Effect of nanoparticles on the nucleation and agglomeration rates of hydrate growth using THF–water clathrates
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Four types of nanoparticles, amorphous carbon, iron III oxide, SiO2, and amino-coated SiO2, were tested to determine changes in tetrahydrofuran–water (THF–water) clathrate hydrate nucleation and agglomeration. Rates were experimentally found to determine their viability for preventing natural gas hydrates from developing during offshore drilling operations. THF–water clathrates were chosen as a model to represent gas hydrate growth at atmospheric pressure. Concentrations of each nanoparticle between 0.15% and 1.0% by weight were tested as a kinetic inhibitor to hydrate formation. Tests were repeated at various temperatures below the formation temperature of 4.4 °C for THF–water clathrate hydrates. Measurements were made to identify how the concentration of THF affects the clathrate hydrates forming under static conditions between 20% and 30% by mole of THF. The primary tests in this study were performed using a 20:80 THF/water ratio. Temperature increases during hydrate nucleation for THF–water were measured between − 5 and 3 °C. The range of ideal nanoparticle concentrations was found to be between 0.15% and 0.45% by weight for optimal static, kinetic inhibition of hydrate nucleation. At approximately 0.3% by weight, the most significant inhibition was observed under static conditions for all four types of nanoparticles tested. We found that functionalized amino-coated SiO2 nanoparticles, across all tests, significantly increased the time required for the formation of THF–water clathrate hydrates compared to the other three non-functionalized nanoparticles. The amorphous carbon and iron III oxide nanoparticles performed similarly across each test and were both the least effective in their inhibition of the clathrate hydrates of the four nanoparticles studied compared to a control.
KeywordsNanoparticles Gas hydrates Clathrates Hydrate inhibition Functionalized nanoparticles
The formation of gas hydrates during offshore drilling in deep waters is a well-recognized operational hazard. Drilling through hydrate-bearing sediments (HBS) is frequently encountered to reach deeper hydrocarbon zones or while drilling through HBS to produce natural gas commercially from methane gas hydrates (Grover 2008). The stability of gas hydrates offshore in specific zones has been seen to extend up to 7000 ft below the mud-line and can begin to grow and plug critical well-control equipment such as the blowout preventer (BOP) stack, choke and kill lines which can lead to severe well-control problems (Kim et al. 2007). Another major issue arises due to the dissociation of HBS during drilling operations when they come in contact or are penetrated by drilling fluids that bring about a change in their temperature and pressure conditions (Kjaer 2014; Zerpa 2013). This leads to severe mud gasification, partial washout and caving, casing running problems and casing subsidence. Models can be used to predict how the drilling fluids will interact with these HBS due to the change in temperature distribution and therefore dissociating more substantial amounts of gas within the wellbore. Wang et al. (2019) proposed an improved thermal model for predicting the temperature distribution in gas-dominated systems by considering the influences of hydrate behavior on the variation in temperature. Once the gas has entered the drilling fluid column, it can then begin to reform into methane hydrates.
There are various ways by which methane hydrates can be prevented. Thermodynamic hydrate inhibitors (THI) can be used to alter the chemical potential of the aqueous or hydrate phase at high THF concentrations. When THI are present, the hydrate dissociation curve displaces to lower temperatures or higher pressures. One of the most popular methods of shifting the thermodynamic equilibrium of hydrate growth is through the use of pumping large amounts of methanol and propylene and ethylene glycols into the troubled zones known for hydrate development (Ameripour 2005). Bishnoi et al. used clathrate hydrate models to determine how electrolytes would be able to affect the rate of hydrate development (Bishnoi and Dholabhai 1999). They found that large concentrations of salts were required. Weight percentages of salt in the fluids ranging from 5% to 15% were required to significantly inhibit the rate of hydrate growth. Many thermodynamic inhibitors were found to have multiplicative effects on the rate of hydrate growth when they combined together. When testing these different hydrate inhibitors, Sloan (1990) tested how each affected the rate of hydrate growth and also combined the different inhibitors to see whether the effects were the same. Their results showed that when each inhibitor was used, they individually performed well at inhibiting the growth of hydrates; however, when they were combined, the effects were magnified and the performance was significantly increased.
