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Pipeline transport of heavy crudes as stable foamy oil
G Model JIEC 3054 No. of Pages 10
Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
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Pipeline transport of heavy crudes as stable foamy oil Jie Suna , Jiaqiang Jinga,b,* , Cheng Wua , Fei Xiaoa , Xiaoxuan Luoa a b
School of Oil & Natural Gas Engineering, Southwest Petroleum University, Chengdu 610500, China Oil & Gas Fire Protection Key Laboratory of Sichuan Province, Chengdu 611731, China
A R T I C L E I N F O
Article history: Received 15 June 2016 Received in revised form 18 August 2016 Accepted 20 August 2016 Available online xxx Keywords: Foamy heavy oil Cold transportation Stability Drag characteristics Viscosity reduction
A B S T R A C T
A new idea was proposed to transport heavy crudes as foamy oil at normal temperature. The effects of foaming agent type and concentration, foam stabilizer type and concentration, oil–water volume ratio and temperature on the formation and stability of foamy oil were evaluated. The foamy oil properties and drag characteristics of foamy oil flow in small diameter pipes were investigated. The results indicate that the prepared stable foamy oil could be characterized as non-Newtonian power law fluid. The predicted pressure drops were in good agreement with the measured ones. The significant dynamic viscosity reduction rates were obtained. ã 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction The steady increase in global demand for oil and the depletion of conventional oil reserves has created a transition from conventional to non-conventional oil [1,2]. In this environment, heavy oil and oil sands are expected to become a major source of energy and could potentially extend the world’s energy reserves by 15 years [3] if they can be recovered and transformed into final products at a rate and price competitive with other energy sources [4]. However, the high viscosity and complicated composition make heavy oil much difficult and expensive to be produced, transported and refined [5]. In recent years, because of the high cost of heavy oil thermal recovery technology [6–10], cold heavy oil production technology has been successfully applied in field and has achieved a good development effect [11–13]. Among the non-thermal recovery techniques, solution-gas drive is one of the most efficient ways [14]. This method involves simultaneous-mixture flow of gas as very tiny bubbles entrained in heavy oil, which was later defined as foamy oil flow [14]. Several heavy oil reservoirs in the west Canada [15], Venezuela [16], China [17] and Oman [18] under solution gas drive have shown high oil production rates, slower production decline rates and higher primary recovery due to the formation of foamy heavy oil [14,19–21].
Many efforts have been made on this promising heavy oil production method for its low cost and high oil recovery [24–30]. However, almost all related researches for foamy oil focus on underground parts, rarely involving subsequent process of aboveground gathering and transportation. Common approaches for heavy oil transportation mainly include heating, dilution, emulsification and upgrading [31–34]. But the oil are generally needed to be wholly treated, and meanwhile they have their respective adaptability and shortage. High energy consumption always follows the heating or upgrading method, and a large amount of light oil or diluent is generally required for the blending method, and low transport efficiency and large treatment volume of waste water have hindered the popularization and application of the emulsification method [33,35]. Considering the great flowability of foamy oil which have improved the production performance of heavy oil reservoirs, and the broad prospects of cold heavy oil production, an idea that heavy oil is transported in the form of foamy oil at normal temperature was proposed. In this study, the factors affecting the formation and stability of the foamy oil were evaluated based on the synergy of the foaming agent and foam stabilizer, the properties of foamy oil and the drag characteristics of foamy oil pipe flow were experimentally investigated, hoping to provide a possibility for the new technology of cold heavy oil transportation in the future.
* Corresponding author at: Southwest Petroleum University, Chengdu 610500, China. Fax: +86 28 83032828. E-mail addresses: [email protected] (J. Sun), [email protected] (J. Jing). http://dx.doi.org/10.1016/j.jiec.2016.08.019 1226-086X/ã 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: J. Sun, et al., Pipeline transport of heavy crudes as stable foamy oil, J. Ind. Eng. Chem. (2016), http://dx.doi.org/ 10.1016/j.jiec.2016.08.019
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Materials and methods Materials The heavy crude oil sample, whose basic compositions and physical properties are listed in Table 1, was collected from J7 well area in Xinjiang oilfield in China. Nitrogen (N2), which is a colorless, odorless, tasteless, nontoxic, noncorrosive and nonflammable inert gas at ambient temperature and pressure, was used to prepare foamy oil in this study. Moreover, the water used is tap water, which was from Chengdu water supply company. According to the company’s water analysis report, the pH value and salinity are 7.32 and 132 mg/l, respectively. The foaming volume and foam stability of the foamy oil were directly influenced by the type and concentration of foaming agent and foam stabilizer. There are four kinds of common foaming agents in industrial applications, i.e. the anionic, the cationic, the nonionic and the amphoteric, in which the amphoteric surfactant is rarely applied for its high cost. In this paper, two kinds of cationic, anionic and nonionic surfactants were selected as foaming agents, and two kinds of anionic and three kinds of nonionic surfactants were selected as foam stabilizers, which are shown in Tables 2 and 3 respectively. Apparatus The simulation installation of foamy heavy oil flow, as shown in Fig. 1, was designed and assembled by the pipeline testing system and purge system. It has a maximum operating pressure of 1.0 MPa and can work between 10 and 90 C. The foamy oil was transported by a multiphase pump (Shanghai, China) with a maximum flowrate of 12 l/min, a maximum operating pressure of 3.0 MPa and a maximum operating temperature of 250 C, which has great suction performance and small pressure fluctuation. There are three 0.82 m long PVC test pipes with different internal diameters (D) of 4, 5 and 8 mm respectively. The purge system mainly includes a compressor, an air tank (a maximum pressure of 0.8 MPa) and a rotameter etc. The foamy oil storage tank, waste
liquid tank, and auxiliary piping were well insulated by glass wool and controlled by a temperature control system. XP-300C image analytical system (Shanghai, China) was used to capture and analyze foam micrographs. Anton Paar Rheolab QC viscometer (Graz, Austria) was adopted to test the rheological behaviors of the foam and the oil. A combined device of a F-400 homo-mixer (Foshan, China) with a maximum stirring rate of 8000 rpm and a CWYF-2 high temperature-high pressure reactor (Nantong, China) with a maximum operating temperature and pressure of 600 C and 50 MPa, was used to prepare foamy oil. A Shangping FA2104S electronic balance with an accuracy of 1/10,000 g (Shanghai, China) was used to weigh various samples. A Zhongxing digital thermostatic water bath (Shijiazhuang, China) was used as a temperature monitoring system when preparing foamy oil. Some measuring cylinders (1000 ml, 100 ml) and pipettes (100 ml) were used to evaluate the foam performance. Experimental procedure Preparation and performance evaluation of foamy oil Based on Waring Blender method, 50 ml water with the required proportion of surfactants and 50 ml heavy oil was added to a stirring cup and evenly mixed by a glass rod as the foaming fluid. Then the mixture was stirred for 15 min at a stirring rate of 4500 rpm in the reactor at 20 C and 0.1 MPa to prepare the desired foamy oil. Due to the poor flowability, the heavy oil was difficult to be wholly transferred from the measuring cylinder to the stirring cup after the volume was measured. Therefore, we measured the density of the heavy oil at 20 C and weighed the mass of the oil in accordance with the required volume in the follow-up experiments. The time for the bubbles coalescing to half of the original volume is recorded as the half-life t1=2 of the foam system [35], and the foaming volume V o and half-life t1=2 were used as two indexes for evaluating foamy oil property. To evaluate the comprehensive influences on the foaming fluid foamability, its foam composite index (FCI) can be calculated by the formula of FCI ¼ 0:75V o t1=2
Table 1 Basic properties and compositions of J7 crude oil. Viscosity at 50 C (mPa s)
Density at 20 C (kg/m3)
Bound water (wt.%)
Asphaltene (wt.%)
Wax (wt.%)
Resin (wt.%)
932
918.9
1.97
3.79
0.71
6.13
Table 2 Foaming agents used for foamy oil preparation. Surfactant
Code
HLB value
Ionicity
Provider
Cetyl trimethyl ammonium bromide Cetyltrimethylammonium chloride Sodium benzenesulfonat Sodium dodecyl sulfate Coconutt diethanol amide Octyl phenol ethoxylate
CATB CATC ABS SDS CDEA OP-10
16 15.8 10.6 40.0 15 14.5
Cation Cation Anion Anion Nonionic Nonionic
Shanghai Chemical Reagent Plant Shanghai Chemical Reagent Plant Chengdu Kelong Chemical Reagent Chengdu Kelong Chemical Reagent Chengdu Kelong Chemical Reagent Chengdu Kelong Chemical Reagent
Factory Factory Factory Factory
Table 3 Foam stabilizers used for foamy oil preparation. Surfactant
Code
Main composition
MW
Ionicity
Provider
Carboxy methyl cellulose sodium Polyacrylamide Hydroxyethyl cellulose Dodecanol SF-1 suspending agent
CMC PAM HEC Dod SF-1
Carboxy methyl cellulose sodium Polyacrylamide Hydroxyethyl Cellulose 1-Dodecanol Acrylic polymer
264.204 3 108 2.5 105 186 –
Anion Anion Nonionic Nonionic Nonionic
Chengdu Kelong Chemical Reagent Chengdu Kelong Chemical Reagent Chengdu Kelong Chemical Reagent Chengdu Kelong Chemical Reagent Guangzhou Feirui Chemical Ltd.
