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Rock-Flow Cell: An Innovative Benchtop Testing Tool for Flow Assurance Studies Jeong-Hoon Sa, Aline Melchuna, Xianwei Zhang, Rigoberto E. M. Morales, Ana Cameirão, Jean-Michel Herri, and Amadeu K. Sum Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01029 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 4, 2019

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Industrial & Engineering Chemistry Research

Rock-Flow Cell: An Innovative Benchtop Testing Tool for Flow Assurance Studies

Jeong-Hoon Sa1, Aline Melchuna1, Xianwei Zhang1, Rigoberto Morales2, Ana Cameirao3, JeanMichel Herri3, and Amadeu K. Sum*1

1Hydrates

Energy Innovation Laboratory, Chemical & Biological Engineering Department, Colorado School of Mines, Golden, CO 80401, USA

2Multiphase

Flow Research Center – NUEM, Graduate Program in Mechanical and Materials

Engineering – PPGEM, Federal University of Technology of Paraná, Curitiba, PR, BRAZIL 3Mines

Saint-Etienne, Univ Lyon, CNRS, UMR 5307 LGF, Centre SPIN, Departement PEG, F 42023 Saint-Etienne, FRANCE

*Corresponding author e-mail: [email protected] 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Flow assurance is a critical component in the design and operation of robust oil/gas production systems. Undesired precipitation of solids (gas hydrates, wax, asphaltenes, scale) reduces the production rate and often leads to costly and hazardous disruptions. Many experimental and modeling efforts have been made to build knowledge of managing such risks. However, a major difficulty is to transfer the laboratory data to the field conditions. Here, we introduce a new experimental system, rock-flow cell, which is compact and requires less resources to build and operate. This system can readily achieve different flow regimes by controlling the liquid loading, water cut, and rocking angle/speed. A sight glass visualizes when, where, how, and how much solids form and precipitate out. Gas hydrate formation tests with anti-agglomerants are presented to demonstrate the capabilities. The rock-flow cell is an innovative testing tool for flow assurance studies by properly capturing thermo-hydraulic conditions in actual flowlines.

Table of Contents Graphic

“Rock-Flow Cell” for Flow Assurance Studies Electric Motor Coolant Out

Cooling Jacket

Visualization

Camera

Rocking

Coolant In

Supporting Table

2 ACS Paragon Plus Environment

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INTRODUCTION In the oil and gas industry, flowlines connecting the wellhead to the production facilities constitute a significant part of the offshore production and transportation systems, and they often encounter a variety of flow assurance problems.1 In particular, the phase changes and precipitation of solids such as gas hydrates, wax, asphaltenes, and scale are critically important, as they can even lead to the catastrophic blockage in flowlines.2,3 The resulting pressure drop and reduced production rate cause severe production losses and safety concerns. It is thus essential to have sufficient knowledge about how to properly manage such flow assurance issues for a sustainable and reliable operation of oil and gas production.4-6 Multiphase flow is of the major component in flow assurance studies, as it governs how the phases (gas, liquid, solid) are distributed in the flowline,7 and is the key to capture the thermohydraulic conditions in flowlines.8,9 The essence of multiphase flow is the distribution of the flowing phases that are characterized by the flow regime such as stratified, stratified-wavy, and slug flow, which are common in oil and gas production system. Such flow regimes determine how phases are dispersed under a given shear, and thus when, where, and how much solids precipitate out when the conditions are appropriate. The main challenge in flow assurance studies is therefore to couple the solid precipitation with multiphase flow characteristics, which can enable better transfer of the laboratory data to field applications.10 Phase change/precipitation is another major component of flow assurance, and it receives significant attention due to the disruption in production caused by solids (hydrates, wax, asphaltenes, scale),3 which are not only from production losses but also related to safety concerns. The management of any of the solids is a complex problem, requiring in-depth knowledge of their thermodynamics, kinetics, and transport dynamics.11 The latter point is closely coupled to 3 ACS Paragon Plus Environment