The other method of preventing hydrates is the application of kinetic hydrate inhibitors (KHIs) which are added at low concentrations and do not affect the thermodynamics of hydrate formation (Meindinyo 2017). Instead, KHIs may delay hydrate nucleation and crystal growth. Anti-agglomerates are also used at low concentrations to prevent the agglomeration of hydrates so that the hydrate crystals are transportable through the well before forming a plug. These inhibitors are generally used in conjunction with each other in the right proportions to compose suitable mixtures to form hybrid inhibitors (Pakulski 2011). At extreme water depths or extremely low mud-line temperatures, thermodynamic inhibitors by themselves are not very useful and hence need to be used in combination with other inhibitors. Such a combination is generally referred to as hybrid inhibitors.
1.1 Hydrate clathrate structure
The objective of this study is to investigate the effect of nucleation and agglomeration times on clathrate hydrate formation in the presence of nanoparticles (NPs) at varying concentrations. Nanoparticles are particles between 1 and 100 nm in size. NPs have been proven to enhance rheological properties of drilling fluids or enhance rheological properties of drilling fluids under high-pressure/high-temperature conditions, increase wellbore stability and to control or prevent loss circulation (Agarwal et al. 2013). They have been proven to be highly effective while drilling through shale formations because fluid invasion is considerably reduced so there is minimal shale swelling (Amanullah and Al-Tahini 2009).
1.2 Functionalized nanoparticles
Apart from the conventional NPs used, a modified NP coated with amino groups was selected to investigate how functionalized particles can affect the rates of hydrate nucleation and agglomeration. Previous studies have identified how different functional groups on molecular inhibitors can drastically affect the rates of hydrate growth (Anderson et al. 2005). As a comparison in this study, standard SiO2 nanoparticles will be compared to functionalized SiO2 nanoparticles that have a double layer of amino groups around the exterior of the particles. Other studies have shown how molecules with organic functional groups have been successfully able to inhibit the formation of hydrates in drilling fluids used for deepwater wells (Halliday 1998).
Tests were conducted using a mixture of tetrahydrofuran (THF) in the presence of deionized water, and the results obtained based on the nucleation time of hydrates will help comprehend the interaction of nanoparticles on the formation of hydrates. This helps develop a guideline on whether currently used nanoparticles lead to inhibition or stability of hydrates over a range of temperatures and concentrations.
1.3 Comparison of methane hydrates to THF hydrates
The primary benefit of using THF–water Type 2 hydrates is that the hydrates are stable at standard pressure and begin growing below 4.4 °C (Lee et al. 2007). It has been reported in previous studies that THF–water hydrates have physical properties very similar to methane hydrates, so they make an excellent analog for testing reagents and materials for inhibiting their formation (Lee et al. 2007). Although each hydrate is of a different structure, Type 1 versus Type 2, their formation and properties are coincidentally similar. This study is to be used as a means of determining possible candidates of hydrate inhibition for methane hydrate experiments. There are so many possible materials that can be used, and therefore, it is important to be able to screen likely candidates to be used as possible additives during drilling operations. Because of the similarity in structure and mechanism of crystal growth, the THF hydrates can help with the screening process for larger-scale studies using methane hydrate flow loops to study natural gas hydrates.
2 Experimental materials
3 Experimental methods
3.1 Preparation of nanoparticle dispersions
To prepare each nanoparticle dispersion, 200 mL DI water and dry nanoparticles, which were measured using a scale, were added to a flask. The flask was submerged in an ice bath to prevent the solution from heating up while sonicated. During sonication, extreme heating occurs due to localized cavitation bubbles. Prolonged production of these cavitation bubbles results in bulk heating of the solution and lead to excessive evaporation (Taurozzi 2012). The nanoparticle solutions were ultrasonicated using the Q-Sonica Q500 ultrasonicator for 45 min at 50% amplitude pulsing for 1 min on and 15 s off. Once dispersed, the water-nanoparticle solution and THF were both cooled to 5 °C using the recirculating bath before combining each in a 325-mL flask using the magnetic stirrer. Cooling the THF and the water-nanoparticle dispersions before mixing was found to be an essential step in the mixing process because the addition of concentrated THF to water is very exothermic. When mixing each solution together at 20 °C, we identified, on average, a 5 mL change in total volume. When cooling both the dispersion and the THF prior to mixing, there was no noticeable loss in the total volume. The nanoparticle dispersion and THF were then mixed for 5 min before transferring the solution back to the recirculating bath to begin observations of clathrate hydrate growth at various temperatures.
