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Wax deposition is a significant flow assurance issue in oil/gas production and transportation, often leading to significant flow constrictions with eventual blockages of flowlines. Wax deposition i...

Gas hydrate deposits formed on the pipe wall can lead to blockages of oil and gas production flowlines. Similar to wax and frost deposits, hydrate deposits formed along the pipe wall also effective...

Type II clathrate hydrates are prevalent in numerous applications including gas production, gas storage, and seawater desalination. However, less focus has been given to understanding the molecular...

Nanobubbles have attracted intense attentions due to their unexpectedly long lifetimes and stabilities in liquid solutions. However, explanations for the unique properties of nanobubbles at the molecular scale are somewhat controversial. Of special interest is the validity of the Young-Laplace equation in predicting the inner pressure of such bubbles. In this work, large-scale molecular dynamics simulations were performed to study the stability and diffusion of nanobubbles of methane in liquid water. Two types of force fields, atomistic and coarse-grained, were used to compare the calculated results. In accordance with predictions from the Young-Laplace equation, it was found that the inner pressure of the nanobubbles increased with decreasing nanobubble size. Consequently, a large pressure difference between the nanobubble and its surroundings resulted in the high solubility of methane molecules in water. The solubility was considered to enable nanobubble stability at exceptionally high pressures. Smaller bubbles were observed to be more mobile via Brownian motion. The calculated diffusion coefficient also showed a strong dependence on the nanobubble size. However, this active mobility of small nanobubbles also triggered a mutable nanobubble shape over time. Nanobubbles were also found to coalesce when they were sufficiently close. A critical distance between two nanobubbles was thus identified to avoid coalescence. These results provide insight into the behavior of nanobubbles in solution and the mechanism of their unique stability while withstanding high inner pressure.

In the flowlines of oil/gas production systems, the formation of gas hydrates, icelike crystalline compounds of water and gas that form at low temperature and high pressure, can disrupt stable flow, often resulting in solid blockages that necessitate remediation and pose safety issues. Low dosage hydrate inhibitor antiagglomerants (LDHI-AAs) are chemical additives used to manage the hydrate slurry flow at relatively low dosage (∼1% of water content) by dispersing hydrates in the continuous liquid hydrocarbon phase. A reliable assessment of LDHI-AA performance is thus required for their optimal use under a given field condition. Rocking cells have been extensively used in the oil/gas industry for LDHI-AA qualifications as it is easy to build and allows full visualization of the cell interior. However, a solid rolling ball, which is put into the conventional rocking cell for mixing, brings significant disturbances to the flow and dispersion of the phases and, in particular, by breaking up the hydrates formed and pushing them to accumulate at the end of the cell, giving a very conservative (worst-case) evaluation of LDHI-AAs. There is a large gap between the testing conditions of rocking cells and the actual field conditions. The recently developed rock-flow cell can provide a much closer representation of the shear, phase dispersion and thus the flow regime in flowlines. Here, we demonstrate the capability of the rock-flow cell as a more robust testing tool for LDHI-AA qualification. The impact of the cooling mode, degree of subcooling, and salinity are well characterized in terms of hydrate aggregation, deposition, and bedding. The test results demonstrate the advantages of the rock-flow cell over the conventional rocking cell to characterize the hydrate slurry by visualization and quantification of the hydrate formation and accumulation. As such, the rock-flow cell can be used as an effective testing tool for LDHI-AA qualification, which can also be applied to many other phase precipitation problems encountered in flow assurance.

To investigate the effect of natural convection on hydrate deposition in gas-filled deadlegs, a series of hydrate deposition experiments were conducted in a water-saturated gas system in a 1 in. de...

Flow assurance is a central component of oil and gas production, concerning all methods applied to ensure stable, safe, and economical transport of hydrocarbons during all stages of production. As ...