multiphase flow, as it is the flow condition that will dictate where and how much of each phase will be accumulated or transported with the fluids. Traditionally, solid precipitation studies at the laboratory for hydrates, wax, asphaltenes, and scale are performed in batch type systems focused mainly on the thermodynamics and more recently on the kinetics.12 The majority of the experimental setups in laboratory disregard the flow conditions, and in order to achieve pipe flow, a flow loop is typically needed. In the cases in which a flow loop is actually used, most of them are small in pipe diameter and typically can only achieve single-phase flow. On the other hand, the typical batch type setups cannot reproduce the flow conditions corresponding to pipe flow, as it is not sufficient to have equivalent shear. The most important flow condition is the flow regime, which determines the dispersion/distribution of the phases. As such, if laboratory scale studies and testing are to be meaningful and closely reproduce pipe flow conditions as encountered in the field, it is necessary for the experimental setup to couple the flow conditions with any other phenomena of consideration, that is, hydrate formation, wax formation, asphaltene precipitation, and scale precipitation, without the expense of a large-scale flow loop. The focus of this paper is to emphasize the importance of the multiphase flow in understanding the various flow assurance problems, especially those related to solid precipitation. We have come a long way as a community to advance the thermodynamics (time-independent) and kinetics (time-dependent) of solid precipitation, but the multiphase flow-dependent phenomena are only gaining traction in recent years.13,14 The remaining of the paper will first give an overview of currently used testing systems and then introduce a new testing rig that aims to bridge the gap between the bench scale testing and large testing facilities and field conditions. While much of the discussion to follow focuses on gas hydrates as solid precipitation, the same 4 ACS Paragon Plus Environment

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concepts can be equally applied to the other flow assurance issues involving wax, asphaltenes, and scale.

Flow Assurance Testing Tools The flowlines in the field are of course the ideal system, but they are not accessible for studies and testing of flow assurance problems, as it would be too costly to operate due to their magnitude in size. In addition, it is often difficult to obtain a lot of information from the field, as only limited data are available, such as temperature, pressure, flow rate, and phase hold-up. While the flowline in the field is the system we would like to reproduce, there is little control on how the different flow assurance issues for solid precipitation arise, and from a fundamental point of view, everything has to be inferred from the limited data. The next best system for testing is a flow loop at the pilot scale,15,16 followed by stirred cells and rocking cells at the lab scale. The smaller the scale, more information can be available as there is more control of the experimental system, however, at the cost of being farther away from representing the actual system in flowlines. A stirred cell is often used for laboratory studies on the thermodynamic and kinetics of solid precipitation.17,18 The stirred cell provides vigorous mixing by mechanical agitation and thus can accomplish fast kinetics of solid formation with fairly high conversion. It is, however, not suitable to achieve proper dispersion of phases that simulates the multiphase flow conditions in flowlines.10 A rocking cell with a rolling ball is also widely used for laboratory studies as it can simply generate the flow by rocking.19,20 Phases in the rocking cell are better dispersed, but the flow is still relatively stagnant, and the rolling ball actually causes more disturbances to the flow resulting in a non-specific flow regime and dispersion of the phases. Table 1 lists the characteristics for each experimental system. 5 ACS Paragon Plus Environment


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Table 1. Characteristics for commonly used experimental system for flow assurance studies. Flow Loop

Stirred Cell

Rocking Cell

 

 

  

          

Semi-batch system Can consider multiphase flow Control mixture velocity, pump rate Control temperature, pressure Pump design and operation are critical Measure pressure drop, gas consumed Measure particle size/distribution Measure flow rate, density Need large quantity of fluids Need large cooling capacity Limited view of fluid/flow Expensive to build and operate Very few available

      

Semi-batch system Mixing determined by impeller shape, size, rpm Small volume of fluid Not clear how to relate shear to actual flow conditions Can do constant pressure and constant volume Can measure relative viscosity, particle size/distribution May have windows for visual inspection Measure gas consumed/water converted Widely used by industry and research laboratories

       

Batch system Limited mixing Mixing determined rolling ball Small volume of fluid Not clear how to relate shear to actual flow conditions Rolling ball can give rough estimate of relative viscosity Glass surface (+ and -) Pass/Fail result Measure gas consumed Can run multiple cells in parallel Widely used by industry and research laboratories

