MITIGATION OF METHANE HYDRATE BLOCKAGE IN SUBSEA PIPELINES USING IONIC LIQUID AS HYDRATE INHIBITOR
Mazuin Jasamai1, Mazlin Idress1, Mohd. Zahdan Bin Arshad1, M Faisal Taha2
1Petroleum Engineering Department,
2Fundamental and Applied Sciences Department, Universiti Teknologi PETRONAS
Email: [email protected] ABSTRACT
Gas hydrates one of a major problem in the oil and gas industry. Formation of gas hydrates causes flow assurance issues as it can plug the pipeline. The use of chemical inhibitors; kinetic hydrate inhibitors and thermodynamic hydrate inhibitors is one of the most feasible ways to solve this problem. However, the problem with thermodynamic inhibitors is that it required in a large dosage and cause environmental issues. Thus, this study emphasises on the usage of ionic liquid as an effective kinetic hydrate inhibitor. The ionic liquid is a green chemical that can be fine-tuned explicitly as a hydrate inhibitor. The aim is to study the effectiveness of 1-Ethyl-3-Methylimidazolium Tetrafluoroborate (EMIMBF4), an ionic liquid as a kinetic hydrate inhibitor at various pressure and concentration. Micro Differential Scanning Calorimeter was used to measure the induction time of methane hydrate. The performance of ionic liquid was tested in different concentration and compared to the commercial kinetic hydrate inhibitor, Polyvinylpyrrolidone (PVP). From the experimental work, it was found that EMIMBF4 shows a dynamic inhibition effect as it can delay the induction time of hydrate. EMIMBF4 shows a higher induction time at a low concentration of 0.1wt%. At the pressure of 60 bar, the effectiveness of EMIMBF4 is comparable with PVP. However, PVP shows superior kinetic inhibition effect at the pressure of 40 bar. This study indicates that an effective, green hydrate inhibitor can be developed to counter hydrate formation problems in offshore subsea pipelines in a more cost-effective and environmentally friendly.
Keywords: Methane Hydrate; Hydrate Inhibitor; Ionic Liquid; EMIMBF4; Micro DSC; pipeline blockage
INTRODUCTION
Flow assurance can be defined as an uninterrupted flow of hydrocarbon from the reservoir to the point of sale. The formation of gas hydrates can cause flow disruption in the pipeline as it agglomerates and plugs the pipeline. It decreases the cross-sectional area of the pipeline giving high backflow pressure and production loss [1]. The Offshore subsea pipeline blockage caused by gas hydrates formation can probably cause a serious flow assurance issues if it is not being treated as it affects the production of hydrocarbon, thus causing a major loss to the company itself. Currently, thermodynamic hydrate
inhibitor, methanol is widely used in the industry to counter hydrate formation problems.
However, the major problem of using methanol is that it is toxic to the environment. As the most feasible and economical method to inhibit the formation of gas hydrates, injection of inhibitor will be tested and further improved by using a green chemical which is the ionic liquid as kinetic hydrate inhibitor.
Gas hydrate is a crystalline solid that is formed when natural gas component is trapped inside water molecules. It comprises of hydrogen-bonded water molecules acting as cages that trap the gas molecules, in the water cavity [2]. Hydrocarbon compound that is
smaller than pentane such as methane, ethane and carbon dioxide is susceptible to be trapped in the water cavities [3].
The structure of gas hydrate depends on the natural gas component itself. There are three structures for gas hydrate, structure I, structure II and structure H, and all come with different size and shape. A common and light gas component such as methane (CH4), ethane (C2H6), and carbon dioxide (CO2) usually form a cubic structure I due to the small size of the guest molecule.
Propane (C3H8), Iso-butane (C4H10), etc. normally form into the other structure due to its bigger molecule size. However, Sloan [4] stated that only structure I and structure II hydrate had been found in oil and gas pipelines. There are four conditions where gas hydrate formation will occur, (i) the presence of water, (ii) natural gas component, (iii) low temperature and (iv) high pressure. Offshore subsea pipeline for oil and gas transportation manage to satisfy all the condition as it is low in temperature and high in pressure. Hydrates are formed inside the pipeline by the slow cooling of hydrocarbon and pressure increase across the valves.
The formation of hydrates inside the pipeline can cause blockage and flow disruption of hydrocarbon inside the pipeline, thus increase in backflow pressure and loss of production. In the mitigation of gas hydrate formation, several approaches have been developed and used in the oil and gas business, both mechanical and chemical way. The basic principle to control hydrate formation is to comprehend the pressure and temperature conditions in which gas hydrate might be formed.
