elasticity of ringwoodite: Implications for detecting the 520 discontinuity and metastable ringwoodite at depths greater than 660 km

The 520 km discontinuity (the 520) and the 660 km discontinuity (the 660) are primarily caused by the wadsleyite to ringwoodite and ringwoodite to bridgmanite + ferropericlase phase transitions, respectively. Global seismic studies show significant regional variations of the 520, which are likely due to chemical and thermal heterogeneities in the Mantle Transition Zone (MTZ). However, the effects of chemical composition and temperature on the detectability of the 520 are unclear. Additionally, it remains unknown whether the possibly existing metastable ringwoodite in the core of the cold and fast subducting slabs could create a detectable seismic signature near the top of the lower mantle. Our understanding of both issues is hindered by the lack of single-crystal elasticity measurements of ringwoodite at simultaneous high pressure-temperature (P-T) conditions. In this study, we measured the single-crystal elasticity of an anhydrous Fe-bearing ringwoodite up to 32 GPa and 700 K by Brillouin spectroscopy, and then modeled the composition-dependent elastic properties of ringwoodite to calculate the compositional effects on the velocity jumps at the 520. We found that opposite to the effect of Fe, water enhances the Vp (P-wave velocity) jump, yet decreases the Vs (S-wave velocity) jump of the 520 across the wadsleyite to ringwoodite transition. Higher temperature increases both Vp and Vs contrasts across the 520. At depths between 660-700 km in the lower mantle, the existence of metastable ringwoodite may only result in ∼1-2% low velocity anomaly, which is seismically difficult to resolve. The low velocity anomaly caused by metastable ringwoodite increases to 5-7% at 750 km depth due to the weak pressure dependence of Vs in ringwoodite at lower mantle conditions, but whether it is seismically detectable depends on the extension of the regions in subducted slabs that are sufficiently cold to host metastable ringwoodite.

Modeling of gas viscosity at high pressure-high temperature conditions: Integrating radial basis function neural network with evolutionary algorithms

Increase in global natural gas production over the last 15 years has led to the use of new and untapped reservoirs including high pressure-high temperature ones in order to meet the consumer demands. As flow characteristics of gas in various environments such as porous media, wells, and pipelines are influenced by its viscosity, it is an important parameter for petroleum engineers. In this work, an immense gas viscosity dataset consisting of 3017 laboratory data points was used for properly implementing two smart techniques: radial basis function neural network with two training algorithms as well as multilayer perceptron neural network with four training algorithms. By using these techniques, various models with high accuracies were developed for viscosities estimation of gas mixture, pure methane, and pure nitrogen at high pressures (5000–25000 psia) and high temperatures (100–1880 °F). The radial basis function (RBF) neural network with ant colony optimization (ACO) (namely RBF-ACO model) is considered as the best model. Average absolute relative errors of the aforementioned model for estimating pure methane, pure nitrogen and gas mixture viscosities are 0.36 %, 0.49 %, and 1.76 %, respectively. RBF-ACO model provides better results comparing with other presented empirical models. Also, RBF neural network optimized by particle swarm optimization (PSO) shows a high error for estimating the viscosities of methane and gas mixture and RBF neural network optimized by genetic algorithm (GA) yields a high error for estimating the viscosity of gas mixture. Afterwards, effects of input parameters on the viscosity value obtained by RBF-ACO model, were investigated using the relevancy factor. Finally, based on numerical simulation, a sensitivity analysis was conducted for measuring the uncertainty of cumulative gas production resulted from gas viscosity estimation for a high pressure-high temperature gas reservoir. This process indicates that accurate estimation of gas viscosity plays an important role in reliable estimation of cumulative gas production.

Structural Diversity of Magnetite and Products of Its Decomposition at Extreme Conditions.