The flow loop is certainly the closest system that approaches the flowline in the field.21,22 However, due to the high cost to build and operate, it is not a widely available system. Moreover, the flow loop is often too large that much of the knowledge gained has to be inferred from pressure drop measurements, making it difficult to understand mechanistically the processes for solid precipitation. The stirred cell and the rocking cell are lab scale systems that have provided valuable information, but flowline flow conditions cannot be considered, making it very difficult to translate the results to field conditions,10 obtaining qualitative results at best. It is recognized that every


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experimental system has its advantages and disadvantages, and one may require a multiscale approach to get a full understanding of the flow assurance problems. For any flow assurance problem, one must consider the following variables: •

Fluid samples and properties

•

Amount of water, oil/condensate, gas (liquid loading, water cut, gas-oil ratio)

•

Temperature (surface and bulk)

•

System pressure

•

Constant volume vs. constant pressure

•

Process conditions (steady state vs. transient)

•

Pipe geometry

•

Flow and flow regime: phase dispersion/distribution

The last bullet is the critical point, and it is based on the multiphase flow characteristics of the system. The flow regimes of interest are: stratified, stratified wavy, slug, dispersed bubble, and annular flow. These are the most common flow regimes encountered in hydrocarbon production, and the flow regime determines the distribution of gas, oil, and water in the flowline, which are at the core in determining when, where, how, and how much solids will precipitate out. As industry moves to a management strategy for the different solids, answers to these questions become increasingly important so that the risk can be quantified based on the production conditions.

New Bench Scale Testing Tool for Flow Assurance To bridge the gap between what can be done at the laboratory scale and what happens in actual 7 ACS Paragon Plus Environment


flowlines, we need an experimental device that can consider, as best as possible, most of the variables just listed above. To that end, a new experimental setup has been developed, to be called rock-flow cell, which is illustrated in Figure 1. The rock-flow cell has a number of features that make it suitable for flow assurance studies including solid precipitation. The setup of the rockflow cell is quite simple: a sealed pipe on a rocking surface. The key to the rock-flow cell is the length over inner diameter ratio, which has to be at least the order of 10 in order to establish the proper flow regime. The fluids in the cell are mixed and dispersed through the rocking motion, allowing one to achieve the desired flow regime by adjusting the liquid loading and rocking conditions (angle and rate). While the displacement of the liquid in the cell is gravity-driven, as opposed to pressure driven in actual flowlines, one still obtains the commonly encountered flow regime in horizontal pipe flow, that is, stratified, stratified-wavy, and slug flows, as will be discussed later.

PC

Motor Controller

(Data recording) Gas

Electric Motor

T

Camera

Cooling Jacket

T

P

Camera Pipe

Glass Window

Supporting Stand Pivot

ControllerPivotWebcamC2PC(Datarecording)Rocking

MotorTTPMotor

Rocking

WindowElectric

Chiller

ChillerSupportingStandCoolingJacketPipeGlass

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 1. A schematic representing the experimental setup of the rock-flow cell system. 8 ACS Paragon Plus Environment

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The key characteristics of the rock-flow cell are: •

Semi-batch system

•

Can consider multiphase pseudo-flow (stratified, stratified-wavy, slug, disp. bubble)

•

Temperature control (surface and bulk)

•

Pressure control

•

Need a small quantity of fluids (order of liter)

•

Can be easily instrumented

•

Can be made to have full visualization of cell contents

•

Can correlate pseudo-flow to fluid velocity

•

Relatively inexpensive to build and operate

•

Can be used for steady-state and transient studies

•

Can be used for studies of hydrates, wax, asphaltenes, scale, corrosion, sand (each

separately or combined)

The rock-flow cell is essentially a flow loop, but much more compact and less expensive to build and operate. One of the major advantages of the rock-flow cell is the amount of fluid (order of 1 liter) required for the tests. Table 2 compares the features of the rock-flow cell with those of flow loops, and it is clear that the rock-flow cell can offer significantly more return despite its limitations. One of the features of the rock-flow cell that is especially beneficial is the ability to easily have access to its interior, unlike the flow loop which would only have small sections with windows or clear pipe. The ability to see the inside of the pipe is particularly useful for solid precipitation studies and testing, and once again is interested in when, where, how, and how much 9 ACS Paragon Plus Environment


solids are formed. Table 2. Comparison of flow loop and rock-flow cell. Feature

FLOW LOOP

ROCK-FLOW CELL

Flow line representation

Flow loop length

Cell length

Length

10-100 m

~1 m

Pump

Multiphase

None

Cooling requirement

Large

Compact

Fluid volume

10-100s liters

~1-5 liter

Visualization

Small windows

Nearly fully visual

Fluids

Gas, Oil/Cond., Water

Gas, Oil/Cond., Water

Flow regimes

Str., Str.-Wavy, Slug

Str., Str.-Wavy, Slug, D. Bub.

