Synthetic Natural Gas Mixture Research Articles

Complex phase behaviours related to gas injection in reservoir fluids

Enhanced oil recovery (EOR) involves several techniques, including the injection of gas (such as natural gas, nitrogen, or CO2) into the reservoir to increase its pressure, thereby displacing oil from one or more injection wells to production wells. For this process to be effective, it is essential that the injected gas and the oil reach a homogeneous state. Particularly, the First-Contact Minimum Miscibility Pressure (FC-MMP) offers a reliable (and safe) initial estimate for the pressure at which the EOR process should be conducted. In this work, an efficient algorithm is developed that allows computing complete pressure(P)-α diagrams (α: percentage of injected gas) at a fixed temperature using traditional cubic equations of state (EoS), including complex cases with three-phase regions. This algorithm is used to study both the qualitative and quantitative behaviour of the P-α diagrams and to analyse how FC-MMP changes with the injection of different gases or gas mixtures. Various reservoir fluids from the literature (with and without asphaltenes and with varying levels of CO2 and methane) are evaluated. In one section, three injection gases are used: CO2, N2, and a synthetic natural gas mixture, comparing their effects on the P-α diagrams. Then, the impact of interaction parameters on phase equilibrium and FC-MMP is analysed, and the role of asphaltene precipitation is discussed. The study concludes with a quantitative comparison between the FC-MMP calculated here and the Multiple-Contact MMP (MC-MMP) reported for the same fluids. Additionally, the problem of whether asphaltenes precipitation should be considered or ignored in the determination of the FC-MMP is analysed and discussed, with different perspectives for conventional and non-conventional shale type reservoirs.

Kinetic Gas Hydrate Inhibition by Alternating Dipeptoids with Optimal Size and Shape N-Substituents.

Current commercial kinetic hydrate inhibitors (KHIs) are all based on water-soluble polymers with amphiphilic alkylamide or lactam groups. The size and shape of the hydrophobic moiety are known to be critical for optimum KHI performance. Proteins and peptides represent an environmentally friendly alternative, especially as bioengineering could be used to manufacture a product predetermined to have optimum KHI performance. Here, we explore a new series of polymers that are alternating dipeptoids where one of the peptide links originates from glycine. The dipeptoids contain n-propyl groups on the nitrogen atom and varying size and shape alkyl side chains on the neighboring carbon atom. Experiments were carried out in high-pressure steel rocking cells using the slow constant cooling (SCC) test method (1 °C/h) and a synthetic natural gas mixture. All the dipeptoids showed good KHI performance with the best result being for that with a glycine-N-propylleucine repeating unit (Poly iC4-Pr), which has pendant iso-butyl groups on the carbon atom. It exhibited the same KHI performance as poly(N-vinyl caprolactam). Dipeptoids with smaller or longer alkyl groups than iso-butyl gave worse performance. It is conjectured that the iso-butyl group is the optimal carbon length for this polymer class. In addition, the end-branching maximizes the van der Waals interaction with open cavities on growing hydrate particles, which must occur without loss of hydrogen-bonding from the neighboring peptide linkage for optimum KHI performance. Thus, the study provides further evidence for the premise that good KHI molecules must contain multiple amphiphilic groups (often as polymers) with optimal size and shape hydrophobic groups adjacent to strong hydrogen bonding groups. The solvent, n-butyl glycol ether, was shown to be a synergist for Poly iC4-Pr, lowering the onset temperature of hydrate formation in SSC tests relative to the polymer alone.

Nonpolymeric Citramide-Based Kinetic Hydrate Inhibitors: Good Performance with Just Six Alkylamide Groups.

