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Modified Asphaltene Capillary Deposition Unit: A Novel Approach to Inhibitor Screening Saugata Gon, and David M Fouchard Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02185 • Publication Date (Web): 14 Jan 2016 Downloaded from http://pubs.acs.org on January 16, 2016

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Modified Asphaltene Capillary Deposition Unit: A Novel Approach to Inhibitor Screening Saugata Gon†*, David M. Fouchard† † Nalco Champion Energy Service, 7705 Highway 90-A, Sugar Land, TX 77478 Keywords: Asphaltene, Capillary Deposition, Inhibitor, Chemical Screening, Fouling

ABSTRACT: Asphaltene deposition in capillaries is a tool which has been used in an attempt to better understand asphaltene deposition in the field. However, data reproducibility and inhibitor ranking present some challenges with this technique. An improved asphaltene capillary deposition unit and a novel experimental protocol were developed to address these problems and are presented here. Using untreated Gulf-of-Mexico oil, the current study generated reproducible amounts of asphaltene deposit inside the capillary. It further identified the fact that residual oil inside the capillary tube can be a limitation to inhibitor selection. Evaluating the amount of asphaltene depositing in the capillary as a function of time proved successful in addressing this issue and led to inhibitor performance differentiation.

Introduction: Asphaltene deposition is considered a problem in oilfields and refineries causing plugging in wells, flow lines, tubing, and damage to equipment. In the oil field, deposition of asphaltenes has been reported to reduce permeability in reservoir perforations leading to loss of production.1,2 It is therefore of paramount importance to accurately predict the risks of asphaltene deposition in the oil field and select the asphaltene inhibitor most suited to reduce this risk. The asphaltene capillary deposition unit was developed with these objectives in mind. A

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crude oil and a suitable asphaltene precipitant are mixed at a target ratio and the mixture is flown through a capillary tube at a constant flow rate. Pressure drop across the capillary is monitored during this test. As asphaltene deposits inside the tube it generates a pressure drop in the system. Research conducted on the asphaltene capillary deposition unit has proven it as a useful tool to study asphaltene deposition under flowing conditions.3-11 The Hagen Poiseulle equation can be used to correlate the pressure drop across the capillary to the amount of asphaltene depositing in the line.

Hagen Poiseulle equation:

∆ =

(1)

Where ΔP is the pressure drop across the capillary, µ is the dynamic viscosity, L is the tube length, Q is the volumetric flow rate, and R is the effective radius of the tube. As asphaltenes deposit onto the capillary wall, the effective radius R diminishes, triggering an increase in the pressure drop. Calculation of effective radius using experimental pressure drop and Hagen Poiseulle equation, however, assumes uniform asphaltene deposition inside the capillary, which is a deposition profile not supported by experimental evidence as shown by research conducted on both crude oil9 and model systems11. A more direct way of quantifying the extent of asphaltene deposition consists of weighing the amount of deposited asphaltenes inside the capillary. This can be accomplished by displacing the oil and heptane mixture at the end of the test, followed by toluene extraction of the deposit and subsequent removal of the toluene by evaporation.12 Although suitable to assess the extent of deposition observed with untreated oil, application of this method alone was found to


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be inadequate for performance differentiation among various asphaltene inhibitors in our experience. A possible explanation for the seemingly incoherent results observed in the presence of inhibitors could be that inhibitors modify the wetting properties of the oil. In such case, different amounts of residual oil would be expected to adhere to the capillary wall at the end of the run in the presence and absence of an inhibitor, hence biasing the blank baseline. The work presented here outlines the exploration of this hypothesis, which ultimately led to the development of a new screening procedure able to differentiate among various inhibitors.

Materials and Methods: Crude Oil, Asphaltene Inhibitors and Asphaltene Precipitant: A Gulf-of-Mexico (GOM) asphaltenic crude oil was used and referenced as Crude Oil A for this study. Three Nalco Champion asphaltene inhibitor chemicals were used. These were referenced as Inhibitor A, Inhibitor B and Inhibitor C. Oil samples were conditioned at 60 °C for an hour prior to injection of 500 ppm of chemical. The treated samples were then capped off and shaken by hand (50 times) for efficient mixing of chemical. The treated samples were then placed in a 60 °C oven for another hour before injection into the capillary deposition unit. Heptane was used as the asphaltene precipitant. Determination of Asphaltene Precipitation Point and Capillary Flow Rate: A custom built automatic titration device, the Flocculation Point Analyzer (FPA) was used to calculate the onset of asphaltene precipitation at atmospheric pressure. 10 ml of crude oil A, conditioned at 60 °C was placed inside a metal cell along with a magnetic stirrer. Heptane titration was carried out at a 0.5 ml/min flow rate, while an NIR sensor (980 nm) was used to measure the transmittance