Magnetite, Fe3O4, is the oldest known magnetic mineral and archetypal mixed-valence oxide. Despite its recognized role in deep Earth processes, the behavior of magnetite at extreme high-pressure high-temperature (HPHT) conditions remains insufficiently studied. Here, we report on single-crystal synchrotron X-ray diffraction experiments up to ∼80 GPa and 5000 K in diamond anvil cells, which reveal two previously unknown Fe3O4 polymorphs, γ-Fe3O4 with the orthorhombic Yb3S4-type structure and δ-Fe3O4 with the modified Th3P4-type structure. The latter has never been predicted for iron compounds. The decomposition of Fe3O4 at HPHT conditions was found to result in the formation of exotic phases, Fe5O7 and Fe25O32, with complex structures. Crystal-chemical analysis of iron oxides suggests the high-spin to low-spin crossover in octahedrally coordinated Fe3+ in the pressure interval between 43 and 51 GPa. Our experiments demonstrate that HPHT conditions promote the formation of ferric-rich Fe-O compounds, thus arguing for the possible involvement of magnetite in the deep oxygen cycle.

Coarsening reduction strategies to stabilize CO2-foam formed with a zwitterionic surfactant in porous media

Micromodel experiments at high-pressure high-temperature conditions were performed to evaluate the impact of different coarsening reduction strategies on CO2-foams stabilized with a zwitterionic surfactant, using silica nanoparticles in small concentration and mixing CO2 with a less water-soluble gas (nitrogen). The CO2-foam stabilized only by the surfactant showed poor behavior in the porous media, while an improvement in both foam texture (bubble density at the inlet of the micromodel) and gas trapping was qualitatively seen in the experiments using a nanofluid containing silica nanoparticles. The strong adsorption of the nanoparticles at the gas-water interface allowed to reduce the coarsening mechanism, reaching a minimum pressure gradient for strong foam generation throughout the micromodel chip. The use of a small concentration of nanoparticles in combination with the zwitterionic surfactant proved to be effective in reducing gas mobility without significant increase in pressure drop, which could be used as a strategy to control the mobility of injected CO2 in high permeability porous media, with reduced loss of injectivity. Coarsening was also reduced by using a mixture of CO2 and N2 as foaming gas, with an even greater gas mobility reduction, achieving a pressure drop 7.6 times higher compared to the CO2-foam stabilized only by surfactant. Furthermore, larger areas with trapped gas were visualized along the micromodel when co-injecting the mixture of gases and the zwitterionic surfactant solution. The large decrease in gas mobility seen for this strategy suggests that using a gas mixture would be suitable to control gas mobility in high-permeability channels and to block thief zones. The combination of using a gas mixture and a nanofluid to decrease CO2-foam destruction in porous media yielded a slight reduction in gas mobility, similar to that obtained with CO2-foam in the presence of nanoparticles, for they had similar pressure-drop values at same injection flow rate. The results obtained in this work demonstrated the importance of interfacial phenomena for tuning of coarsening and coalescence mechanisms associated with CO2-foam stability to the desired pore-scale application.

Synthesis of paracrystalline diamond.

Solids in nature can be generally classified into crystalline and non-crystalline states1-7, depending on whether long-range lattice periodicity is present in the material. The differentiation of the two states, however, could face fundamental challenges if the degree of long-range order in crystals is significantly reduced. Here we report a paracrystalline state of diamond that is distinct from either crystalline or amorphous diamond8-10. The paracrystalline diamond reported in this work, consisting of sub-nanometre-sizedparacrystallites that possess a well-defined crystalline medium-range order up to a few atomic shells4,5,11-13, was synthesized in high-pressure high-temperature conditions (for example, 30 GPa and 1,600 K) employing face-centred cubic C60 as a precursor. The structural characteristics of the paracrystalline diamond were identified through a combination of X-ray diffraction, high-resolution transmission microscopy and advanced molecular dynamics simulation.The formation of paracrystalline diamond is a result of densely distributed nucleation sites developed in compressed C60 as well as pronounced second-nearest-neighbour short-range order in amorphous diamond due to strong sp3 bonding. The discovery of paracrystalline diamond adds an unusual diamond form to the enriched carbon family14-16, which exhibits distinguishing physical properties and can be furthered exploited to develop new materials. Furthermore, this work reveals the missing link in the length scale between amorphous and crystalline states across the structural landscape, having profound implications for recognizing complex structures arising from amorphous materials.

