Low dosage hydrate inhibitors (LDHIs) are a recent and alternative technology to thermodynamic inhibitors for preventing gas hydrates from plugging oil and gas production wells and pipelines. LDHIs are divided into two main categories, kinetic inhibitors (KHIs) and anti-agglomerants (AAs), both of which are successfully being used in field applications. This paper reviews the research and development of LDHIs with emphasis on the chemical structures that have been designed and tested. The mechanisms of both KHIs and AAs are also discussed.

History of the Development of Low Dosage Hydrate Inhibitors

A new equation of state for the thermodynamic properties of natural gases, similar gases, and other mixtures, the GERG-2008 equation of state, is presented in this work. This equation is an expanded version of the GERG-2004 equation. GERG-2008 is explicit in the Helmholtz free energy as a function of density, temperature, and composition. The equation is based on 21 natural gas components: methane, nitrogen, carbon dioxide, ethane, propane, n-butane, isobutane, n-pentane, isopentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, hydrogen, oxygen, carbon monoxide, water, hydrogen sulfide, helium, and argon. Over the entire composition range, GERG-2008 covers the gas phase, liquid phase, supercritical region, and vapor–liquid equilibrium states for mixtures of these components. The normal range of validity of GERG-2008 includes temperatures from (90 to 450) K and pressures up to 35 MPa where the most accurate experimental data of the thermal and caloric properties are represented to within their accura...

The GERG-2008 Wide-Range Equation of State for Natural Gases and Other Mixtures: An Expansion of GERG-2004

[1] Methane gas hydrates, crystalline inclusion compounds formed from methane and water, are found in marine continental margin and permafrost sediments worldwide. This article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate-bearing sediments. Formation phenomena include pore-scale habit, solubility, spatial variability, and host sediment aggregate properties. Physical properties include thermal properties, permeability, electrical conductivity and permittivity, small-strain elastic P and S wave velocities, shear strength, and volume changes resulting from hydrate dissociation. The magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate-bearing sediments to changes in mechanical, thermal, or chemical boundary conditions. These predictions are vital for mitigating borehole, local, and regional slope stability hazards; optimizing recovery techniques for extracting methane from hydrate-bearing sediments or sequestering carbon dioxide in gas hydrate; and evaluating the role of gas hydrate in the global carbon cycle.

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Physical properties of hydrate-bearing sediments

The stability of submarine gas hydrates is largely dictated by pressure and temperature, gas composition, and pore water salinity. However, the physical properties and surface chemistry of deep marine sediments may also affect the thermodynamic state, growth kinetics, spatial distributions, and growth forms of clathrates. Our conceptual model presumes that gas hydrate behaves in a way analogous to ice in a freezing soil. Hydrate growth is inhibited within fine-grained sediments by a combination of reduced pore water activity in the vicinity of hydrophilic mineral surfaces, and the excess internal energy of small crystals confined in pores. The excess energy can be thought of as a “capillary pressure” in the hydrate crystal, related to the pore size distribution and the state of stress in the sediment framework. The base of gas hydrate stability in a sequence of fine sediments is predicted by our model to occur at a lower temperature (nearer to the seabed) than would be calculated from bulk thermodynamic equilibrium. Capillary effects or a build up of salt in the system can expand the phase boundary between hydrate and free gas into a divariant field extending over a finite depth range dictated by total methane content and pore-size distribution. Hysteresis between the temperatures of crystallization and dissociation of the clathrate is also predicted. Growth forms commonly observed in hydrate samples recovered from marine sediments (nodules, and lenses in muds; cements in sands) can largely be explained by capillary effects, but kinetics of nucleation and growth are also important. The formation of concentrated gas hydrates in a partially closed system with respect to material transport, or where gas can flush through the system, may lead to water depletion in the host sediment. This “freeze-drying” may be detectable through physical changes to the sediment (low water content and overconsolidation) and/or chemical anomalies in the pore waters and metastable presence of free gas within the normal zone of hydrate stability.

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Formation of natural gas hydrates in marine sediments 1. Conceptual model of gas hydrate growth conditioned by host sediment properties

The development of van der Waals cubic equations of state and their application to the correlation and prediction of phase equilibrium properties is presented and analyzed. The discussion starts with a brief account of the contributions to equation of state development during the years before van der Waals. Then, the original equation proposed in the celebrated thesis of van der Waals in 1873 and its tremendous importance in describing fluid behavior are analyzed. A chronological critical walk through the most important contributions during the first part of the 1900s is made, to arrive at the proposal that I consider to be the most outstanding since van der Waals:  the equation proposed by Redlich and Kwong in 1949. The contributions after Redlich and Kwong to the modern development of equations of state and the most recent equations proposed in the literature are analyzed. The application of cubic equations of state to mixtures and the development of mixing rules is put in a proper perspective, and the ...

