Unusual Self-Preservation of Methane Hydrate in Oil Suspensions

Article pubs.acs.org/EF

Unusual Self-Preservation of Methane Hydrate in Oil Suspensions Andrey S. Stoporev,† Andrey Yu. Manakov,*,†,‡ Lubov’ K. Altunina,§ Andrey V. Bogoslovsky,§ Larisa A. Strelets,§ and Eugeny Ya. Aladko† †

Nikolaev Institute of Inorganic Chemistry SB RAS, 3 Academician Lavrentiev Avenue, Novosibirsk 630090, Russian Federation Novosibirsk State University, 2 Pirogova Street, Novosibirsk 630090, Russian Federation § Institute of Petroleum Chemistry SB RAS, 4 Akademichesky Avenue, Tomsk 634021, Russian Federation ‡

S Supporting Information *

ABSTRACT: The effect of self-preservation of methane hydrate particles with a characteristic size of a few tens of micrometers was found in suspensions of the hydrate in four crude oils. For example, an ice−hydrate suspension in one of the oils at atmospheric pressure and a temperature of −20 °C loses only 4% of its gas in 5 h. No self-preservation occurs in suspensions of the hydrate in decane with similarly sized hydrate particles. We consider that the observed self-preservation of such small particles is mainly due to the formation of insulating ice shell on the surface of the hydrate particles. Our results show that the adsorbed medium influences the self-preservation of gas hydrates and allows for this phenomenon to be managed.

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So-called “thermodynamic inhibitors” (methanol, glycols, etc.) that shift the hydrate equilibrium curve toward lower temperatures and higher pressures are most commonly used to prevent hydrate-plug formation.2,4,6 Nowadays, two types of low-dosage inhibitors are also used: kinetic inhibitors, which increase the induction time of GH formation, and antiagglomerates, which prevent aggregation of hydrate particles.4,7 Kinetic inhibitors also limit growth of GH after nucleation. In some cases, crude oil already contains natural kinetic inhibitors and anti-agglomerates.8−12 Methods for combining the transportation of oil and associated gas with the use of nonagglomerating GH suspensions are discussed in the literature.13−15 Thus, studies of the formation, properties, and stability of GH suspensions in crude oils are topical. A review of some works performed in this area is given below. A model assuming the rapid formation of hydrate shells on droplets of emulsified water followed by slow hydrate growth inward of droplets has been developed in detail.16 The model accounts for stages of methane transfer through the gas−oil boundary and its diffusion through the oil and hydrate shell at the surface of a water droplet. The influence of stirring intensity, degree of system oversaturation, and water cut on the initial rate of hydrate formation have also been studied. It has been shown that, as any of these parameters becomes larger, the rate becomes greater. Detailed studies of hydrate formation from water-in-oil emulsions were performed using calorimetric methods.17,18 For dilute emulsions (20 vol % water), the authors suggest a mathematical model that well describes the kinetics of the process. Initially, independent hydrate formation on individual water droplets of emulsion (each droplet working as an independent reactor) was assumed.17 Later, the model was modified to anticipate the appearance of a hydrate nucleus at one of the droplets, with subsequent spreading of the process

INTRODUCTION A substantial portion of hydrocarbon resources in the Earth’s crust exists in the form of gas hydrates (GHs). In addition, in many cases, GH formation occurs during production and transportation of natural gas and oil, which complicates these technological processes.1−5 Thus, the cost of preventing GH formation in the U.S. petroleum industry amounts to almost 200 million dollars per year.4 That is the reason for the great interest of the petroleum industry and researchers in investigations of GHs and processes of GH formation. GHs are clathrate compounds with the crystalline host framework formed by hydrogen-bonded water molecules. Guest molecules of gases or volatile liquids are included in the cavities of the framework. The type of hydrate framework is mainly determined by the size of the guest molecules. Methane, ethane, and carbon dioxide form GH with a so-called “cubic structure I” (CS-I, sI) type framework. Small diatomic molecules (nitrogen and oxygen) and larger molecules (propane, sulfur hexafluoride, and tetrahydrofuran) form hydrates of “cubic structure II” (CS-II, sII). Detailed reviews of different aspects of GH science are available from the literature.1,2 Some conceptual ideas on the mechanism of formation of hydrate suspensions in oils and hydrate plugging of pipelines are discussed in the literature.4,5 According to data presented in these works, the first stage of the process is the formation of water-in-oil emulsions. In the presence of dissolved gas, at suitable temperature and pressure, the surface of a water droplet is quickly covered by hydrate film with a thickness of 10−30 μm. Subsequent growth of the hydrate occurs “inside” a particle and is limited by gas diffusion through the GH shell formed. Then, adhesion of the formed particles and formation of hydrate plugs occur. Notably, hydrate plugs can be generated by conversion of just a small fraction of the water into the hydrate state, with unconverted liquid water in that case being trapped within GH shells formed earlier. © 2014 American Chemical Society