Many unanswered questions still exist at the molecular level to understand the nucleation process and mechanism of clathrate hydrates, especially for larger guest molecules that would result in the structure II crystal. Here, we report on molecular dynamics simulations for propane and water to describe the molecular mechanism leading to a structure II system. Through a large number (30) of long (5 μs) and coupled annealing (20 μs) simulations, we detail the prenucleation, nucleation, growth, and annealing of propane clathrate hydrate structures at 250 K and 1800 bar. The results demonstrate the equal importance of the empty and occupied cages in the nucleation of propane hydrates. The critical nucleus size is identified to be eight cages. While separate distinct clusters may exist during the prenucleation period, only one survives to grow beyond the critical nucleus size, with the others remaining subcritical. From the annealing simulations, it is clear that solid rearrangement is a very slow process, and 20 μs is still not long enough to capture long-range ordering resembling the structure II crystal. These results, along with the developed analysis method, have a significant impact in advancing our understanding of the nucleation process for unlike molecules.

Clathrate hydrates are ubiquitous in deep-sea sediments and the permafrost on earth, as well as abundant in interstellar environments. Despite many potential applications of gas hydrates for gas storage/transportation, carbon sequestration, and water treatment, the fundamental mechanism for gas hydrate nucleation and growth are still poorly understood due to the difficulty in spatial and temporal measurements that can probe specific structural properties. Here, high resolution measurements for xenon hydrate formation from ice spheres by X-ray computed tomography are reported. For the first time, the nucleation and growth of hydrates can be accurately and directly quantified from microscale measurements, resulting in new estimates for the intrinsic reaction rate constant of xenon hydrate formation, and the effective diffusion coefficient of xenon in the hydrate. The measured activation energy of hydrate formation is 71.11 kJ/mol and the diffusivity of xenon in hydrate ranges from 2.8 × 10–15 to 4.3 × 10–14 m2/s. These results are of fundamental value in developing a comprehensive understanding on the mechanism of gas hydrate formation, which is essential in their application for energy solutions and increasingly important in astrophysical science.

This paper presents the development and application of an electrical impedance sensor allied with an optical fiber Bragg grating (FBG) sensor whereby both are integrated in fiberglass composite material. The use of composite material allows the integration of the sensors according to the desired geometry. In the field of oil and gas production it is of great importance to monitor the formation of clathrate hydrates (crystalline structures) because such solids may disrupt the flow causing risks and increasing extraction costs. As there is no fully established method for this sensing application, impedance spectroscopy allied to temperature technique is explored for the purpose. Hence, impedance spectra from 1 kHz to 5 MHz along with temperature readings are obtained during an exemplary hydrates experiment, showing that the developed system is able to monitor the hydrates formation in pipes.

Deadlegs are defined as pipe sections in intermittent use for production or special services in oil/gas production systems. Due to the absence of flow, deadlegs are commonly much colder than the ma...

Exploration of oil fields with high concentrations of carbon dioxide brings new challenges to the oil and gas industry. The high pressures reached in production pipelines can cause the condensation...

Significant portions of the oil reserves in Brazil are located in deep or ultradeep waters. Oil production from these reservoirs implies a constant awareness of gas hydrate formation. The petroleum...

Transient operations in oil and gas production can result in conditions with a high potential for the formation of hydrate plugs. In restart operations, the shear flow and the increased pressure ca...

The shortage of fresh water is among the most serious issues in the world. A representative technology to overcome the problem is desalination, but most conventional methods (RO membrane or thermal distillation) have been focused on the treatment of relatively low salinity water, such as seawater or brackish water. To strengthen water security, in this study, we introduce a possibly economic technology for desalination of high salinity water (over-saturated concentration, in this study, a 30 wt% NaCl system) via gas hydrate formation by coupling LNG waste cold energy. First, the thermodynamic effects of NaCl on CH4 (methane), SF6 (sulfur hexafluoride), and HFC-134a hydrates were investigated. Based on the phase equilibrium of each hydrate, experimental pressures for kinetic experiments were selected under vapor pressure boundaries as follows: 4.5 MPa for CH4, 0.75 MPa for SF6, and 0.16 MPa for HFC-134a at 258.15 K (assuming the use of LNG waste cold energy). The results of the formation kinetics on the basis of gas moles consumed for hydrates showed the order CH4 > HFC-134a > SF6; however, after considering the hydration numbers and structures for each hydrate, surprisingly, the conversion rate of water to gas hydrates showed the order HFC-134a > CH4 > SF6, even though the experimental pressure condition for HFC-134a was very mild (0.16 MPa) compared to CH4 (4.5 MPa). For this interesting phenomenon, we suggest a possible mechanism through visual observations during hydrate formation. We believe these thermodynamic, kinetic, and morphological results show potential as an alternative desalination technology, especially for saturated salinity water, with lower energy consumption.

