Inhibiting effect of a composite formulation of kinetic and thermodynamic inhibitors for gas hydrate formation in high water cut oil–water emulsion

In oil and gas exploitation and transportation, it is essential to avoid hydrate blockage in the wellbores and pipelines. Using kinetic inhibitors in composite with thermodynamic inhibitors can reduce the operational expenditure for oil and gas pipelines under high water content conditions. The phase equilibria of pure water natural gas system, oil–water natural gas system, and oil–water monoethylene glycol (MEG) natural gas system were first investigated by experiments and combined with software predictions in this study. The results showed that both mineral oil and MEG shifted the phase equilibrium of the system to high‐pressure and low‐temperature directions. The oil phase increased the effective temperature range of the compounding inhibitor. Then the hydrate inhibition performance and natural gas hydrate generation characteristics of the composite formulation of Luvicap 55w and MEG in oil–water emulsions and natural gas mixtures were investigated. With the increased subcooling, the induction time decreased sharply, and the gas consumption increased. The induction time increased with increasing MEG concentration, which verified the synergistic effect of MEG in oil–water emulsion for Luvicap 55w. The hydrate generation characteristics did not change much with increasing Luvicap concentration at the same temperature. In contrast, the hydrate induction time rose from 142.8 to >3500 min as the MEG concentration increased from 10 to 20 wt%.