Fluid control

LL, WC, GOR

LL, WC, GOR

Test conditions

Pseudo-SS, transient

Steady-state, transient

Cost to build

~$1M

~$10K

Cost to operate

$1000s per day

$100s per day

The following section details the use of the rock-flow cell for gas hydrate studies as an example of how it is used to highlight its advantages.

EXPERIMENTAL SECTION Apparatus Figure 1 shows the schematic diagram of the rock-flow cell. For the discussion pertaining to the work to be presented here, we used a stainless steel pipe with 2-inch in inner diameter and 2-feet in length, and maximum operating pressure is 100 bar. The main pipe piece is covered by a stainless steel cooling jacket, which is connected to a chiller unit to control the temperature. Both pipe ends are sealed with glass windows to get access to the interior, and here a webcam records the video during the experiments. This experimental setup was manufactured by SejinYoungTech


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(Korea). An LED light strip is attached on the top inner surface to provide the proper lighting. The main pipe is placed on a supporting table to which the rocking is applied by an electric motor. Its rotation is transformed into the rocking motion through a connecting metal shaft. Therefore, the control of liquid loading, water cut, and rocking angle and speed can achieve different flow regimes. A pressure transmitter (WIKA®) and three thermocouples (OMEGA®) are placed in the cell to monitor gas, liquid, and chiller temperatures, which are recorded by LabView® software.

Test Procedure Deionized water with blue color dye and clear oil (mineral oil called CP70T, specific gravity 0.853, viscosity: 10.45 cP at 40 °C, from STE Oil Co.) with yellow color dye are first mixed in a glass beaker. This mixture is then loaded into the rock-flow cell. After assembling the glass windows at both ends, the main pipe is placed on the rocking table. The rocking angle is controlled by the distance between the center pivot and the connecting part of a metal shaft, which is linked to an electric motor. The rotation speed of this electric motor determines the rocking speed. For the gas hydrate formation tests, the cell is pressurized with 10 bar of ethane (C2H6) gas (99% purity from General Air) at 10 °C. Considering its solubility in the oil phase, the C2H6 is refilled until the pressure is stabilized. Then, the temperature is decreased to 1 oC to initiate hydrate formation. Here, the temperature has to be at least above 0 oC to avoid ice formation. The onset of hydrate formation is indicated by a large temperature spike and a start of sudden pressure drop. The pressure drop is used to estimate the amount of water consumed for hydrate formation.

RESULTS AND DISCUSSION Flow Regimes Obtained in the Rock-Flow Cell 11 ACS Paragon Plus Environment


In the rock-flow cell set-up, a gravity-driven flow is generated by rocking the main pipe, so different flow regimes can be achieved by controlling the liquid loading, water cut, and rocking conditions (angle and rate). While the flow in actual flowlines is driven by pressure, the liquid velocity of gravity-flow in the rock-flow cell is determined by the rocking angle. The rocking speed influences in different ways depending on the flow condition. At slow rocking, the liquid tends to stay at the end of the pipe, so increasing the rocking speed results in stratified flow. However, if the rocking speed is too fast, liquids are not sufficiently mixed, and they never reach the end of the pipe. A proper combination of experimental conditions thus needs to be applied to get the desired flow regime. Some images showing the stratified, stratified-wavy, and slug flows are given below (Figure 2). The Supporting Material contains additional videos demonstrating the actual “flow” of the liquids for these different flow regimes. These results are obtained with a clear pipe version of the rock-flow cell used to demonstrate the flow. In the following section, a stainless steel pipe is used where the views will only be from the end of the pipe. We should note and emphasize that in the rock-flow cell, the gas is stagnant, and it is only displaced due to the motion of the liquid. Also, the liquid flow velocity is relatively low (between about 0.2 to 1.0 m/s) as it is only driven by gravity, while the pressure-driven flow in actual flowlines is usually faster. In the rock-flow cell, the change of direction causes the liquid to turn at the ends, which introduces energy and mixing. This is equivalent to the pump in a flow loop, which is the mechanism by which the fluids flow.