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inside the cell. An initial increase in transmittance was noted due to dilution effect from heptane titration followed by a sharp decrease in transmittance as asphaltene aggregates grew large enough to be detected. The volume of heptane at the peak transmittance was used to calculate the instantaneous asphaltene precipitation onset value for oil A. An asphaltene onset value of 53.5 % heptane was observed for crude oil A, and was therefore used to calculate the capillary flow rates. The total flow rate was maintained at 0.2 ml/min (oil flow rate 0.093 ml/min and heptane flow rate 0.107 ml/min) throughout the study. Capillary Deposition Unit: A custom built asphaltene capillary deposition unit was used to carry out the tests for this study. A schematic diagram of the system is presented in Figure 1. The valves are marked as 1 through 20 and the pressure sensors are marked as P1 through P5. The system is similar to the deposition unit used by Wang et al., 2004 8, with a few key differences that are highlighted here. For example, mixing of oil and heptane was achieved using a static mixing manifold M1 (1/8” manifold with 6 inlets purchased from Valco Instruments Co. Inc.), whose use was inspired by the concept of a coaxial jet mixer under laminar flow condition as reported by Scampavia et al., 199513. This was then placed inside a 40 °C oven. Crude oil and heptane were poured into transfer cylinders D and E respectively and injected into the mixing manifold using Isco pump A and B respectively to assure continuous fluid delivery into the system. A 75 foot-long 316 SS tubing with an ID of 0.02” purchased from Valco Instruments Co. Inc. was used as capillary. The difference between pressure sensors P3 and P4 was used to calculate the pressure drop across the capillary. The pressure sensors P1 through P5 could monitor fouling across the entire delivery line. All the pressure sensors were calibrated at the beginning of each test by filling the line with xylene and using Isco pump C. An air compressor coupled with a back


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pressure regulator was used to deliver 50 psi back pressure into the system. The back pressure was applied to ensure that the pressure lines were filled with fluid during the experimental run. The pressure sensors were equipped with gate valves and exit lines to efficiently fill up different sections of the line with the test fluid and systematically flush out any entrapped air in the system. Detailed description of various components of the system (temperature and pressure sensors along with the valves and pumps) are included in the supporting information section. Operating procedure for the deposition unit: The startup procedure was devised to minimize error in pressure and weight of total deposit collected at the end of the test. The system was filled with the target crude oil A at the beginning of the test while the heptane delivery line was filled with solvent. Back pressure was applied to the system, and the crude and heptane flow rates were set to target conditions. Data recording was initialized immediately after the start of heptane flow through the capillary. The system was thoroughly cleaned between each run using toluene, isopropanol and xylene. The thorough cleaning of the capillary was verified using the Isco pump C’s pressure head observed with the new capillary as a reference. The reader is referred to the supporting information section for more details. The same capillary was used to generate the entire data set for consistency and better data reproducibility. Shutdown and deposit data collection: At the end of the test the flow of crude and heptane through the capillary was stopped, and the system was slowly brought back to atmospheric pressure. The capillary tubing was then detached and the capillary line was flushed with nitrogen (0.05 cm3/min flow rate) for 3 hours to slowly displace the residual oil-heptane mixture. The weight of the empty capillary tubing was subtracted from the weight of the flushed capillary to obtain the weight of the total deposit. The deposit was then extracted out of the capillary with toluene and the solvent was removed using a R-205 BÜCHI Rotavapor and a standard vacuum


