Thermodynamic Hydrate Inhibitors Research Articles

Natural Gas Evolution in a Gas Hydrate Melt: Effect of Thermodynamic Hydrate Inhibitors.

Natural gas extraction from gas hydrate sediments by injection of hydrate inhibitors involves the decomposition of hydrates. The evolution of dissolved gas from the hydrate melt is an important step in the extraction process. Using classical molecular dynamics simulations, we study the evolution of dissolved methane from its hydrate melt in the presence of two thermodynamic hydrate inhibitors, NaCl and CH3OH. An increase in the concentration of hydrate inhibitors is found to promote the nucleation of methane nanobubbles in the hydrate melt. Whereas NaCl promotes bubble formation by enhancing the hydrophobic interaction between aqueous CH4 molecules, CH3OH molecules assist bubble formation by stabilizing CH4 bubble nuclei formed in the solution. The CH3OH molecules accumulate around the nuclei leading to a decrease in the surface tension at their interface with water. The nanobubbles formed are found to be highly dynamic with frequent exchange of CH4 molecules between the bubble and the surrounding liquid. A quantitative analysis of the dynamic behavior of the bubble is performed by introducing a unit step function whose value depends on the location of CH4 molecules with respect to the bubble. It is observed that an increase in the concentration of thermodynamic hydrate inhibitors reduces the exchange process, making the bubble less dynamic. It is also found that for a given concentration of the inhibitor, larger bubbles are less dynamic compared to smaller ones. The dependence of the dynamic nature of nanobubbles on bubble size and inhibitor concentration is correlated with the solubility of CH4 and the Laplace pressure within the bubble. The effect of CO2 on the formation of nanobubble in the CH4-CO2 mixed gas hydrate melt in the presence of inhibitors is also examined. The simulations show that the presence of CO2 molecules significantly reduces the induction time for methane nanobubble nucleation. The role of CO2 in the early nucleation of bubble is explained based on the interaction between the bubble and the dissolved CO2 molecules.

Methane hydrate-liquid-vapour-equilibrium phase condition measurements in the presence of natural amino acids

This work reports the thermodynamic effect of five amino acids on methane hydrate phase boundary. The studied amino acids are glycine, alanine, proline, serine and arginine. To effectively investigate the impact of selected amino acids on methane hydrates formation, the methane hydrate-liquid-vapour-equilibrium (HLwVE) curve is measured in amino acids aqueous solutions. Experiments are performed at concentration range of 5–20 wt% by employing the isochoric T-cycle method in a sapphire hydrate cell reactor at pressures and temperatures range of 3.86–9.98 MPa and 276.50–286.00 K, respectively. Results suggests that, all studied amino acid inhibits methane hydrate formation. Glycine showed the highest inhibition effect with an average depression temperature of 1.78 K at 10 wt%. The impact of inhibition is due to amino acids hydrogen bonding energies, confirmed via COSMO-RS predictions and side group alkyl chain. The inhibition impact of glycine is found to be in the range of some ionic liquid (OH-EMIM-Cl) and slightly higher than ethylene glycol (a conventional thermodynamic hydrate inhibitor) at 10 wt%. The methane hydrate dissociation enthalpies in the presence of amino acids are calculated using Clausius–Clapeyron equation, which suggests that, amino acids do not take part in methane hydrate cage occupation during hydrate formation.

Reliable and Repeatable Evaluation of Kinetic Hydrate Inhibitors Using a Method Based on Crystal Growth Inhibition

Low dosage hydrate inhibitors (LDHIs)—antiagglomerants (AAs) and kinetic hydrate inhibitors (KHIs)—are increasingly used as an alternative to the traditional thermodynamic hydrate inhibitors for controlling gas hydrate problems in the petroleum industry. As nucleation inhibitors, KHIs induce an extended induction time (ti) at particular subcoolings before nucleation of hydrate can proceed to growth. However, due to the stochastic nature of nucleation, gaining repeatable and transferrable induction time results can be very challenging and time-consuming. To overcome this problem, study on KHI crystal growth inhibition (CGI) has yielded a KHI evaluation technique which is considerably simpler than induction time measurements. Moreover, this study shows that KHIs create a number of well-defined hydrate CGI regions as a function of subcooling which are completely repeatable and transferrable between different setups. These regions range from complete inhibition (CIR), to slow growth region (SGR), and ultimate...