Received: August 8, 2013 Revised: December 28, 2013 Published: January 1, 2014 794

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Table 1. Composition and Properties of the Crude Oils and Emulsions Used in This Work oil field Urubcheno-Tokhomskaya (UTOF) content of asphaltene (mass %) content of paraffin (mass %) content of resin (mass %) solidification temperature (°C) density (kg/m3) viscosity (mPa s) density of the emulsion (kg/m3) viscosity of the emulsion (mPa s) average size of water droplets in the emulsion (μm) (standard deviation of the average size)/size range (μm)

absent 7.6 below −30 816 55.8 908 109 47 (40)/10−260

to neighboring droplets.18 GH formation was also studied in high water cut emulsions and oil-in-water emulsions.19,20 The presence of free water results in noticeable agglomeration and phase inversion during formation and decomposition of hydrates. The size of the water droplets emulsified in oil and the sizes of hydrate particles in suspensions obtained from these emulsions were studied.21−23 Experiments were performed with the use of laboratory reactors21,23 and a flow loop.22 It was demonstrated that particle sizes in suspension are a bit larger than those of the initial water droplets.21 CCl3F hydrate formation/decomposition in water-in-oil and aqueous NaCl solution-in-oil emulsions were studied by differential scanning calorimetry (DSC).24,25 The heat emission and rate of hydrate formation were maximal immediately after hydrate nucleation. Dielectric and proton nuclear magnetic resonance (1H NMR) methods were also applied to study GH formation in water-inoil emulsions.26,27 Application of the 1H NMR microimaging method to investigation of GH formation in water droplets discovered some features of the process that partially contradict observations discussed above.28,29 In the context of this work, the most interesting features are the long-term coexistence of hydrate and liquid water in the same droplet, coexistence of several hydrate crystallites in the same droplet, and (in some cases) formation of hydrate particles in the central part of the water droplet. The results of these works show that development of more sophisticated models of GH formation in droplets is necessary. Comprehensive discussions of various properties of some oils in the context of flow assurance are available.30,31 These papers exemplify an integrated approach to the problem. Analysis of published reviews1,2,4,32,33 shows that the formation and especially decomposition of hydrate suspensions in oils at temperatures below 0 °C have been studied little until now. To the best of our knowledge, there is the only work on rheological properties of GH suspensions in oil at subzero temperatures.34 At the same time, self-preservation of GH occurs at temperatures below 0 °C. The most known practical aspect of the self-preservation phenomenon is GH transportation of natural gas.35,36 The phenomenon of GH selfpreservation represents a significant (by many orders of magnitude) decrease of the GH decomposition rate because of an ice shell forming on the hydrate surface.37−39 It was also suggested that the surface process of GH decomposition at temperatures below 0 °C results in the formation of gas and metastable liquid water with subsequent crystallization of the ice shell.40 The self-preservation effect has been studied in numerous works.41−47 Self-preservation appears to be temper-

Verkhnechonskaya (VOF)

Gerasimovskaya (GOF)