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oven. Elemental analysis (C, H, N, S) was conducted on select deposit samples to check the nature of the asphaltene deposits. The pressure drop across the capillary (ΔP) was obtained by subtracting the calibration-corrected data measured at P4 from that measured at P3. Contact angle study: The contact angle study was conducted with a Ramé Hart Goniometer (model 295-F4) on a 316 SS flat coupon. Crude oil A (subsampled into 10 ml vials) was conditioned at 60 °C for half an hour. Chemical A was injected at different dosages and the sample was hand-shaken. The treated oil was conditioned in a 60 °C oven for another hour. 4 microliter of oil was injected onto the coupon using a graduated 100 µL glass syringe. A picture of the oil droplet was taken using the goniometer immediately after the oil was dispensed onto the coupon. The contact angle of the drop was measured using DROPimage Advanced Software. Results and Discussion: Data Reproducibility and Interpretation from benchmarking study: Initial evaluation of the asphaltene deposition unit used for this study was conducted on untreated crude oil A with 5 replicate runs. Figure 2 shows the ΔP profile across the capillary tube for a representative run marked as run 3. As the capillary was filled with crude oil at the beginning of the test, starting the flow of heptane increased the overall flow rate for the capillary, leading to the pressure drop spike seen in Figure 2. As the oil inside the capillary was displaced by the less viscous oilheptane mixture, the pressure drop returned to a flat baseline. This point in the run was marked as start time as shown in Figure 2. The closing of valves 13-15 marks the end of the run. Under the run’s conditions the fluid’s residence time inside the capillary is 23 minutes. This value aligns well with the 25 minutes required for the initial pressure spike to form a flat baseline after the initiation of heptane flow. Deposited asphaltenes can be dislodged from the wall and


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momentarily clog the capillary, resulting in a pressure drop spike as shown in Figure 2 around 900 minutes. This phenomenon plays a major role in the amount of noise typically observed in the pressure drop data collected on capillary-based asphaltene deposition systems. Compared to an earlier prototype, however, the set up used in this study displayed a significantly improved noise to signal ratio, as illustrated by the first 550 minutes of the 5 replicated runs shown in Figure 3. The very gradual ∆P increase shown during this period suggested a relatively slow deposition of asphaltenes inside the capillary. The ∆P profiles significantly differ from each other past the 550 minutes mark, however. Asphaltenes depositing inside the capillary slowly narrows the capillary’s inner diameter over time. A smaller diameter inside the capillary leads to an increase in the frequency of temporary blockades caused by particles dislodged from the wall (or even precipitating out of the liquid phase). Similar observations were reported by other researchers9-10. Eventually, these dislodged particles will no longer be able to be pushed out of the capillary, resulting in complete plugging of the line. The non-uniform asphaltene deposition profile and the stochastic nature of the dislodging events lead to the complete blockage of the line occurring at random run times. For this reason, we decided to rely on gravimetric analysis of the deposit formed inside the capillary for the quantitative aspect of the study, while the ∆P profiles were only used for qualitative analysis. For instance, since it was necessary for our study to displace the oil-heptane mixture at the end of our test to calculate the weight of deposited asphaltenes, the pressure data was monitored to ensure that the capillary was not clogging during the run. Monitoring the pressure data at various positions in the system further enabled us to monitor potential deposition upstream of the capillary unit. In order to obtain accurate gravimetric measurements of deposited asphaltenes, great care was put into flushing the oil-heptane mixture out of the capillary with nitrogen immediately after


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the end of each run. Visual inspection of the eluent did not reveal any flocculated asphaltenes coming out of the capillary during the nitrogen flushing. The deposition rates were calculated using equation (2). =

! "

(2)

The average value for the deposition rates for total and dry deposits are presented in Figure 4. Standard error was calculated based on equation (3). =

#!# $#! %! "&' # !

(3)

A low standard error was observed for both the total deposit and dry deposit. The error associated with both data sets being similar, we elected to use the total deposition rate as metric for the remainder of the study. Elemental analysis study: A control run was performed by flowing crude oil A through the capillary unit without any heptane for 16 hours. The oil was displaced from the capillary following the protocol described above and the oil film adhering to the capillary was extracted with toluene. The oil extract was then subjected to rotary evaporator and dried in a vacuum oven. Elemental analysis was done on the resulting solid and compared with a field asphaltene deposit sample and dried deposit samples obtained from various test runs in the presence of heptane. Figure 5 shows the hydrogen to carbon atom ratio comparison for these samples. As per literature evidence, the hydrogen to carbon atom ratio of crude oil asphaltenes varies within a 1.1 to 1.3 range14,15, with an average value around ~1.1516 The elemental analysis of the field deposit of asphaltene considered for this study showed H/C atom ratio around 1.14. However, the asphaltene deposits from blank benchmarking runs showed higher hydrogen to carbon ratio