A new method for the replacement of CH4 with CO2 in natural gas hydrate production

Natural gas hydrate (NGH), as a clean energy, has a great development potential in the future for its immense reserves. The CO2 replacement method has dual effects in energy exploitation and greenhouse gas (GHG) sequestration, but its CH4 replacement rate is too low. When an inhibitor is added, CH4 hydrate and CO2 hydrate are different in terms of their thermodynamic stability. In this regard, a new method was proposed and experimentally proved for NGH production. This new method combines CO2 replacement method with thermodynamic hydrate inhibitor technology to accelerate the decomposition of CH4 hydrate. Three common thermodynamic CH4 hydrate inhibitors (i.e. methanol, sodium chloride and magnesium chloride) in two categories were compared by means of core displacement experiments. It is shown that, when methanol solution is used as the inhibitor, the decomposition rate of CH4 hydrate reaches 0.01194 mol/h, which is much higher than that of electrolyte solution (0.00086 mol/h and 0.00141 mol/h respectively). Therefore, methanol solution is used to accelerate hydrate decomposition. The decomposition ratio of CH4 hydrate was over 92% when methanol solution plug was injected at the early stage and CO2 was continuously injected at the late stage. Moreover, CO2 was sequestrated in the form of hydrate. Eventually, the generated CO2 hydrate accounted for 16–27% of the initial total CH4 hydrate.

Dual inhibition effects of diamines on the formation of methane gas hydrate and their significance for natural gas production and transportation

Dual (thermodynamic and kinetic) inhibition effects of piperazine (PZ) and hydrazine (HZ), two representative diamines, on the formation of CH4 hydrate were investigated, primarily focusing on thermodynamic, spectroscopic, and computational analyses for their potential application in natural gas production and transportation. The phase behavior demonstrated that the addition of PZ and HZ shifts the CH4 hydrate equilibrium lines to higher pressure or lower temperature regions depending on diamine concentrations. The 13C NMR and Raman spectra of the CH4+diamine hydrates revealed that PZ and HZ are not enclathrated in the cages of CH4 hydrate and do not affect the hydrate structure. The time-dependent growth patterns and the induction time of CH4 hydrate in the presence of diamines were observed via in-situ Raman spectroscopy. HZ showed a more significant thermodynamic inhibition effect, slower hydrate growth, and longer induction time than PZ did. Interaction energy calculation using density functional theory (DFT) indicated that diamine molecules could disrupt the hydrogen bonding networks of hydrate cages, leading to destabilization of gas hydrate and retardation of hydrate nucleation and growth. Experimental and computational results demonstrated that both PZ and HZ can function as both kinetic and thermodynamic hydrate inhibitors for CH4 hydrate.