Summary: Activation of inositol-requiring enzyme (IRE1α) is an indispensable step in remedying the cellular stress associated with lipid perturbation in the endoplasmic reticulum (ER) membrane. IRE1α is a single-spanning ER transmembrane protein possessing both kinase and endonuclease functions, and its activation can be fully achieved through the dimerization and/or oligomerization process. How IRE1α senses membrane lipid saturation remains largely unresolved. Using both computational and experimental tools, we systematically investigated the dimerization process of the transmembrane domain (TMD) of IRE1α and found that, with help of the serine 450 residue, the conserved tryptophan 457 residue buttresses the core dimerization interface of IRE1α-TMD. BiFC (bimolecular fluorescence complementation) experiments revealed that mutation on these residues abolished the saturated fatty acid-induced dimerization in the ER membrane and subsequently inactivated IRE1α activity in vivo. Therefore, our results suggest that the structural elements of IRE1α-TMD serve as a key sensor that detects membrane aberrancy. : Cho et al. demonstrate that the conserved tryptophan residue on the transmembrane domain of IRE1α is crucial for palmitate-induced IRE1α dimerization and/or oligomerization in vivo. Mutation of this residue inhibits the enzymatic activity of IRE1α, suggesting that the intrinsic feature of the TMD sequence senses membrane lipid saturation. Keywords: IRE1α, UPR, dimerization, membrane lipid saturation

International audience; This study proposes a criterion for safe transportability of gas hydrate slurries in oildominant flowlines. Fluids chemistry plays a role on how the particles agglomerate, which occurs in the time window the particles take to decrease their porosity because of crystallization in the capillary walls or to seal the water within the pores by the action of chemical additives, then completely preventing any water in the outer surface of the particle and avoiding liquid bridge formation (agglomeration). Hydrodynamic aspects come from the lift vs buoyancy/weight forces that tend to suspend/settle the particles, as well as the collision and disruption rates of particles that play a role on the agglomeration process. The criterion is rather simple and shows the importance of the subcooling of crystallization, water cut, mixture velocity, and the oil-water interfacial tension that can be lowered by the use of additives. A simple chart for assuring safe, fully suspended slurry flow (low plugging risk) is proposed, called slurry phase map, and directives of its use for flowline design and management are discussed. Discussion is also given upon how to scale up laboratory measurements into field conditions by the proposal of a new dimensionless group, called Bassani number.

In light of the 60-year anniversary of the publishing of "Clathrate Solutions" by van der Waals and Platteeuw in 2019, we present a critical review of the famed solid solution model first disclosed in 1959. First, we lay out the groundwork in the 1950s aimed at the development of a phenomenological approach to clathrate modeling. Then we review the statistical thermodynamics fundamentals of the model, considering van der Waals and Platteeuw's earlier works, to obtain a consistent interpretation of the model. We turn our focus to clathrate hydrates and discuss the major contributions that led to the current state-of-the-art of gas hydrate thermodynamic modeling. Finally, we present some of the areas in clathrate thermodynamics that we foresee as the new frontiers in this subject. We expect this review to help newcomers to clathrate science in elucidating some subtle aspects of the model and to intrigue clathrate experts with a fresh look on this well-established solid solution model.