In the effort of producing methane from gas hydrate from the reservoir, depressurisation and thermal induction are commonly used to dissociate gas hydrate. However, this method is not suitable to be used in the subsea pipeline due to its limitation.
Hence, the use of chemical inhibitors was introduced.
Currently, there are two types of hydrate inhibitor mainly used: Thermodynamic hydrate inhibitor (THI) and kinetic hydrate inhibitor (KHI). THI works by shifting the equilibrium hydrate dissociation/stability curve to lower temperatures and high pressures whereas gas hydrate will not have formed. Also, methanol
(MeOH) was the most widely used THI, but it has been replaced with monoethylene glycol due to health, safety, and environment (HSE) issues [5]. However, the significant limitation of THI is that it requires up to 60% of the weight of water that cost around $500 million annually to prevent gas hydrate formation [6].
Tohidi [7] also mentioned that huge volume of THI is required in more challenging operating conditions that will end up with a significant effect to the capital expenditure (Capex) and operational expenditures (Opex).
On the contrary, KHI only requires less than one per cent (<1%) of the weight of water, thus lead to significant cost-saving compared to THI [6]. Unlike THI, KHI is a polymeric chemical, such as Polyvinylpyrrolidone, PVP that slow down the nucleation (growth) of hydrate by disrupting water molecules with hydrogen bonding strong adsorption to the hydrate surface. In designing a new KHI, it must be extremely soluble in water, must not hydrolyse any compounds and have strong adsorption to hydrate surface [6]. Conventionally, KHI is normally combined with Anti-Agglomerates (AA) to create what is known as Low Dosage Hydrate Inhibitor (LDHI). Tariq [8] stated that Anti-agglomerates do not prevent the formation of gas hydrate particles;
however, they inhibit the agglomeration of these particles from forming bigger clusters. The existing kinetic inhibitors, however, are still not believed to give an economical solution especially at high pressure and large degrees of supercooling [9].
The ionic liquid is a green chemical that is recently known as dual-function gas hydrate inhibitor as it can be designed to become both THI and KHI. It slowed down the nucleation/growth of methane hydrate and shifted the hydrate liquid-vapour equilibrium (HAVE) [10]. Furthermore, the ionic liquids are organic salt that exists in liquid form at room temperature; it has strong electrostatic charges and able to exhibit hydrogen bond with water, thus making it a suitable hydrate inhibitor [9]. This is due to the anion groups of an ionic liquid that disrupt the hydrogen bonding of water molecules. With this in mind, the choosing criteria for the type ionic liquid as an effective KHI are soluble in water (highly hydrophilic) and abundance
with the high electronegativity anions group that can exhibit hydrogen bond with water. 1-Ethyl-3- Methylimidazolium Tetrafluoroborate (EMIMBF4) is an ionic liquid that was tested in the experiment believed to satisfied the criteria as a kinetic hydrate inhibitor.
EMIMBF4 is a dialkyl imidazolium-based ionic liquid with BF4- anion. It is air and water stable, room temperature ionic liquid with an empirical formula of C6H11BF4N2 and molecular weight of 197.97 kg kmol-1. Furthermore, EMIMBF4 is a hydrophilic ionic liquid due to the short alkyl chain which enables it to be soluble in water. The hydrophobicity of ionic liquids increased with the increasing length of alkyl chain [8].
On the other hand, anion for the ionic liquid is BF4-.
The anion is the main component that interacts with water in the prevention of hydrate formation [11]. This enables it to exhibit bond with water cations through hydrogen bonding, thus making it suitable as a kinetic hydrate inhibitor.
Acquisition
HP Micro Gas Panel
Figure 1 Micro DSC Diagram
MATERIALS AND METHOD
SETARAM High-Pressure Micro DSC (μDSC) was used as the main equipment to investigate the formation of methane hydrate. μDSC is build up with two vessels that can contain up to 0.5cm3 volume of samples. This instrument can provide high pressure up to 400 bar (pressure deviation of ±0.5 bar) and temperature range of -45°C to 120°C (±0.1°C and ±0.02 of
accuracy and precision). Furthermore, μDSC heating and cooling rate are programmable from 0.001-5°C/
min. μDSC provides heat flow information that can be used to detect the methane hydrate.