Factory Factory Factory Factory
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3
Fig. 1. Simulation installation of foamy heavy oil flow.
[36], where the units of V o and t1=2 are milliliter (ml) and minute (min) separately. The larger the FCI value, the stronger its comprehensive performance. According to calculated FCI values, the comprehensive performance of foam system can be divided into four grades, i.e. superior grade (>15,000), good grade (10,000– 15,000), medium grade (5000–10,000), poor grade (<5000) [37]. Simulation experiment of flow characteristics The flow characteristics of foamy oil flowing through the three test sections were investigated at different temperatures and flow rates. Pressure drops and flow rates were monitored and acquired in real time during the testing by a Rosemount differential pressure transmitter and a Krohne mass flow meter. In order to reduce accidental errors, the temperatures at the inlet and outlet of the test sections were measured by the temperature control device and the average values of the two were taken as the final effective values. After the end of each experiment, appropriate diesel was pumped into the pipe for 3–5 min to fully dissolve the residual oil on the inner pipe wall, and then the pipe was further cleaned by the purge system. Data processing The liquid volume in foamy oil V l is the original volume of the foaming fluid. The foaming volume V o is the sum of gas volume V g and liquid volume V l . Therefore, the gas volume fraction in foamy oil, namely the foamy oil quality G can be calculated by Eq. (1):
G¼
100V g 100ðV o V l Þ %¼ % Vo Vg þ Vl
ð1Þ
In order to reduce accidental errors, 3 sets of experimental data for each test point were obtained and the average value was taken as the effective foamy oil quality. Results and discussion
of 1.5 g/l and oil–water volume ratio of 1:1 at 20 C was preliminarily prepared based on the experience. The effects of different types of foaming agents on foaming capacity of 100 ml foaming fluid are shown in Table 4. Both nonionic and cationic surfactants (OP-10 and CDEA, CATB and CATC) show poor foaming capacity, the foaming volumes are small and the bubbles have an uneven distribution. On the contrary, SDS and ABS have good compatibility with J7 heavy oil, both of the foaming volumes are more than 360 ml and the produced foamy oil demonstrates dispersive spherical bubbles. What’s more, the drainage hardly adheres to the wall of the measuring cylinder after experiments and the two anionic surfactants show good lubricating effects. Under the same condition, SDS shows a little larger foaming volume than ABS, but ABS presents a longer half-life and bigger FCI value, which indicates that the foamy oil with ABS is probably more stable than that prepared with SDS. Therefore, ABS is used as the foaming agent in the following experiments. Effect of foaming agent concentration The effect of ABS concentration on foamy oil performance was investigated with oil–water volume ratio of 1:1 at 20 C. As shown in Fig. 2, with the increase of ABS concentration, the foaming volume continually increases and reaches the maximum at 1.5 g/l, and then levels off. The half-lives at 1 and 1.5 g/l are close to each other, while the FCI values increase to the maximum at 1.5 g/l and then rapidly decrease. The reason may be that when the concentration of the additive surfactant is less than that of the critical micelle concentration (CMC), the adsorbing amount of the surfactant in gas bubble film is also low. As the foaming agent increases, the adsorbing amount increases and the surface tension will be reduced, which eventually results in an enhancement in foaming capacity and foam stability of the foaming fluid. When the surfactant concentration continues to increase, the adsorption reaches
Composition of foaming fluid Evaluation of foaming agent type In the experiments, parts of the selected foaming agents exhibited lower comprehensive performances at higher concentrations. Thus the foamy oil with each foaming agent concentration
Table 4 Performance of 100 ml foaming fluid with different types of foaming agents. Parameter
Foaming volume V0 (ml) Half-life t1/2 (min) FCI value (ml min)
Foaming agent CATC
CATB
OP-10
CDEA
ABS
SDS
120 – –
110 – –
110 – –
112 – –
375 78 21938
385 38 10973 Fig. 2. Effect of foaming agent concentration on foamy oil performance.
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saturation and the surface tension can not be further reduced, the foaming volume almost remains unchanged. But when the surfactant concentration is too high, the liquid content in bubble film decreases, the “fragility” of foam increases and the Marangoni effect gradually weakens, finally resulting in reduced foam stability. Therefore, the concentration of ABS for preparing foamy oil in the following experiments is determined as 1.5 g/l. Evaluation of foam stabilizer type High viscosity of heavy oil itself can enhance the film strengths of the bubbles in foamy oil. The addition of different types of foam stabilizers often leads to electrostatic attraction or repulsion, chemical and physical interactions between the heavy oil, foaming agent and foam stabilizer. The foaming fluid with foam stabilizer concentration of 2.0 g/l was prepared. The effect of different types of foam stabilizers on foamy oil performance are shown in Fig. 3. As can be seen in Fig. 3, all the foaming volumes reduce with the addition of five foam stabilizers in the foaming fluid. The main reason may be that the additive foam stabilizers limit the free movement of the foaming agent molecules to some extent. The tackifying stabilizers (CMC and PAM), instead of increasing the stability of the foamy oil, make the FCI values greatly reduced. This is mostly likely that the action mechanism of these two foam stabilizers is to improve the stability of the bubble by increasing the viscosity of the liquid film. But the effects are not obvious because the liquid film viscosity has been high enough, and the compatibility between the foaming agents and J7 oil is poor. In addition, when HEC and Dod were added, the changes on half-life were not noticeable, and the FCI values also reduced. Amazingly, the half-life of the foamy oil with SF-1 can increase to 170 min and the FCI value can reach more than 30,000 (superior grade). Therefore, SF-1 is used as the foam stabilizer in follow-up study. Effect of foam stabilizer concentration The SF-1 concentration required for stable foamy oil was investigated with ABS concentration of 1.5 g/l and oil–water volume ratio of 1:1 at 20 C. As shown in Fig. 4, with the increase of SF-1 concentration, the foaming volume continues to reduce, and the half-life and FCI value increase at first and then decrease. The reason may probably lie in that, at low concentration of SF-1, the interaction of SF-1 with the oil and foaming agent improved the arrangement of active molecules in liquid film. But when the concentration is too high, some stable micelles can easily form in the internal of the foamy oil, which will weaken the self-recovering ability of bubble film. This is to say when the bubble film is deformed, but the active molecules are bound in micelles, and can not move to the deformation place to repair the bubble film, which ultimately results in the decrease of the foam oil stability. Comparatively speaking, the foamy oil shows a better stability at SF-1 concentration of 2.5 g/l.