3.2 Distinguishing the phases of hydrate clathrate development
The time required for nucleation can be artificially reduced by introducing a seed crystal or promoting their growth by scratching the inside glass of the container the solution is in. Artificially inducing this rapid phase shift can be done within a few seconds using either of these two methods when below 4.4 °C. Previous experiments have shown that the length of the nucleation time is dependent on the apparatus setup, the presence of substrate material, the history of the water, water and gas composition, pressure and temperature, cooling rate, and if it is in a dynamic or static condition (Birkedal 2009). As a result, the nucleation time is sometimes considered a stochastic process. To ensure reproducibility in our study and prevent external influences, each of the tests measuring nucleation times involved allowing the solutions to remain isolated without any external influences once placed in the recirculating bath until they had completely agglomerated. Each test was repeated four times to ensure consistency in results.
3.3 Effect of THF–water molar ratio on nucleation time
For THF–water clathrate hydrates to form, the required minimum molar ratio of THF to water is 1:17 (Lee et al. 2007). The thermodynamics of clathrate hydrate nucleation will become unfavorable at high concentrations of THF relative to water (Nesterov and Reshetnikov 2015). For this study, it was essential to determine whether there was a significant difference between the nucleation times for each nanoparticle solution at different molar ratios of THF to water. Three molar ratios of THF to water, 20:80, 25:75, and 30:70, were selected, and each was tested using the four types of nanoparticles at 0 °C. Initial tests performed using the required minimum molar ratio of 1:17 was found to require very long periods of time before nucleation occurred. When we increased the THF concentration, we found the nucleation times to be much more reasonable for conducting multiple tests. The 20:80 molar ratio was found to be sufficient for the amount of time required and was used for further tests in experiments.
3.4 Temperature increase due to clathrate hydrate nucleation
When clathrate hydrates begin to nucleate, there is an initial increase in the temperature of the fluid sample before the crystals begin to anneal and form into a solid plug. To determine the expected temperature changes during crystal formation, a 1:5 THF to water solution, without any nanoparticles, was first mixed using the blender before slowly cooling down to − 5, − 3, − 2, 0, 2, and 4 °C. By gradually and slowly cooling the samples to these temperatures, we can prolong the time required before initial nucleation of the hydrates begins until the desired temperatures have been reached. Once the temperature was reached, a seed crystal was introduced to induce nucleation. The temperature of the samples was monitored using a digital temperature probe. To compare each set of data, the temperatures recorded were set relative to the initial time of nucleation for each sample. The measurements continued for 40 min to observe the decrease in temperature after the initial increase. These tests were conducted to determine the change in temperature during the nucleation period. The results of these tests can be used to explain the time period between the initial nucleation and the beginning of the agglomeration phase of hydrate growth.
3.5 Nucleation times for different nanoparticles
The time which is required to initiate clathrate hydrate crystal nucleation is known as the nucleation time. Once the crystals start to nucleate, it causes a cascade of crystal production throughout the sample where both solid and liquid phases of the solution exist before the agglomeration phase of hydrate growth begins. For these tests, 20:80 THF–water solutions with 1% by weight of each nanoparticle solution were observed at both − 6 and 0 °C. This test will be used to identify how each of the four nanoparticles differs by inhibiting the formation of hydrates at two temperatures below the required temperature for THF–water hydrate nucleation.
