Determination of Asphaltene Stability in Crude Oils Using a Deposit Level Test Coupled with a Spot Test: A Simple and Qualitative Approach.

The main goal of this study is to monitor the stability of crude oils in terms of both precipitation and deposition magnitude with respect to time. To achieve this goal, two experimental techniques which include a deposit level test and a spot test were integrated and applied simultaneously. The method was implemented using six crude oils, namely A, B, D, E, F, and G, and tests were performed at different times which split them into short duration tests and long duration tests. All crude oils were found to exhibit potential for asphaltene precipitation and subsequent deposition at different rates. Crude oils B, G, and D were observed to have started asphaltene precipitation and subsequent deposition relatively quicker. Similarly, crude oils B, A, and F exhibit a higher potential for producing asphaltene deposits in terms of deposition level. Crude oil E produces relatively fewer deposits at comparatively slower rates. The overall result indicates that crude oil B was found to be the most risky crude oil as it produces a higher quantity of deposits at higher rates, while crude oil E proved to be the least risky. Sensitivity analysis was also performed via the computing relevancy factor to determine the relative importance of two input parameters, namely the specific gravity of crude oil and the time for two-output precipitation intensity and deposition level. Precipitation intensities were found by the implementation of an image-processing tool on spot test results. The relationship between time and precipitation intensity was found to be negligible; however, the correlation between time and deposition level was found to be strongly positive with a relevancy factor value of approximately 0.521. Similarly, the relationship of the specific gravity of oil with precipitation intensity and deposition level was found to be moderately negative and very close to each other, i.e., -0.228 and -0.247, respectively. The integration of the deposit level test with the spot test allows the continuous and simultaneously reliable monitoring of both asphaltene precipitation and deposition at different times without involving cost, complex instrumentation, or interpretation, irrespective of the type of oil. The method enables the successful determination of stability ranking of different crude oils both in terms of precipitation and deposition.

Introduction

Asphaltene deposition in oil fields remains a challenging problem for the Upstream Petroleum Industry.[1] This problem could arise anywhere in the production system, i.e., at the reservoir or at subsurface and surface facilities.[2] Asphaltene deposition could impact oil companies technically in terms of the deployment of complex treatment methods and economically with respect to interruption of sustainable hydrocarbon production and implementation of expensive treatment techniques.[3−7]

Crude oil is usually characterized into SARA (Saturates, Aromatics, Resins, and Asphaltenes) fractions.[8,9] Among the SARA fractions, asphaltene is regarded as the heaviest and most polar component of crude oil.[9,10] Asphaltene, by definition, is that component of crude oil which is found to be soluble in aromatic solvents like xylene, toluene, etc. while being insoluble in n-alkanes, especially n-pentane and n-heptane.[10,11] The asphaltene molecule is composed of mainly carbon and hydrogen and some amount of heteroatoms and metallic compounds.[6,12,13] The composition of asphaltene is considered an important stability parameter because it provides information regarding the asphaltene precipitation risk in crude oils. It was reported by several researchers that asphaltene stability in crude oils increases as the H/C ratio of asphaltene increases and aromaticity and heteroatoms decreases.[1] Previously in the literature, various values of asphaltene molecular weights were presented by different researchers. However, the probable value of asphaltene was found to be 750 g/mol.[12−15]

With respect to structural architecture, asphaltene does not have a definite structure. Continental, archipelago, anionic continental, and Yen–Mullins models are some of the most accepted and famous structural models of asphaltene.[9]Figure shows the aforementioned structural models.[14,17] According to Figure , the archipelago model possesses several aromatic rings joined together through various aliphatic branches.[9,16,18] Alternatively, the continental model consists of a large central cluster of aromatic rings in an asphaltene molecule that is joined together through several aliphatic chains.[9,18] The anionic continental model has a structure similar to that of the continental model, but the major difference lies in the presence of a negative charge onto the aliphatic chains, which is further joined to the main structure.[9] Finally, the last, most accepted, and recent model is the Yen–Mullins model. The model describes asphaltene structure in terms of both size and behavior which have an influence on crude oil properties that asphaltene displays. According to the Yen–Mullins model, in lighter crude oils, asphaltenes exist as small polyaromatic hydrocarbon compounds and possess an average diameter of 1.5 nm. In this scenario, the asphaltene concentration found to comparatively low with constant asphaltene size. In black oils, the asphaltene concentration found to be relatively high, and it exists as nanoaggregates with an average diameter of 2 nm, which is comparatively higher than that of the lighter oils. In heavy oils with extremely low API gravity, the asphaltene wa found to exist in higher concentrations with an average diameter of 5 nm in the form of cluster.[9,15]

Figure 1

(a) Archipelago asphaltene structure. Reprinted from ref (17). Copyright 2008 American Chemical Society. (b) Continental asphaltene structure. Reprinted from ref (17). Copyright 2008 American Chemical Society. (c) Anionic continental asphaltene structure. Reprinted from ref (17). Copyright 2008 American Chemical Society. (d) Yen–Mullins asphaltene model. Reprinted from ref (14). Copyright 2010 American Chemical Society.