Fig. 3. Performance of 100 ml foaming fluid with different types of foam stabilizers.
Fig. 4. Effect of foam stabilizer concentration on foamy oil performance.
Effect of oil–water volume ratio Fig. 5 shows the effect of oil–water ratio on foaming volume, half-life and FCI value of 100 ml foaming fluid at 20 C. The foaming volume continues to increase with the decrease of oil–water ratios. This can be attributed to that high water content makes both the viscosity of foaming fluid and the energy required for preparing foamy oil decrease, thus the foaming volume will inevitably increase when same mechanical energy is introduced. With oil– water ratio decreasing, the half-life and FCI value increase at first and then decrease rapidly. Both the half-life and FCI value reach the maximum at oil–water ratio of 5:5. Based on the foregoing analysis, the foamy oil prepared with 1.5 g/l ABS, 2.5 g/l SF-1 at 1:1 oil–water volume ratio shows the best stability. What’s more, combining with the test method of rheometer and flow loop (L = 40 m, D = 25 mm) [38], we found that the measured phase inversion point of J7 oil–water emulsion was around 60% at different temperatures. Meanwhile, based on the subsequent analysis of foamy oil microstructure, it is indicated that the foaming fluid with oil–water ratio of 5:5 behaves as water in oil emulsion, which may be due to the high viscosity and complicated compositions of the heavy crude oil and the effects of the added surfactants. Currently, the crude oil are often produced with a large amount of water at the late stage production of many oilfields, which provides favorable conditions for the field application of heavy oil transported in the form of foamy oil. Effect of temperature The stability of the foamy oil is greatly influenced by temperature T. As shown in Fig. 6, the foaming volume is nearly unchanged, but the half-life and FCI value continually decrease with the increase of temperature. The main reason may be that, on
Fig. 5. Effect of oil–water volume ratio on foamy oil performance.
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Fig. 6. Effect of temperature on foamy oil performance.
the one hand, the increase of temperature makes the spaces between molecules of the foaming fluid increased and enhances the molecular motion, which ultimately results in the weakening of intermolecular forces and dispersive arrangement of surfactant molecules. On the other hand, as temperature increases, the viscosity of foaming solution decreases, which furtherly accelerates the drainage rate of the bubble film. For foamy oil, the higher the temperature, the better the mobility, but the worse the stability. Foamy oil property Foamy oil quality Blauer et al. first divided the foam fluid into three regions based on foam quality, i.e. dispersion region (0–0.52), interference (0.52–0.74) and deformation (0.74–1.00) [39]. Afterwards, he found the Mitchell quality range existed between 0.5236 and 0.9999 and preferably from 0.60 to 0.85, in which the foam is a homogenous mixture with uniform bubble sizes and could be stable for several hours [40]. Foamy oil qualities under different conditions were calculated according to Eq. (1) and shown in Table 5. Only the anionic surfactants (ABS and SDS) show good foaming capacity. With the increase of ABS concentration, the foamy oil quality exceeds 73% at 1.5 g/l and then levels off. The addition of the foam stabilizer makes the foamy oil quality decrease, and the higher the foam stabilizer concentration, the lower the foamy oil quality. But the quality is still more than 60% when SF-1 concentration reaches 3.0 g/l. What’s more, as the oil–water ratio decreases, the foaming volume increases gradually. Moreover, the temperature has smaller effect on the foamy oil quality. Rheological behavior The rheological curves of the foamy oil were measured under the shear rate range from 0 to 100 1/s at 20–30 C, which shows the
5
rheological behavior of a non-Newtonian fluid. As shown in Table 6, the curves can be well described by power law mode at 20 and 25 C, and all the correlation coefficients R2 are more than 0.99. But when the temperature reaches 30 C, the correlation coefficient slightly decreases. It may be that the stability of the foamy oil decreases with increase of temperature, and rotation of the rotor in rheometer accelerates drainage of foam films. Overall, the test results are in poor agreement with Bingham model, and the maximum correlation coefficient is only 0.9872. The apparent viscosities at 100 1/s calculated according to the obtained fitted rheological equations are less than 120 mPa s for power law model. In addition, for both power law model and Bingham model, all the apparent viscosities basically decreases with the increase of temperature and all the viscosity reduction rates of the foamy oil are more than 97%. The sources of foamy oil viscosity are main from the internal friction from relative motion of liquid layers and the collision and extrusion between dispersed bubbles. Studies have also shown that bubbles could be elongated and gradually tend to be neatly arranged after the foamy oil being sheared, which could reduce the probability of bubbles colliding with each other and then reduce the apparent viscosity [41]. Microstructure The micrographs of the foamy oil were captured by XP-300C image analytical system, and the microstructures in different vision fields are shown in Fig. 7. Based on the different light transmittance, it can be deduced that the white spherical bubbles are gas and water, and the black parts are heavy oil drops. The bubbles are linked together at numerous points. Two-dimensional diameters and areas of the bubbles in pictures were collected by using the MiVnt image analysis system, and then the equivalent three-dimensional average diameter and number of bubbles could be obtained. The bubble size is in accord with normal distribution and the bubble diameter are basically between 60 and 300 mm. The bubbles with a diameter of about 150 mm are the most, while the probability of occurrence are the highest (Fig. 8). At present, the decay mechanism of foam fluid is generally believed to lie in two aspects: the liquid film drainage and the diffusion of gas through liquid film [42]. The pressure in small bubbles is higher than that in large bubbles, which will make the gas in small bubbles diffuse to large ones until the small bubbles disappear, and large bubbles continue to expand and rupture. Therefore, the more uniform the bubble size distribution in foamy oil, the lower degree the bubble diffusion occurs, and then the longer the life of the bubble and the more stable the foamy oil. The normal distribution curve of the bubble in foamy oil is “tall”, which shows the bubble size distribution is narrow. The bubble size is relatively uniform and the diffusion rate between bubbles is slow. This may also be one of internal factors for the good stability of the foamy oil prepared in this study. Characteristics of foamy oil flow
Table 5 Foamy oil quality under different conditions. Foaming agent (1.5 g/l)
CATC
CATB
OP-10
CDEA
ABS
SDS
Foamy oil quality Γ (%) ABS concentration (g/l) Foamy oil quality Γ (%) SF-1 concentration (g/l) Foamy oil quality Γ (%) Oil–water volume ratio Foamy oil quality Γ (%) Temperature ( C) Foamy oil quality Γ (%)
16.7 0.5 25.9 0 73.3 7:3 34.2 20 63.0
9.1 1.0 65.5 1.5 68.1 6:4 47.4 30 59.2
9.1 1.5 73.3 2.0 64.9 5:5 63.0 40 58.3
10.7 2.0 73.0 2.5 63.0 4:6 70.1 50 57.4
73.3 3.0 72.6 3.0 62.0 3:7 73.8 60 –
74.0 – – – – – – – –
Rheological model Generally, the rheology of foamy oil in pipe flow is much different with that tested by a rheometer. Thus the rheological model of foamy oil flow should be investigated first in order to predict its frictional resistance. As we all know, it is much difficult to accurately describe the rheological property of flowing foamy oil for its multi-phase composition and changing structure. At present, there are main two models, namely power law model [43] and Bingham model [44], that can be used to describe the rheological behavior of foamy fluid. Furtherly, assuming dispersive distribution of bubbles in the oil, most of researchers use
Please cite this article in press as: J. Sun, et al., Pipeline transport of heavy crudes as stable foamy oil, J. Ind. Eng. Chem. (2016), http://dx.doi.org/ 10.1016/j.jiec.2016.08.019
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Table 6 Fitting results of rheological curves of the foamy oil. T
Power law model
20 25 30
Bingham model
Rheological equation
R
AV
VRR
Rheological equation
R2
AV
VRR
t ¼ 3:1658g_ t ¼ 0:7679g_ 0:5743 t ¼ 0:8186g_ 0:5321
0.9943
114.42
98.90
0.9410
152.82
98.53
0.9986
108.12
98.33
0.9550
140.48
97.83
0.9863
94.90
97.83
t ¼ 9:7615 þ 0:0552g_ t ¼ 7:0782 þ 0:0697g_ t ¼ 7:1868 þ 0:0382g_
0.9872
110.07
97.48
0:2790
2
Note: T, temperature ( C); AV, apparent viscosity at 100 1/s (mPa s); VRR, viscosity reduction rate (%).