The use of kinetic hydrate inhibitors (KHIs) is a well-known method for preventing gas hydrate formation in oil and gas production flow lines. The main ingredient in KHI formulations is one or more polymers with amphiphilic groups. Here, we report a series of citramide-based nonpolymeric KHIs. The KHI performance of these citramide derivatives has been studied using a synthetic natural gas mixture (forming structure II hydrate as the thermodynamically preferred phase) in slow constant cooling (ca. 1 °C/h starting from 20.5 °C) high-pressure (76 bar) rocking cell experiments. Isobutyl-substituted alkyl chains in the mono/bis(trialkyl citric acid) amide derivative gave better KHI performance than n-propyl-substituted citramide derivatives. Moreover, biscitramides with six alkylamide functional groups gave better performance than the equivalent monocitramides with three alkylamide groups. A solution of 2500 ppm of bis(tributyl citric acid) amide gave an average gas hydrate onset temperature (To) of 8.4 °C compared to 8.9 °C for a low molecular weight N-vinyl pyrrolidone/N-vinyl caprolactam 1:1 copolymer. For the bis(tributyl citric acid) amide, addition of liquid hydrocarbon (n-decane) lowered further the average To value to 6.2 °C, although this is at least partly due to lowering of the hydrate equilibrium temperature. This study demonstrates that good KHI performance can be obtained from molecules with as little as six amphiphilic alkylamide groups.

Rapid Gas Hydrate Formation—Evaluation of Three Reactor Concepts and Feasibility Study

Gas hydrates show great potential with regard to various technical applications, such as gas conditioning, separation and storage. Hence, there has been an increased interest in applied gas hydrate research worldwide in recent years. This paper describes the development of an energetically promising, highly attractive rapid gas hydrate production process that enables the instantaneous conditioning and storage of gases in the form of solid hydrates, as an alternative to costly established processes, such as, for example, cryogenic demethanization. In the first step of the investigations, three different reactor concepts for rapid hydrate formation were evaluated. It could be shown that coupled spraying with stirring provided the fastest hydrate formation and highest gas uptakes in the hydrate phase. In the second step, extensive experimental series were executed, using various different gas compositions on the example of synthetic natural gas mixtures containing methane, ethane and propane. Methane is eliminated from the gas phase and stored in gas hydrates. The experiments were conducted under moderate conditions (8 bar(g), 9–14 °C), using tetrahydrofuran as a thermodynamic promoter in a stoichiometric concentration of 5.56 mole%. High storage capacities, formation rates and separation efficiencies were achieved at moderate operation conditions supported by rough economic considerations, successfully showing the feasibility of this innovative concept. An adapted McCabe-Thiele diagram was created to approximately determine the necessary theoretical separation stage numbers for high purity gas separation requirements.

Phase envelope calculations of synthetic gas systems with a crossover equation of state

We describe, in this work, a procedure for the calculation of the phase envelope of multi-component systems using a crossover Equation of State (EoS) based on the renormalization group theory and two different algorithms. The first algorithm utilizes pressure and temperature as natural variables, while the second uses volume and temperature. Our comparison shows that the second method is more suitable to a crossover EoS, as it avoids solving for the volume roots of the EoS, which is computationally intensive and potentially hampers the widespread use of such models in engineering applications. Moreover, we compare the simulated phase envelopes with the experimental data of 30 synthetic natural gas mixtures. The results indicate that the crossover SRK EoS has a similar performance to the classical model. Although larger deviations are observed for the representation of the bubble and dew-point curves, the crossover EoS yields a superior description of the critical points.

Speed of sound data, derived perfect-gas heat capacities, and acoustic virial coefficients of a calibration standard natural gas mixture and a low-calorific H2-enriched mixture

This work aims to address the technical aspects related to the thermodynamic characterization of natural gas mixtures blended with hydrogen for the introduction of alternative energy sources within the Power-to-Gas framework. For that purpose, new experimental speed of sound data are presented in the pressure range between (0.1 up to 13) MPa and at temperatures of (260, 273.16, 300, 325, and 350) K for two mixtures qualified as primary calibration standards: a 11 component synthetic natural gas mixture (11 M), and another low-calorific H2-enriched natural gas mixture with a nominal molar percentage xH2=3%. Measurements have been gathered using a spherical acoustic resonator with an experimental expanded (k = 2) uncertainty better than 200 parts in 106 (0.02%) in the speed of sound. The heat capacity ratio as perfect-gas γpg, the molar heat capacity as perfect-gas Cp,mpg, and the second βa and third γa acoustic virial coefficients are derived from the speed of sound values. All the results are compared with the reference mixture models for natural gas-like mixtures, the AGA8-DC92 EoS and the GERG-2008 EoS, with special attention to the impact of hydrogen on those properties. Data are found to be mostly consistent within the model uncertainty in the 11 M synthetic mixture as expected, but for the hydrogen-enriched mixture in the limit of the model uncertainty at the highest measuring pressures.