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(around 1.4). This value was lower than the value observed for the control run data for oil extract (1.65). Based on the elemental analysis, it was concluded that the asphaltene deposit obtained inside the capillary contained some amount of residual crude oil. Surface wetting properties of inhibitor-treated oil: Asphaltene and other components in crude oil have been reported to be able to adsorb onto mineral surfaces and alter the wetting characteristics of the oil17,18. Additionally, research in enhanced oil recovery has shown that chemical treatment can alter wetting characteristics of the formation in water-oil systems19,20, while another study has shown that chemical treatment can alter wettability in gas-oil systems.21 Based on these studies and the results of our elemental analysis study, it seemed plausible that asphaltene inhibitors might change the wetting characteristics of oil inside a metal capillary. To understand the effect of chemical treatment on the amount of residual oil left on the capillary wall after nitrogen flush, a contact angle study was performed with Inhibitor A at dosages of 50, 500 and 2000 ppm using a 316 SS coupon. The data is presented in Figure 6. It was observed that as the dosage rate was increased, the contact angle of the oil droplet decreased. It has been reported in literature that surface wettability of a liquid drop over a solid surface increases with decrease in contact angle.22 As chemical treatment could alter the wetting nature of the crude oil; it could lead to larger amounts of residual oil in the capillary after nitrogen flush. To further confirm this finding, a capillary control run experiment was conducted in the absence of heptane using oil treated with 500 ppm of chemical A. The treated oil was allowed to flow through the capillary unit for 15 hours. The oil was then displaced by nitrogen at the end of the test and the weight of the residual oil left behind in the capillary at the end of the flush was noted. The value was compared with a similar experiment conducted with untreated oil. The amount of oil remaining in the capillary in the case of the treated sample (0.342 g) was found to be higher than


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the amount remaining after an untreated oil run (0.261 g). This data further confirmed that treatment with asphaltene inhibitor altered the amount of residual oil adhering to the capillary’s walls at the end of the nitrogen flush. This study highlighted the importance of residual oil correction for asphaltene deposit quantification. Although the contact angle study was used as preliminary evidence for our assumption, we did not rely on the contact angle study to quantify asphaltene inhibitor performance. Accounting for inhibitor-dependent surface wetting tendencies of oil: The problem posed by the inhibitor and dosage-dependent amount of residual oil left behind after nitrogen flush could be solved by conducting increasingly longer capillary runs and plotting the total amount of deposit obtained in each run as a function of the run time. In these experiments, the run time was varied from 0 to almost 18 hours. The start time data points were collected by stopping the test 30 minutes after the start of heptane flow. Linear rate of deposition profiles were observed for both treated and untreated samples as illustrated in Figure 7. Such linear correlations were also reported by Tavakkoli et al.23 and Carmichael et al.,24. The slope of the linear regression correlates to the asphaltene deposition rate, while the intercept represents the amount of residual oil. It was assumed that at zero time there would not be any asphaltene deposition inside the capillary hence at zero time the mass retained inside capillary can be a direct contribution from the residual oil. A residual oil-corrected plot is shown in Figure 8. It was generated by subtracting the corresponding intercept value from the total amount of deposit obtained at the end of a given run. The percent inhibition displayed by the respective inhibitors was calculated using equation (4). Where mB refers to the slope of the untreated crude and mT refers to the slope of a treated oil. The data is reported in table 1.


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% ) =

*"+ ,"- .×011 "+

(4)

This approach allowed us to differentiate among the performances of the three inhibitors selected for this study. It presented inhibitor C as the best candidate for this crude oil while inhibitor A and inhibitor B did not show good inhibition. Statistical Significance of Deposition Rate: The residual oil correction for the asphaltene deposition unit was based on the linear deposition rate observed for the treated oil systems studied here. Total deposition would be a function of asphaltene deposition and residual oil. Let us consider that the residual oil would not be a function of time while asphaltene deposition would be a linear function of time. Hence if we calculated deposition rate from different runs (conducted with the same treated systems but ran for different amounts of time) with and without residual oil correction, we should see a decrease in error for the residual oil-corrected deposition rate calculations. Table 1 shows that we did in fact observe significantly larger errors for the deposition rate calculations conducted without residual oil correction. The standard errors reported in table 1 were calculated using the deposition rates obtained from the individual data points in Figure 7. Details on the calculation can be found in supporting information. Conclusions: After initial experimentation with an earlier version of our capillary deposition unit, improvements were made to address issues regarding precise control over precipitant and crude delivery as well as efficient mixing of crude oil and precipitant. The experiments conducted on this unit following a modified operating procedure showed good data reproducibility in terms of quantification of asphaltene deposit mass. However, one limitation of the system used for this