Application of Cellulosic Fibrous Material from Agro Waste as Filtrate Loss Modifier in Aqueous Mud-Morphology at HTHP Conditions

Controlling the filtration characteristics of any drilling fluid does not only include the control of the filtrate volume penetrating into the formation; but also the ability of the mud to deposit a thin low-permeability filter cake on the wall of the wellbore quickly. The permeability of the filter cake is very dependent on the particle size because when small size particles are used, the permeability decreases, because of the fact that colloidal particles get packed very tightly. This study investigated the filtration and filter cake characteristics of water-based mud (WBM) using fibrous cellulose from Tiger Nut waste and the guar gum. American Petroleum Institute (API) Recommended Practice Standard Procedure for Field Testing Drilling Fluids, API RP 13B-1 was applied during the analysis. Statistical analysis of the experimental data was conducted and the R2 value (0.99) showed that the experimental method adopted was replicable. Mud samples B2 and C2 gave an optimum result and their mud cakes developed under High-Temperature High-Pressure (HTHP) condition were dried and further analyzed with scanning electron microscope (SEM). The results showed that the filter cakes SEM structure for cellulosic fibrous from agro waste and guar gum exhibits similar characteristics; and the mud cakes was firm after soaking with 15% HCl for thirty minutes but started dissolving after one hour. The SEM analysis inferred that the filter cake morphology shows a good particle-pore interlocking for sample C2 than B2.

Preparation of CoGe2-type NiSn2 at 10 GPa

Abstract An unprecedented NiSn2 intermetallic with CoGe2-type crystal structure has been recovered (at ambient conditions) after high-pressure high-temperature treatment of a Ni33Sn67 precursor alloy at 10 GPa and 400 °C. The orthorhombic structure with Aeam space group symmetry is pseudotetragonal. Based on the evaluation of powder X-ray diffraction data, lattice parameters of a = b = 6.2818 Å and c = 11.8960 Å have been determined. Complicated line broadening and results of a further microstructure analysis, however, imply a defective character of the crystal structure. First-principles calculations with different model structures and a comparison with structural trends in the literature suggest that at the high-pressure high-temperature conditions a CuAl2-type crystal structure might be stable, which transforms to the recovered CoGe2-type crystal structure upon cooling or the release of pressure.

Simultaneous Control of Formation and Growth of Asphaltene Solids and Wax Crystals Using Single-Walled Carbon Nanotubes: an Experimental Study under Real Oilfield Conditions

Asphaltene and wax depositions in oil production systems have been recognized as the most serious flow assurance concern of petroleum engineering. Despite the several chemicals offered for treatment of issues induced by these organic deposits, proposing a single efficient chemical to control simultaneously both asphaltene and wax precipitation has not been attended yet. Here, single-walled carbon nanotubes (SWCNTs) were synthesized to assess their effects on controlling the formation and size growth of asphaltene particles and wax crystals in live oil under realistic production conditions. SDS experiments showed that during depressurization at 125 °C, the asphaltene onset pressure decreased from 5946 psi in untreated live oil to 4375 in treated live oil with 100 ppm SWCNTs. High-pressure microscopy revealed that SWCNTs efficiently controlled the growth of asphaltene aggregates and reduced the size of asphaltenes in live oil during depressurization at 125 °C. This is ascribed to the strong π–π interactions between the aromatic rings of SWCNTs and electron clouds of asphaltenes. Results of cross-polarized microscopy showed that SWCNTs at an optimum concentration of 75 ppm postponed the wax formation temperature and gelation point from 32.6 and 21.5 °C to 23.2 and 14.7 °C, respectively. In addition, the morphology of the wax crystals with an average size of 756 nm in untreated oil changed from dense spherical-like into thin needle-like with an average size of 86 nm in treated oil with SWCNTs at 75 ppm. The changes in the wax crystal size and structure via X-ray powder diffraction analysis after crude oil treatment with SWCNTs confirm interactions between the carbon nanotubes and active sites of the wax crystals. The findings of this research prove that synthesized carbon nanotubes can be used as efficient inhibitor and dispersant agents for stabilizing the waxes and asphaltenes in live oil with controlling the rate of aggregation under high-pressure–high-temperature conditions.