| INTRODUCTION
Natural gas hydrates are compounds formed by cage-like structures formed by water molecules trapping other molecules (such as methane, ethane, cyclopentane, etc.) in a low-temperature, high-pressure environment. 1 In 1810, hydrates were first discovered in a laboratory. 2 However, it was not until the 1930s that petroleum engineers first determined that blockages in natural gas transmission pipelines were caused by the formation of hydrates. 3 In oil and gas two-phase mixed transmission pipelines, water is often present. Hence, three phases are generally current: liquid hydrocarbons, water, and natural gas; oil and gas pipelines in high latitudes and deep sea often operate under high-pressure and lowtemperature environments, and all these make hydrates easily form, resulting in the blockage of pipeline hydrates. Unblocking hydrates in subsea production/ transmission systems is costly and has the possibility of causing safety issues. [4][5][6] Therefore, preventing hydrate formation in oil and gas pipelines is crucial. There are four main methods to avoid hydrate blockage: heating, depressurization, water removal, and injection of inhibitors, and the former three methods are often difficult to apply in marine long pipeline oil and gas development systems and platforms due to economic or technical reasons, in contrast to adding hydrate inhibitors, which is an easy method. 5,[7][8][9][10] Most of the oil industry injects inhibitors today in composite with other techniques, such as pressure monitoring, moisture detection, periodic pipeline cleaning, and downhole throttling to avoid hydrate plug formation. 11 Hydrate inhibitors are classified into three categories according to their inhibition mechanism: thermodynamic hydrate inhibitors (THIs), kinetic hydrate inhibitors (KHIs), and antiaggregation agents (AAs). 12 Typically, THIs is methanol (MeOH), monoethylene glycol (MEG), or triethylene glycol (TEG), and some salts. Among TEG, MeOH, and MEG, MeOH is the most effective THI. However, because of its high toxicity and volatile and flammable nature, it is used decreasingly, and salt inhibitors often corrode pipelines, so MEG is practically preferred as the THI in engineering. 10,13 Nevertheless, MEG, as a THI, requires complex treatment processes and bulky facilities. 14 It produces significant inhibition of hydrate generation only when the injection concentration reaches 20 to 60 wt% of the output water. 15,16 Therefore, in addition to not applying to small offshore platform applications, the use of THIs is also a major operational expenditure for offshore oil and gas extraction. It is estimated that the cost of injecting THIs into subsea pipelines can reach 50% of the total operating cost of an offshore field. 17 Considering the high cost due to the large injection of THIs, low-dose hydrate inhibitors (LDHIs) were explored in the 1990s, which can significantly inhibit hydrate nucleation and growth at low concentrations (<3 wt%). LDHIs are classified into KHIs and AAs based on their mechanism of action. The AAs are a type of surfactant consisting of hydrophilic and hydrophobic groups, which can prevent the binding of hydrate grains so that they can flow in the pipeline without blockages. The durable subcooling of AAs is higher than KHIs, but it should be used with a water cut of less than 50%. 18,19 Otherwise, AAs may fail and cause the hydrate slurry to become too viscous, finally causing plugging, and most of them are highly toxic. KHIs are mostly water-soluble polymers, and researchers have proposed several microscopic inhibition mechanisms, but no unified conclusion is available. 20 Macroscopically, it is mostly accepted that the mechanism of KHIs is to delay the hydrate formation with inhibition of hydrate crystalline nucleation and growth. 8,21 From the first generation of KHI polyvinylpyrrolidone (PVP) studied by CSM in 1995 to the second generation of various KHIs (e.g., polyvinylcaprolactam [PVCap], N-vinylpyrrolidone/N-vinylcaprolactam), which are also amides but more effective, and to the recent development of biopolymer inhibitors, such as polysaccharides, antifreeze proteins (AFPs), pectins, and dual functional inhibitors, such as ionic liquids, 8,9,[22][23][24][25][26][27][28][29] researchers have never stopped their search for new types of KHIs. However, the industrial application of these inhibitors has never been reported, and today commercial KHIs are still mainly formulated or polymerized with second-generation KHIs as the key components, so the study of the effectiveness of amide-based KHIs is significant for practical engineering. The cloud point (T cl ) of various amide inhibitors in salt solutions has also been investigated. 30 Clark et al. studied the use of corrosion inhibitors (CIs) together with KHI and found that KHI and CI do not affect each other. 31 The additives in the oil and gas pipeline are complicated, and subcooling is high in most cases when KHI is used alone, which makes it easy to invalidate. Consequently, it is crucial to investigate the combined use of KHI and THI. Kim et al. studied the performance of PVP-A in conjunction with MeOH. 32 Mozaffar et al. investigated the effect of various alcohols on PVCap using crystal growth inhibition. 33 Daraboina et al. first studied the effect of polyethylene oxide and salt on Luvicap and later on the effect of crude oil and salt on the inhibitory properties of various amide-based KHIs. 34,35 Metaxas et al. demonstrated the ability to effectively manipulate formation probability boundaries via a combination of THI and KHI. 36 Cha et al. investigated the synergistic inhibitory effect of MEG and PVP. 37 Most of the studies mentioned above are nonoil systems, while in actual oil and gas pipelines, water is often mixed with oil in the form of emulsions. 38 In addition, these studies are primarily constant rate cooling methods to test the hydrate inhibition performance. The experimental results of this method vary significantly according to the set cooling rate, and there is no uniform standard for the evaluation of KHI. Constant temperature and constant capacity methods are more scientific.
In this study, the phase equilibrium of the oil-water natural gas system with different concentrations of MEG was measured by using magnetic stirred cells. Then the constant temperature and pressure experiments were carried out for oil-water emulsions with different concentrations of MEG and Luvicap 55 W according to the phase equilibrium results, to study the hydrate generation characteristics of oil-water emulsions with different subcooling and different composites of inhibitor concentrations, hoping to give helpful information for practical engineering.

| Materials
Dalian Special Gases Co., Ltd. supplied the natural gas mixture, Table 1, used in this work. Lab self-prepared deionized water was obtained with an ultrapure water polishing system (Aquapro2S, Aquapro International Company LLC). The THI is MEG (>99.8% purity, USD 1747.2 per ton) supplied by Shanghai Macklin Biochemical Co., Ltd. The KHI used in this work is Luvicap 55w (41% PVCap aqueous solution, USD 20,384.4 per ton) from BASF. The oil phase's medium was mineral oil obtained from Shenzhen Jinmei Lubricant Co., Ltd. Table 2 shows the analyzed components of this mineral oil by using gas chromatography-mass spectrometry.