Fig. 7. Microstructures of the foamy oil at 20 C.
homogenous flow model to predict the pressure drop of foamy oil flow. In this study, both power law model and Bingham model were used to explore a more appropriate rheological model for the foamy heavy oil flow. (1) Power law model Assuming the foamy oil is a power law fluid, and its constitutive equation is shown as follows: n du t¼K ð2Þ dr where t is the internal friction stress per unit area, K is the consistency coefficient, n is the flow pattern index, u is the velocity of the pipe flow and r is the arbitray radius of the pipe. Substituting Eq. (2) into the shear stress equation t ¼ D2Lpr, and the velocity distribution equation can be calculated as follows: 1 du 1 Dp r n ¼ dr K L 2
ð3Þ
where Dp is the pressure drop, L is the length of liquid column.
Integrating the above equation, the velocity calculation formula can be derived as: u¼
Dp
1n Z
2KL
r R
1
rn ¼
Dp
1n
2KL
h i nþ1 nþ1 n R n r n nþ1
ð4Þ
where R is the pipe radius. And the flow rate calculation formula can be furtherly integrated as blew: Z Q¼
R 0
u2prdr ¼
1n
pnR3 DpR
ð5Þ
3n þ 1 2KL
where Q is the volumetric flow rate. For the average velocity in the cross-section v ¼ QA , the relation between pressure drop and average velocity can be obtained as following: DpD 8v 1 þ 3n n ¼K ð6Þ 4L D 4n where A is the cross-sectional area and D is the internal pipe diameter. Taking the logarithm on both sides of Eq. (6) DpD 1 þ 3n n 8v ¼ lg K þ nlg ð7Þ lg 4L 4n D Based on the measured pressure drops Dp and flow rates Q, the fitted curves can be plotted in terms of the wall-shear stress pD lg D4L and the pipe flow parameter lg 8v D . The slope of the line is 0
defined as n0 , and the intercept is defined as k . 8 0 3n þ 1 n < K ¼ 10k = 4n : n ¼ n0
ð8Þ
The generalized Reynolds number Re0 for power law fluid flow can be defined as below, and 2000 is taken as the critical value. Re0 ¼
Dn v2n r m
ð9Þ
Fig. 8. Distribution of bubble diameters in foamy oil at 20 C.
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where m is a self-defined parameter, which can be calculated by n and r is the density of the fluid in pipe. the formula m ¼ K8 6nþ2 n (2) Bingham model The constitutive equation of Bingham fluid is shown as follows: du t ¼ t 0 þ hp ð10Þ dr where t 0 is the limiting shear stress, hp is the structural viscosity. Substituting Eq. (10) into the shear stress equation t ¼ D2Lpr, and the velocity distribution equation can be calculated as follows: du Dp ¼ ðr r 0 Þ dr 2Lhp
ð11Þ
7
Integrating Eq. (11), and the velocity distribution equation in the gradient region can be derived as: u¼
i Dp h ðR r0 Þ2 ðr r0 Þ2 4Lhp
ð12Þ
Then the flow rate in the gradient region Q1 can be furtherly obtained as below: Z R u2prdr Q1 ¼ r0 # " pDp R4 2R3 r0 5r0 4 2 2 3 ð13Þ R r0 þ 2Rr0 ¼ 4Lhp 2 3 6
where r0 is the radius of the core region of the flow.
Fig. 9. Foamy oil flow characteristic curves fitted by power law and Bingham models.
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When r ¼ r0 , the velocity in the core region v0 can be expressed as: v0 ¼
Dp ðR r0 Þ2 4Lhp
ð14Þ
And then integrating the above equation, the flow rate in the core region Q 0 can be obtained as follows: Q 0 ¼ pr 0 2 v 0 ¼
pDp 2 2 R r0 2Rr0 3 þ r40 4Lhp
ð15Þ
The total flow rate Q is the sum of the flow rates in the gradient and core region. pR4 Dp 4 r0 1 r40 þ 1 ð16Þ Q ¼ Q0 þ Q1 ¼ 8Lhp 3 R 3 R4 For Dp0 R ¼ Dpr0, Eq. (16) can be furtherly deformed as the following equation: pR4 Dp 4 Dp0 1 Dp0 4 þ Q¼ 1 ð17Þ 8Lhp 3 Dp 3 Dp4 where Dp0 is the initial pressure drop. When the core region of
the
pipe
flow
is
small,R4 r40 ,Dp4 Dp40 , so the simplified equation can be obtained as following by neglecting the higher order terms. pR4 4 Dp Dp0 ð18Þ Q¼ 3 8Lhp Similarly, for the average velocity in the cross-section v ¼ QA , Eq. (18) can be deformed as below:
Dp L
¼
32hp D2
16 t 0 vþ 3 D
ð19Þ
Substituting the tested values of pressure drop Dp and flow rate Q into Eq. (19), the fitted curves can be constructed in the pressure drop gradient DLp versus the average velocity v. The slope of the line 00
is defined as n00 , and the intercept is defined as k . 8 2 00 > > < hp ¼ D n 32 00 > > : t 0 ¼ 3Dk 16
ð20Þ
The generalized Reynolds number for plastic fluid flow Re0 can be defined as below, and 2000 is also taken as the critical value. Re0 ¼
rvD hp 1 þ 6th0pDv
ð21Þ
As shown in Fig. 9, the rheological behavior of the foamy oil in pipe flow can be better described by power law model than Bingham model, all the correlation coefficients of the curves fitted by the former model are more than 0.99, but most of the correlation coefficients of the curves fitted by the latter model basically maintain at about 0.97. At the same pipe diameter, with the increase of temperature, the consistency coefficient K decreases and the flow pattern index n varies irregularly when the prepared foamy oil are treated as power law fluid. Meanwhile, both of the limiting shear stress and structural viscosity reduce when the foamy oil regarded as Bingham fluid. At the same temperature, with increasing the pipe diameter, neither of the consistency coefficient and flow pattern index show any regularity in variation for the power law fluid, but the limiting shear stress decreases and the structural viscosity increases for the Bingham fluid. In addition, all the calculated generalized Reynolds numbers at different experimental conditions are less than 400, which demonstrates a laminar flow. Drag characteristic The friction drag coefficient is a key parameter for the calculation of resistance loss of fluid flow. The Darcy friction coefficient l or the Fanning friction coefficient f (f ¼ l=4) are generally used in engineering and laboratory experiments.
l¼
8t w rv 2
ð22Þ
where t w is the shear stress at the pipe wall. The friction loss is actually the energy loss of the sheared fluid on the macro-performance, thus the shear rate at the pipe wall g_ w is also a significant parameter for measuring the drag characteristics of fluid flows. For power law fluid, the wall-shear rate can be calculated as: 3n þ 1 n 8v n g_ w ¼ K ð23Þ 4n D For Bingham fluid, the wall-shear rate can be expressed as: 4 3
g_ w ¼ t 0 þ hp
8v D
ð24Þ
Fig. 10. Friction drag coefficient versus wall-shear rate.