Barite-Scaling Risk and Inhibition at High Temperature

Summary Barite (BaSO4) is one of the common scales in oil-and-gas production. Extensive work has been conducted to study barite nucleation and inhibition at temperatures below 100°C. However, with the advance in deepwater exploration and production (E&P) which can encounter high-temperature (HT) conditions, a better understanding of barite-scaling risk at HT (e.g., > 150°C) becomes essential. In this paper, a systematic study was conducted to explore barite nucleation kinetics from 70 to 200°C in synthetic brines containing phosphonate (0–10 ppm) or polymeric (5–10 ppm) scale inhibitors. A 2-hour protection time with or without any detectable barite nucleation was used to define the scaling risk. To detect barite nucleation, two novel apparatuses were developed—a modified dynamic flow loop and a batch reactor. The modified dynamic flow loop has a retention time of up to 4 hours and is ideal to carry out experiments at higher than 100°C. Ba concentrations in the effluents were monitored to determine barite nucleation more precisely compared with traditional “tube blocking” technique. The new batch reactor uses our newly developed laser-detection method, a transparent pressure tube, and an oil bath. The transparent pressure tube allows laser light to pass through and can withstand 150-psi pressure at 175°C, therefore providing an efficient approach to study the precipitation kinetics of scales and to evaluate inhibition efficiency of inhibitors at HT. Constant inhibitor-concentration isopleths of diethylenetriamine pentamethylene phosphonic acid (DTPMP) for barite inhibition were constructed on the basis of our experimental data. Finally, a semiempirical model that is based on data of barite nucleation and inhibition kinetics from this study and previous work was built to predict scaling risk of barite at different physicochemical conditions. This model covers a wide range of temperature (from 4 to 200°C) and brine compositions. It also covers the effect of Ba2+–SO42− ratio in solution, common cations (e.g., Ca2+), and thermodynamic hydrate inhibitors on barite precipitation. Model precipitations were found to be consistent with field observations. The results of this study can guide the design of barite-scale treatment for HT oil-and-gas production.

Effect of alcohols and diols on PVCap-induced hydrate crystal growth patterns in methane systems

Methanol (MeOH), mono-ethylene-glycol (MEG) and ethanol (EtOH) are the most widely used thermodynamic hydrate inhibitors (THIs) for hydrate inhibition in hydrocarbon production operations. However, effective use of THIs often requires large quantities, which demands large storage, regeneration, and injection facilities which increase CAPEX/OPEX, in addition to environmental concerns. As a result, low dosage Kinetic Hydrate Inhibitors (KHIs) are seeing increasing use as a potential alternative for hydrate prevention, either partly or wholly replacing THIs. Combining KHIs with THIs offers a potential means to increase the sub-cooling to which KHIs can be used (THI acts as a ‘top-up’ inhibitor), while KHIs can be potentially used to reduce THI volumes. If the benefits of such combinations are to be realised, a better understanding of combined performance is required. Here, we report the results of experimental studies of combined KHI and THI methane hydrate inhibition performance using a Crystal Growth Inhibition (CGI) method previously developed at this laboratory. The KHI used for all tests is PVCap (poly-n-vinylcaprolactam) and the initial THIs investigated were MEG, MeOH and EtOH. For a more comprehensive understanding of the effect alcohols and diols (mainly the effect of the alkyl ‘tail’ carbon number of these chemicals) on PVCap performance, three other alcohols (n-propanol (n-POH), i-propanol (i-POH) and n-butanol (n-BOH)) and two other diols (1,3-propanediol (1′3-PDO) and 1,4-butanediol (1,4-BD)) have also been examined in the presence of this KHI. Results show that neither MeOH nor EtOH act as full ‘top-up’ thermodynamic inhibitors for PVCap: KHI-induced CGI regions are consistently reduced to lower sub-coolings as THI concentration is increased in both cases, although a negative effect is seen, which is more pronounced with EtOH than MeOH. Furthermore, n-POH and i-POH have slight negative effects on PVCap performance, while n-BOH has not shown this negative behaviour and has not reduced any of the CGI regions to lower sub-coolings. In contrast, MEG was found to consistently act as a full ‘top-up’ thermodynamic inhibitor to PVCap: CGI regions were larger or equal to those for PVCap alone, and were present on top of the thermodynamic inhibition offered by MEG up to concentrations of 50 mass%. Furthermore, MEG has an increasingly synergistic effect on PVCap as the concentration is increased, reducing hydrate growth rates in CGI regions where growth does occur. However, the findings indicate neither 1,3-PDO nor 1,4-BD seem to offer the same good synergism as MEG, and that the favourable properties of the latter are not ubiquitous to all diols: increasing central alkyl chain length of glycols progressively eliminates top-up properties.