| Experimental apparatus
The experimental apparatus used in this work is shown in Figure 1, which mainly consists of two pressure cells with high-pressure resistant glass windows (Shandong, ShiyiTechnology Co., Ltd., in China), two magnetic stirrers (EYELA, Japan), a refrigeration circulation system, a vacuum pump (China Zhengzhou Century Shuangke Experimental Instrument Co.), a gas cylinder, and a data acquisition system. The two pressure cells with an adequate volume of 240 ml were designed to work safely under the pressure of 10 MPa. A UNIK5000 pressure sensor with a measuring range of 0 to 25 MPa and a model PT100 temperature probe with a measuring range of −50 to 100°C were located at the top and middle of the cell, respectively. The magnetic stirrer can work in a speed range of 0 to 1600 RPM. The refrigeration circulation system comprises a circulator and a water bath. The circulator (VIVO RT4, JULABO Technology GmbH, Germany) can provide a temperature range of −25 to 50°C with a control accuracy of 0.1°C.

| Experimental procedure
The solutions to be tested were prepared by weight method on balance with an accuracy of ±0.0001 g before the start of the experiment, except for the pure water experiment in the phase equilibrium experiment, where the volume ratio of oil to water in all solutions was controlled to be 4:6.

| Phase equilibrium test
For THI and KHI composite experiments, it is critical to control the experimental subcooling, so the phase equilibrium of the system is measured using the T A B L E 1 Natural gas composition.

Component
Mole fraction isovolumetric pressure search (IPS) method 13,39,40 to set the subcooling for composite inhibitor experiments accurately. Before each experiment, the inner wall of the cell was washed with detergent and tap water to remove the remained mineral oil, then washed with deionized water three times and dried. Take 120 mL of the prepared solution into the cell and encapsulate it. Inject 5 MPa nitrogen and keep pressure for 1 h to check the leak-proofness of the cell. Then release the nitrogen and vacuum the cell for 15-20 min until the cell's pressure remains stable at −0.10 MPa. Close the pumping valve and set the initial temperature of the water bath at 20°C. When the temperature in the cell no longer changes, start the data acquisition, and then open the cylinder valve and control the regulator to inject the target pressure of the experimental gas into the cell. Turn on the magnetic stirrer to set the speed of 600 rpm (at this speed, oil and water can be thoroughly mixed to form an emulsion, to ensure the contact between the gas and the water phase) for 20 min to ensure that the solution is fully dissolved in the experimental gas. The temperature does not change after 20 min. Close the cell inlet valve to make it a closed system. Set the water bath temperature to the hydrate formation region. After the hydrate is fully formed, turn off the water bath and let the water bath slowly ramp up at an ambient temperature (about 23°C). After the complete decomposition of hydrate in the cell, a phase equilibrium point can be obtained. The above experimental steps were repeated at different pressures and for various systems to get the corresponding phase equilibrium points.

| Experimental hydrate generation in emulsions with composite inhibitors
The constant temperature and the pressure method 41,42 were used for the experiments, and the steps are as follows: 1. The apparatus cleaning and the leak picking procedure were the same as in Section 2.3.1, and then 120 mL of the oil-water solution of the composite inhibitor was measured into the cell and sealed. 2. On the basis of phase equilibrium data obtained from Section 2.3.1 experiments and PVTsim (a professional petroleum PVT simulation software) predictions, the phase equilibrium temperature of the system at 4.5 MPa was determined to set the initial temperature. The composite concentration, experiment subcooling, and the estimated cost of treatment of 1 ton of water are shown in Table 3. The experiments were repeated three times for each condition except 10 # to improve the accuracy (Figure 1).