Synthesis of Ilmenite-type ε-Mn2O3 and Its Properties.

In contrast to the corundum-type A2X3 structure, which has only one crystallographic site available for trivalent cations (e.g., in hematite), the closely related ABX3 ilmenite-type structure comprises two different octahedrally coordinated positions that are usually filled with differently charged ions (e.g., in Fe2+Ti4+O3 ilmenite). Here, we report a synthesis of the first binary ilmenite-type compound fabricated from a simple transition-metal oxide (Mn2O3) at high-pressure high-temperature (HP-HT) conditions. We experimentally established that, at normal conditions, the ilmenite-type Mn2+Mn4+O3 (ε-Mn2O3) is an n-type semiconductor with an indirect narrow band gap of Eg = 0.55 eV. Comparative investigations of the electronic properties of ε-Mn2O3 and previously discovered quadruple perovskite ζ-Mn2O3 phase were performed using X-ray absorption near edge spectroscopy. Magnetic susceptibility measurements reveal an antiferromagnetic ordering in ε-Mn2O3 below 210 K. The synthesis of ε-Mn2O3 indicates that HP-HT conditions can induce a charge disproportionation in simple transition-metal oxides A2O3, and potentially various mixed-valence polymorphs of these oxides, for example, with ilmenite-type, LiNbO3-type, perovskite-type, and other structures, could be stabilized at HP-HT conditions.

Dynamical Immiscibility of Aqueous Carbonate Fluid in the Shortite–Water System at High-Pressure–Temperature Conditions

Dynamical Immiscibility of Aqueous Carbonate Fluid in the Shortite–Water System at High-Pressure–Temperature Conditions

Experimental Investigation of the Rheological Behavior of an Oil-Based Drilling Fluid with Rheology Modifier and Oil Wetter Additives.

Drilling issues such as shale hydration, high-temperature tolerance, torque and drag are often resolved by applying an appropriate drilling fluid formulation. Oil-based drilling fluid (OBDF) formulations are usually composed of emulsifiers, lime, brine, viscosifier, fluid loss controller and weighting agent. These additives sometimes outperform in extended exposure to high pressure high temperature (HPHT) conditions encountered in deep wells, resulting in weighting material segregation, high fluid loss, poor rheology and poor emulsion stability. In this study, two additives, oil wetter and rheology modifier were incorporated into the OBDF and their performance was investigated by conducting rheology, fluid loss, zeta potential and emulsion stability tests before and after hot rolling at 16 h and 32 h. Extending the hot rolling period beyond what is commonly used in this type of experiment is necessary to ensure the fluid’s stability. It was found that HPHT hot rolling affected the properties of drilling fluids by decreasing the rheology parameters and emulsion stability with the increase in the hot rolling time to 32 h. Also, the fluid loss additive’s performance degraded as rolling temperature and time increased. Adding oil wetter and rheology modifier additives resulted in a slight loss of rheological profile after 32 h and maintained flat rheology profile. The emulsion stability was slightly decreased and stayed close to the recommended value (400 V). The fluid loss was controlled by optimizing the concentration of fluid loss additive and oil wetter. The presence of oil wetter improved the carrying capacity of drilling fluids and prevented the barite sag problem. The zeta potential test confirmed that the oil wetter converted the surface of barite from water to oil and improved its dispersion in the oil.