3.6 Agglomeration times for different nanoparticles
The agglomeration time is the time required for the clathrate hydrates to form a solid plug starting after crystal nucleation has caused a dramatic phase shift in the solution. This period begins when the solid crystals agglomerate at the outer perimeter of their containing flask and expand resulting in the development of a solid hydrate crystal. It is essential to understand how the agglomeration rate is impacted by different nanoparticles. When the hydrates exist in their free crystalline form, the hydrates can flow through subsea production pipes, but the agglomeration process allows the free crystals to begin to grow leading to blockages. Understanding the rate of agglomeration for hydrates is a significant factor when deciding to employ subsea pipe cleaning measures which are typically used to sweep wax depositions. It can be costly to use these techniques because they slow down production, but if the subsea pipes are not regularly cleaned, there is a greater possibility of hydrates completely blocking the flow within the pipes. Each of the tests done to determine the agglomeration times was determined by using the same samples used to find the nucleation time for each nanoparticle solution at − 6 and 0 °C.
3.7 Nucleation times for different concentrations of nanoparticles
In testing the capability for each of these nanoparticles to act as a kinetic inhibitor, concentrations from 0.15% to 0.6% by weight of each nanoparticle were tested to determine whether there is an optimal concentration in their applications. Nanoparticles of design can be expensive in their uses. Therefore, optimal concentrations are important for their deployment in field applications. It has been well reported that different concentrations of nanoparticles will affect the rate of clathrate hydrate growth. To fulfill the kinetic inhibition requirements of < 1% by weight for the nanoparticle concentrations in each sample, four solutions were tested using 0.15, 0.3, 0.45, and 0.6 percent by weight for each nanoparticle.
3.8 Agglomeration times for different concentrations
The time required to complete the agglomeration process of the free clathrate hydrate crystals was measured using the same nanoparticle solutions tested for nucleation times. Each solution was allowed to continue the hydrate growth process by remaining in the recirculating bath until each sample developed into a solid hydrate crystal at 0 °C. The recorded time-lapse videos were analyzed to find both the total time required for complete agglomeration as well as the rate of growth in cm/h.
4 Results and discussion
4.1 The effect of concentration of THF and type of nanoparticles on nucleation times
4.2 Temperature increase during clathrate hydrate nucleation
4.3 Time required for clathrate nucleation using different nanoparticles
4.4 Time between nucleation and the agglomeration period to begin
4.5 Time required for complete clathrate agglomeration using different nanoparticles
Agglomeration rates in cm/h of clathrate hydrates of THF–water (20:80) solutions with 1% by weight of nanoparticles at 0 and − 6 °C
Agglomeration rate, cm/h
4.6 Clathrate nucleation times for different nanoparticle concentrations
4.7 Clathrate agglomeration times for different nanoparticle concentrations
The average rate of THF–water clathrate hydrate agglomeration in cm/h for each nanoparticle solution at 0 °C
Nanoparticle concentration, wt%
Average agglomeration rate, cm/h
The use of nanoparticles as a kinetic inhibitor of hydrate growth significantly increases the times required for hydrate nucleation and agglomeration to begin, and the required time for complete agglomeration of THF–water hydrate clathrates.
Compared to a control without nanoparticles, the time required for nucleation of THF–water hydrate clathrates significantly increases when nanoparticles are added to THF–water solutions. At 0.15% by weight of nanoparticles, we observed an increase in the time required for nucleation to begin. The greatest increase was found at 0.30% by weight of nanoparticles compared to the control. As the nanoparticle weight percent increases beyond 0.30, the time required before hydrates begin to develop decreases.
Amino-coated SiO2 nanoparticles outperform all other nanoparticles when used as an additive for inhibiting the growth of clathrate hydrates. With comparable concentrations in solutions, these nanoparticles provide an increase in the nucleation times for clathrate hydrates.
At the moment of THF and water clathrate hydrates induction, there is a sudden and increasingly significant rise in temperature within the solution. We predict that this increase in temperature when nucleation begins will affect the time required for the agglomeration period to begin.
For preventing methane hydrates, it is recommended based on our findings using THF–water clathrates as a model that amino-coated SiO2 nanoparticles, and potentially other amino-coated nanoparticles, can be used as very effective hydrate inhibitors. The presence of nanoparticles at 0.30% by weight will cause a significant increase in the nucleation time, increase the time before agglomeration, and increase the total time before the agglomeration period commences.
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