Kinetic Promotion and Inhibition of Methane Hydrate Formation by Morpholinium Ionic Liquids with Chloride and Tetrafluoroborate Anions

Ionic liquids of 1-hydroxyethyl-1-methylmorpholinium chloride (HEMM-Cl) and 1-hydroxyethyl-1-methylmorpholinium tetrafluoroborate (HEMM-BF4) were synthesized to investigate the different effects of anions in the ionic liquids on the methane hydrate formation kinetics. HEMM-Cl and HEMM-BF4 acted as the kinetic hydrate promoter and inhibitor, respectively. The induction time of HEMM-BF4 solutions increased in proportion to the HEMM-BF4 concentration, and both ionic liquids showed thermodynamic hydrate inhibition effect on methane hydrate formation. The X-ray diffraction pattern of hydrates formed in the presence of both ionic liquids showed that there was no influence on, and no incorporation of ionic liquids into, the crystal structure. Using the dynamic light scattering (DLS) technique, HEMM-Cl was revealed not to form micelles, which implies that HEMM-Cl is nonamphiphilic and its hydrate promotion mechanism is different from that of amphiphilic surfactant promoters. HEMM-Cl might distort the rigid hydrat...

Experimental and DFT Approach on the Determination of Natural Gas Hydrate Equilibrium with the Use of Excess N2 and Choline Chloride Ionic Liquid as an Inhibitor

This work presents the characterization of hydrate-forming conditions of a Qatari natural gas-type mixture, QNG-S1, obtained using two different experimental methods, namely, a benchtop reactor and a gas hydrate autoclave. The obtained experimental results were found to be in agreement with each other. Another mixture in which the QNG-S1 sample was diluted with nitrogen (N2) in a 1:1 ratio was also characterized for hydrate dissociation conditions using a rocking cell apparatus only. The thermodynamic hydrate inhibition effect of a biocompatible ionic liquid, choline chloride (ChCl), was tested for both QNG-S1 and QNG-S1+N2 at two concentrations (1 and 5 wt %) using the rocking cell apparatus. It was found that the ChCl shows a typical classical thermodynamic inhibitor behavior for both tested mixtures QNG-S1 and QNG-S1+N2 by shifting the hydrate equilibrium toward lower temperature and higher pressure. Likewise, the interaction between ChCl and model hydrate cages was analyzed using density functional th...

Performance of Polymer Hydrogels Incorporating Thermodynamic and Kinetic Hydrate Inhibitors

This study investigates the hydrate formation characteristics of hydrogel particles containing thermodynamic or kinetic hydrate inhibitors. Hydrate onset time, hydrate fraction in the liquid phase, and torque changes were determined from the experiments using a high pressure autoclave. To simulate the mixture of water and liquid hydrocarbon encountered in subsea pipelines, decane was added corresponding to an initial 60% water-cut. In the liquid phase a high hydrate fraction was observed in the pure water + decane mixture with a local maximum in torque. For the hydrogel decane mixture, a lower hydrate fraction was observed, and the torque remained stable during the hydrate formation. The addition of 0.5 wt % of a commercial kinetic hydrate inhibitor (Luvicap) to the aqueous phase (with no hydrogel) delayed the hydrate onset time, but several spikes in the torque were observed during the hydrate formation, which suggests that segregation and deposition of hydrate particles is occurring in the liquid phase....