NEW CORRELATIONS FOR THE PSEUDOCRITICAL PARAMETERS AND THE ACENTRIC FACTOR S OF THE HYDROCARBON COMPONENT OF THE DNIEPER-DONETS PETROLEUM BASIN’S NATURAL GAS

NEW CORRELATIONS FOR THE PSEUDOCRITICAL PARAMETERS AND THE ACENTRIC FACTOR S OF THE HYDROCARBON COMPONENT OF THE DNIEPER-DONETS PETROLEUM BASIN’S NATURAL GAS

Kinetic Hydrate Inhibition of Glycyl-valine-Based Alternating Peptoids with Tailor-Made N-Substituents

A series of alternating peptides with glycine-N-substituted valine dipeptide repeating units were synthesized. The N-substitution included methyl, ethyl, n-propyl, and hydroxyl groups. The performance of the polypeptides as kinetic hydrate inhibitors (KHIs) was evaluated in high pressure rocking cells with a synthetic natural gas mixture using the slow constant cooling method. All the polypeptides gave better KHI effect than a low molecular weight poly(N-vinylpyrrolidone). The polypeptide with N-substituted n-propyl groups, PA-Pep, performed the best, the same as a commercial poly(N-vinyl caprolactam) (PVCap) at the same concentration. The KHI performance of PA-Pep improved as the concentration of the solution was increased from 1000 to 5000 ppm. At 5000 ppm the average (10 experiments) onset temperature for hydrate formation was 9.8 °C (ca. ΔT = 10.4 °C).

Investigation of Solvent Synergists for Improved Kinetic Hydrate Inhibitor Performance of Poly(N-isopropyl methacrylamide)

The synergistic effect of a range of solvents on the kinetic hydrate inhibitor (KHI) performance of poly(N-isopropyl methacrylamide) (PNIPMAm) has been investigated in slow (ca. 1 °C/h) constant cooling high-pressure (76 bar) rocking cell experiments using a structure II forming synthetic natural gas mixture. We have tested the synergistic effects of several monoglycol ethers and compared them to those of corresponding diglycol ethers and alcohols. Compounds containing lactate, carboxylate, ammonium, and pyrrolidone headgroups have also been investigated as synergists. In general the monoglycol ethers were found to function best as synergists with PNIPMAm, and the performance improved going from n-butyl to isobutyl, cyclopentyl, and cyclohexyl alkyl substituents. The best performing synergist in this study was found to be 2-(cyclohexyloxy)ethanol (CHexGE). Addition of 5000 ppm CHexGE to 2500 ppm PNIPMAm-II (made in isopropyl alcohol and precipitated out, Mn = 8100 g/mol) dropped To from 6.8 °C for pure po...

A Systematic Study of Cubic Equations of State with van der Waals Mixing Rules and Different Combining Rules in Predicting the Densities of LNG and Natural Gas Model Systems

Cubic equations (EoSs) of state are successfully used in petroleum and natural gas industry. In order to extend these equations to mixtures, van der Waals mixing rules with Lorentz-Berthelot (LB) combining rules are often employed; however, the accuracies of these EoSs in predicting the densities of hydrocarbon mixtures are not adequate. The main objective of this study was comparing 13 EoSs coupled with 10 combining rules in predicting the densities of hydrocarbon mixtures. Binary and ternary liquid mixtures, LNG mixtures and synthetic natural gas mixtures comprising 752 data points were collected and used in this study. Results revealed that for predicting the liquid densities of binary and ternary mixtures, the Schmidt and Wenzel (SW) EoS coupled with Hudson-McCoubrey (HMC) or LB combining rules are the best among the others. The SW EoS coupled with the LB combining rules were also the best in predicting the densities of the LNG mixtures. Additionally, the LB combining rules are the best in predicting natural gas mixtures densities using the Patel and Teja (PT), SW and Patel-Teja-Valderrama (PTV) EoSs. In general, it was found that the Redlich-Kwong (RK) family EoSs, SW and Trebble-Bishnoi-Salim (TBS) EoSs are best coupled with the LB combining rules. However, the Peng and Robinson (PR) family EoSs with Halgren (HHG) combining rules were in better agreement with experimental data. The Waldman-Hagler (WH) combining rules lacked predictability when coupled with the most EoSs.