International audience; In the third part of this series, we introduce the mathematical model for the agglomeration of gas hydrates in oil-continuous flow. The aim is to develop an expression for the agglomeration efficiency that considers the existence of wet or dry particles. If the particle is wet, then water is available at its outer surface, thus allowing the formation of a liquid bridge that holds the aggregate together. The criterion for a wet or dry particle was developed in part II of this series and arises from competitions between water permeation through the porous hydrate particle and water consumption caused by crystallization in the particle’s outer surface. The new expression for the agglomeration efficiency is coupled with a population balance solved through the method of moments and considering simple expressions for the collision rate and the shear rate induced by the flow arising from Smoluchowski’s and Kolmogorov’s theory, respectively. When compared to experimental data, the model stays within the ±40% deviation range and proves capable of predicting smaller agglomerate sizes for higher subcooling and lower interfacial properties (use of surfactant additives). The influence of subcooling on changing the porous medium parameters (especially the porous medium interconnectivity) proved important for the determination of the time taken for the particle to dry out. The model is simplified for engineering purposes considering gases much more soluble in oil than in water (hydrocarbon gases) in oil-continuous flow, and a simple criterion is proposed to predict if the system behaves as dispersed (slurry) or if it agglomerates after the onset of hydrate formation.

Among the many different types of molecules that form clathrate hydrates, H

The current knowledge of guest–guest interactions and co-occupation in clathrate hydrates is exclusive for the same guests (H2 or N2) at moderate pressure. Here, we introduce the unusual co-occupation of distinct guests in the metastable state of hydrates. With controlled hydrate fraction, particle size, and intensification of the sintering of SF6 hydrate particles formed from water and SF6 gas as a help gas, we observed an abnormal but unique synchronous behavior in Raman intensities of two guest molecules (SF6 and N2/H2) in hydrates consistently and repeatedly; over time, the scattering intensity for the guests (i) increases, (ii) decreases, and (iii) finally reaches the stable level. Without a concentration change of SF6, this abnormal behavior must arise from the possible changes in the scattering cross section of the molecules, suggesting that N2/H2 strongly interacts with SF6 in the large cages, resulting in a possible co-occupation during the metastable transition. These observations on the metasta...

Gas hydrates cold flow can be defined as flow of non-adhesive and non-cohesive hydrate particles dispersed in the production fluids flowlines. Implementation of cold flow related hydrate management...

In this study, the van der Waals and Platteeuw model was coupled with the Cubic Plus Association (CPA) equation of state (EoS) for equilibrium calculations in systems with gas hydrates. It has been applied to simple and complex multicomponent systems involving methane, ethane, propane, isobutane, carbon dioxide, nitrogen and hydrogen sulfide. Methanol, ethanol, monoethylene glycol, calcium chloride, sodium chloride and potassium chloride were contemplated as thermodynamic hydrate inhibitors. The calculations were performed in the presence of single and mixed inhibitors. The mole fraction of components in all phases were determined using flash algorithm procedures to improve the calculations accuracy. To evaluate the ability of the methodology, the prediction of hydrate phase behavior in the presence and absence of inhibitors was compared with the experimental data. Additionally, the binary interaction coefficients were considered as a linear function of temperature. Furthermore, the Langmuir constants were optimized for each gas hydrate former. The results obtained with the proposed approach agreed well with the experimental data, predominantly for hydrate systems comprising mixture of different inhibitors as salts and alcohols.

Phase equilibrium properties of gas hydrates in the presence of thermodynamic hydrate inhibitors (THIs) like alcohols, glycols, and salts are essential in developing thermodynamic prediction models to identify their physicochemical features and phase behavior for gas production from hydrates and operation of oil/gas production systems. Though a significant amount of experimental data are available in the literature and models have been developed based on those data, a reliable method to assess the thermodynamic consistency of the data has not been established. Here, we provide a rigorous method to assess the thermodynamic consistency of hydrate phase equilibrium data in inhibited systems. Fundamental thermodynamic relations are used to derive the assessment criteria for thermodynamic consistency, including the heat of dissociation for solid hydrate phase and the activity for liquid water. The quantitative assessment of CH4 hydrate + THIs, including salts, glycols, methanol, and amino acids are performed for demonstration, and guidelines on how to assess the thermodynamic consistency are provided.