| Phase equilibrium data collation
This section compares the experimentally obtained phase equilibrium data with the calculated results of the PVTsim. Figure 2 shows the results of an IPS test, where a phase equilibrium point can be obtained from the P-T curve.
To use the PVTsim 20.0 hydrate phase equilibrium calculation module, the mass concentration of MEG needs to be converted to the concentration of MEG occupying the aqueous phase (excluding the oil phase).
In Formulas (1) and (2), η m/m+w is the mass ratio of MEG to the aqueous phase, and η meg and η water are the concentration of MEG and water in the solution, respectively. The 0.4 and 0.6 in Formula (2) were determined according to the volume ratio of oil to water used in the experiment. ρ mineral oil and ρ water were 0.817 and 0.997 g/mL, the densities of mineral oil and deionized water were obtained by taking the average three measurements at a laboratory temperature of about 23°C. According to Formulas (1) and (2)  temperature and pressure experiment, so the main concern for the investigation was the induction time of hydrate generation. Conventionally, the induction time of hydrate formation is defined as the time lapsed from hydrate nucleation to the nuclei growth reaching a critical size. 43 As shown in Figure 3, the traditional induction time should be from the start of gas injection to the beginning of pressure drop due to hydrate nucleation. Still, at higher subcooling, as shown in Figure 4, the pressure drops at the end of gas injection, making it difficult to determine the induction time. So in this study, the induction time is considered to be the period from the start of gas injection to the time when the hydrate starts to be produced in large quantities resulting in a sudden increase in temperature in the cell. In addition to the induction time, it is necessary to analyze the properties of hydrates once the generation starts, and here we explore the amount and rate of hydrate generation. The hydrate conversion chemical reaction relationship is shown in Formula (3) where M represents the hydrate guest and N is the hydrate theoretical number with a value of 5.75 for structure I hydrate and 5.67 for structure II hydrate. 44 The gas-forming hydrates used in this experiment are generally of II structure, 24 so N in this study is 5.67. Since the solution was proportioned by the total mass method in this experiment, the amount of water contained in the system differed when the concentration of MEG in the solution was different, so the water conversion rate (the ratio of the amount of water converted to hydrate to  the total amount of water in the system) was used to measure the amount of hydrate produced. Water conversion rate calculation using Formula (4): where WC is the water conversion rate, n Δ is the number of moles of gas consumed, and n w0 is the initial number of moles of water.
Calculate the gas consumption during hydrate formation using the Formula (5): where R is the gas constant (8.314 J/(mol K)), T and P are the temperature and pressure of the gas, and Z is the actual gas compression factor calculated by entering the temperature and pressure data in REFPROP software. The subscript 0 and t represent the moment when the gas first starts to produce a significant pressure drop due to hydrate generation and a certain moment after the gas begins to be consumed, respectively. The average gas consumption rate is also essential to measure the hydrate generation characteristics. To eliminate the effect on the average rate of hydrate generation due to hydrate blocking gas mass transfer, the average rate of hydrate consumption is taken as the average rate of hydrate consumption between the time the gas is consumed and the time when the consumption reaches 90% of the maximum consumption. The calculation is shown in Formula (6): where n Δ 90 is 90% of the final amount of gas consumed and t 90 is the time taken from the start of consumption to 90% of the total gas consumed.