Gas hydrate formation probability distributions: Induction times, rates of nucleation and growth

An improved, high pressure, stirred automated lag time apparatus (HPS-ALTA) was used to determine gas hydrate nucleation and growth rates from induction time measurements at fixed subcooling. The improved HPS-ALTA uses multiple thermoelectric elements to allow rapid, radially symmetric heating and cooling, which together with the automatic detection of hydrate formation, makes feasible the measurement of statistically significant induction time distributions. Four induction time probability distributions, with between 83 and 312 points, were measured at subcoolings ranging from (6 to 9.7) K for water with a synthetic natural gas mixture at pressures around 12 MPa. The induction time data measured at each subcooling were exponentially distributed and were fit using a model derived from Classical Nucleation Theory based on the mononuclear nucleation mechanism. The nucleation rates obtained in this work were consistent within a factor of three or better with recent literature measurements made using a completely different apparatus. Hydrate growth rates measured at fixed subcooling using the improved HPS-ALTA were also found to be stochastic but consistent with previous measurements of the kinetic growth rate in a binary gas mixture. The results obtained here establish the need to improve theoretical models of hydrate formation to reconcile predicted nucleation rates with the much slower ones observed experimentally.

Synthesis and modification of Zeolitic Imidazolate Framework (ZIF-8) nanoparticles as highly efficient adsorbent for H2S and CO2 removal from natural gas

In this work, synthesis and modification of some ZIF-8 nanoparticles in different functionalization media using ethylenediamine (ED) were investigated for H2S and CO2 efficient removal form synthetic natural gas mixture. Characterization of the prepared ZIF-8 nanoparticles was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), and N2 adsorption/desorption analysis. XRD and FTIR analysis of the ZIF-8 nanoparticles before and after H2S adsorption revealed that their structure does not considerably change. CO2 and H2S removal efficiencies of the nanoparticles were evaluated at 25 °C and their Co-adsorption performance was also measured in static and dynamic modes at 25 °C and 0–13 bar. The unmodified ZIF-8 nanoparticles (WS-ZIF-8) adsorption capacities for CH4, CO2 (20 bar and 25 °C) and H2S (10 bar and 25 °C) were found as 4.4, 10.8, 18.8 mmol/g, respectively. Those for modified ZIF-8 nanoparticles (ED-ZIF-8) increased by 15%, 22% and 3 times, respectively. Mixed gas H2S adsorption of ED-ZIF-8 nanoparticles measurements up to 3 mol. % H2S approved their improved acid gases removal efficiencies and also structural stability confirmed by XRD analysis. The unmodified ZIF-8 nanoparticles breakthrough times for H2S and CO2 adsorbed from mixed gas of CH4 / CO2 / H2S /He as 88.8 / 7.3 / 3 / 1 mol. % at 25 °C and 2 bar were found as ˜ 190 and 308 s, respectively, while those for modified ZIF-8 nanoparticles considerably improved ˜ 400 and 531 s, respectively.

