Precipitation, aggregation and breakup of asphaltene particles are studied under natural depletion and nitrogen injection processes by means of high pressure filtration and a photographic procedure coupled with image analysis. Filtration results show that nitrogen destabilizes asphaltenes extremely and the problem is more severe for heavier crude samples. Bimodal histograms of particle size distribution show two agglomeration mechanisms; cluster aggregation dominant around bubble point pressure and diffusion limited far away. Fractal structure of aggregates is also altered by gas injection; it is observed that the flocs grow in size and become more compact and organized when the cluster aggregation overcomes.
Keywords: Nitrogen Injection /Asphaltene Aggregation/ High Pressure Filtration/ High Pressure Microscopy/ Fractal Dimensions.
Asphaltenes are defined as the components of crude oil that are soluble in aromatics such as benzene or toluene, but are insoluble in n-pentane, n-hexane, or n-heptane . It is well known that the injection of CO2, N2 and hydrocarbon gases changes the solubility of the reservoir fluid and causes asphaltene instability. Most of published studies are focused on hydrocarbon gas and CO2 injection [2-5]. Idem et al.  used titration technique to evaluate the kinetics of CO2-induced asphaltene precipitation for Saskatchewan crude oils under isothermal and isobaric reservoir conditions. Takahashi et al.  investigated effect of CO2 concentration on asphaltene precipitation at reservoir temperature for a range of pressures. Hu et al.  studied the effects of operating pressure, injected CO2 concentration, and multiple-contact on the onset and amount of asphaltene precipitation. Negahban et al.  discussed experimental work associated with evaluation of asphaltene precipitation due to injection of a synthetic gas for a field in UAE.
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Nitrogen injection is a common EOR process applied worldwide [10-12]. However, little work is done on investigation of asphaltene precipitation due to nitrogen injection .
Kinetics of aggregation and particle size distribution (PSD) for colloids has been investigated extensively in several industrial processes like liquid-liquid extraction, polymerization, fluidized bed, and water treatment. However, the mechanism of asphaltene flocculation has not been studied in detail . Sheu and Storm  concluded that asphaltene agglomeration occurs in a manner similar to that in surfactant systems but with less uniformity and more polydispersity in the structure. Rahmani et al. [16-17] measured floc size distribution using optical microscopy and image analysis. The aggregation behavior of asphaltenes was investigated by monitoring the size distribution of flocs for various intensities of agitation (i.e., shear rate), solvent composition (i.e., ratio of toluene to n-heptane in the solution) and particle contents (i.e., volume fraction of particles). Khoshandam and Alamdari  investigated the asphaltene particle size distribution in a heptane-toluene (Heptol) mixture using a nano-particle size analyzer to measure the size of the asphaltene particles in the range of 1 nm to 6 μm. Solaimany-Nazar and Rahimi  studied effects of shear rate, solvent composition and initial particle size on the average diameter for asphaltene samples.
Most of related researches are conducted for extracted asphaltenes in synthetic Heptol samples at standard conditions. In the present work, effects of pressure drop and miscible nitrogen injection on asphaltene precipitation and particle size distribution are studied by means of a high pressure filtration and a high pressure microscopy (HPM) technique. Afterwards, experimental results are analyzed and interpreted thoroughly; fractal dimensions of the aggregates are studied and finally, the results of image analysis are compared to the filtration technique.
2. Experimental setup and procedures
2.1. Preliminary characterization of samples
Two different bottom-hole samples are characterized for API, molecular weight, solution gas oil ratio and composition (Tables 1-3). SARA test is also performed as described by Institute of Petroleum handbook  to determine saturate (S), aromatic (A), resin (R) and asphaltene (A) content of the samples.
2.2. Asphaltene precipitation experiments without gas injection
Natural depletion is conducted at reservoir temperature to examine whether asphaltene precipitation could be happening in the absence of gas injection. Schematic diagram of the experimental setup is shown in Figure 1. The main part of the system is a visual JEFRI equilibrium cell connected to a high pressure filter. The working temperature range of the cell is 243-473 °K and the maximum working pressure is 69 MPa.
Experimental procedure is described as follows:
The cell is cleaned and maintained at reservoir temperature.
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A known volume of single-phase oil is charged into the cell at reservoir pressure. A magnetic stirrer agitates the sample overnight to accelerate the equilibrium process. To remove any possible solid particles present in the oil, the feed oil is filtered first.
