Article pubs.acs.org/EF
Experimental Investigation of Asphaltene Deposition in Capillary Flow Kazeem A. Lawal,† John P. Crawshaw,*,‡,§ Edo S. Boek,‡,§ and Velisa Vesovic† †
Department of Earth Science and Engineering, Imperial College London, London, SW7 2AZ, United Kingdom Department of Chemical Engineering, Imperial College London, London, SW7 2AZ, United Kingdom § Qatar Carbonates and Carbon Storage Research Centre (QCCSRC), Imperial College London, London, SW7 2AZ, United Kingdom ‡
ABSTRACT: This paper reports a set of experiments carried out to examine asphaltene deposition in a glass microcapillary pipet, as a proxy to asphaltene deposition in reservoir pores. A new capillary-flow experiment was designed to ensure capillarydominated flow, low inertial forces, negligible dispersion, and insignificant gravity effects, and the end-effect was limited to 0.1% of capillary length. This was achieved by maintaining the flow rates in the range 5 ≤ QT ≤ 60 μL/min. The asphaltene precipitation was induced by bringing into contact a heavy oil sample, diluted with toluene, with a number of different precipitants (n-pentane, n-heptane, and n-octane). The deposition of asphaltene was monitored by imaging the capillary tube and by measuring the pressure drop across it. A new, simple model has been developed to relate the pressure drop to the change in the thickness of the deposited layer and subsequently to the change in permeability. The model indicates that the thickness varies as one over the fourth power of the pressure drop, while the change in permeability is proportional to the square root of the pressure drop. A series of experiments has been carried out to examine the effects of different flow rates, precipitant concentration, and the nature of the precipitant on asphaltene deposition. The deposit growth was generally monotonic and reasonably uniform, indicating the lack of erosion and entrainment. The results indicate the deposition rate increases with decreasing carbon number of the precipitant, while it is not unduly influenced by changes in a flow rate. No noticeable deposition was observed when the precipitant/solvent ratio was 1.22 despite the ratio exceeding the minimum threshold value of 1.08, obtained from gravimetry. deep bed filtration continuum models.6,7 However, these are all phenomenological models, the parameters of which are difficult to relate to the physics of the underlying deposition process. Progress was made toward a more physical understanding of the deposition process in terms of a continuum model.8 In a recent paper,9 the deposition of precipitated asphaltene, redispersed in toluene, in a glass capillary was investigated. The experimental results were compared with colloidal computer simulations by calculating the dimensionless conductivity of the capillary, and good agreement was found. From this study, the effective value of the interaction potential well depth of the colloidal asphaltene particles was estimated. In a subsequent study,10 the deposition of asphaltenes from crude oil in capillary flow experiments was investigated, using heptane as a precipitant for the asphaltenes. By comparing the experimental dimensionless conductivity with computer simulation results, it was concluded that the precipitated asphaltenes are more “sticky” than the crude oil asphaltenes. In the current study, we build on the experience of previous capillary flow experiments and extend our work to various different precipitants, including n-pentane, n-heptane, and n-octane. The deposition of asphaltene was monitored by imaging the capillary tube and by measuring the pressure drop across it. Here, we develop a new, simple model relating the
1. INTRODUCTION The deposition of asphaltenes may cause problems in both downstream and upstream operations. In downstream operations, the precipitated asphaltene may deposit in pipelines, production facilities, and heat exchangers. This usually happens under high Reynolds number conditions. Substantial research effort is currently directed to address this problem. In upstream operations, on the other hand, the asphaltene precipitates may deposit in rock pores, leading to reduced porosity and permeability, as well as wettability alteration. Much less is known about these processes, which generally occur under low Reynolds number conditions. This is the problem we are addressing in the current paper. A number of capillary flow experiments have been carried out to investigate asphaltene deposition. Broseta and co-workers1 have calculated an effective hydrodynamic thickness of a deposited asphaltene layer in flow experiments in a metal capillary, assuming a uniform thickness of the layer deposited. Also, it was assumed in this model that the thickness of the deposit is very small in comparison with the capillary radius. In this paper, we will investigate whether these assumptions are valid. Wang and co-workers2 studied experimentally the deposition of asphaltene on metallic surfaces using the homogeneous deposition hypothesis. There are several papers in the literature related to the modeling of asphaltene deposition in flowing systems. These include the flow and deposition of asphaltene in production pipe lines3,4 and formation damage due to deposition in the reservoir. The latter include network models5 and Darcy scale © 2012 American Chemical Society
Received: November 28, 2011 Revised: March 12, 2012 Published: March 19, 2012 2145
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Figure 1. Simplified set-up for the asphaltene deposition experiment in a capillary tube. (Modified with permission from ref 7. Copyright 2011, Springer.)