Characterization of asphaltenes requires the extraction of asphaltene from crude oil and then utilization of various analytical techniques.[19] According to the definition of asphaltenes, different standard protocols are developed for the extraction of asphaltenes from crude oils. These include ASTM D-4124-01, ASTM D-3279-07, WRI, ASTM D-4124-09, ASTM D-6560-00, ASTM D-2007-03, etc.[12] The extracted asphaltene sample is then used for the determination of various properties. X-ray diffraction (XRD)[20] and nuclear magnetic resonance (NMR)[21,22] can be employed for the determination of average molecular parameters of asphaltenes. Mass spectrometry (MS) techniques, vapor pressure osmometry (VPO), and size-exclusion chromatography (SEC) can be used to determine the molecular weight distribution of asphaltenes.[23−26] MS technology and particularly ultrahigh resolution (UHR) mass spectrometers can be utilized for analyzing the molecular composition and chemical properties of extracted asphaltenes.[27−29] Electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance–mass spectrometry (FT-ICR–MS) can be used to analyze the composition of heteroatomic compounds in asphaltenes and their corresponding aromatics, saturates, and resin contents.[30,31] Gel-permeation chromatography (GPC) with inductively coupled plasma–mass spectrometry (ICP–MS) facilitates in the identification and quantification of sizes associated with various Ni, V, and S compounds in asphaltenes.[32] Atomic force microscopy (AFM) is an important technique which enables the direct observation of asphaltene molecules on an individual scale.[33]

The asphaltene precipitation and its subsequent deposition are governed by various factors. The factors mainly include variations in pressure, crude oil composition, temperature, and electrokinetic effects.[34] The terms asphaltene precipitation, asphaltene flocculation, and asphaltene deposition are different in terms of mechanism and definition.[35] Precipitation can be explained as asphaltene changes its phase from the liquid to the solid state by the process of aggregation.[35] Kinetically, the aggregation process in a colloidal system is either controlled by an RLA (reaction-limited aggregation) process or a DLA (diffusion-limited aggregation) process. In initial phase of aggregation, the RLA process is found to be dominant, whereas the later stage of aggregation is controlled by a DLA process.[36−38] After precipitation begins, these precipitated asphaltenes grow their size to about 1 μm through the process of clustering. This stage is referred as asphaltene flocculation.[35] Lastly, a deposition phase starts after the flocculation stage where flocculated asphaltene attains its maximum size and starts depositing at the surface of the metal.[35] It is worthy to note here that asphaltene precipitation does not always guarantee asphaltene deposition and is not considered as undesirable as asphaltene deposition; however, it might facilitate the deposition process significantly, which needs to be controlled by implementing various treatment methods.[35] The treatment methods may include operational, mechanical, chemical, biological, ultrasonic, and thermal.[34,39]