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Figure 2. Images obtained from the rock-flow cell showing stratified, stratified-wavy, and slug flow regimes under various water (yellow) or water/oil (blue) flow conditions. See Supporting Material for videos for each condition. In flow assurance studies, especially those concerning with phase change, the dispersion of the phases is key to properly capture the phase changes. In the rock-flow cell, one can clearly see that the dispersion of the gas and liquid phases are much more closely represented than in stirred cells and the conventional rocking cell. Another advantage of the rock-flow cell is the ability to model it with Computational Fluid Dynamics (CFD) so that detailed understanding can be obtained of the flow conditions and their coupling with any solid precipitation. The compact size of the rock-flow cell makes CFD simulations to be efficient, and because of the control of the test conditions, one can obtain detailed information that can be used to tune CFD models and generate 13 ACS Paragon Plus Environment


further insight into the mechanistic process associated with solid precipitation. The computational effort to do such CFD simulations of a flow loop would be too expensive, and not enough data would be available for such comparison. This is work in progress, and it will be reported in a later publication.

Gas Hydrate Formation in Water and Oil Mixture In order to demonstrate the capability of our rock-flow cell, the formation of C2H6 hydrates (structure I) was first tested without any additive (Figure 3). A 50% by volume water and oil mixture was loaded into the cell with 30% liquid loading (by volume), and C2H6 gas was pressurized up to 10 bar. The rocking angle and speed are ±8º and 20 rpm, respectively. The flow pattern at this particular experimental condition is shown in Figure 2e, and the corresponding video is available in the Supporting Material. At the onset of hydrate formation around 1 °C, sudden large temperature spikes of both gas and liquid phases are observed (Figure 3a). The pressure immediately starts to decrease and then becomes stabilized within 3 hours. This pressure drop can be used to estimate the amount of water consumed to form hydrates, and the final water consumption after reaching a steady-state is around 15% (Figure 3b). The water consumption calculated here would be slightly underestimated. As hydrates form, a pressure decrease may allow the C2H6 gas dissolved in the oil phase to liberate. The exact amount of water consumed to form hydrates would be at slightly higher than 15%. The formation of C2H6 hydrates in the rock-flow cell is also visualized by a webcam placed in front of the side glass window. At the very beginning, before hydrate formation, clear oil is well dispersed in the blue colored water phase (Figure 4a). Under this specific experimental condition, this water and oil mixture presents a flow regime in between stratified and stratified14 ACS Paragon Plus Environment

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wavy flow. At the onset, small chunks of hydrates rapidly form at the bottom, and the remaining liquids flow above the hydrate chunks (Figure 4b). At this stage, only a small amount of water condenses on the top surface. Hydrates continuously grow over time with decreasing pressure. Upon further growing up, however, more hydrates spread out along the pipe and are stuck to the bottom with negligible liquid flowing above them (Figures 4c and 4d). As mentioned above, only around 15% of liquid water was consumed to form hydrates (Figure 3b), so the rest of the free water and oil are considered to be trapped in the hydrates. While the amount of water and gas consumed to form hydrates are small, they can result in severe concerns in flowlines in terms of transportability. As shown here, our rock-flow cell system enables a clear visualization of when, where, and how hydrates form in the pipe, providing qualitative and quantitative knowledge, which are essential to establish proper management strategies for gas hydrate formation and any other solid precipitation.

Figure 3. (a) Temperature/pressure changes over time and (b) the fraction of water consumed for hydrate formation without additive under 30% liquid loading, 50% water cut, and ±8º/20 rpm.