Figure 2. Experimental rig. The simplified setup for the asphaltene deposition experiment is illustrated in Figure 1, while Figure 2 depicts the experimental rig. The host medium (tube) was a cylindrical glass microcapillary pipet (71900-10 from Sigma-Aldrich, Germany) with internal volume of 10 μL ± 0.5%. The length and internal diameter (ID) of the microcapillary were 127 mm and 0.32 mm, respectively. All of the capillaries used in the experiments were from the same batch, so as to minimize differences in the internal surface roughness, which is likely to have an impact on deposition behavior. Two dedicated constant-rate programmable syringe pumps (BS-8000/9000 from Braintree Scientific, U.S.A.), with designed throughput of 0.73 μL/h to 2120 mL/h, were used for the precipitant and the crude/solvent streams. Other workers used three pumps,1,2 one each for the crude, solvent, and precipitant, but this was presumably influenced by the relatively high mobility (viscosity and density below 60 mPa·s and 883 kg/m3) of their crudes under laboratory conditions. In the current case of 200 Pa·s crude, it became imperative to premix the solvent and the oil prior to injection, hence eliminating a pump. Each of the pumps was equipped with a gastight syringe, through which it discharged the preset flow rates. The syringes had barrels of volume 10 mL and ID 14.6 mm (81601 from Hamilton, U.S.A.). Imaging was performed with a camera-mounted Zeiss microscope (Axio Observer.A1m from Carl Zeiss, Germany), connected to a computer for image analysis (see Figures 1 and 2). More details are available elsewhere.7 A well-calibrated 0−5 V (1 bar range) pressure transducer, with output to a computer, was used to measure pressure at the capillary entrance. Because the capillary was made to discharge to the atmosphere, the recorded gauge pressure was taken to be the pressure-drop across the capillary. In our discussions on the hydraulic design, we shall show that the 1 bar range was adequate for all the runs investigated.
pressure drop to the change in the thickness of the deposited layer. This model is more general than previous models in the sense that it is also valid for deposit thickness approaching the capillary radius. The model indicates that the thickness varies as one over the fourth power of the pressure drop, as is the case in the well-known Hagen−Poiseuille equation, while the change in conductivity is proportional to the square root of the pressure drop.
2. EXPERIMENTAL SECTION In this section, we discuss a set of capillary-flow experiments conducted to improve our understanding of the mechanism of asphaltene deposition in reservoir pores. Specifically, we examine the issues of deposition profile, predeposition kinetics and the effects of some process variables on asphaltene deposition. Although this study employs n-pentane, n-heptane and n-octane as precipitants, in principle, most of the deductions reached should be relevant for other precipitants. 2.1. Materials and Procedure. The extra-heavy oil sample, identified here as crude X, was from a field in South America. As n-heptane is insoluble, its asphaltene content is about 18 wt %.11 At 1 bar and 293 K (20 °C), its density and viscosity are approximately 990 kg/m3 and 200 Pa·s, respectively. To achieve reasonable flow rates under atmospheric conditions and for the purpose of obtaining visible images (the crude is very dark), the oil was diluted with toluene (solvent).12−14 In the range of dilution ratios examined, the oil viscosity was reduced from about 200 Pa·s to an average of 20 mPa·s (298 K, 0.01 s−1 shear rate). n-Heptane, pentane, and octane were used as the asphaltene precipitants. The purity of the toluene and precipitants exceeded 95% (Sigma-Aldrich, U.K.). At 298 K, their densities are 865 and 680 kg/m3, respectively. 2146
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to constitute one stream, which we call the “experimental oil”. This was then injected through a dedicated line for onward dynamic mixing with the precipitant stream, which was injected through another line (see Figure 1). The mixing and reaction of the coinjected streams commenced just at the entrance of the microcapillary and continued throughout its length. Essentially, the system approximated a plug-flow reactor.17 Except when stated otherwise, all the runs had the experimental oil prepared to honor the toluene fraction in the desired solvent-precipitant ratio and the fixed crude/precipitant−solvent volumetric ratio of 1:40. Under the assumptions of steady-state and incompressible fluids, simple material balance yields the following expressions for the flow rates of the crude, Qc, solvent, Qs, and precipitant, Qp, respectively.