Application of COSMO-RS in Investigating Ionic Liquid as Thermodynamic Hydrate Inhibitor for Methane Hydrate

Gas hydrate formation in oil and gas pipelines has resulted in flow assurance issues. To mitigate hydrate formation, recently, ionic liquid (IL) inhibitors have been studied frequently using experimental method. However, experimental testing alone is insufficient to examine all potential ionic liquid combinations due to large amounts of cation and anion. Therefore, in this work, the thermodynamic inhibition ability of ILs have been predicted based on its fundamental property, or specifically hydrogen bonding energy. For this, Conductor-Like Screening Model for Real Solvent (COSMO-RS) software is used to simulate and study the fundamental property of IL-hydrate system. Relationship between IL inhibition ability and hydrogen bonding energy is then justified. The pattern of relationship is next applied to rank the IL inhibition ability. Through this method, pre-screening of ineffective ILs can be conducted and hence narrows down the scope of ILs waiting to be tested experimentally. As consequence, effective thermodynamic hydrate inhibitor could be discovered faster and be applied in industry.

Experimental Evaluation of a Novel Thermodynamic Inhibitor for CH4 and CO2 Hydrates

In natural gas transmission and processing, gas hydrate formation is a major flow assurance challenge which led scientists towards conducting new and more detailed studies on different aspects of gas hydrates inhibitors. Ionic liquids (IL) recently revealed as novel hydrate inhibitors due to their unique properties like electrostatic charges together with ability to form hydrogen bonding with water molecule lead them viable research area in the field of gas hydrate mitigation. This paper highlighted the experimental evaluation of thermodynamic measurements of tetra methyl ammonium hydroxide (TMAOH) for Methane (CH4) and Carbon Dioxide (CO2) gas hydrates. TMAOH belongs to ammonium based ionic liquids (AILs) which is comparatively economical ILs among the other ILs families. Traditional T-cycle technique with isochoric step heating method was adopted for determining thermodynamic inhibition in this work. Results reveal that TMAOH effectively shift the hydrate equilibrium curve to upper pressure and lesser temperature regions for CH4 + TMAOH + water system and CO2 + TMAOH + water system. The average reduced temperature obtained for CH4 + TMAOH + water system is around 1.06 oC while for CO2 + TMAOH + water system, the inhibition effect found to be around 2.09 oC. Therefore, this study provides roadmap for superior alternative for the development of novel thermodynamic hydrate inhibitor, which can efficiently control the gas hydrate formation.

Biotreatment of Hydrate-Inhibitor-Containing Produced Waters at Low pH

Summary With proper treatment to remove organics and inorganics, one can use the produced water (PW) generated during oil-and-gas extraction as process water. Biotreatment is generally regarded as the most cost-effective method for organics removal, and although widely used in industrial wastewater treatment, PW biotreatment installations are limited. This paper follows up to an earlier paper published in the SPE Journal (Janson et al. 2014). Although the earlier paper assessed the biotreatability of PW from a Qatari gas field from the summer season, this paper focuses on assessing the biotreatability of PW during the winter season [i.e., containing the thermodynamic hydrate inhibitor monoethylene glycol (MEG) and a kinetic hydrate inhibitor (KHI)]. Tests were conducted in batch and continuous reactors under aerobic mixed-culture conditions without pH control during 31 weeks. The results indicated that one could remove >80% of the chemical oxygen demand (COD) and total organic carbon (TOC) through biological treatment of PW with 1.5% MEG added. In contrast, biotreatment can remove only ≈43% of COD and TOC present in PW when 1.5% KHI was added as a hydrate inhibitor; 2-butoxyethanol, a solvent in KHI, is extremely biodegradable; it was reduced in concentration from >5000 to <10 mg/L by biotreatment; the KHI polymer though was only partially biodegradable. Cloudpoint tests conducted on PW with 1.5% KHI added showed only an 8°C increase in cloudpoint temperature (from 35 to 43°C). The target cloudpoint temperature of >60°C was not achieved. Although the feed to the reactors (PW with either KHI or MEG) was at pH 4.5, the reactors stabilized at a pH of 2.6, considered extremely acidic for aerobic bioactivity. The successful operation of an aerobic biological process for an extended period of time at a pH of 2.6 was unexpected, and published reports of bioactivity at that pH are limited. After extensive analytical tests, it was concluded that the pH decrease was caused by the production of an inorganic acid. A mechanism by which hydrochloric acid could be produced biologically was proposed; however, further research in this area by the academic community is recommended.