For petroleum companies, it is always preferred to adopt preventive measures rather than going for mitigation of asphaltene deposition as it prevents them from costly shutdowns and deployment of expensive treatment techniques. Therefore, under these circumstances, it becomes very important to assess the stability of crude oils. In the literature, there are various stability assessment methods proposed to determine the asphaltene deposition risk potential in crude oils. Among them, one of the methods is based on determining asphaltene stability in crude oil on the basis of crude oil SARA values.[1,40] The basic concept behind this method is that crude oil is found to be more risky in terms of asphaltene deposition if it contains higher contents of saturates and asphaltene and lower contents of resins and aromatic contents.[1,22] Different models like the colloidal instability index (CII), stability index (SI), colloidal stability index (CSI), Stankiewicz plot (SP), ANJIS Model, qualitative–quantitative analysis (QQA), and stability cross plot (SCP) were developed and applied in the past, which works well on this concept.[41−49] Although these models are very easy and simple to apply as it involves less computational work, their reliability and accuracy are debatable.[1,40] These indices suffer inaccuracies due to the absence of the incorporation of other factors like asphaltene and resins compositional and structural properties and operational conditions such as temperature and pressure conditions which are equally important for judging asphaltene stability in crude oils.[1] In addition, these parameters take SARA values as input which are determined through various techniques and found to have significant differences in values from each other; therefore, in this context it may produce different stability outcomes for same crude oil.[1,40,41,44] Moreover, recent studies reported that these SARA-based parameters produce biased predictions, i.e., predict certain classes better, and because of this a significant difference is usually found in the prediction outcomes of these models.[1,40] The second way of determining asphaltene stability in crude oils is to use thermodynamic models.[1] In the literature, several models have been reported for the modeling of asphaltene precipitation behavior in hydrocarbon mixtures. The modeling methods are generally categorized in five groups. These include equation of state (EOS) models, polymer solubility models, colloidal approaches, thermodynamic micellization models, and molecular thermodynamic models.[50] This technique is the best method; however, it involves complex computations and requires considerable compositional data and experimental points for calibration.[1] The experimental point can be obtained from various high-pressure high-temperature (HPHT) experimental techniques which include the gravimetric method, acoustic resonance technique, light-scattering technique, filtration method, and high-pressure microscopy.[1] The determination of experimental points is difficult because it requires time and effort for performing long duration experiments, high cost, and procurement of a high quality pressurized bottom–hole crude oil sample free from contamination.[1,5,6] Moreover, experimental results may be found to be different for the same oil at the same conditions obtained from different equipment which has different sensitivities.[6] Furthermore, some of the deterministic tools are found to be effective within particular process conditions and depend on linear system identification models, whereas the asphaltene precipitation and deposition mechanism is strongly a nonlinear phenomenon with respect to the process and thermodynamic parameters.[50] Third, the quick and most reliable methods to determine asphaltene stability in crude oil are through experiments. The experiments include the Heithaus parameter, toluene equivalence, Bureau of Mines Correlation Index–toluene equivalence, oil compatibility model, microscopy, and spot test.[42,45] These experiments involve the addition of precipitant (usually n-heptane) in crude oils to detect the asphaltene onset point and then performance of calculations for determining the stability. The quality of the results depends upon the applied methodology, equipment sensitivity, and analyst ability.[42] Although, these tests are very useful but they show a discrepancy in the results, are prone to human error, are time-consuming, and are not continuous.[1] The fourth way to judge asphaltene stability in crude oil is through asphaltene compositional and structural characteristics. Previous studies reported that the H/C ratio of asphaltene,[1] asphaltene solubility profile distribution,[51,53] asphaltene aromaticity and density,[52] polarity of asphaltene,[54,57,58] and electrical charges on asphaltenes[59] can provide information regarding the chance for asphaltene stability in crude oils. However, this method required the extracted asphaltene sample, which may lead to uncertainties in results due to asphaltene extraction methodology applied and the absence of the effect of other crude oil components such as resins, aromatics, and saturates. It was also reported that asphaltenes collected from the field are more polar than asphaltene extracted from the same crude under laboratory conditions.[56] Lastly, the geological settings, presence of tar mat, and biomarker correlation of the crude oil and the source rock also facilitate the determination of asphaltene stability in oils.[55] However, these methods are still not very common or mature and add difficulty to the interpretations of instability and stability from the obtained results.

In the literature, the majority of experimental stability tests proposed were based on the determination of the onset asphaltene precipitation point as the function of precipitant consumption volume by crude oil.[42,45,46] The crude oils are considered to be more stable when they require a higher volume of precipitant to trigger asphaltene precipitation.[42,45,46] The interpretation of results obtained from these tests mainly depends upon the type of methodology followed, sensitivity of equipment, and type of precipitant used.[42] Moura et al.[42] recently reported that these tests could not detect onset points for oils having lower asphaltene content (≤0.60% w/w) and higher saturate content. Moreover, these tests do not incorporate the magnitude of deposition and asphaltene precipitation kinetics after the inception of the onset point, which is extremely important for the assessment of asphaltene stability in crude oil in terms of operational point of view. That is why some studies could be found in the literature related to commercial software development and implementation which have incorporated the information on asphaltene precipitation and deposition kinetics as key input parameters for the modeling of asphaltene deposition.[60−63] There is another type of experiment involved in the stability determination. This class of test requires the passing of light through the supernatant at different times, and the magnitude of light absorbance evaluates the dispersion efficiency.[45,63] The accuracy of these tests depends upon the light wavelength used and offers complex instrumentation and interpretation.[6] These experimental tests also do not provide information regarding the quantity of deposition and assume that higher dispersion efficiency offers higher deposition control, which according to recent studies is not always true particularly between precipitation tests and deposition tests.[6] In addition, field operators are more interested to determining stability in terms of deposition rather than dispersion. Therefore, there is a need for an effective extended duration experimental approach that could incorporate both asphaltene precipitation kinetics and magnitude of asphaltene deposition to provide reliable and accurate results for asphaltene stability in crude oils.

In this research study, a new method is proposed, namely, a deposit level test coupled with a spot test to monitor the asphaltene stability in crude oil kinetically in terms of both precipitation and deposition. The method does not involve complex instrumentation and is flexible and continuous. The integration of two methods not only improves the reliability of stability results but also provides deep insight on the mechanisms involved and the behavior of asphaltene precipitation and deposition rates.