Impact of Chelating Agent Salt Type on the Enhanced Oil Recovery from Carbonate and Sandstone Reservoirs

In this paper, chelating agents were introduced as standalone fluids for enhancing the oil recovery from carbonate and sandstone reservoirs. Chelating agents such as glutamic acid di-acetic acid (GLDA), ethylene-diamine-tetra acetic acid (EDTA), and hydroxyl-ethylethylene-diamine-tri-acetic acid (HEDTA) were used. Chelating agents can be found in different forms such as sodium, potassium, or calcium salts. There is a significant gap in the literature about the influence of salt type on the hydrocarbon recovery from carbonate and sandstone reservoirs. In this study, the impact of the salt type of GLDA chelating agent on the oil recovery was investigated. Potassium-, sodium-, and calcium-based high-pH GLDA solutions were used. Coreflooding experiments were conducted at high-pressure high-temperature (HPHT) conditions using carbonate and sandstone cores. The used samples had porosity values of 15–18%, and permeability values were between 10 and 75 mD. Seawater was injected as a secondary recovery process. Thereafter, a GLDA solution was injected in tertiary mode, until no more oil was recovered. In addition to the recovery experiments, the collected effluent was analyzed for cations concentrations such as calcium, magnesium, and iron. Moreover, dynamic adsorption, interfacial tension, and contact angle measurements were conducted for the different forms of GLDA chelating agent solutions. The results of this study showed that incremental oil recovery between 19% and 32% of the Original Oil in Place (OOIP) can be achieved, based on the salt type and the rock lithology. Flooding carbonate rocks with the calcium-based GLDA chelating agent yielded the highest oil recovery (32% of OOIP), followed by that with potassium-based GLDA chelating agent, and the sodium-based GLDA chelating agent yielded the lowest oil recovery. The reason behind that was the adsorption of the calcium-based GLDA on the rock surface was the highest without reducing the rock permeability, which was indicated by the contact angle, dynamic adsorption, and flooding experiments. The outcome of this study will help in maximizing the oil recovery from carbonate and sandstone reservoirs by suggesting the most suitable salt type of chelating agents.

Optimizing Sour Gas Qualification Testing: Determination and Verification of H2S Solubility from H2S/N2 Gas Mixtures up to 20,000 psig at 70°F

Traditionally, the H2S partial pressure (PH2S) of the gas/hydrocarbon phase has been used as the primary sour severity metric for material qualification and selection under ANSI/AMPP (NACE) MR0175/ISO 15156 guidelines. While the PH2S is appropriate for characterizing low total pressure systems, the strict, or ideal, Henry’s Law approach leads to over estimation of the dissolved H2S concentration (CH2S) for high-pressure, high-temperature (HPHT) wells by up to approximately 20 times at 70°F (21°C). Alternatively, the Ensemble Henry’s Law equation corrects for the non-ideal phase behavior of H2S at HPHT conditions and avoids over-estimation of CH2S. Given the industry’s reliance on using thermodynamic models to evaluate sour HPHT systems, an investigation was initiated to determine the accuracy of these model calculations. An empirical program was undertaken to verify CH2S predictions for the H2S-N2-H2O system. Multiple 2.7 L C-276 lined autoclaves were charged with a fixed inventory of H2S in N2 at multiple total pressure steps, with increasing N2 pressure, between 30 psig and 20,000 psig (3 bar and 1,380 bar) at 70±5°F (21±3°C). Per total pressure step, H2S levels in both the headspace gas and liquid phases were measured using common H2S sampling techniques (H2S-specific colorimetric tubes and iodometric titration, respectively), following ANSI/NACE TM0177-2016 guidelines. The results were used to calculate total pressure corrected (apparent) H2S solubility coefficients (AkH2S). Very good agreement was observed between empirically and computationally derived AkH2S values.