Experimental Measurement of Phase Equilibrium of Hydrate in Water + Ionic Liquid + CH4 System

With the goal of discovering a more effective type of thermodynamic hydrate inhibitors (THIs), the phase equilibrium conditions of CH4 hydrates were examined in the presence of morpholinium and piperidinium ionic liquids (ILs) at a mass fraction of 0.1. It was observed that the addition of ILs shifted the hydrate equilibrium conditions toward higher pressure and lower temperature compared with those of hydrates formed from pure water. Both cationic and anionic species influenced the equilibrium conditions of the CH4 hydrate. The piperidinium ILs showed better inhibition effect than did the morpholinium ILs at the same species of counteranions. The result may be due to the more hydrophobic nature of piperidinium ILs, which have a higher affinity for CH4 molecules. It was also seen that the inhibition effect of BF4– ions was stronger than that of Br– ions for both piperidinium and morpholinium ILs. Thus, the inhibition effect became stronger in the order: N-ethyl-N-methylpiperidinium tetrafluoroborate ([EMP...

Equilibria of cyclopentane hydrates with varying HLB numbers of sorbitan monoesters in water-in-oil emulsions

This study investigated the effect of hydrophilic-lipophilic balance (HLB) numbers of sorbitan monoesters on equilibrium conditions of cyclopentane hydrates in water-in-oil emulsions with or without NaCl. HLB numbers of the surfactants (Span 20, 40, 60, and 80), which have the same hydrophilic group, are determined by the length and shape of alkyl chain and the HLB number refers to hydrophobicity of surfactant. The equilibrium state of the hydrate phase can be affected by the HLB of the surfactants whose number is the lowest in Span 80 (4.3) and the highest in Span 20 (8.6). The emulsion size is proportional to the increasing HLB number of sorbitan monoesters. Equilibrium dissociation temperatures of cyclopentane hydrate in the water-in-oil emulsion also rise as the HLB number increases, which was determined in micro differential scanning calorimetry (micro-DSC). In the presence of 3.5 wt % NaCl in the aqueous phase, hydrate equilibrium temperatures were almost constant regardless of the HLB number because the salt ions diminished the interaction between water and sorbitan esters as a thermodynamic hydrate inhibitor. This HLB-dependent behavior provides basic insights into the relationship between clathrate hydrate equilibria and hydrophobicity in the hydrate-forming emulsion system.

Experimental measurements and modelling of carbon dioxide hydrate phase equilibrium with and without ethanol

In this study, three-phase equilibrium conditions of liquid water-hydrate-vapour (LW-H-V) were experimentally determined for carbon dioxide hydrates with and without ethanol, a thermodynamic hydrate inhibitor, at temperatures ranging from 275.65 to 281.65 K and pressures up to 3.5 MPa. A thermodynamic model for predicting hydrate dissociation conditions was developed. The model was based on the van der Waals–Platteeuw statistical model for the solid hydrate phase, and on the Cubic Plus Association (CPA) equation of state for the vapour and liquid phases. The Kihara potential parameters for the hydrate model were estimated using experimental and literature (LW-H-V) equilibrium data. The agreement between the experimental and the predicted dissociation pressure was in general acceptable, with an average absolute deviation of about 2.10%.

Phase equilibria of CH4 and CO2 hydrates formed from aqueous solutions of glutaric acid and malonic acid

The equilibrium boundaries of CH4 and CO2 hydrates were examined in the presence of di-carboxylic acids, glutaric acid and malonic acid, at a mass fraction of 0.1 and 0.01. Including di-carboxylic acids in the water phase shifted the phase boundaries to higher pressure and lower temperature conditions. We attributed this to the formation of hydrogen bonding between the inhibitors and water molecules rather than to the formation of water structures serving as hydrate precursors. In addition, the inhibition effect of shorter carbon-chained malonic acid was better than that of longer-chained glutaric acid due to its higher mole fraction in water phase. This inhibition effect was compared with that of conventional thermodynamic hydrate inhibitors (THIs) used in commercial processes. The comparison showed that the effect of malonic acid was close to that of tri-ethylene glycol for inhibiting the CH4 hydrate.