Methods and Materials

In this research study, the stability of six crude oil samples, namely A, B, D, E, F, and G, having a specific gravity of 0.85, 0.83, 0.84, 0.86, 0.92, and 0.81, respectively, is determined at different times for 1 day. A mixture of n-heptane and a crude oil sample in the ratio of 20:1 is prepared in a graduated centrifuge tube having 10 mL volume. The deposit level test and spot test are conducted simultaneously at 2, 15, 30, and 50 min as a short duration test and at 19, 22, and 24 h as a long duration test. The description of two tests applied in this study is given below:

Deposit Level Test

In the deposit level test, a mixture of n-heptane and oil in a centrifuge tube at some higher proportion is prepared. The tube is then monitored for asphaltene deposition at different times, and finally, the level of deposition in the tube is determined.[64] Moreover, the appearance of supernatant can also provide information about whether all asphaltene particles have settled or if they in the dispersed phase.[64]

Spot Test

This test is used for the detection of asphaltene precipitation in crude oils. The crude oil and n-heptane are mixed in certain proportions to initiate asphaltene precipitation in a tube. Sample drop at the upper portion of the supernatant is then allowed to drop at a filter paper for spot formation.[45] After drying, the spot is observed and identified on the basis of the types as presented in Table :[45]

Table 1

Spot Reference and Its Description[45],a

spot reference no.	description of spot
1	homogeneous spot without inner ring
2	faint or poorly defined inner ring
3	well-defined thin inner ring, only slightly darker than the background
4	well-defined thin inner ring, slightly thicker than the ring in reference spot no. 3 and somewhat darker than the background
5	very dark solid or nearly solid area in the center; central area is darker than the background

Reprinted with permission from ref (45). Copyright 2017 Elsevier.

According to the spot test, if a spot fall comes under the category of spot reference no. 3 or higher, then it indicates that asphaltene precipitation and flocculation have occurred and particles of asphaltene exist in a dispersed state in the supernatant.[45]Figure shows the schematic of the experiment performed in this study.

Figure 2

Schematic of experiment performed in this study.

Image Processing of Spot Test, Statistical Analysis, and Sensitivity Analysis

After experimental performance, the deposition level of asphaltene deposits produced at different time intervals was noted. However, for the quantitative determination of precipitation intensity at different times, all spot test results were analyzed through the image processing tool. These two obtained outputs, i.e., precipitation intensity and deposition level, were further used for sensitivity analysis. The sensitivity of two input parameters, namely specific gravity of crude oils and time, were determined against each output using relevancy factor given below as eq (65)where inp and are the ith value and the average value of the lth input variable, respectively (lth = time and specific gravity of oil). out and are the ith value and the average value of the outcome (precipitation intensity and deposition level), respectively.

The higher negative value of relevancy factor represents a strong negative relationship between an input and output variable, whereas the high positive value indicates a strong positive correlation between an input factor and output variable.[65]

Figure shows the overall workflow followed in this research study

Figure 3

Overall workflow followed in this research study.

Results and Discussion

Oil Sample A

Oil A has been monitored at different times. Referring to Figure a–d, the spot generated at time 2 min clearly indicates that asphaltene precipitation had started. Qualitatively, the appearance of dark inner ring inside the spot clearly indicates that the precipitation is intense. The same spot was also observed at times 15, 30, and 50 min, which depicts that precipitates are still in the solution at a dispersed state. Looking at Figure a,b, the visual inspection indicates that at 30 and 50 min the deposition has not occurred for oil A and all of the asphaltene formed are in dispersed phase as is evident from its dark color appearance of supernatant. It is important to note here that the inner ring appearance in the spot gets brighter as time progresses from 2 to 50 min because with increasing time the asphaltene particles get bigger and moves toward the lower portion of the centrifuge tube and the upper portion of the supernatant where fluid has been taken for spots that hold less precipitate. Now discussing the long duration test at time 19, 22, and 24 h, the spot test clearly depicts that at all times the dispersed asphaltene particles are not in the dispersed state because of the absence of inner ring in spots generated at these times (Figure e–g). The observation seems true when we refer to Figure c,d, which shows a visual appearance of fluid in centrifuge tube. It clearly shows that before the starting time of long duration tests the total deposition has occurred as the deposition level is found to be at a constant level, i.e., no change in deposition level. Moreover, the appearance of supernatant becomes brighter, which indicates that all dispersed asphaltene particles have been settled at these times (Figure c,d).

Figure 5

Spot test results for oil A at (a) 2 min, (b) 15 min, (c) 30 min, (d) 50 min, (e) 19 h, (f) 22 h, (g) 24 h.

Figure 4

Deposit level test results for oil A at: (a) 15 min, (b) 50 min, (c) 19 h, and (d) 24 h.