| Effect of oil phase and THI on phase equilibrium
Understanding the phase equilibrium is crucial for using KHIs in composite with THIs. KHIs are generally used in small amounts and do not change the system's phase equilibrium. This study's primary focus is on the phase equilibrium of MEG in emulsions to provide a basis for determining the subcooling in the KHI and THI composite experiments. The pure water system, the oil-water system, the 10 wt% MEG oil-water system, and the 20 wt% MEG oil-water system was tested, and five phase equilibrium points were obtained for each system in the pressure range of 1.9 to 5.92 MPa and temperature range of −3.2 to 16.3°C.
As shown in Figure 5, the 40 vol% of mineral oil shifts the phase equilibrium of the system toward high temperature and low pressure compared with the pure water system. It is due to the addition of the oil phase reducing the water activity. 45,46 In mineral oil and water systems with MEG, as the MEG concentration increases, the water content in the system decreases, and the water activity is further reduced, so the phase equilibrium curve shifts again in the direction of difficult hydrate formation. Comparing the experimental data with the software predicted, it can be observed that the calculated results of PVTsim software are close to the experimental test results. It means that the prediction of the phase equilibrium of this experimental apparatus using PVTsim is reliable. For oil-water emulsions with MEG concentrations of 20 and 10 wt% and oil-water ratios of 4:6 vol, the predicted phase equilibrium temperatures were 2.8°C and 7.05°C at 4.5 MPa.

| Effect of subcooling on the hydrate generation characteristics of emulsion composite inhibitor systems
The composite inhibitor has different performance in the oil-water solution at different subcooling. Figure 6 F I G U R E 5 Phase equilibrium test results of pure water, oilwater emulsions, and MEG oil-water emulsions compared with the phase equilibrium calculated by PVTsim. MEG, monoethylene glycol.
compares the hydrate generation characteristics of the composite experiments of 1 # , 2 # , and 3 # conditions in Table 3. Figure 6A shows the gas consumption for these three experiments at different subcooling. In this work, each gas consumption curve is one of three sets of repeated experiments under one condition and removed the constant pressure process. The gas consumption at a subcooling of 8.8°C is lower than that at 10.8°C and 12.8°C subcooling conditions, and the final gas consumption at different subcooling degrees is 0.085, 0.108, and 0.136 mol, respectively. It can also be observed that the gas consumption rate reaches its peak (when the gas consumption curve is most inclined) earlier with higher subcooling. Still, the time of maximum consumption increases with increasing subcooling, which may be due to the faster hydrate generation under high subcooling conditions, where the hydrate hinders the contact between gas and water, thus slowing down the rate of gas consumption in the late-stage hydrate generation. Figure 6B shows the average gas consumption rate and induction time at different subcooling degrees. The average gas consumption rate is obtained by Formula (6), and the induction time is the average of three repeated experiments. The average gas consumption rate increased when the subcooling degree increased from 8.8°C to 10.8°C. At the same time, it decreased instead when the subcooling degree increased from 10.8°C to 12.8°C, probably due to the blockage of gas-water contact, where the induction time decreased from 747.7 to 142.8 min when the subcooling increased from 8.8°C to 10.8°C, and from 142.8 to 59.1 min when the subcooling increased from 10.8°C to 12.8°C. This is because the KHI will fail rapidly at high subcooling. The trend is consistent with previous studies in pure water. 43,47 Figure 7 shows the hydrate generation characteristics of experiment conditions 4 # , 5 # , and 6 # in Table 3. The figure shows that the MEG concentration of 20 wt% behaves similarly to the MEG concentration of 10 wt% at different subcooling. However, the final gas consumption is different because the gas content is fixed at 50% for each experiment, so when the MEG concentration increases from 10 to 20 wt%, the water content of the liquid phase decrease, leading to a significant reduction in the final gas consumption, with the final gas consumption from 8.8°C to 12.8°C subcooling, respectively, 0.054, 0.084, and 0.103 mol. Figure 8 shows the hydrate generation characteristics of experiment conditions 7 # , 8 # , and 9 # in Table 3. In Figure 8A, with the increase of Luvicap 55w concentration, the characteristics of gas consumption are similar, and the final gas consumption rates are all about 0.07 mol, so it is presumed that Luvicap 55w concentration in the range of 0.5 to 1.5 wt% has little effect on the amount of the final hydrate production. Moreover, in the early stages of hydrate generation, the gas consumption of the system is faster at 0.5 wt% concentration of Luvicap 55w compared with 1.0 wt%, but the difference is not significant at 1.0 and 1.5 wt% concentrations. Figure 8B shows the variation of induction time and average gas consumption rate with Luvicap 55w concentration, where induction time increases with increasing Luvicap 55w concentration from 347.5 to 518 min, indicating that increasing the Luvicap 55w concentration at this subcooling can delay the hydrate generation. Still, the delay is limited and far from achieving the engineering requirements for the hydrate inhibitor inhibition effect. Figure 9 compares the hydrate generation characteristics for experiment conditions 2 # and 5 # in Table 3.  Figure 9A shows that the gas consumption is more at an MEG concentration of 10 wt% than at 20 wt%, which is caused by the different water content in the system. Figure 9B shows the system's final water conversion rate and induction time at different MEG concentrations, where the water conversion rate was calculated using Formula (4). As shown in the figure, the final water conversion rate for an MEG concentration of 10 wt% is more significant than that for an MEG concentration of 20 wt%, which is consistent with the final gas consumption. The induction time increases with MEG concentration, which may be due to the synergistic effect of MEG for Luvicap 55w. 33,48 Figure 10 shows the experimental conditions for two cases, 2 # and 10 # , in Table 3, with different MEG concentrations and subcooling but the same temperature of −3.8°C. The temperature and pressure data for three sets of condition 2 # and one set of condition 10 # . It can be observed that the pressure of these three sets of repeat experiments decreases rapidly shortly after the end of the gas injection. At the same time, the temperature increased, which means the rapid generation of hydrate with an induction time of 142.8 min on average. In contrast, the experiment for condition 10 # shows no significant changes in temperature and pressure within 3500 min, which means that this concentration of composite inhibitor completely inhibits hydrate formation under this subcooling.