Figure 4. Images obtained from hydrate test without additive under 30% liquid loading, 50% water cut, and ±8º/20 rpm (blue dye in water, clear oil) at (a) 0.0 h, (b) 22.7 h, (c) 23.0 h, and (d) 32.0 h. Videos are available in the Supporting Material. Gas Hydrate Formation with Additive One strategy for hydrate management in flowlines is the injection of hydrate inhibitors. Thermodynamic inhibitors (e.g. methanol, ethylene glycol, salts) shift the phase equilibria to lower temperature and higher pressure regions to completely avoid hydrate formation,23 but they require a large amount of dosage (~10 to 50 wt%), causing safety and cost issues. Kinetic inhibitors retard the formation of hydrates with a relatively low concentration (~1 wt%) for hydrate management,24,25 but their effect is limited under high subcooling environment. Anti-agglomerants (AAs) allow the formation of hydrates but prevent the agglomeration by producing a flowable slurry with good transportability, while still requiring a low dosage (~1 vol%).26,27 The performances of AAs on hydrate formation are well suited for testing using the rockflow cell. This was done with a commercial AA, to be denoted AA-A, to illustrate the features of


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the rock-flow cell. As shown in Figure 5a, at the onset of C2H6 hydrate formation with 3.0 wt% AA-A, temperature spikes for liquid and gas phases are observed, as seen from the test without additive (Figure 3a). In this test, hydrates readily start to form before the temperature reaches 1 °C. In the presence of AA-A, which acts as a surfactant, the mass transfer between water and gas phases are improved, readily inducing the initiation of hydrate formation. However, the rate of hydrate growth and water consumption are considerably reduced (Figure 5b), indicating that this AA is effective in delaying hydrate formation. The water consumption at the end is around 7%, which is even less than half of that obtained from the test without AAs.

a

b Pressure

Onset

Gas Liquid

Chiller

Figure 5. (a) Temperature/pressure changes over time and (b) the fraction of water consumed for hydrate formation with 3 wt% AA-A under 30% liquid loading, 50% water cut, and ±8º/20 rpm. According to the visual observation, at the onset, small hydrate particles rapidly form rather than hydrate chunks, and they flow with water and oil (Figure 6a). They then grow into large hydrate chunks over time. The majority of gas hydrates are stuck to the bottom, and some are deposited on the side wall (Figure 6b). As hydrate formation further proceeds, hydrate deposits on the side wall fall down and accumulate on the bottom surface as a hydrate bed (Figures 6c and 6d). Based on the pressure drop, only 7% of liquid water is consumed to form hydrates, so this hydrate



bed is thought to soak all the remaining free water like a sponge. Only the transparent oil phase flows above it. While the hydrate bed formed in this test still allows the oil to flow well, it is stuck to the bottom and does not flow at all. If the flowlines contain more water and gas to form hydrates, the resulting hydrate bed would result in a significant reduction of transportability, and this would eventually lead to the blockage. Based on the experimental result, this AA does not appear to perform well under the given test conditions. It should be emphasized that the result of the test with AA is only valid for the chosen test conditions (i.e., liquid loading, water cut, subcooling, AA type and concentration, rocking conditions); changing any of the test conditions may result in different results, in particular changes to the rocking conditions that correspond to different mixing and dispersion of the phases. As such, it is critical that every test result must be properly qualified with the test conditions. The rock-low cell provides the best option to control the most important variable for the qualification of hydrate formation with and without additives.

Figure 6. Images obtained from hydrate test with 3 wt% AA-A under 30% liquid loading, 50% water cut, and ±8º/20 rpm (blue dye in water, clear oil) at (a) 0.6 h, (b) 2.0 h, (c) 4.0 h, and (d) 11.0 h. Videos are available in the Supporting Material.


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Figure 7 shows the results for another AA sample, denoted as AA-B, used to demonstrate the features of the rock-flow cell for a case where hydrate slurries are formed. In the presence of 3.0 wt% AA-B, liquid and gas temperature spikes observed at the onset of C2H6 hydrate formation, which is similar to the other tests (Figure 7a). However, the rate of hydrate formation and the amount of hydrates formed are even lower (Figure 7b) than with the other case shown in Figure 5b. The final water consumption is estimated to be less than 5%. As mentioned above, the fraction of water consumed for C2H6 hydrate formation reported in this paper is a little underestimated, but the relative comparison of those numerical values should be valid. It can thus be concluded that AA-B is slightly more efficient to delay the formation kinetics of C2H6 hydrates than AA-A under the tested condition.