Electronic structure of the bond disproportionated bismuthate Ag2BiO3

We present a comprehensive study on the silver bismuthate Ag$_2$BiO$_3$, synthesized under high-pressure high-temperature conditions, which has been the subject of recent theoretical work on topologically complex electronic states. We present X-ray photoelectron spectroscopy results showing two different bismuth states, and X-ray absorption spectroscopy results on the oxygen $K$-edge showing holes in the oxygen bands. These results support a bond disproportionated state with holes on the oxygen atoms for Ag$_2$BiO$_3$. We estimate a band gap of $\sim$1.25~eV for Ag$_2$BiO$_3$ from optical conductivity measurements, which matches the band gap in density functional calculations of the electronic band structure in the non-symmorphic space group $Pnn2$, which supports two inequivalent Bi sites. In our band structure calculations the disproportionated Ag$_2$BiO$_3$ is expected to host Weyl nodal chains, one of which is located $\sim$0.5~eV below the Fermi level. Furthermore, we highlight similarities between Ag$_2$BiO$_3$ and the well-known disproportionated bismuthate BaBiO$_3$, including breathing phonon modes with similar energy. In both compounds hybridization of Bi-$6s$ and O-$2p$ atomic orbitals is important in shaping the band structure, but in contrast to the Ba-$5p$ in BaBiO$_3$, the Ag-$4d$ bands in Ag$_2$BiO$_3$ extend up to the Fermi level.

Modeling LiF and FLiBe Molten Salts with Robust Neural Network Interatomic Potential.

Lithium-based molten salts have attracted significant attention due to their applications in energy storage, advanced fission reactors, and fusion devices. Lithium fluorides and particularly 66.6%LiF-33.3%BeF2 (Flibe) are of considerable interest in nuclear systems, as they show an excellent combination of favorable heat transfer, neutron moderation, and transmutation characteristics. For nuclear salts, the range of possible local structures, compositions, and thermodynamic conditions presents significant challenges in atomistic modeling. In this work, we demonstrate that atom-centered neural network interatomic potentials (NNIPs) provide a fast method for performing molecular dynamics of molten salts that is as accurate as ab initio molecular dynamics. For LiF, these potentials are able to accurately reproduce ab initio interactions of dimers, crystalline solids under deformation, crystalline LiF near the melting point, and liquid LiF at high temperatures. For Flibe, NNIPs accurately predict the structures and dynamics at normal operating conditions, high-temperature-pressure conditions, and in the crystalline solid phase. Furthermore, we show that NNIP-based molecular dynamics of molten salts are scalable to reach long time scales (e.g., nanosecond) and large system sizes (e.g., 105 atoms) while maintaining ab initio density functional theory accuracy and providing more than 3 orders of magnitude of computational speedup for calculating structure and transport properties.

Effect of salinity, pressure and temperature on the solubility of smithsonite (ZnCO3) and Zn complexation in crustal and upper mantle hydrothermal fluids

Modelling the reservoirs and fluxes of Zn in Earth's crust and mantle requires data on the solubility of its mineral hosts and ores in coexisting fluids, as well as on the complexation of Zn in these fluids as a function of fluid composition, pressure, and temperature. However, due to experimental challenges, the availability of such data is limited to pressures below 1 GPa, which are only representative of upper crust conditions.Here, we report the effects of salinity (0–4.5 m total Cl), pressure (0.5–6 GPa) and temperature (25–400 °C) on the solubility of smithsonite (ZnCO3) and speciation of Zn in aqueous fluids. Solubilities at mineral-fluid equilibria and Zn speciation in the coexisting aqueous fluids were determined in situ at high pressure-temperature (P-T) conditions by synchrotron X-ray fluorescence (XRF) and X-ray absorption spectroscopy (XAS) using resistively heated diamond anvil cells (RH-DAC). The solubility of smithsonite increases with salinity, pressure, and temperature. In agreement with previous studies, conducted at lower pressures (below 1 GPa), we observed a gradual transition from octahedral hydrated [Zn(H2O)6]2+ to tetrahedral hydrated and chlorinated [Zn(H2O)4-nCln]2-n (n = 1–4) complexes with increasing salinity and temperature. Our results suggest that these tetrahedral complexes remain stable under the conditions relevant to cold slab dehydration. This change of coordination further enhances the solubility of smithsonite in Cl-rich fluids and provides a likely mechanism for the efficient uptake of Zn by slab-derived fluids.