| Summary of cost and performance for hydrate inhibitor composite experiments
On the basis of the costs in Table 3 and the experimental results above, the different experimental conditions in this study are summarized. Experimental conditions 1 # , 2 # , and 3 # were less costly, but the induction time of this concentration of composite inhibitor is short. Experimental conditions 4 # , 5 # , and 6 # show a slight increase in induction time compared with 1 # , 2 # , and 3 # at the same subcooling but still do not completely inhibit hydrate formation. In experimental conditions 7 # , 8 # , and 9 # , the cost increased with the concentration of Luvicap 55w, which was cheaper than the cost of increasing the MEG concentration in experimental conditions 1 # , 2 # , and 3 # to 4 # , 5 # , and 6 # , but with little increase in induction time. The experimental conditions 10 # are less costly than 8 # and 9 # and can completely inhibit hydrate formation, which is the recommended condition for using composite inhibitors in this study.

| CONCLUSION
The oil phase significantly affects the phase equilibrium of the system, which can move the phase equilibrium boundary toward low temperature and high pressure, making it more difficult to generate hydrates. On the basis of the data obtained from the phase equilibrium study, the experiments of composite MEG with Luvicap 55w under an oil-water emulsion system were designed. The comparison experiments were conducted for different subcooling at the same composite concentration, different THI composite concentrations at the same subcooling, and different KHI composite concentrations at the same temperature. The experimental data were processed for the induction time as well as the amount and the average gas hydrate rate during hydrate generation. The results show that the final hydrate generation increases when the subcooling increases, the average gas consumption rate increases and then decreases with increasing subcooling, and the induction time decreases sharply with subcooling. The induction time increased with MEG concentration at the same subcooling, indicating that MEG has a synergistic effect on Luvicap 55w. At high subcooling, the inhibition effect of the composite inhibitor was sharply weakened, and increasing the concentration of Luvicap 55w had little effect on prolonging the induction time. In the end, a comprehensive analysis of the results and costs of the composite experiments is presented.
In conclusion, the composite use of KHI and THI must precisely set the subcooling. It is the key to achieving cost reduction while inhibiting hydrate generation.