Systematic Study of Polyglyoxylamides as Powerful, High-Cloud-Point Kinetic Hydrate Inhibitors

A series of polyglyoxylamides (PGAms) have been synthesized and investigated for performance as kinetic hydrate inhibitors (KHIs) in slow constant cooling high-pressure rocking cell experiments using a structure II-forming synthetic natural gas mixture at 76 bar initial pressure. We found that the KHI performance improved as the size of the alkyl pendant group was increased. The best performing PGAm, poly(pyrrolidinyl glyoxylamide) (PPyGAm-I), gave an onset temperature of 8.2 °C at 2500 ppm. The KHI performance was improved by increasing the polymer concentration or by adding the high-flash-point solvent n-butyl glycol ether (nBGE) as a synergist. For example, using 7500 ppm of PPyGAm-I, the onset temperature was lowered to 3.8 °C (giving a subcooling of 15.7 °C, compensating for the drop in pressure at To). A mixture of 2500 ppm PPyGAm-I and 7500 ppm nBGE gave an onset temperature of 5.1 °C. Combined with a high cloud point (TCl = 79 °C), this makes PPyGAm-I a strong candidate for potential industrial us...

Gasification of wood biomass with renewable hydrogen for the production of synthetic natural gas

In this article we argue that the broad diffusion of renewables such as solar and wind power in the global energy system will probably require the development of efficient systems for the storage of excess electricity produced during off-peak hours. We also point out that in many places on Earth wood is a type of biomass that is available in large quantities as by-product of agricultural and industrial activities, which could be valorized through gasification processes that produce syngas. We combine these two observations by presenting an analysis of a power-to-gas system in which hydrogen obtained via electrolysis is used for the hydrogasification of wood. This process produces a synthetic natural gas (SNG) mixture that consists mostly of methane with only minor impurities of hydrogen. We show that the SNG generated in this sustainable way represents an efficient means for renewable energy storage. We also study wood hydrogasification processes with hydrogen production methods alternative to electrolysis, such as the partial oxidation of biomass, the water gas shift reaction, and the Sabatier reaction. We determine the process operating conditions that maximize the SNG yield and energy efficiency. Through an economic assessment we examine the applicability of the process to a plant located in Northern Italy. In all process types SNG exhibits characteristics that comply with methane grid quality requirements. We conclude that SNG production through wood hydrogasification can compete with methane market prices when using zero-cost renewable electricity produced during off-peak hours.

Comparative Study of Equations of State for the Dew Curves Calculation in High Pressure Natural Gas Mixtures

The success during the operation of natural gas processing plants depends on the correct estimation of thermodynamic properties of the system. This paper calculates the equilibrium curves of real and synthetic natural gas mixtures means of three Equations of State (EOS). These equilibrium curves were constructed and compared with experimental data found in the literature covered. The results showed that, above 4 MPa the Peng-Robinson equation presented a considerable deviation with respect to the experimental data, reaching an absolute error of 4.36%; therefore, the GERG2008 equation is recommended for systems that operate at high pressures when the components present in the mixture apply.

Accurate experimental (p, ρ, T) data of natural gas mixtures for the assessment of reference equations of state when dealing with hydrogen-enriched natural gas

The GERG-2008 equation of state is the approved ISO standard (ISO 20765-2) for the calculation of thermophysical properties of natural gas mixtures. The composition of natural gas can vary considerably due to the diversity of origin. Further diversification was generated by adding hydrogen, biogas, or other non-conventional energy gases. In this work, high-precision experimental (p, ρ, T) data for two gravimetrically prepared synthetic natural gas mixtures are reported. One mixture resembled a conventional natural gas of 11 components (11 M) with a nominal mixture composition (amount-of-substance fraction) of 0.8845 for methane as the matrix compound with the other compounds being 0.005 for oxygen, 0.04 for nitrogen, 0.015 for carbon dioxide, 0.04 for ethane, 0.01 for propane, 0.002 each for n- and isobutane, and ultimately 0.0005 each for isopentane, n-pentane, and n-hexane. The other mixture was a 13-component hydrogen-enriched natural gas with a low calorific value featuring a nominal composition of 0.7885 for methane, 0.03 for hydrogen, 0.005 for helium, 0.12 for nitrogen, 0.04 for carbon dioxide, 0.0075 for ethane, 0.003 for propane, 0.002 each for n- and isobutane, and 0.0005 each for neopentane, isopentane, n-pentane, and n-hexane. Density measurements were performed in an isothermal operational mode at temperatures between 260 and 350 K and at pressures up to 20 MPa by using a single-sinker densimeter with magnetic suspension coupling. The data were compared with the corresponding densities calculated from both GERG-2008 and AGA8-DC92 equations of state to test their performance on real mixtures. The average absolute deviation from GERG-2008 (AGA8-DC92) is 0.027% (0.078%) for 11 M and 0.095% (0.062%) for the 13-component H2-enriched mixture, respectively. The corresponding maximum relative deviation from GERG-2008 (AGA8-DC92) amounts to 0.095% (0.127%) for 11 M and 0.291% (0.193%) for the H2-enriched mixture.