Oil Sample B

For oil B, the spots were observed for early times at 2, 15, 30, and 50 min. Looking at Figure a–d, which shows spots at the above-mentioned times, it can be clearly observed by the presence of an inner dark ring in the spot that precipitation has begun before the completion of 2 min. At the same time, deposition was also found to occur in oil B as evident by the visual inspection of the presence of deposits in the centrifuge tube at time 15 min (Figure a). The spot test indicates that after 15 min the precipitates in the supernatant settle at the bottom of the tube as the inner ring in the spot was found to be brighter in appearance at times 30 and 50 min. The claim seems to hold true for the presence of the brighter color of the supernatant and the presence of a clear interface surface between the deposit and supernatant. Referring to Figure c,d, for long duration tests at 19, 22, and 24 h, it can be clearly seen that total deposition had occurred at time 19 h and no more deposition occurred after this time. Moreover, these two tests also indicate that no asphaltene particles are in the dispersed phase at later times because of the absence of an inner dark ring from the spot and the appearance of a brighter supernatant color (Figure e–g).

Figure 7

Spot test results for oil (B) at (a) 2 min, (b) 15 min, (c) 30 min, (d) 50 min, (e) 19 h, (f) 22 h, and (g) 24 h.

Figure 6

Deposit level test results for oil B at (a) 15 min, (b) 50 min, (c) 19 h, and (d) 24 h.

Oil Sample D

For oil sample D, the spot tests were carried out at four early times. According to Figure a, it was found that precipitation has begun before time 2 min as evident by the presence of an inner dark ring in the spot taken at 2 min. At 15 min, it was observed that the inner ring darkness was slightly increased compared to the former inner ring spot taken at 2 min, which indicates that the precipitation intensity has increased (Figure b). After 15 min, the darkness of inner ring decreases (Figure c,d), which shows signs of settling of the majority of precipitates at the bottom of the tube due to their size growth. This interpretation was proved to be true by observing the deposition test results which clearly show that at a time of 50 min the deposition of asphaltene particles has occurred in the centrifuge tube (Figure a,b). At later test times (from 19 to 24 h), it was found that all dispersed precipitated asphaltenes are deposited as is evident by the clear appearance of supernatant (Figure c,d) and the absence of an inner dark ring from the spot (Figure e–g).

Figure 9

Spot test results for oil D at (a) 2 min, (b) 15 min, (c) 30 min, (d) 50 min, (e) 19 h, (f) 22 h, and (g) 24 h.

Figure 8

Deposit level test results for oil D at (a) 15 min, (b) 50 min, (c) 19 h, and (d) 24 h

Oil Sample E

The results of the spot test of oil sample E as illustrated in Figure clearly indicate that precipitation in oil E progresses slowly. According to Figure a, the spot contains a very bright inner ring which is a clear sign of very low precipitation at time 2 min. However, as the time progresses from 15 to 50 min, the inner ring of the spot get darker, indicating that the magnitude of precipitation has increased gradually in the supernatant (Figure b–d). On the other hand, the results of the deposition test indicate that deposition of precipitates does not occur until 50 min (Figure a,b). The possible reason for this observation might be the slow asphaltene precipitation and subsequent aggregation kinetics in oil sample E. Oppositely, the results of long duration tests from 19 to 24 h show the disappearance of an inner dark ring from the spot (Figures e–g) and a considerable increase in the deposition level (Figures c,d). This condition clearly signifies that asphaltene particles formed during early hour test (2 min until 50 min) do not remain in a suspended form in the supernatant and now they are deposited at the bottom of the centrifuge tube.

Figure 11

Spot test results for oil E at (a) 2 min, (b) 15 min, (c) 30 min, (d) 50 min, (e) 19 h, (f) 22 h, and (g) 24 h.

Figure 10

Deposit level test results for oil E at (a) 15 min, (b) 50 min, (c) 19 h, and (d) 24 h.

Oil Sample F

According to the Figure a, it can be clearly observed that precipitation starts before 2 min as the appearance of the inner ring is present in the spot; however, the color is not too dark, which means that the precipitation was not intense at this time. The color of the inner ring appears to get darker as time progresses from 15 to 50 min (Figure b–d). The visual inspection of oil sample F suggests that no deposition occurred from 2 to 50 min (Figure a,b). Oppositely, at long duration hour tests, all formed asphaltene precipitates are deposited at the bottom of the centrifuge tube as it can be observed by the disappearance of dark inner ring from the spot and visual monitoring of the tube (Figure c,d and e–g). It is worth noting here that the supernatant appearance of oil F is found to be quite dark as compared to other oil samples tested particularly at long duration test. This observation might lead to a misleading understanding of the presence of asphaltene particles in the dispersed phase. Therefore, spot test results found at later stages confirm that all dispersed asphaltene particles are deposited as evident from the absence of an inner ring from the spot at later times.

Figure 13

Spot test results for oil F at (a) 2 min, (b) 15 min, (c) 30 min, (d) 50 min, (e) 19 h, (f) 22 h, and (g) 24 h.

Figure 12

Deposit level test results for oil F at (a) 15 min, (b) 50 min, (c) 19 h, and (d) 24 h.