We present pure CH4 and CH4/C3H8 mixed hydrate phase equilibria formed from a mixture of NaCl (10 wt %) and monoethylene glycol (MEG, 10 and 30 wt %) solutions. As expected for thermodynamic inhibitors, the mixture of salt and glycol causes the hydrate phase equilibrium boundary to shift to lower temperatures and higher pressures, and on increasing the MEG concentration, the hydrate stable region shifted more. The measured experimental data are also compared with a thermodynamic model recently developed, named the Hu–Lee–Sum correlation, showing that the data match well with the predictions. The experimental data were used to calculate the enthalpy of hydrate dissociation. The enthalpies of CH4 hydrates in the mixture of 10 wt % NaCl brine and 10 or 30 wt % MEG were found to be ∼58.7 and 54.63 kJ/mol, respectively, corresponding to structure I hydrates, whereas for the CH4/C3H8 (91.98:8.02 mol %) mixed gas system, the enthalpies of dissociation were found to be ∼101.10 kJ/mol (10 wt % NaCl + 10 wt % MEG) ...

Pipe blockage due to gas hydrate formation is a main concern in the oil and gas industry due to the revenue losses caused by either production impairments or interruptions, and to the high costs associated to the elimination of such blockages. Assuming a hydrate formation rate in the gas-water interface based on the system subcooling, the present work models the transition from two-phase liquid-gas to three-phase solid-liquid-gas flows when hydrates form. The multiphase flow is assumed to be within the slug flow pattern region, as this is the prevailing flow regime in offshore production scenarios. The model couples mass, momentum and energy balances for the slug flow unit cell. The hydrate phase is assumed as homogeneously dispersed in the water. The gas and water consumption rates due to hydrate formation are modeled as source terms in the mass balance equations. The exothermic characteristic of the hydrate formation is taken into account in the energy conservation equation. The model provides analytic expressions for temperature and pressure distributions along the pipeline. However, the unit cell geometry is solved by numerical integration and the model closure is achieved only when empirical correlations for the slug frequency, the unit cell translational velocity and the slug aeration are used. The results from the numerical simulations are presented for the same input parameters for cases with and without hydrate formation. The discussion focuses on the influence of hydrate formation in the slug flow hydrodynamics and heat transfer. The main mechanisms affecting the mixture temperature and pressure distributions, the mixture heat transfer coefficient, the superficial velocities of the phases, the liquid loading, the slug flow frequency and the unit cell geometry are presented.

In offshore oil production, scheduled or emergency shutdowns of the production system may occur due to maintenance, equipment failure, production issues, and weather-related events. During the shut-in, the fluids may enter into the hydrate zone because of the thermal energy transfer to the cold ocean waters, increasing the risk of hydrate formation during the shut-in period and upon the restart of the production system. This work is focused on identifying some parameters that influence the hydrate formation in shut-in and restart conditions. Using a rock-flow cell with visual capabilities, the hydrate formation and the phenomena involved upon the restart were investigated. The main results of the experiments showed that the high shear conditions applied at the restart promoted water dispersion into the oil phase and contributed to the dispersion of hydrates. In addition, the low shear rate favored the formation of hydrate deposits while hydrate agglomerate was formed at high shear rate. Besides the shear rate, the subcooling proved to be an important parameter that favors the hydrate formation under sheared conditions. Conditions with high subcooling and low water cut promoted the formation of small hydrate particles (~2 mm) that can be dispersed in the oil-phase even without the anti-agglomerant. A high subcooling promoted the formation of “dry” hydrates, preventing the particles to stick with each other (agglomerates). The results indicated that subcooling is an important parameter to ensure a safe restart in the pipeline. Lastly, the addition of an anti-agglomerant reduced the water–oil interfacial tension, which promoted the water/hydrate dispersion; the effectiveness of the anti-agglomerant was clearly affected by the shear and the water cut.

Sloughing of gas hydrates from deposits formed on the pipe wall is a phenomenon that can cause hydrate accumulation and blockage of the flow in oil/gas pipelines. While hydrate sloughing has been recognized as an important mechanism leading to hydrate blockage, its observation and measurements have not been reported. Experiments performed in a visual rocking cell to emulate multiphase flow conditions with a methane–ethane gas mixture, fresh water, and non-emulsifying oil or condensate as hydrocarbon liquid demonstrated that hydrate sloughing occurs at a wide range of subcooling and temperature gradient conditions. However, sloughing was not detected in a narrow operational window defined by both subcooling lower than 4 °C and temperature gradient in the cell lower than 1 °C. The potential existence of an operational window for conditions without sloughing might be valuable for the development of hydrate management strategies for blockage-free production.