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Fig. 11. Friction drag coefficient versus generalized Reynolds number. Table 7 Viscosity reduction effect of J7 heavy oil at 30 C. Different oils
Emulsified heavy oil (50 vol.% water) Heavy oil Foamy oil Foamy oil Foamy oil Foamy oil
Viscosity testing device
Rotational rheometer Rotational rheometer Rotational rheometer Simulation installation (D = 8 mm) Simulation installation (D = 5 mm) Simulation installation (D = 4 mm)
The changing regularity that the friction drag coefficient l varies with the wall-shear rate g_ w is similar for the power law and Bingham fluid at the same condition. Therefore, the changing curves for power law fluid are taken as example and shown in Fig. 10. All the friction drag coefficients gradually decrease with the increase of wall shear rates. In addition, as shown in Fig. 10a, the friction drag coefficient reduces with increasing temperature at the same pipe diameter. This can be contributed to that the friction drag coefficient is positively correlated with pressure gradient, and the pressure gradient of foamy oil flow reduces with increasing temperature. Fig. 10b shows the effect of pipe diameter on the friction drag coefficient. At the same temperature and flow velocity, the flow pattern index n deceases with deceasing the internal diameter D, thus the smaller the pipe diameter, the larger the wall shear rate. But at the same temperature and inner diameter, the wall shear rate increases with increasing the flow velocity. What’s more, the l decreases with the increase of internal diameter at the same wall shear rate. This is due to that the larger the pipe diameter, the higher the average velocity in the cross-section, thus the smaller the friction drag coefficient. Numerous studies have been conducted on the prediction of the friction drag coefficient l [45–47]. It is generally believed that the l for the power law and Bingham fluid at laminar flow can be 64 calculated be the theoretical formula l ¼ Re 0 . In this experiment, we found that the fitting equation was in very good agreement with the theoretical formula when the foamy oil was treated as Power law fluid (Fig. 11a), but the goodness of fit decreased obviously when we considered the foamy oil as Bingham fluid (Fig. 11b). Substituting the fitted friction drag coefficient into the Dp equation l ¼ 2D rLv2 , 189 sets of pressure drops of foamy oil flow
under different flow conditions were calculated. Comparatively speaking, the prediction accuracy of the power-law model is higher
Viscosity or apparent viscosity at different shear rates (mPa s) 100 1/s
1000 1/s
3000 1/s
12961.4 4367.4 94.9 46.5 30.4 30.3
9342.3 4367.1 35.4 35.6 23.3 23.0
7991.1 4366.3 22.2 33.2 21.7 20.4
than the Bingham model, the absolute maximum relative errors of the two models can be up to 13.4% and 35.8% respectively, and their respective absolute average relative errors are 4.3% and 6.3%. It is furtherly proved that the properties of the foam oil are more close to that of the power law fluid. Viscosity reduction effect The apparent viscosity of foam fluid is affected by shear rate, foam quality, temperature, pressure, bubble size, etc. In this study, viscosities of J7 heavy oil, and apparent viscosities of the emulsified heavy oil and foamy oil at 30 C were measured or calculated at different shear rates (Table 7). As shown in Table 7, there exist some differences in apparent viscosities of the foamy oil tested by rheometer and small-scale flow loop (D 8 mm), but the apparent viscosity of the foamy oil prepared by Waring Blender method is far lower than that of heavy oil, and the minimum viscosity-reducing rate exceeds 97%. The foamy oil prepared in this experiment contains 50 volume percent of water, and all the apparent viscosities tested by rheometer or flow loop apparatus are less than 100 mPas. Comparing with the emulsified J7 heavy oil with the same water content, the viscosity reduction rates are more than 99.5%. This is probably because the surfactants and bubbles changed the internal structure and greatly improved the flowability of the foamy oil. Conclusions Based on the synergy of foaming agents and foam stabilizers, a kind of stable foamy heavy oil was prepared with 1.5 g/l ABS, 2.5 g/l SF-1, oil–water volume ratio of 1 and nitrogen volume fraction of 63% at 20 C and 0.1 MPa. The foaming volume and half-life of 100 ml foaming fluid can reach 270 ml and 274 min respectively. The foamy oil quality is in the Mitchell quality range of foam fluid. In addition, the bubbles, namely the gas bubbles and water
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droplets in the oil are dispersed in spherical shape and the bubble size distribution is mainly maintained at about 150 mm. Through rheological and flow loop experiments, it is found that the prepared foamy oil can be characterized as non-Newtonian power law fluid. The friction drag coefficient decreases with increasing the shear rate, temperature and pipe diameter, and its fitted empirical formula is in very good agreement with the theoretical equation l ¼ 64=Re0 . The pressure drops predicted by the power-law model and the measured values coincide well with each other, and the absolute relative errors are within 13.4%. What’s more, compared with heavy crudes, the viscosity of the foamy oil decreases significantly, and the viscosity-reducing rates are more than 97%. For plenty of microbubbles existing in the foamy oil, the efficiency of pipeline transportation of heavy oil may be reduced at the same flow rate, but subsequent degassing and dehydration problems still need to be solved. Overall, this study confirms the effectiveness of transportation of heavy crudes as stable foamy oil in small diameter pipes, and the dosage of additive may be greatly reduced compared with the emulsification method. Future investigations will focus on the maintenance of dynamic stability of foamy oil in large diameter pipeline over long distances and its popularization and application in the oil field. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant no. 51074136), Research Fund for the Doctoral Program of Higher Education of China (Grant no. 20115121110004) and Science and Technology Project of Sichuan Province (Grant no. 2015JY0099). References [1] [2] [3] [4]
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Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Pipeline transport of heavy crudes as stable foamy oil Jie Suna , Jiaqiang Jinga,b,* , Cheng Wua , Fei Xiaoa , Xiaoxuan Luoa a b
School of Oil & Natural Gas Engineering, Southwest Petroleum University, Chengdu 610500, China Oil & Gas Fire Protection Key Laboratory of Sichuan Province, Chengdu 611731, China
A R T I C L E I N F O
Article history: Received 15 June 2016 Received in revised form 18 August 2016 Accepted 20 August 2016 Available online xxx Keywords: Foamy heavy oil Cold transportation Stability Drag characteristics Viscosity reduction
A B S T R A C T
A new idea was proposed to transport heavy crudes as foamy oil at normal temperature. The effects of foaming agent type and concentration, foam stabilizer type and concentration, oil–water volume ratio and temperature on the formation and stability of foamy oil were evaluated. The foamy oil properties and drag characteristics of foamy oil flow in small diameter pipes were investigated. The results indicate that the prepared stable foamy oil could be characterized as non-Newtonian power law fluid. The predicted pressure drops were in good agreement with the measured ones. The significant dynamic viscosity reduction rates were obtained. ã 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction The steady increase in global demand for oil and the depletion of conventional oil reserves has created a transition from conventional to non-conventional oil [1,2]. In this environment, heavy oil and oil sands are expected to become a major source of energy and could potentially extend the world’s energy reserves by 15 years [3] if they can be recovered and transformed into final products at a rate and price competitive with other energy sources [4]. However, the high viscosity and complicated composition make heavy oil much difficult and expensive to be produced, transported and refined [5]. In recent years, because of the high cost of heavy oil thermal recovery technology [6–10], cold heavy oil production technology has been successfully applied in field and has achieved a good development effect [11–13]. Among the non-thermal recovery techniques, solution-gas drive is one of the most efficient ways [14]. This method involves simultaneous-mixture flow of gas as very tiny bubbles entrained in heavy oil, which was later defined as foamy oil flow [14]. Several heavy oil reservoirs in the west Canada [15], Venezuela [16], China [17] and Oman [18] under solution gas drive have shown high oil production rates, slower production decline rates and higher primary recovery due to the formation of foamy heavy oil [14,19–21].