0.1 2.3 19.7 −43 858 19.3 919 184.9 20 (9)/6−69

2.2 5.1 5.1 +6 863 25.1 931 130.8 16 (4)/6−25

Usinskaya (UOF) 9.9 1.1 31.1 965 7062.0 990 5060 37 (20)/5−82

ature-dependent. The hydrate decomposition rate rises steadily when the temperature increases from −78 to −33 °C; further temperature increases up to −2 °C result in significant decreases in the rate.41 The minimum rate was registered at −5 ± 1 °C. It was found that, at −84 °C, the decomposition process slows abruptly after the appearance of ice Ih on particle surfaces.42,43 The coefficient of methane diffusion through an ice film was determined to be 2.2 × 10−11 m2/s at −84 °C and 8.7 × 10−11 at −75 °C. It was shown by X-ray powder diffraction that the formation of plate-like ice crystals promotes self-preservation of methane hydrate.46 The self-preservation behavior of mixed methane + ethane hydrates with different methane/ethane molar ratios was also studied. 47 The composition of the hydrate appears not to be the only factor influencing the self-preservation of this hydrate. In particular, impurities of the untransformed water in the hydrate sample promoted the self-preservation of this sample. The formation of the ice layer on the surface of the hydrate and its morphology were studied with the use of electron microscopy.48,49 Only a few works have been devoted to studying the effect of hydrate particle size on the self-preservation process. Self-preservation occurs for particles with a characteristic size of 200−300 μm at a pressure of 0.7 MPa.44 According to the data of ref 45, at atmospheric pressure, all hydrate particles with sizes below approximately 250 μm dissociated at about 210 K. However, larger hydrate particles (above 1 mm) have a high-temperature regime, in which the hydrate fraction decreased very little with increased temperature. For particle sizes of 1−1.4 mm, 20% of the hydrate remained even at 263 K.45 Self-preservation of carefully powdered samples of GHs formed by different hydrate formers has been studied with use of X-ray diffraction.50 Most likely, these samples were compacted during preparation of the sample for powder diffraction experiments. It makes for a difficult comparison of these data to the data presented in other papers. It was recently demonstrated that it is possible to control the self-preservation of GHs by choosing appropriate hydrophobic and hydrophilic media.51 In summary, analysis of the literature data and the experience of the authors suggest that self-preservation is a very complex phenomenon, with its effectiveness being dependent upon the size of the hydrate sample, method and rate of hydrate decomposition, method of the sample preparation (freely sifted powder, compacted powder, monolithic piece of hydrate, etc.), hydrate hosting medium, and guest material. Although we know the most common factors that can affect self-preservation (e.g., better self-preservation of compacted hydrate samples), significant details of self-preservation remain unclear. 795

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portion of the suspension (0.3−0.5 g) at the temperature of liquid nitrogen was placed into a specially designed cell connected to a gas buret. Then, the cell was warmed at a rate of 1−2 K/min, and the current temperature of the cell was registered. Heating the cell from −160 to −30 °C took 1 h, and heating the cell from −30 to 5 °C also took 1 h (also see the Supporting Information). The emitting gas was collected into a graduated buret filled with a saturated aqueous solution of NaCl. The NaCl solution was used to avoid dissolution of the emitting gas in the liquid. The content of GH in samples of the suspensions was calculated from the volume of the gas emitted and the mass of the sample after decomposition. Ice was used instead of hydrate as a control. The respective curve can be found in Figure 1. In several experiments, we loaded gas-saturated emulsions without hydrate to ensure an accurate interpretation of the gas emission curve.

Overall, we may conclude that the properties of hydrate suspensions at temperatures below 0 °C have been studied insufficiently. In particular, there is no information about the self-preservation behavior of hydrate particles in suspensions. In this work, we present results on the decomposition kinetics of GH suspensions in different media at subzero temperatures.