a

b

Pressure

Gas Onset

Liquid Chiller

Figure 7. (a) Temperature/pressure changes over time and (b) the fraction of water consumed for hydrate formation with 3 wt% AA-B under 30% LL, 50% WC, and ±8º/20 rpm. Through the side glass window, water and oil look much better mixed with each other probably due to the presence of AA-B, resulting in a milky liquid phase (Figure 8a). With this AAB, hydrates are dispersed in the liquid and flow instead of agglomerating. The hydrate growth after the onset is limited by the mass transfer, as inferred from a slow heat dissipation (Figure 7a). As



hydrate formation proceeds further, hydrate slurries are still well dispersed in the liquid flow, and this flow becomes more viscous based on the visual observation (Figures 8b and 8c). The temperature was kept constant during the test, so this viscosity change is caused by the increasing amount of hydrates formed in the slurry. Some water droplets condensed on the top surface and the side wall convert into hydrates as well, but they do not grow into hydrate deposits. Hydrate formation is continued several more hours until the pressure is stabilized, but no significant visual changes are found (Figure 8d). The experimental result reported here indicates that the sample AA-B has the potential to be used as a hydrate dispersant. Hydrates can still form in the presence of AA-B while it reduces the rate of hydrate growth, but they result in dispersed hydrate slurries, thus making them flow well. This AA-B can also prevent existing hydrates on the wall from growing into deposits, ensuring nice transportability in flowlines. Though the performances of two AA samples reported in this paper indeed require further assessments, the experimental results demonstrate that our rock-flow cell system can be used to readily examine the effect of AAs in dispersing hydrates and reducing the risk of hydrate plugging. In addition, the precipitation of other solids including wax, scale, and asphaltenes can also be studied with this rock-flow cell.


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Figure 8. Images obtained from hydrate test with 3 wt% AA-B under 30% liquid loading, 50% water cut, and ±8º/20 rpm (blue dye in water, clear oil) at (a) 1.2 h, (b) 3.0 h, (c) 5.0 h, and (d) 8.0 h. Videos are available in the Supporting Material.

CONCLUSION In this paper, we introduced the rock-flow cell, which is an innovative testing tool for flow assurance studies. Unlike the other experimental setups, the rock-flow cell can readily provide the proper shear and dispersion of the phases involved, thus can better reproduce a variety of flow regimes to capture the thermo-hydraulic conditions in flowlines. While the control of the flow regime has been partly achieved by a large scale flow loop, the new rock-flow cell is much more compact and thus can be easily instrumented with less cost. In addition, it requires much less fluids and no additional equipment to operate. This experimental setup provides easy access to the interior that allows visual observation of the formation and precipitation of solids including gas hydrates, wax, asphaltenes, scale, corrosion, and sands. The rock-flow cell can also be used to



simulate transient conditions (shut-in and restart) by simply stopping and resuming the rocking. Using the rock-flow cell, the understanding of the formation and precipitation of solids coupled with multiphase flow characteristics can better transfer the laboratory data to field applications.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/acs.iecr.xxxxxxx. Videos showing the flow regimes (stratified, stratified-wavy, and slug flows) obtained in the rockflow cell and hydrate formation test results with or withouts anti-agglomerants (AVI) Video #1: Figure2a_15LL_100WC_12deg_10rpm.avi Video #2: Figure2b_30LL_100WC_4deg_30rpm.avi Video #3: Figure2c_90LL_100WC_4deg_20rpm.avi Video #4: Figure2d_30LL_50WC_8deg_10rpm.avi Video #5: Figure2e_30LL_50WC_8deg_20rpm.avi Video #6: Figure2f_30LL_50WC_8deg_50rpm.avi Video #7: Figure2g_90LL_50WC_4deg_20rpm.avi Video #8: Figure4_30LL_50WC_8deg_20rpm.avi Video #9: Figure6_3wt_AA-A_30LL_50WC_8deg_20rpm.avi Video #10: Figure8_3wt_AA-B_30LL_50WC_8deg_20rpm.avi

ACKNOWLEDGEMENTS


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The three academic groups make the Joint International Research Program (JIRP) on Gas Hydrates and Multiphase Flow, which works with industry to advance innovations in flow assurance research.

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Rock-Flow Cell: An Innovative Benchtop Testing ... - ACS Publications

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