Many efforts have been made on this promising heavy oil production method for its low cost and high oil recovery [24–30]. However, almost all related researches for foamy oil focus on underground parts, rarely involving subsequent process of aboveground gathering and transportation. Common approaches for heavy oil transportation mainly include heating, dilution, emulsification and upgrading [31–34]. But the oil are generally needed to be wholly treated, and meanwhile they have their respective adaptability and shortage. High energy consumption always follows the heating or upgrading method, and a large amount of light oil or diluent is generally required for the blending method, and low transport efficiency and large treatment volume of waste water have hindered the popularization and application of the emulsification method [33,35]. Considering the great flowability of foamy oil which have improved the production performance of heavy oil reservoirs, and the broad prospects of cold heavy oil production, an idea that heavy oil is transported in the form of foamy oil at normal temperature was proposed. In this study, the factors affecting the formation and stability of the foamy oil were evaluated based on the synergy of the foaming agent and foam stabilizer, the properties of foamy oil and the drag characteristics of foamy oil pipe flow were experimentally investigated, hoping to provide a possibility for the new technology of cold heavy oil transportation in the future.
* Corresponding author at: Southwest Petroleum University, Chengdu 610500, China. Fax: +86 28 83032828. E-mail addresses: [email protected] (J. Sun), [email protected] (J. Jing). http://dx.doi.org/10.1016/j.jiec.2016.08.019 1226-086X/ã 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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Materials and methods Materials The heavy crude oil sample, whose basic compositions and physical properties are listed in Table 1, was collected from J7 well area in Xinjiang oilfield in China. Nitrogen (N2), which is a colorless, odorless, tasteless, nontoxic, noncorrosive and nonflammable inert gas at ambient temperature and pressure, was used to prepare foamy oil in this study. Moreover, the water used is tap water, which was from Chengdu water supply company. According to the company’s water analysis report, the pH value and salinity are 7.32 and 132 mg/l, respectively. The foaming volume and foam stability of the foamy oil were directly influenced by the type and concentration of foaming agent and foam stabilizer. There are four kinds of common foaming agents in industrial applications, i.e. the anionic, the cationic, the nonionic and the amphoteric, in which the amphoteric surfactant is rarely applied for its high cost. In this paper, two kinds of cationic, anionic and nonionic surfactants were selected as foaming agents, and two kinds of anionic and three kinds of nonionic surfactants were selected as foam stabilizers, which are shown in Tables 2 and 3 respectively. Apparatus The simulation installation of foamy heavy oil flow, as shown in Fig. 1, was designed and assembled by the pipeline testing system and purge system. It has a maximum operating pressure of 1.0 MPa and can work between 10 and 90 C. The foamy oil was transported by a multiphase pump (Shanghai, China) with a maximum flowrate of 12 l/min, a maximum operating pressure of 3.0 MPa and a maximum operating temperature of 250 C, which has great suction performance and small pressure fluctuation. There are three 0.82 m long PVC test pipes with different internal diameters (D) of 4, 5 and 8 mm respectively. The purge system mainly includes a compressor, an air tank (a maximum pressure of 0.8 MPa) and a rotameter etc. The foamy oil storage tank, waste
liquid tank, and auxiliary piping were well insulated by glass wool and controlled by a temperature control system. XP-300C image analytical system (Shanghai, China) was used to capture and analyze foam micrographs. Anton Paar Rheolab QC viscometer (Graz, Austria) was adopted to test the rheological behaviors of the foam and the oil. A combined device of a F-400 homo-mixer (Foshan, China) with a maximum stirring rate of 8000 rpm and a CWYF-2 high temperature-high pressure reactor (Nantong, China) with a maximum operating temperature and pressure of 600 C and 50 MPa, was used to prepare foamy oil. A Shangping FA2104S electronic balance with an accuracy of 1/10,000 g (Shanghai, China) was used to weigh various samples. A Zhongxing digital thermostatic water bath (Shijiazhuang, China) was used as a temperature monitoring system when preparing foamy oil. Some measuring cylinders (1000 ml, 100 ml) and pipettes (100 ml) were used to evaluate the foam performance. Experimental procedure Preparation and performance evaluation of foamy oil Based on Waring Blender method, 50 ml water with the required proportion of surfactants and 50 ml heavy oil was added to a stirring cup and evenly mixed by a glass rod as the foaming fluid. Then the mixture was stirred for 15 min at a stirring rate of 4500 rpm in the reactor at 20 C and 0.1 MPa to prepare the desired foamy oil. Due to the poor flowability, the heavy oil was difficult to be wholly transferred from the measuring cylinder to the stirring cup after the volume was measured. Therefore, we measured the density of the heavy oil at 20 C and weighed the mass of the oil in accordance with the required volume in the follow-up experiments. The time for the bubbles coalescing to half of the original volume is recorded as the half-life t1=2 of the foam system [35], and the foaming volume V o and half-life t1=2 were used as two indexes for evaluating foamy oil property. To evaluate the comprehensive influences on the foaming fluid foamability, its foam composite index (FCI) can be calculated by the formula of FCI ¼ 0:75V o t1=2
Table 1 Basic properties and compositions of J7 crude oil. Viscosity at 50 C (mPa s)
Density at 20 C (kg/m3)
Bound water (wt.%)
Asphaltene (wt.%)
Wax (wt.%)
Resin (wt.%)
932
918.9
1.97
3.79
0.71
6.13
Table 2 Foaming agents used for foamy oil preparation. Surfactant
Code
HLB value
Ionicity
Provider
Cetyl trimethyl ammonium bromide Cetyltrimethylammonium chloride Sodium benzenesulfonat Sodium dodecyl sulfate Coconutt diethanol amide Octyl phenol ethoxylate
CATB CATC ABS SDS CDEA OP-10
16 15.8 10.6 40.0 15 14.5
Cation Cation Anion Anion Nonionic Nonionic
Shanghai Chemical Reagent Plant Shanghai Chemical Reagent Plant Chengdu Kelong Chemical Reagent Chengdu Kelong Chemical Reagent Chengdu Kelong Chemical Reagent Chengdu Kelong Chemical Reagent
Factory Factory Factory Factory
Table 3 Foam stabilizers used for foamy oil preparation. Surfactant
Code
Main composition
MW
Ionicity
Provider
Carboxy methyl cellulose sodium Polyacrylamide Hydroxyethyl cellulose Dodecanol SF-1 suspending agent
CMC PAM HEC Dod SF-1
Carboxy methyl cellulose sodium Polyacrylamide Hydroxyethyl Cellulose 1-Dodecanol Acrylic polymer
264.204 3 108 2.5 105 186 –
Anion Anion Nonionic Nonionic Nonionic
Chengdu Kelong Chemical Reagent Chengdu Kelong Chemical Reagent Chengdu Kelong Chemical Reagent Chengdu Kelong Chemical Reagent Guangzhou Feirui Chemical Ltd.
Factory Factory Factory Factory
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Fig. 1. Simulation installation of foamy heavy oil flow.