Cell pressure is lowered in subsequent predefined steps. At each point, the filtration is conducted with a 0.2µ filter paper. During the filtration process, it is important that the sample remains monophasic as it passes through the filter manifold. High pressure helium is used to maintain a back-pressure on the downstream of the filter. Below bubble point pressure (Pb), the evolved gas in released first at constant pressure and then the oil is allowed to flow towards the filter manifold from top of the PVT cell.
Filtered oil is flashed in a separator and asphaltene content of residual oil is measured by the standard IP procedure. The difference between asphaltene content of original oil and filtered oil at each pressure determines the weight percent of precipitated asphaltene.
2.3. Asphaltene precipitation due to gas injection
Subsequent to natural depletion, effect of miscible nitrogen injection on asphaltene instability is studied as follows :
Before performing filtration tests, a swelling experiment is conducted to determine variation of mixture bubble point pressure with gas injection (results are presented in Figure 2).
Oil sample is fed into the cell at desired temperature while the pressure is maintained above mixture saturation pressure to avoid phase separation during transfer and recombination of oil with the injected gas.
A preset amount of nitrogen is introduced into the cell under isothermal conditions. The mixture is allowed to equilibrate and settle down overnight to ensure full asphaltene precipitation. High pressure filtration is then performed to quantify asphaltene precipitation at different pressures. The sampling, filtration and evaluation processes are similar to natural depletion process.
To examine effect of nitrogen injection on asphaltene instability, 10 mole percent of nitrogen was recombined with sample A at reservoir temperature of 250 °F. Although gas injection showed negative effects on asphaltene precipitation, it was not clearly quantified since the asphaltene content of this sample was very low (< 1 wt %). As a result, this sample was discarded and gas injection was repeated for sample B with of 10, 20 mole percents of nitrogen at reservoir temperature of 185 °F.
2.4. Determination of aggregate size distribution
A High-Pressure Microscope is connected to the equilibrium cell. The microscopic cell is designed to study asphaltenes at high pressure (up to 69 MPa) and over a wide range of temperature (from -40°C up to 180°C).
Procedures of oil feeding, gas recombination and pressure drop are similar to filtration tests for natural depletion and gas injection processes; the only difference is that the fluid is passed through the microscope cell instead of the filter. A thin layer of fluid (0.3mm) is examined under the microscope; then, thanks to computer processing of image, shapes, distribution and quantities can be studied.
3. Results and discussion
3.1. High pressure filtration
Results of high pressure filtration are presented in Tables 4, 5. It is observed that nitrogen injection increases asphaltene precipitation compared to natural depletion. Swelling results in Figure 2 show that nitrogen raises bubble point pressure of the heavier sample (B) more compared to the lighter one (A) .The lower the initial dissolved gas, the higher the effect of nitrogen on phase and solubility behavior of the crude oil, which in turn, causes more instability of asphaltenes.
3.2. High pressure microscopy
Captured images are processed based on the contrast difference between crude oil and aggregates. Figures 3, 4 show analyzed photos from natural depletion and gas injection experiments near the bubble point pressure of the corresponding fluids (2900 and 7500 psi respectively). It seems that nitrogen influences the mechanism of asphaltene precipitation and structure of aggregates. Number, length, width, perimeter and area of the flocs at different operating conditions are summarized in Table 6.
Bimodal histogram of particle size distribution in Figures 5, 6 shows that two different types of aggregates are present in the mixture. For the original reservoir fluid, most of particles are smaller than 10μm at 4500 psi. By lowering pressure to 3200 psi, the larger clusters show up while the number of small particles is still increasing. It shows that the diffusion limited precipitation is still dominant at this point. Around bubble point pressure, at 2900 psi, small particles decrease in count and agglomerated clusters reach their maximum occurrence. Improvement of crude solubility below Pb, breaks up the composite clusters and increases the population of basic particles. Similarity of histograms above and below saturation pressure shows that asphaltene aggregation is totally reversible at these conditions.
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Figure 6 shows aggregate size distributions for the gas injection process. In this case, most of histograms are bimodal that shows a cluster dominant mechanism almost everywhere. Similar to natural depletion, larger clusters have the most incidence near bubble point pressure and decrease in size and count in upper and lower regions. Evaporation of nitrogen and light hydrocarbons at low pressures (e.g. 2000 psi) improves the solubility of the crude and breaks down the complex clusters. It can be concluded that immiscible nitrogen injection at low reservoir pressures would not be challenging for the asphaltene aggregates.