Formation of gas hydrates in oil/gas pipelines has to be properly managed as they can often lead to plugging, primarily by deposition, causing safety issues and significant expenses for repair and recovery. Early detection of hydrate deposition is thus critical for managing such risks and establishing stratagies for hydrate mitigation and remediation. Here, a permittivity probe is applied to an 1-inch vertical pipe system in order to detect hydrate deposition. The vertical pipe system simulates a deadleg, which is a pipe section used for intermittent services and maintenance in hydrate management. Hydrate deposition under water saturated gas environment is monitored by measuring the dielectric constant of the hydrate layer, which is considered as a three-component mixture of hydrates, gas, and water. The permittivity responses upon hydrate formation and dissociation are observed, and their physical interpretations are also provided. By applying appropriate models, thickness, wetness, and porosity of hydra...

Gas hydrates phase equilibria for structure I and II hydrates with chloride salts (NaCl, CaCl 2 , KCl and MgCl 2 ) were measured at high salt concentrations and up to 200 MPa. The measured equilibrium data represent three-phase (Solution – Hydrate – Vapor) or four-phase (Solution – Hydrate – Salt precipitated – Vapor) equilibrium depending on the salt concentration. The hydrate phase boundary with salts was shifted to lower temperatures and higher pressures when the experimental system was below the salt saturation concentration, while the boundaries were unchanged at salt concentrations above saturation, corresponding to quadruple points. The experimental data were compared with hydrate equilibrium predictions calculated by commonly used predictive tools to assess the reliability of these tools for the brines and conditions considered. The comparison demonstrates that predictive tools exhibit large deviation to the measured data, especially at high pressures and high salinity conditions.

In the present study, Raman spectroscopic measurements were performed using clathrate hydrate samples composed of ethanol, propan-1-ol, or propan-2-ol + deuterated methane + deuterium oxide. The temperature of the samples was stepwise increased from 93 K to 263 K in steps of 10 K or 20 K to decompose the clathrate hydrate samples during the measurements. The Raman spectra of the alcohols + deuterated methane hydrate sample in the C–H symmetric vibration region were similar to that of pure vapor alcohols, which indicates the weak guest alcohol–host water interaction although alcohols are hydrophilic compounds. In the propan-2-ol + deuterated methane + deuterium oxide system, the formation of two compounds having different composition from structure II propan-2-ol + deuterated methane clathrate hydrate was observed.

Deadlegs are the pipe sections used for specific services in production and transportation of oil and gas, and they often encounter hydrate management challenges. Despite stagnant fluids in deadlegs, warm water vapor readily condenses on the cold pipe wall, resulting in a risk of hydrate blockages by deposition. Proper management of hydrates in deadlegs is therefore required for economic and safety reasons. Here, we discuss the development of an 1-in. vertical pipe system that is designed to study hydrate deposition from water saturated gas. From a series of hydrate formation and dissociation, the hydrate deposits are characterized to obtain gas/water consumption, thickness/volume hydrate deposit distribution, hydrate morphology, and hydrate porosity and wetness. These characteristic properties are correlated with the header temperature and the time duration for hydrate deposition. Qualitative and quantitative information obtained from the present study contribute to our understandings of hydrate depositi...

In this work, we study the rheology of hydrate slurries in a mixture of water and THF (tetrahydrofuran, C4H8O). This hydrocarbon is miscible in water, and forms hydrates at ambient pressure and temperatures above 0 °C. Rheological tests, constant shear rate, flow curve, creep, and oscillatory, are carried out for different concentrations. Transient and steady-state results are obtained, suggesting that the rheology is strongly affected by agglomeration and breakage of hydrate crystals that seem to happen simultaneously, when the slurry is sheared during hydrate formation.