Figure 1 shows the Diagram of Micro DSC attached to the data acquisition. Data were analysed using Calisto software to integrate the area of a peak, normalise the noise, and detect the offset temperature. The temperature was controlled using the advanced Peltier cooling and heating principles which creates a difference in the temperature by applying a voltage between two electrodes connected to a sample of semiconductor material (Peltier element). The advantages of this technique are to transfer heat from one medium to another on a small scale, and no refrigerant fluid is required. A pressure gauge with an accuracy of ±0.1 MPa was used to measure the cell gauge pressure. Induction time is defined as the time
elapsed until the nucleation (growth) of methane hydrate at a detectable volume [12]. Pure methane (99.9%) are used in this experiment to obtain accurate results. Deionised water is used to prepare 10mL sample solutions.
Figure 2 shows the molecular structure of the ionic liquid used in this study. EMIMBF4 is tested in the different concentration of 0.1wt%, 0.5wt%, and 1.0wt%.
Figure 2 Molecular Structure of EMIMBF4
Results and Discussion
The induction time of methane hydrate is determined from the exothermic reaction onset time. Generally, there will be two exothermic peaks due to the formation of methane hydrate and ice. Usually, the peaks can be easily differentiated as hydrate peak will be smaller compared to the peak of ice formation due to the same latent heats of formation. Since it is known that ice melting point temperature is at 0°C, dissociation peak for ice can easily be identified.
In some cases, only one exothermic peak can be observed as ice and methane hydrate formed at the same time.
Figure 3 Induction Time measurement at 40 Bar According to Figure 3, 0.1 wt% EMIMBF4 shows the
longest induction time of 79.2 minutes, 13.4 minutes’
difference with the blank sample. This indicates that EMIMBF4 can delay the induction time of methane hydrate. It is due to the presence of BF4- anions that interact with water molecules. The interaction creates
hydrogen bond that disrupts the structure of water molecules and creates strong adsorption to the hydrate surface. Various concentration of EMIMBF4 is required to be tested to determine the relationship between the ionic liquid concentration and induction time of hydrate formation.
Furthermore, the induction time for 1.0wt% PVP concentration is 136 min. The induction time difference is very high about 56.8 min. Therefore, at 40 bar PVP effectiveness as a kinetic hydrate inhibitor is superior compared to EMIMBF4.
At higher pressure of 60 bar, EMIMBF4 also shows kinetic inhibition effect when compared to the induction time of the blank sample. From Figure 4, 0.1wt% EMIMBF4 show the longest induction time of 77.1 minutes, 9.2 minutes’ difference with the blank sample. The result indicates that the induction time increased with the decrement in concentration. It shows the same optimum level of 0.1wt% even at the pressure of 60 bar. At different pressure of 40 and 60 bar, EMIMBF4 shows a quite comparable induction time of methane hydrate at the same concentration. Furthermore, the
1.0wt% PVP yield the highest induction time of 80.4 min compared to the other samples. Therefore, at 60 bar it is proven that EMIMBF4 can be an effective kinetic hydrate inhibitor in a lower concentration compared to the commercial kinetic hydrate inhibitor, PVP due to its ability to delay the induction time of methane hydrate.
Figure 4 Induction Time measurement at 60 Bar
CONCLUSION
In conclusion, EMIMBF4 is proven to be effective as a kinetic hydrate inhibitor. In other words, it can delay the induction time of methane hydrate at both pressures of 40 and 60 bar. At a pressure of 40 bar, EMIMBF4 shows higher induction time compared to the pressure at 60 bar. EMIMBF4 is a more effective inhibitor at the concentration of 0.1wt%. EMIMBF4 induction time is comparable to PVP at the pressure of 60 bar.
ACKNOWLEDGEMENT
The authors would like to acknowledge Flow Assurance Lab UTP and PETRONAS Ionic Liquid Centre for their support to use the equipment and chemicals. The authors also would like to thank Mr Hazri Shahpin, Lead Technologies who assist during the lab work procedure.
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AUTHORS' INFORMATION
Mazuin Jasamai is a lecturer in
Petroleum Engineering Department. She received her
Master Degree in Reservoir Geosciences Engineering from École Nationale Supérieure du Pétrole et des Moteurs (IFP School).
Her area of Specialization is in gas hydrate inhibitor and reservoir rocks properties.
Mazlin Idress is a lecturer of Petroleum Engineering. She received her Master Degree in Petroleum Engineering from the University of Adelaide. Her area of specialisation is in gas hydrate and unconventional hydrocarbon.
Mohd. Zahdan Bin Arshad is a recent graduate with a Bachelor Degree in Petroleum Engineering from Universiti Teknologi PETRONAS.
Mohd Faisal Taha is a lecturer of Fundamental and Applied Sciences Department. He received his Master Degree by the University of Manchester. His area of specialisation is in the characterisation of ionic liquids and activated carbon.