Cubic Plus Association Equation of State for Flow Assurance Projects

Thermodynamic hydrate inhibitors such as methanol, ethanol, (mono) ethylene glycol (MEG), and triethylene glycol (TEG) are widely used in the oil and gas industry. On modeling these compounds, we show here how the CPA equation of state was implemented in an in-house process simulator as an in-built model. To validate the implementation, we show calculations for binary systems containing hydrate inhibitors and water or hydrocarbons using the Cubic Plus Association (CPA) and Soave–Redlich–Kwong (SRK) equation of states, also comparing against experimental data. For streams containing natural gas and water, CPA was applied to calculate the loss of the inhibitor to the vapor phase as a function of temperature and pressure. Simulations of dehydration units using TEG were conducted, and the CPA results were compared with that of two commercial simulators which used their available thermodynamic packages for glycol applications, proving that the CPA calculations are in good agreement with these models and showin...

Synergism of thermodynamic hydrate inhibitors on the performance of poly (vinyl pyrrolidone) in deepwater drilling fluid

The influence of common thermodynamic hydrate inhibitors including NaCl, glycol and glycerol on the hydrate inhibition performance of PVP (poly (vinyl pyrrolidone)) in deepwater drilling fluid was experimentally studied, and the synergism of NaCl on the performance of PVP was further investigated. Results of tests demonstrated that as the representative of the first generation of kinetic hydrate inhibitors, PVP could prolong the induction time and lower the rate of hydrate growth. NaCl showed strong synergistic effect on the performance of PVP, while glycol and glycerol were not the right synergists in comparison with NaCl. The presence of NaCl enhanced both the hydrate nucleation inhibition and growth inhibition strength of PVP, thus giving a several time longer induction time with lower pressure drop rate. For deepwater drilling fluid containing 0.5 wt% PVP, the addition of 10 wt% NaCl could prolong the induction time from approximately 60 min to more than 1000 min at either the same water depth condition (2 °C, 10 MPa) or same subcooling (14.2 °C). At higher subcooling (16.4 °C), the synergistic effect was still strong, and the combination of 10 wt% NaCl and 1.5 wt% PVP could give a 600 min induction time. The long induction time is considerable for safe deepwater drilling at different water depth.

Experimental investigation on the dissociation conditions of methane hydrate in the presence of imidazolium-based ionic liquids

In this work, the performance of nine ionic liquids (ILs) as thermodynamic hydrate inhibitors is investigated. The dissociation temperature is determined for methane gas hydrates using a high pressure micro deferential scanning calorimeter between (3.6 and 11.2)MPa. All the aqueous IL solutions are studied at a mass fraction of 0.10. The performance of the two best ILs is further investigated at various concentrations. Electrical conductivity and pH of these aqueous IL solutions (0.10 mass fraction) are also measured. The enthalpy of gas hydrate dissociation is calculated by the Clausius–Clapeyron equation. It is found that the ILs shift the methane hydrate (liquid+vapour) equilibrium curve (HLVE) to lower temperature and higher pressure. Our results indicate 1-(2-hydroxyethyl) 3-methylimidazolium chloride is the best among the ILs studied as a thermodynamic hydrate inhibitor. A statistical analysis reveals there is a moderate correlation between electrical conductivity and the efficiency of the IL as a gas hydrate inhibitor. The average enthalpies of methane hydrate dissociation in the presence of these ILs are found to be in the range of (57.0 to 59.1)kJ⋅mol−1. There is no significant difference between the dissociation enthalpy of methane hydrate either in the presence or in absence of ILs.