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EXPERIMENTAL SECTION

Methane (purity of 99.98%; produced by Moscow Gas Refinery Plant, Moscow, Russia), distilled water, decane (pro analysi; produced by “REAKHIM” at Novocherkassk Plant of Synthetic Products, Russia), four crude oils (Table 1), a sample of asphaltene−resin−wax deposit (ARWD) from the Irelyahskaya oilfield (asphaltene, 4.1 mass %; resin, 12.6 mass %), and surfactant SPAN-80 (Sigma-Aldrich) were used. A suspension (solution) of ARWD in decane was prepared so that its total content of asphaltenes and resins approximately matched that of Gerasimovskaya oil. Water emulsions in oils from the Verkhnechonskaya, Gerasimovskaya, and Usinskaya oilfields and an ARWD decane suspension at a mass ratio of 50:50 were prepared at room temperature using an electrical mixer (hand mixer ETO 2046 Promise, made in Czech Republic; 800 rpm) for 20 min without surfactant additives. The samples [200.0 (0.1) g of oil and 200.0 (0.1) g of water] were stirred in the mixer. The resulting emulsion was placed into a separating funnel for 2 days. As a rule, only 1−2 droplets of water separated out; therefore, the composition of the emulsion was practically unchanged. The emulsions remained stable for a long time (months). A water-inoil emulsifier SPAN-80 was used to prepare emulsions of water in decane and water in Urubcheno-Tokhomskaya oil. In both cases, the water content was 50 wt % and the concentration of SPAN-80 in water was 1 wt %. These samples were cooled by melting ice and mixed by an ultrasound disperser immediately prior to experiments. The emulsions were stable for 24 h. An optical microscope was used to determine the size of the emulsion droplets and particle size in suspensions. Information about the size of water droplets in the emulsions is presented in Table 1 and in the Supporting Information. In one of the experiments, the sizes of water droplets in the initial emulsion and hydrate particles in the resulting suspension were determined using a TM-1000 Hitachi scanning electron microscope. To take pictures, emulsion samples were frozen by quenching in liquid nitrogen to preserve the hydrate particles, crushed, and placed on a metallic sample holder cooled to the temperature of liquid nitrogen. The exposure was limited by the heating dynamics of the sample. Suspensions were placed in a high-pressure cell equipped by a shutoff valve and a manometer. The internal diameter of the cell is 58 mm, and the height of the cell is 110 mm, with a working pressure of up to 20 MPa. Synthesis of GHs in the emulsions was performed without agitation. The emulsion (20−30 mL of 50 wt % emulsion of water in decane, water in oil, or an aqueous dispersion of ARWD in decane) was put into the cell. The cell was purged with methane to remove any residual air. Afterward, the cell was filled with methane up to 6−9 MPa and kept at ∼1 °C. Hydrate formation was registered by the pressure drop at a constant temperature. To achieve a high degree of water transformation into a hydrate, the synthesis was carried out for a long time (up to 2 months) under constant methane pressure. The entire apparatus was cooled to the temperature of liquid nitrogen before sample recovery. A monolith of quenched hydrate suspension obtained in this way was crushed into fragments using a chisel and hammer that had been cooled with liquid nitrogen. Pieces of frozen hydrate suspensions with characteristic sizes of 3−4 mm were stored in liquid nitrogen. The molar ratio of unreacted water and GH was different in different pieces of frozen suspensions. This value was determined with use of thermovolumetric analysis (see below). Samples of frozen hydrate suspensions with a smaller characteristic size of particles were obtained by crushing these pieces in mortar cooled to a liquid nitrogen temperature. Different fractions were selected by sieving the resulted powders in a bath filled with liquid nitrogen. A thermovolumetric method52 was used to determine the volume of gas emitted from a sample as a function of its temperature. A small

Figure 1. Thermovolumetric curve and powder X-ray diffraction patterns of methane hydrate suspension in decane (experiment decane 1/1, ●). Thermovolumetric curves for pure methane hydrate (○) and ice (baseline, □) are given for comparison. The X-ray diffraction study was performed using synchrotron radiation at the fourth beamline of the VEPP-3 storage ring (Siberian Synchrotron and Terahertz Radiation Center, Budker Institute of Nuclear Physics, SB RAS, Novosibirsk, Russian Federation). Debye− Scherrer geometry with short-wave synchrotron radiation (λ = 0.3685 Å)53 was applied. The diffraction picture was registered by a MAR 345 imaging plate detector. The distance between the sample and the detector amounted to approximately 370 mm, as determined with the use of the NaCl calibrant. The sample was pre-prepared as described previously, then quenched in liquid N2, crushed, and placed into a cooled aluminum cell. After that, the cell was mounted on a sample holder and heated at a rate of ≈3 °C/min, starting from the temperature of liquid nitrogen. Diffraction patterns were obtained at different temperatures in the range from −150 to 0 °C. Each diffraction pattern was accumulated for 4 min, and the resulting data were associated with the temperature averaged over this time interval. Integration and processing of the powder diffraction patterns were performed by the FIT2D program.54 A portion of the results was obtained with the use of a Bruker D8 Advance diffractometer equipped with a TTK 450 Anton Paar low-temperature device. Powder diffraction patterns were recorded at 2θ = 5−45° in 2θ scanning mode. Samples were placed in a holder previously cooled to −120 °C. The positions of diffraction peaks corresponding to CS-I methane hydrate, ice Ih, and solid decane were calculated with the use of reference data on space group and unit cell parameters of the respective compounds.

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RESULTS AND DISCUSSION The experiments were performed according to the following scheme: (a) synthesis of hydrate, (b) recovery of a quenched sample from the apparatus, (c) thermovolumetric experiments, and (d) low-temperature X-ray powder diffraction studies of 796

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Table 2. Summary of the Data Obtained in Thermovolumetric Experiments and Some Characteristics of the Samples dispersive medium decane UTOF

VOF

GOF

UOF suspension of ARWD in decane

numbera

tb

Sc (mm)

αd

1/1 1/1 1/2 1/3 2/4 1/1 1/2 2/3f 2/4f 1/1 1/2 1/3 1/4 1/5 1/6 1/7 1/1 1/2 1/1

1 1 1 1 29 24 24 169 169 72 72 72 72 72 72 72 21 21 59