Conventional and machine learning improved prediction of hydrocarbon density using volume-translation at high-pressure high-temperature conditions

ABSTRACT Non-cubic equations of state (nCEOS) are increasingly showing improved performance at predicting volumetric properties of hydrocarbons, nitrogen, and carbon dioxide at high-pressure high-temperature over volume-translation-based cubic equations of state (VT-CEOS). However, since nCEOS are rather complex, a less mathematically complex and more accurate CEOS is desired. Hence, in this study, we have explored different techniques, including conventional (non-linear regression) and machine learning-based approaches (random forest) to predict a more accurate molar volume deviation term of the VT-CEOS. We used an extensive high-pressure and high-temperature PVT dataset ranging from 50 to 150 MPa and 300 – 500 K respectively in this study. The VT was modeled as a function of reduced temperature only as well as reduced temperature and molecular weight/critical pressure of the pure hydrocarbon components. Statistical analyses and graphs displayed high performance of the developed predictive models over existing VT-CEOS models applied to HPHT and PC–SAFT. More specifically, the machine learning model gave 99% accuracy while the accuracy of the conventional approach ranged from 60-98%. To the best of the knowledge of the authors, the application of machine learning to estimating volume-translation based on CEOS for pure hydrocarbon components of natural gas and heavy hydrocarbons is nonexistent. This paper presents the first application of physics-based machine learning and the use of features that honors thermodynamic principles for prediction of hydrocarbon density.

Ab initio lattice thermal conductivity of MgSiO3 across the perovskite-postperovskite phase transition

Lattice thermal conductivity ($\kappa_{lat}$) of MgSiO$_3$ postperovskite (MgPPv) under the Earth's lower mantle high pressure-temperature conditions is studied using the phonon quasiparticle approach by combing \textit{ab initio} molecular dynamics and lattice dynamics simulations. Phonon lifetimes are extracted from the phonon quasiparticle calculations, and the phonon group velocities are computed from the anharmonic phonon dispersions, which in principle capture full anharmonicity. It is found that throughout the lowermost mantle, including the D" region, $\kappa_{lat}$ of MgPPv is ~25% larger than that of MgSiO$_3$ perovskite (MgPv), mainly due to MgPPv's higher phonon velocities. Such a difference in phonon velocities between the two phases originates in the MgPPv's relatively smaller primitive cell. Systematic results of temperature and pressure dependences of both MgPPv's and MgPv's $\kappa_{lat}$ are demonstrated.

Phase Stability and Hydroxyl Vibration of Brucite Mg(OH)2 at High Pressure and High TemperatureSupported by the National Key Research and Development Program of China (Grant No. 2019YFA0708502).

Brucite Mg(OH)2 is an archetypal hydrous mineral and it has attracted a great deal of attention. However, little is known about the evolution of hydroxyl groups in brucite with respect to subduction fluids. We carried out Raman measurements up to 15.4 GPa and 874 K via an externally heated diamond anvil cell, investigating the stability of brucite under the conditions relevant to subducting slabs. The hydroxyl vibration mode A 1_g(I) of brucite is weakened under simultaneous high pressure-temperature conditions. Meanwhile, the presence of carbonated solution can destabilize the hydroxyl groups of brucite at low pressure. Our results suggest that brucite releases water when reacting with hydrogen carbonate ion to form magnesite MgCO3 in subduction zones. This implies that the global water cycle is largely coupled with the deep carbon cycle in Earth’s interior.