Oil Sample G

The behavior of oil G is found to be similar to oil sample B. According to Figure a–d, precipitation starts before 2 min and is found to be more intense at time 15 and 30 min because of the darker inner ring presence in the spot, which shows that a higher number of precipitates are in the supernatant at these times. However, at 50 min the number of precipitates decreases in the supernatant as the inner ring becomes brighter. The visual inspection of sample, according to Figure b, suggests that deposition starts before 15 min. The combination of spot test and deposit level test results shows that both precipitation and deposition occur at early times of the test. At long duration tests at 19, 22, and 24 h, the total precipitates formed at early times are deposited as the inner ring of the spot at these times disappeared and the color of the supernatant got brighter (Figures c,d and 15e–g).

Figure 15

Spot test results for oil G at (a) 2 min, (b) 15 min, (c) 30 min, (d) 50 min, (e) 19 h, (f) 22 h, and (g) 24 h.

Figure 14

Deposit level test results for oil G at (a) 15 min, (b) 50 min, (c) 19 h, and (d) 24 h.

We now discuss the oil samples with respect to RLA, DLA, and sedimentation mechanisms. The pictorial representation of the mechanism is represented in Figure . According to Figure , in a colloidal system, aggregation can be generally described by a process that is either controlled by diffusion, which is termed as the DLA model, or by considering that there exists an energy barrier for the process of aggregation, which is known as the RLA model.[37,38] According to the literature, the beginning phase of asphaltene aggregation is dominated by the RLA model, and the later stage of the aggregation process is controlled by the DLA model, which facilitates the increase of asphaltene aggregate size.[37,38] At the end, the aggregate size grows to the extent where it begins to settle down, and consequently, the RLA and DLA phases are followed by the sedimentation phase.[37,38] According to Table , oil samples A, E, and F are found to have RLA and DLA mechanisms that are more dominant at all stages of the short duration tests because precipitation and aggregation of asphaltene particles are found to start without deposition. On the contrary, oil samples B and G are found to possess RLA and DLA mechanisms along with a sedimentation process at short duration tests. In short duration tests, for these oils, precipitation and enough subsequent aggregation of asphaltene particles occur that cause deposition. Oil sample D was initially followed by the RLA and DLA mechanisms before 15 min, however; afterward, both the DLA and RLA mechanisms were found at short duration tests along with sedimentation. During long duration tests, all samples were found to have sedimentation mechanisms.

Figure 16

Behavior of reaction-limited aggregation and diffusion-limited aggregation. Reprint from ref (38). Copyright 2019 American Chemical Society.

Table 2

Mechanism Identified at Different Times of Tests for Various Oil Samplesa

	time
	short duration test				long duration test
oil sample	2 min	15 min	30 min	50 min	19 h	22 h	24 h
A	RLA+DLA	RLA+DLA	RLA+DLA	RLA+DLA	S	S	S
B	RLA+DLA+S	RLA+DLA+S	RLA+DLA+S	RLA+DLA+S	S	S	S
D	RLA+DLA	RLA+DLA	RLA+DLA	RLA+DLA+S	S	S	S
E	RLA+DLA	RLA+DLA	RLA+DLA	RLA+DLA	S	S	S
F	RLA+DLA	RLA+DLA	RLA+DLA	RLA+DLA	S	S	S
G	RLA+DLA+S	RLA+DLA+S	RLA+DLA+S	RLA+DLA+S	S	S	S

“S” stands for sedimentation.

Comparing all samples, it can be clearly stated here that asphaltene precipitation rates were found to be higher in oil samples B, G, A, and D and lower in samples E and F. In terms of deposition, higher deposition occurs in B, A, and F samples, followed by G and in the last D and E. Although asphaltene precipitation kinetics and asphaltene deposition are competitive mechanism, the higher kinetic rates do not means higher deposition or vice versa. Deposition is the consequence of the aggregation of asphaltene particles to a size which can settle down. The aggregation process might be governed by factors such as crude oil SARA composition and asphaltene composition and properties like polarity, dielectric constant, etc.[1,51−59] On the other hand, the amount of asphaltene deposition might depend upon the overall asphaltene weight percent in the crude oil in the dissolved phase. An oil sample having a higher potential of generating asphaltene deposits at faster rates is more risky than an oil sample that produced a lower amount of asphaltene at slower rates. In our opinion, we believe that if oil samples are producing considerably varying amounts of deposition then kinetics become more important. The claim can be understood in such a way that oil that can produce significant asphaltene deposition at lower rates can be controlled by treatment with a reliable oil sample that can produce considerable amounts of deposits at faster rates because of the lower time of the treatment method deployment. In this context, oil sample B is the most dangerous oil sample as it produced greater amounts of asphaltene deposits at comparatively faster rates. G and D oil samples may be considered as the second and third most risky crude oils, respectively, followed by A, F, and E oil samples at the fourth, fifth, and sixth position, respectively.