Measurement and Prediction of Hydrocarbon Dew Points of Synthetic Natural Gas Mixtures

It is very important to predict the condensation of liquid hydrocarbons when transporting natural gas with pipelines in industry. Using a high-pressure transparent sapphire cell, the hydrocarbon de...

As Green As It Gets: An Abundant Kinetic Hydrate Inhibitor from Nature

We report the development of a kinetic hydrate inhibitor (KHI) based on a cheap, abundantly available natural product that is readily biodegradable and low in toxicity. The optimal product, called KHI530, was determined using KHI screening tests by the slow constant cooling method in multiple rocking cells. The performance was better than a low molecular weight poly(N-vinylpyrrolidone) for a synthetic natural gas mixture (SNG) forming primarily structure II hydrate and about the same performance for structure I methane hydrate. Isothermal tests using the SNG mixture indicate that an active dosage of 0.5 wt % can protect against hydrate formation for 48 h at a subcooling of 6–7 °C. KHI530 exhibits no cloud point up to 95 °C in deionized water or NaCl brines up to 7 wt %. KHI530 can be biostabilized in monoethylene glycol (MEG) or using a biocide such as tetrahydroxymethylphosphonium sulfate (THPS), which does not affect the KHI performance. KHI530 was found to be fully compatible at all ratios with thermod...

Optimizing the Kinetic Hydrate Inhibition Performance ofN-Alkyl-N-vinylamide Copolymers

The generation of gas hydrates in gas and multiphase flowlines can cause blockages, leading to downtime, economic losses, and even potential accidents. Injecting kinetic hydrate inhibitors (KHIs) is an effective way to prevent gas hydrate formation. Most KHI formulations are built around water-soluble polymers containing amide groups. On the basis of past work on N-alkyl-N-vinylamide polymers from our group, we have now been able to become much closer to designing the optimum KHI for this class of polymers. In this study, we have synthesized four N-alkyl-N-vinylamide monomers, where the alkyl group is n-propyl, isopropyl, n-butyl, and isobutyl. These have been copolymerized successfully with N-methyl-N-vinylacetamide and N-vinylformamide to form a series of copolymers with low molecular weights. We have investigated their KHI performance using a slow constant cooling method with a synthetic natural gas mixture in high-pressure rocker cells at 76 bar. All of the new N-vinylamide copolymers show good KHI pe...

Equations of State with Group Contribution Binary Interaction Parameters for Calculation of Two-Phase Envelopes for Synthetic and Real Natural Gas Mixtures with Heavy Fractions

Three equations of state with a group contribution model for binary interaction parameters were employed to calculate the vapor-liquid equilibria of synthetic and real natural gas mixtures with heavy fractions. In order to estimate the binary interaction parameters, critical temperatures, critical pressures and acentric factors of binary constituents of the mixture are required. The binary interaction parameter model also accounts for temperature. To perform phase equilibrium calculations, the heavy fractions were first discretized into 12 Single Carbon Numbers (SCN) using generalized molecular weights. Then, using the generalized molecular weights and specific gravities, the SCN were characterized. Afterwards, phase equilibrium calculations were performed employing a set of (nc + 1) equations where nc stands for the number of known components plus 12 SCN. The equations were solved iteratively using Newton's method. Predictions indicate that the use of binary interaction parameters for highly sour natural gas mixtures is quite important and must not be avoided. For sweet natural gas mixtures, the use of binary interaction parameters is less remarkable, however.