The making-up of the assembly of capillary, PEEK Y-connector (from Upchurch, U.S.A.), and microscope glass slide holder have been detailed by Boek and co-workers.7 However, in contrast to their set-up, which employed a rectangular capillary, ours was cylindrical. Hence, our design had some obvious differences that are peculiar to the current capillary geometry. The assembly was subsequently mounted onto the microscope stage, and the injection lines (0.40 mm ID transparent plastic tubing) connected via the Y-connector to the capillary. For each run, a new capillary was used ,while the precipitant and crude/solvent lines were never interchanged. However, before commencing a new run, the injection lines were adequately cleaned to remove accumulations from the previous runs. The experimental-oil line, while being isolated from the heptane line, was cleaned by flowing toluene through it before emptying it using vacuum. The microscope stage was movable, allowing for image-capture along the length of the capillary. The relative movement of the capillary and stage was eliminated by fastening the capillary to a rectangular glass slide which fitted perfectly into the stage. A rubber band was used to hold both the capillary and slide firmly unto the holder (Figure 2a). Because the slide was transparent, the distortion of transmitted light to the capillary was minimal. In this work, images were taken at 10, 50, and 100 mm along the capillary, taking the stage about 1 s to move between points. Apart from end effects (discussed shortly), the geometry of the capillary holder precluded taking images outside the 10−100 mm interval of the 127-mm long capillary (Figure 2a). The precipitant and crude/solvent injection lines had dead volumes of 250 and 170 μL, respectively. In determining the effective reacting volumes (elapsed times) in the capillary, these dead volumes were discounted. 2.2. Asphaltene Precipitation Onset (APO). Using gravimetric analysis, Li11 established the APO for the sample crude with respect to n-pentane, n-heptane, and n-octane as precipitants. However, rather than the standard ASTM D2007 method15 of mixing 1 vol oil with 40 vol precipitant, Li11 used 1 vol oil and 40 vol of the mixture of toluene and precipitant, where toluene served as an oil solvent. As illustrated in Figure 3, the APO is approximately 52 vol % precipitant
QT
Qc =
R+1
Qs =
(R + 1)(f + 1)
Qp =
(1)
RQT (2)
fRQT (R + 1)(f + 1)
(3)
where R is the volumetric ratio of solvent and precipitant to crude, f is precipitant-to-solvent volumetric ratio, and QT is the total flow rate. In line with eqs 1−3, for a given QT, R, and f, the precipitant was injected at the rate Qp, while the experimental oil was prepared at crude/toluene ratio of Qc/Qs, and then injected at the rate of (Q c + Q s). Although we have studied diluted crudes in these experiments, the resulting states may not be too different (in terms of viscosity and flow regime) from the likely in situ conditions in a reservoir undergoing either a thermal flood or miscible injection, or some combination of these exploitation techniques. 2.4. Hydraulic Design. To come up with an optimal design of the capillary setup, it was necessary to determine the appropriate flow rates (QT) through the capillary and to rate the pressure gauge. As a design philosophy, the hydraulics were governed by ensuring the following:
• • • •
capillary-dominated flow (Nca = μQT/Aσ < 0.01); low inertial forces (NRe = ρcQTD/Aμ < 0.5); negligible dispersion (NPe = QTDa/Aαm > 5); insignificant gravity effects (NG = (ρa − ρ)gADa2/18μQT < 0.0001); • end-effect limited to 0.1% of capillary length (Le = 0.035 ρcQTD2/Aμ < 0.001 L). The quantities Nca, NRe, NPe, and NG refer to the capillary, Reynolds, Peclet, and gravity numbers, respectively. Le is the minimum entrance (exit) length required to attain a fully developed Hagen−Poiseuille flow.18 The definitions of the other variables and the corresponding values used in this study are presented in Table 1.11,19−22 It is worth
Table 1. Numerical Values of Some Hydraulic Design Parameters
Figure 3. Asphaltene precipitation curves for the crude oil.11 to 48 vol % toluene of the 40-vol precipitant/solvent mixture to 1-vol crude. In the current case, the APO is reasonably independent of the precipitant. However, because of the uncertainties associated with the measurements on one hand,16 and the disparity among various methods of determining the APO on the other hand,13 we take the n-heptane/toluene volumetric ratio of 55:45 as the “minimum” APO for our studies, which consider n-heptane as the reference case. 2.3. Preparation of Experimental Oil. Given the APO of 55 vol % n-heptane (in mixture with toluene), which, in principle, is the lower bound of precipitation, we investigated higher precipitant concentrations. To enhance its flow rate, the crude was premixed with toluene 2147
quantity
value
viscosity, μ (Pa·s) crude density, ρc (kg/m3) interfacial tension, σ (N/m) capillary length, L (m) capillary ID, D (m) capillary flow area, A (m2) dispersion coefficient, αm (m2/s) asphaltene floc diameter, Da (m) asphaltene density, ρa (kg/m3) asphaltene content, xa (wt %) maltene density, ρ (kg/m3)
0.02 990 0.029 0.127 3.2 × 10−4 8 × 10−8 1 × 10−10 5 × 10−7 1160 18 865
ref/comment
19
20 21 22 11 assuming toluene is the continuous phase, as its density is between n-C7 and the crude.
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noting that the dispersion refers to the asphaltene particles rather than the bulk flow, hence the use of floc diameter Da and not capillary internal diameter D. Furthermore, we used the stream viscosity of 20 mPa·s, which applied to the solvent/crude mixture. Ideally, the precipitant would further lighten (∼ 2 mPa·s) this mixture, but from a conservative viewpoint (maximum pressure gauge rating), this effect was ignored. The flow rates satisfying the various constraints are indicated in Table 2. From these results, it was straightforward to deduce that the most suitable range of total injection rate QT, was 5 ≤ QT ≤ 60 μL/min.
and limiting deposit thickness. However, to obtain a fixed concentration of precipitable asphaltene in practice, some extracted asphaltenes can be dissolved in the dilute solutions.9,12 Nevertheless, caution should be exercised with this option, as some workers have argued that extracted asphaltenes tend to aggregate faster (more sticky) than in situ asphaltenes;10 hence, they may show different overall kinetics. 2.6. Pressure Sensor Rating. As a guide to the selection of a suitable pressure gauge, we employ the Hagen−Poiseuille equation,18
ΔP