3.3. Fractal structure of aggregates
The structure and density of the aggregates influence their strength and characterize their behavior during fluid flow through porous media and wellbore. However, structure of asphaltene particles under high pressure conditions has not been thoroughly investigated. Fractal dimensions relate aggregate size to some property in n-dimensions, where n = 1, 2, or 3 and Dn is the fractal dimension in n dimensions. For example, fractal dimension of aggregates imbedded in two dimensions is:
where l is the characteristic length scale and A is the cross-sectional area of an aggregate . 2D fractal dimension quantifies the rate at which the aggregate projected area increase with respect to the characteristic length; higher values of D2 indicate that the aggregates are more dense. A perimeter-based fractal dimension is defined d based on the measured perimeter (P) and the projected area (A) of the flocs as follows:
The value of Dpf is related to the surface morphology since Dpf varies between 1 (the projected area of a solid sphere, a circle) and 2 (a line, e.g., a chain of particles). Values of Dpf greater than one have been found for synthetic fractals and a variety of natural objects such as snow particles, clouds and lakes [21-22]. The interpretation of Dpf > 1 is that as the objects become larger, i.e., as A increases, P increases more rapidly than for Euclidean objects, and the boundary becomes more convoluted .
Two-dimensional and perimeter-based fractal dimensions are obtained from the log-log plots of aggregate area vs. length and perimeter respectively (Figure 7). Results of fractal dimension analyses are summarized in Tables 7, 8. It is observed that above bubble point pressure, D2 increases with decreasing pressure, reaches a maximum around Pb and decreases afterwards. It means that the aggregates have the most compact structure near the bubble point pressure and are porous elsewhere.
Surface roughness of aggregates seems to be correlated to their compaction. Minimum of Dpf around bubble point pressure shows that composite clusters have more regular boundaries. It is also concluded that the aggregates of gas injection process are more compact than the natural depletion's at high pressures, but become more porous and irregular at low pressures (e.g. 2000 psi). This is in agreement with the presented histograms of particle size distribution (Figures 5, 6 ).
3.4. Comparison of filtration and microscopy results
Filtration results include weight percent of precipitated asphaltene versus pressure (Table 4). Data from image analysis must be converted to Wt% for the comparison. Total captured area of the microscope is 13283070 μm2 which is equivalent to 0.00398 cc of sample when considering the channel thickness of 0.3 mm. Oil volume at each pressure is converted to oil mass when multiplied by measured oil density (3rd and 4th columns of Table 9).
Total area occupied by the aggregates is converted to mass by assuming constant asphaltene density of 1.22 gr/cc. Weight percent of precipitated asphaltene is then estimated by dividing asphaltene mass to the oil mass at each pressure (8th to 4th column of Table 9). Results show good agreement with filtration data of Table 4.
The filtration process incorporates all particles larger than 0.22 μm as the precipitated asphaltene. Some of fine particles may not be detected by the microscope. As a result, it is unsurprising that image analysis underestimate the total quantity of aggregates. However, this technique is widely applicable in structural characterization of aggregates which is an important issue in transport and deposition of asphaltenes.
A high pressure filtration procedure is used to investigate effect of miscible nitrogen injection on asphaltene precipitation at reservoir conditions. It is observed that nitrogen injection destabilizes asphaltenes significantly and the problem is more severe for the heavier crudes.
Results of image processing shows that for the natural depletion process, the dominant mechanism is cluster agglomeration around bubble point pressure and diffusion-limited elsewhere. Similar histograms above and below saturation pressure confirm the reversibility of precipitation and aggregation phenomena.
Although presence of nitrogen intensifies formation of clusters at high pressures, the reversible nature of aggregation remains unchanged. Evaporation of nitrogen at low pressures improves the capability of crude to overcome association of the flocs and breaks down the complex clusters. As a result, immiscible nitrogen injection would not be challenging for the structure of asphaltene aggregates.
Investigation of 2D fractal dimensions shows that aggregates have the most compact structure near the bubble point pressure and become more porous elsewhere. Surface roughness of aggregates seems to be correlated to their compaction. Minimum value of perimeter-based fractal dimension (Dpf) around bubble point pressure shows that composite clusters have more regular boundaries.
Image analysis may underestimate the total quantity of precipitated asphaltene when compared to filtration. However, it is applicable in structural characterization of aggregates which is an important issue in transport and deposition of asphaltenes.