Image Processing of Spot Test Results and Statistical Analysis

To conduct sensitivity analysis, the qualitative interpretation of all the tests results discussed in the previous section need to be converted in numeric value. As far as the deposition level value is concerned, it can easily be acquired by noting the asphaltene deposition mark on the test tubes obtained at different times. However, the transformation of the spot test results to a numeric value is quite challenging. Although assigning of a numerical value to each spot on the basis of the appearance of inner dark ring as mentioned in Table can be done, it may lead to misleading interpretation due to human error and use of only five numeric real values for different types of spots produced. Therefore, a reliable and accurate technique is needed that can determine the exact intensities of spots formed at different time intervals.

In this research study, image processing software is implemented on all spot test results for the evaluation of exact precipitation intensities at different time intervals for each oil sample. In each intensity chart, six numeric data or information were obtained. Their description is given below:

count represents number of readings analyzed

mean represents mean color intensity

StdDev represents standard deviation

min represents minimum value of color intensity

max represents maximum value of color intensity

mode represents maximum number of repeated value

Each spot is characterized as a hill-type structure by image processing tool having either two peaks, a rough single or multiple shorter peaks, or a sharp single peak with smooth dipping edges on either side. Two peaks on the intensity chart were characterized by the presence of color with mainly two major different intensities. This structure indicates that the spot has a dark inner ring surrounded by a lighter bright ring which indicates that the precipitation phase has been started and precipitates formed a exist at the upper portion of supernatant from where the solution is taken to produce spots. Rough or multiple short peaks on the intensity chart indicate that precipitation has been initiated but its intensity is not severe. Finally, the last class, which is a sharp single peak with smooth dipping edges on either side, indicates that either precipitation has not yet started or precipitation phase has been finished and all the precipitates formed were settled at the bottom of the tube.

The outcomes of the spot test results image processing for each oil sample at different times are shown in Figures S1–S6. According to these figures, each image chart is placed along with its corresponding spot marked with yellow circle which encompasses all the readings analyzed by image processing software. It can be clearly observed that during the early duration tests from 2 to 50 min all oil samples image charts show either two peaks or a rough multiple peak hill-type structure. This means that all oil samples have started asphaltene precipitation during this time period with different intensities. Comparing the image charts of all oils, it can be depicted that asphaltene precipitation in oil samples A, B, D, and G has started intensely during the 2 min test as their image chart contains a hill-type structure with two clear long separated peaks. However, at later times, these oils show a decrease in asphaltene precipitation because during 50 min test time the two peaks were transformed into multiple shorter peaks. Alternatively, in oil samples E and F, the asphaltene precipitation trend is found to be reverse. These oil samples image charts contain rough multiple shorter peaks during a 2 min test, whereas during the 50 min time test the appearance of two clearly separated long peaks was observed. This condition clearly indicates that in oils E and F the asphaltene precipitation kinetics is slow. It is important to note here that during the time period from 19 to 24 h all oil samples yield image charts having hill-type structures which contain a sharp single peak with smooth dipping edges on either side except for one image chart obtained for oil sample B at time 22 h having two peaks. The reason for this outcome might be the incorrect spotting procedure due to human error. The overall behavior of image processing results at later stages of tests indicates that dominancy of precipitation phase has been reduced or ceased and all the precipitates formed have been settled down at the bottom of the centrifuge tube. This interpretation of image processing results was found in excellent agreement with the qualitative description of spot test results already discussed in previous section.

Sensitivity Analysis

To investigate the effect of input parameters (i.e., specific gravity of crude oil and time) on asphaltene precipitation intensity and deposition level, a sensitivity analysis was carried out. For this purpose, a relevancy factor (r) was employed to determine the influence degree of each input factor on two outputs. To implement relevancy factor, the numeric values of output must be known. Therefore, as discussed in previous section, the deposition level value at different times for each oil sample was simply obtained by noting the deposition level mark. However, for precipitation intensity values, the data obtained through intensity charts from spot test results between time intervals at 2 min until 50 min were used. The precipitation intensity at each time interval was calculated by taking the difference of maximum and minimum color intensity values. For computation purposes, it was assumed that the minimum value of the color intensity represents the color of the area surrounding the inner dark ring, whereas the maximum value of color intensity denotes the color intensity of the inner dark ring.

Parts a and b of Figure show the relevancy factor results obtained. According to Figure a, it can be clearly observed that precipitation intensity has a negative relationship with both specific gravity of oil and time. The relationship was found to be slightly strong for specific gravity (approximately −0.227) than time (approximately −0.0171). According to previous studies, it was reported extensively that lighter crude oil are more prone to asphaltene precipitation than higher crude oils.[66,67] Lighter crude oils contains more saturates and less resins. The addition of n-alkanes in lighter crude oil promotes destabilization at faster rates because they need to dissolve less resin contents which act as protective shield for asphaltene precipitation.[1,8] On the other hand, the relationship of time with precipitation intensity was found to be almost negligible because in the majority of oils (i.e., A, D, B, and G) both the increment and decrement in precipitation intensity (i.e., increase followed by decrease) were observed during the time period between 2 and 50 min. Similarly, according to Figure b, it can be clearly observed that the specific gravity of crude oil has almost the same relationship, i.e., −0.247, with a deposition level as it has with precipitation intensity, i.e., moderate inverse relation. However, the time shows quite strong positive correlation with deposition level yielding approximately over +0.521 relevancy factor. This indicates that with the increase of time more and more deposition will occur.