[36], where the units of V o and t1=2 are milliliter (ml) and minute (min) separately. The larger the FCI value, the stronger its comprehensive performance. According to calculated FCI values, the comprehensive performance of foam system can be divided into four grades, i.e. superior grade (>15,000), good grade (10,000– 15,000), medium grade (5000–10,000), poor grade (<5000) [37]. Simulation experiment of flow characteristics The flow characteristics of foamy oil flowing through the three test sections were investigated at different temperatures and flow rates. Pressure drops and flow rates were monitored and acquired in real time during the testing by a Rosemount differential pressure transmitter and a Krohne mass flow meter. In order to reduce accidental errors, the temperatures at the inlet and outlet of the test sections were measured by the temperature control device and the average values of the two were taken as the final effective values. After the end of each experiment, appropriate diesel was pumped into the pipe for 3–5 min to fully dissolve the residual oil on the inner pipe wall, and then the pipe was further cleaned by the purge system. Data processing The liquid volume in foamy oil V l is the original volume of the foaming fluid. The foaming volume V o is the sum of gas volume V g and liquid volume V l . Therefore, the gas volume fraction in foamy oil, namely the foamy oil quality G can be calculated by Eq. (1):
G¼
100V g 100ðV o V l Þ %¼ % Vo Vg þ Vl
ð1Þ
In order to reduce accidental errors, 3 sets of experimental data for each test point were obtained and the average value was taken as the effective foamy oil quality. Results and discussion
of 1.5 g/l and oil–water volume ratio of 1:1 at 20 C was preliminarily prepared based on the experience. The effects of different types of foaming agents on foaming capacity of 100 ml foaming fluid are shown in Table 4. Both nonionic and cationic surfactants (OP-10 and CDEA, CATB and CATC) show poor foaming capacity, the foaming volumes are small and the bubbles have an uneven distribution. On the contrary, SDS and ABS have good compatibility with J7 heavy oil, both of the foaming volumes are more than 360 ml and the produced foamy oil demonstrates dispersive spherical bubbles. What’s more, the drainage hardly adheres to the wall of the measuring cylinder after experiments and the two anionic surfactants show good lubricating effects. Under the same condition, SDS shows a little larger foaming volume than ABS, but ABS presents a longer half-life and bigger FCI value, which indicates that the foamy oil with ABS is probably more stable than that prepared with SDS. Therefore, ABS is used as the foaming agent in the following experiments. Effect of foaming agent concentration The effect of ABS concentration on foamy oil performance was investigated with oil–water volume ratio of 1:1 at 20 C. As shown in Fig. 2, with the increase of ABS concentration, the foaming volume continually increases and reaches the maximum at 1.5 g/l, and then levels off. The half-lives at 1 and 1.5 g/l are close to each other, while the FCI values increase to the maximum at 1.5 g/l and then rapidly decrease. The reason may be that when the concentration of the additive surfactant is less than that of the critical micelle concentration (CMC), the adsorbing amount of the surfactant in gas bubble film is also low. As the foaming agent increases, the adsorbing amount increases and the surface tension will be reduced, which eventually results in an enhancement in foaming capacity and foam stability of the foaming fluid. When the surfactant concentration continues to increase, the adsorption reaches
Composition of foaming fluid Evaluation of foaming agent type In the experiments, parts of the selected foaming agents exhibited lower comprehensive performances at higher concentrations. Thus the foamy oil with each foaming agent concentration
Table 4 Performance of 100 ml foaming fluid with different types of foaming agents. Parameter
Foaming volume V0 (ml) Half-life t1/2 (min) FCI value (ml min)
Foaming agent CATC
CATB
OP-10
CDEA
ABS
SDS
120 – –
110 – –
110 – –
112 – –
375 78 21938
385 38 10973 Fig. 2. Effect of foaming agent concentration on foamy oil performance.
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saturation and the surface tension can not be further reduced, the foaming volume almost remains unchanged. But when the surfactant concentration is too high, the liquid content in bubble film decreases, the “fragility” of foam increases and the Marangoni effect gradually weakens, finally resulting in reduced foam stability. Therefore, the concentration of ABS for preparing foamy oil in the following experiments is determined as 1.5 g/l. Evaluation of foam stabilizer type High viscosity of heavy oil itself can enhance the film strengths of the bubbles in foamy oil. The addition of different types of foam stabilizers often leads to electrostatic attraction or repulsion, chemical and physical interactions between the heavy oil, foaming agent and foam stabilizer. The foaming fluid with foam stabilizer concentration of 2.0 g/l was prepared. The effect of different types of foam stabilizers on foamy oil performance are shown in Fig. 3. As can be seen in Fig. 3, all the foaming volumes reduce with the addition of five foam stabilizers in the foaming fluid. The main reason may be that the additive foam stabilizers limit the free movement of the foaming agent molecules to some extent. The tackifying stabilizers (CMC and PAM), instead of increasing the stability of the foamy oil, make the FCI values greatly reduced. This is mostly likely that the action mechanism of these two foam stabilizers is to improve the stability of the bubble by increasing the viscosity of the liquid film. But the effects are not obvious because the liquid film viscosity has been high enough, and the compatibility between the foaming agents and J7 oil is poor. In addition, when HEC and Dod were added, the changes on half-life were not noticeable, and the FCI values also reduced. Amazingly, the half-life of the foamy oil with SF-1 can increase to 170 min and the FCI value can reach more than 30,000 (superior grade). Therefore, SF-1 is used as the foam stabilizer in follow-up study. Effect of foam stabilizer concentration The SF-1 concentration required for stable foamy oil was investigated with ABS concentration of 1.5 g/l and oil–water volume ratio of 1:1 at 20 C. As shown in Fig. 4, with the increase of SF-1 concentration, the foaming volume continues to reduce, and the half-life and FCI value increase at first and then decrease. The reason may probably lie in that, at low concentration of SF-1, the interaction of SF-1 with the oil and foaming agent improved the arrangement of active molecules in liquid film. But when the concentration is too high, some stable micelles can easily form in the internal of the foamy oil, which will weaken the self-recovering ability of bubble film. This is to say when the bubble film is deformed, but the active molecules are bound in micelles, and can not move to the deformation place to repair the bubble film, which ultimately results in the decrease of the foam oil stability. Comparatively speaking, the foamy oil shows a better stability at SF-1 concentration of 2.5 g/l.
Fig. 3. Performance of 100 ml foaming fluid with different types of foam stabilizers.
Fig. 4. Effect of foam stabilizer concentration on foamy oil performance.
Effect of oil–water volume ratio Fig. 5 shows the effect of oil–water ratio on foaming volume, half-life and FCI value of 100 ml foaming fluid at 20 C. The foaming volume continues to increase with the decrease of oil–water ratios. This can be attributed to that high water content makes both the viscosity of foaming fluid and the energy required for preparing foamy oil decrease, thus the foaming volume will inevitably increase when same mechanical energy is introduced. With oil– water ratio decreasing, the half-life and FCI value increase at first and then decrease rapidly. Both the half-life and FCI value reach the maximum at oil–water ratio of 5:5. Based on the foregoing analysis, the foamy oil prepared with 1.5 g/l ABS, 2.5 g/l SF-1 at 1:1 oil–water volume ratio shows the best stability. What’s more, combining with the test method of rheometer and flow loop (L = 40 m, D = 25 mm) [38], we found that the measured phase inversion point of J7 oil–water emulsion was around 60% at different temperatures. Meanwhile, based on the subsequent analysis of foamy oil microstructure, it is indicated that the foaming fluid with oil–water ratio of 5:5 behaves as water in oil emulsion, which may be due to the high viscosity and complicated compositions of the heavy crude oil and the effects of the added surfactants. Currently, the crude oil are often produced with a large amount of water at the late stage production of many oilfields, which provides favorable conditions for the field application of heavy oil transported in the form of foamy oil. Effect of temperature The stability of the foamy oil is greatly influenced by temperature T. As shown in Fig. 6, the foaming volume is nearly unchanged, but the half-life and FCI value continually decrease with the increase of temperature. The main reason may be that, on
Fig. 5. Effect of oil–water volume ratio on foamy oil performance.