The state of the cubic equations of state

Preface. Nomenclature. Phase Behaviour Fundamentals. Reservoir Fluid Composition. Phase Behaviour. Pure compound. Corresponding states. Multicomponent mixture. Classification of Reservoir Fluids. Dry gas. Wet gas. Gas condensate. Volatile oil. Black oil. References. Exercises. PVT Tests and Correlations. Fluid Sampling. Well preparation. Sample collection. PVT Tests 38. Dry gas. Wet gas. Black oil. Gas condensate. Volatile oil. Emperical Correlations. Black oil. (Bubble point pressure. Gas in solution. Oil formation volume factor. Total formation volume factor. Oil density. Oil viscosity). Natural gas. (Volumetric data. Gas viscosity). Formation water. (Water content of hydrocarbon phase. Hydrocarbon solubility in water. Water formation volume factor. Compressibility of water. Water density. Water viscosity). References. Exercises. Phase Equilibria. Criteria for Equilibrium. Chemical potential. Fugacity. Activity. Equilibrium Ratio. Raoult's law. Henry's law. Emperical correlations. References. Exercises. Equations of State. Viral EOS and its Modifications. Starling-Benedict-Webb-Rubin EOS. Cubic Equations of State. Two-parameter EOS. (Soave-Redlich-Kwong EOS. Peng-Robinson EOS. Volume shift). Three-parameter EOS. (Schmidt-Wenzel EOS, Patel-Teja EOS). Attracting term temperature dependency. Mixing Rules. Random mixing rules. Non-random mixing rules. References. Exercises. Phase Behaviour Calculations. Vapour-Liquid Equilibrium Calculations. Root selection. Rapid flash calculations. Stability Analysis. Stability limit. Critical Point Calculations. Compositional Grading. Equilibrium assumption. Non-equilibrium fluids. Heat of transport. Significance. References. Exercises. Fluid Characterisation. Experimental Methods. Distillation. Gas chromatography. Critical properties. Lee-Kesler correlations. Riazi-Daubert correlations. Perturbation expansion correlations. Description of Fluid Heavy End. Single carbon number function. Continuous description. Direct application. References. Exercises. Gas Injection. Miscibility Concepts. Miscibility in Real Reservoir Fluids. Experimental Studies. Slim tube. Rising bubble apparatus. Contact experiments. Prediction of Miscibility Conditions. First contact miscibility. Vaporising gas drive. Condensing-vaporising gas drive. References. Exercises. Interfacial Tension. Measurement Methods. Prediction of Interfacial Tension. Parachor method. Corresponding states correlation. Comparison of predictive methods. Water-Hydrocarbon Interfacial Tension. References. Exercises. Application in Reservoir Simulation. Grouping. Group selection. Group properties. Composition retrieval. Comparison of EOS. Phase composition. Saturation pressure. Density. Gas and liquid volumes. Robustness. Tuning of EOS. Fluid characterisation. Selection of EOS. Experimental data. Selection of regression variables. Limits of tuned parameters. Methodology. Dynamic Validation of Model. Relative permeability function. Viscosity prediction.

PVT and Phase Behaviour of Petroleum Reservoir Fluids

Following our previous communications on the impact of the amount of water phase on the time required at each temperature-step 1 and the limitations of visual techniques in determining hydrate dissociation points, 2 a series of tests were conducted in this laboratory to investigate the impact of measuring techniques, mixing efficiency, heating method and heating rate on the accuracy of hydrate dissociation point measurements. The results showed that nonvisual techniques combined with stepwise heating and satisfactory mixing could save a significant amount of time while providing accurate hydrate dissociation data.

Improving the accuracy of gas hydrate dissociation point measurements

Abstract: Following our previous communications on the impact of the amount of water phase on the time required at each temperature‐step 1 and the limitations of visual techniques in determining hydrate dissociation points, 2 a series of tests were conducted in this laboratory to investigate the impact of measuring techniques, mixing efficiency, heating method and heating rate on the accuracy of hydrate dissociation point measurements. The results showed that nonvisual techniques combined with stepwise heating and satisfactory mixing could save a significant amount of time while providing accurate hydrate dissociation data.

Ali Danesh

Papers

PVT and Phase Behaviour of Petroleum Reservoir Fluids

Improving the accuracy of gas hydrate dissociation point measurements

Improving the accuracy of gas hydrate dissociation point measurements

Is subcooling the right driving force for testing low-dosage hydrate inhibitors?

Prediction of VL and VLL equilibria of mixtures containing petroleum reservoir fluids and methanol with a cubic EoS