Figure 17

Relevancy factor of each input parameter with (a) precipitation intensity and (b) deposition level.

Sources of Errors or Uncertainties

Although, this integrated test found to be very simple, effective, cheap, and quick but there following points need to be consider cautiously in order to obtain quality and reliable results:

For the spot test, it is important to draw sufficient liquid for producing a spot from the supernatant from the same top portion of centrifuge tube at the required times during all tests; otherwise, it may yield inconsistencies in outcomes.

Investigators must take extra care while reading the asphaltene deposition level, particularly in dark fluids because in dark fluids it is often difficult to read the deposition mark.

The presence of an inner dark ring in the spot could indicate the existence of precipitation phase, intensity of precipitation phase, or suspension of particles in the supernatant; however, asphaltene precipitation concentration could not be exactly measured by observing the appearance of inner ring of spot.

The compaction of asphaltene deposits over time might yield significant error in reading the deposit level.

The precipitant to crude oil ratio and underlying conditions must be kept constant during all tests to achieve reliable results.

Testing must be done on fresh and contaminant-free oil samples. Previous studies reported that long-term storage of samples under different conditions and oil sample contamination might create differences in stability results.

Recommendations for Future Studies

The new experimental approach adopted in this study is quite efficient and flexible. Future studies must be directed toward adoption of the same methodology by involving various factors and effects like dilution ratio and precipitant type variations, conduction of experiments at more and equal time intervals, blending of crude oils, and high temperature conditions. Both asphaltene kinetics and deposition have a strong relationship with temperature; therefore, varying temperature conditions must affect the asphaltene stability in crude oils. Another avenue to apply this procedure is to implement this methodology on deposition tests carried on metallic surfaces preferably in dynamic state and at high-pressure and high-temperature conditions. The deposition tests are usually found to simulate conditions close to real scenarios. In addition, previous studies have reported that outcome yields from deposition tests are quite different from those obtained by precipitation tests. It is a well-known fact that asphaltene precipitation in field conditions are usually triggered as a function of pressure rather than through the precipitant. In this context, the applied methodology must be implemented on live oil under HPHT conditions where asphaltene formation occurs due to depressurization of crude oils. Finally, the applied methodology could easily be implemented for evaluating the performance efficiency of chemical additives both in terms of asphaltene deposition control as well as retardation of asphaltene precipitation kinetics.

Conclusion

In this research study, the stability of six crude oil samples was determined using deposit level tests integrated with spot tests. The two tests were performed simultaneously at short time durations (i.e., at 2, 15, 30, and 50 min) and at long time durations (i.e., at 19, 22, and 24 h). The coupling of the two tests allows the determination of crude stability both in terms of precipitation kinetics and level of deposition. For quantitative interpretation of spot test results, an image processing was performed. According to the results, all crude oils were found to possess potential for asphaltene precipitation upon addition of precipitant. Precipitation was found to start during short duration tests at different rates for all crude oils and all precipitates formed during early time were found to deposit completely during long duration tests in all oil samples. In terms of asphaltene deposition level, crude oil B, A, and F were found to produce a higher volume of asphaltene deposits, followed by G and in the last D and E. Sedimentation in crude oils B, G, and D starts during the short time duration test, while in sample A and F deposition begins after 50 min. Alternatively, with respect to asphaltene precipitation kinetics rates, crude oils B, G, and D were observed to destabilize quickly and start asphaltene deposition during the short time duration tests. Among all crude oil samples tested, crude oil B was found to be the most risky crude oil in terms of asphaltene problems as in these oil samples higher volumes of deposits were produced at quick rate. Oppositely, oil sample E was found to be the relatively least risky crude oil as it produces asphaltene deposits at slowest rates. Moreover, sensitivity analysis was also carried out to determine the relative importance of input parameters which include specific gravity and time on output variables, i.e., precipitation intensity and deposition level. It was found that the specific gravity of the crude oil has a negative moderate correlation with both precipitation intensity and deposition level. Similarly, time shows a negligible negative relationship with precipitation intensity (during short duration tests) because intensity, in most cases, increases and decreases during the test time. Alternative, the time shows strong positive correlation with deposition level (both short duration and long duration reading taken). The integration of the spot test with the deposit level test not only increases the overall strength of this test but also allows researchers to monitor precipitation and deposition continuously and simultaneously without involving complex instrumentation and interpretation. The implementation of this approach is easy and flexible and can be helpful for both the upstream and downstream petroleum industry for evaluating the stability of crude oils under different environments and conditions.