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study was found to be use of pressure drop data across the capillary tube. This limitation was addressed by focusing our efforts on the mass of deposited asphaltenes. Elemental analysis conducted on the deposit samples showed presence of residual oil in the deposit sample. Further contact angle study using a standard goniometer showed that asphaltene inhibitors could change the wetting characteristics of oil inside the capillary. This highlighted that different asphaltene inhibitors might alter the wetting characteristics of the oil in different ways, resulting in variations in residual oil amount inside the capillary unit after it is flushed with nitrogen. Correcting the total amount of deposit collected in a run by the amount of residual oil led to inhibitor performance differentiation. Implementing a residual oil correction factor also reduced the standard error associated with the deposition rate calculation from individual runs by an order of magnitude. This study suggested that for inhibitor testing, generating asphaltene deposition rate profiles was advantageous when compared to the generation of single data points. Although the device and methods presented herein appear to generate reproducible data and allow for assessment of asphaltene inhibitor performance, further in depth studies would be required to fully validate the use of the method as an inhibitor selection tool. Furthermore, the amount of time, crude oil and labor required to screen a single inhibitor do not make the asphaltene capillary deposition unit a suitable tool for inhibitor screening. Other methods, such as the coupon deposition test24, continue to offer more robust options for inhibitor screening and selection.


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ASSOCIATED CONTENT Supporting Information: Capillary cleaning procedure, error calculation for understanding statistical significance of deposition rate, detailed part list for the system used for this research. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION * E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank Nalco Champion (An Ecolab Company) for permission to publish the results of this study. The authors would also like to thank Prof. Jill S. Buckley and Dr. Tianguang Fan of New Mexico Institute of Mining and Technology and Dr. Christopher Russell, Dr. Andrew T. Yen, Jose Macias, Nicholas Sadeghi and Rogelio Banda of Nalco Champion for their time on discussions.


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Spiecker, P. M.; Gawrys, K. L.; Trail, C. B.; Kilpatrick, P. K. Colloid Surface A. 2003, 220, 9-27


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Tables: Table 1: Analysis of Asphaltene deposition profile of treated runs Asphaltene deposition rate (slope), g/h

Residual oil (intercept), g

% Inhibition

Standard error of deposition rate post residual oil correction

Standard error of deposition rate without residual oil correction

0.020

0.210

-

0.002

0.01

Inhibitor A (500 ppm)

0.024

0.223

-20

0.002

0.01

Inhibitor B (500 ppm)

0.0199

0.255

2.0

0.0006

0.002

Inhibitor C (500 ppm)

0.0140

0.269

31

0.0006

0.003

Blank


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Figures:

Figure 1. Schematic of asphaltene capillary deposition unit. The gate valves are denoted as 1 through 20. The pressure sensors are marked as P1 through P5, the temperature sensors are marked as T and the manifolds are marked as M1 through M3.


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Figure 2. Pressure data for a representative blank run (Run 3)

Figure 3. Comparison of blank pressure data on 5 replicate runs


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Figure 4. Comparison of average asphaltene deposition rate from untreated oil

1.80 1.60

H/C atom ratio

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1.65 1.4

1.40

1.14

1.20 1.00 0.80 0.60 0.40 0.20 0.00

Average Untreated Deposit

Untreated oil wo Field Deposit heptane

Figure 5. Elemental analysis of deposits. The H/C ratio values are mentioned above the respective columns


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Energy & Fuels

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Figure 6. Contact angle study. A, B, C and D refer to pictures of oil droplet over the test surface for untreated and treated oil samples (treated with 50 ppm, 500 ppm and 2000 ppm inhibitor A) respectively. E refers to the plot of contact angle of the oil droplets against their treatment dosages.

Figure 7. Deposition profile against time


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Energy & Fuels

Figure 8. Asphaltene deposition profile post oil entrainment correction


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Modified Asphaltene Capillary Deposition Unit: A ... - ACS Publications

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