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Fig. 6. Effect of temperature on foamy oil performance.
the one hand, the increase of temperature makes the spaces between molecules of the foaming fluid increased and enhances the molecular motion, which ultimately results in the weakening of intermolecular forces and dispersive arrangement of surfactant molecules. On the other hand, as temperature increases, the viscosity of foaming solution decreases, which furtherly accelerates the drainage rate of the bubble film. For foamy oil, the higher the temperature, the better the mobility, but the worse the stability. Foamy oil property Foamy oil quality Blauer et al. first divided the foam fluid into three regions based on foam quality, i.e. dispersion region (0–0.52), interference (0.52–0.74) and deformation (0.74–1.00) [39]. Afterwards, he found the Mitchell quality range existed between 0.5236 and 0.9999 and preferably from 0.60 to 0.85, in which the foam is a homogenous mixture with uniform bubble sizes and could be stable for several hours [40]. Foamy oil qualities under different conditions were calculated according to Eq. (1) and shown in Table 5. Only the anionic surfactants (ABS and SDS) show good foaming capacity. With the increase of ABS concentration, the foamy oil quality exceeds 73% at 1.5 g/l and then levels off. The addition of the foam stabilizer makes the foamy oil quality decrease, and the higher the foam stabilizer concentration, the lower the foamy oil quality. But the quality is still more than 60% when SF-1 concentration reaches 3.0 g/l. What’s more, as the oil–water ratio decreases, the foaming volume increases gradually. Moreover, the temperature has smaller effect on the foamy oil quality. Rheological behavior The rheological curves of the foamy oil were measured under the shear rate range from 0 to 100 1/s at 20–30 C, which shows the
5
rheological behavior of a non-Newtonian fluid. As shown in Table 6, the curves can be well described by power law mode at 20 and 25 C, and all the correlation coefficients R2 are more than 0.99. But when the temperature reaches 30 C, the correlation coefficient slightly decreases. It may be that the stability of the foamy oil decreases with increase of temperature, and rotation of the rotor in rheometer accelerates drainage of foam films. Overall, the test results are in poor agreement with Bingham model, and the maximum correlation coefficient is only 0.9872. The apparent viscosities at 100 1/s calculated according to the obtained fitted rheological equations are less than 120 mPa s for power law model. In addition, for both power law model and Bingham model, all the apparent viscosities basically decreases with the increase of temperature and all the viscosity reduction rates of the foamy oil are more than 97%. The sources of foamy oil viscosity are main from the internal friction from relative motion of liquid layers and the collision and extrusion between dispersed bubbles. Studies have also shown that bubbles could be elongated and gradually tend to be neatly arranged after the foamy oil being sheared, which could reduce the probability of bubbles colliding with each other and then reduce the apparent viscosity [41]. Microstructure The micrographs of the foamy oil were captured by XP-300C image analytical system, and the microstructures in different vision fields are shown in Fig. 7. Based on the different light transmittance, it can be deduced that the white spherical bubbles are gas and water, and the black parts are heavy oil drops. The bubbles are linked together at numerous points. Two-dimensional diameters and areas of the bubbles in pictures were collected by using the MiVnt image analysis system, and then the equivalent three-dimensional average diameter and number of bubbles could be obtained. The bubble size is in accord with normal distribution and the bubble diameter are basically between 60 and 300 mm. The bubbles with a diameter of about 150 mm are the most, while the probability of occurrence are the highest (Fig. 8). At present, the decay mechanism of foam fluid is generally believed to lie in two aspects: the liquid film drainage and the diffusion of gas through liquid film [42]. The pressure in small bubbles is higher than that in large bubbles, which will make the gas in small bubbles diffuse to large ones until the small bubbles disappear, and large bubbles continue to expand and rupture. Therefore, the more uniform the bubble size distribution in foamy oil, the lower degree the bubble diffusion occurs, and then the longer the life of the bubble and the more stable the foamy oil. The normal distribution curve of the bubble in foamy oil is “tall”, which shows the bubble size distribution is narrow. The bubble size is relatively uniform and the diffusion rate between bubbles is slow. This may also be one of internal factors for the good stability of the foamy oil prepared in this study. Characteristics of foamy oil flow
Table 5 Foamy oil quality under different conditions. Foaming agent (1.5 g/l)
CATC
CATB
OP-10
CDEA
ABS
SDS
Foamy oil quality Γ (%) ABS concentration (g/l) Foamy oil quality Γ (%) SF-1 concentration (g/l) Foamy oil quality Γ (%) Oil–water volume ratio Foamy oil quality Γ (%) Temperature ( C) Foamy oil quality Γ (%)
16.7 0.5 25.9 0 73.3 7:3 34.2 20 63.0
9.1 1.0 65.5 1.5 68.1 6:4 47.4 30 59.2
9.1 1.5 73.3 2.0 64.9 5:5 63.0 40 58.3
10.7 2.0 73.0 2.5 63.0 4:6 70.1 50 57.4
73.3 3.0 72.6 3.0 62.0 3:7 73.8 60 –
74.0 – – – – – – – –
Rheological model Generally, the rheology of foamy oil in pipe flow is much different with that tested by a rheometer. Thus the rheological model of foamy oil flow should be investigated first in order to predict its frictional resistance. As we all know, it is much difficult to accurately describe the rheological property of flowing foamy oil for its multi-phase composition and changing structure. At present, there are main two models, namely power law model [43] and Bingham model [44], that can be used to describe the rheological behavior of foamy fluid. Furtherly, assuming dispersive distribution of bubbles in the oil, most of researchers use
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Table 6 Fitting results of rheological curves of the foamy oil. T
Power law model
20 25 30
Bingham model
Rheological equation
R
AV
VRR
Rheological equation
R2
AV
VRR
t ¼ 3:1658g_ t ¼ 0:7679g_ 0:5743 t ¼ 0:8186g_ 0:5321
0.9943
114.42
98.90
0.9410
152.82
98.53
0.9986
108.12
98.33
0.9550
140.48
97.83
0.9863
94.90
97.83
t ¼ 9:7615 þ 0:0552g_ t ¼ 7:0782 þ 0:0697g_ t ¼ 7:1868 þ 0:0382g_
0.9872
110.07
97.48
0:2790
2
Note: T, temperature ( C); AV, apparent viscosity at 100 1/s (mPa s); VRR, viscosity reduction rate (%).
Fig. 7. Microstructures of the foamy oil at 20 C.
homogenous flow model to predict the pressure drop of foamy oil flow. In this study, both power law model and Bingham model were used to explore a more appropriate rheological model for the foamy heavy oil flow. (1) Power law model Assuming the foamy oil is a power law fluid, and its constitutive equation is shown as follows: n du t¼K ð2Þ dr where t is the internal friction stress per unit area, K is the consistency coefficient, n is the flow pattern index, u is the velocity of the pipe flow and r is the arbitray radius of the pipe. Substituting Eq. (2) into the shear stress equation t ¼ D2Lpr, and the velocity distribution equation can be calculated as follows: 1 du 1 Dp r n ¼ dr K L 2
ð3Þ
where Dp is the pressure drop, L is the length of liquid column.
Integrating the above equation, the velocity calculation formula can be derived as: u¼
Dp
1n Z
2KL
r R
1
rn ¼
Dp
1n
2KL
h i nþ1 nþ1 n R n r n nþ1
ð4Þ
where R is the pipe radius. And the flow rate calculation formula can be furtherly integrated as blew: Z Q¼
R 0
u2prdr ¼
1n
pnR3 DpR
ð5Þ
3n þ 1 2KL
where Q is the volumetric flow rate. For the average velocity in the cross-section v ¼ QA , the relation between pressure drop and average velocity can be obtained as following: DpD 8v 1 þ 3n n ¼K ð6Þ 4L D 4n where A is the cross-sectional area and D is the internal pipe diameter. Taking the logarithm on both sides of Eq. (6) DpD 1 þ 3n n 8v ¼ lg K þ nlg ð7Þ lg 4L 4n D Based on the measured pressure drops Dp and flow rates Q, the fitted curves can be plotted in terms of the wall-shear stress pD lg D4L and the pipe flow parameter lg 8v D . The slope of the line is 0
defined as n0 , and the intercept is defined as k . 8 0 3n þ 1 n < K ¼ 10k = 4n : n ¼ n0
ð8Þ
The generalized Reynolds number Re0 for power law fluid flow can be defined as below, and 2000 is taken as the critical value. Re0 ¼
Dn v2n r m
ð9Þ
Fig. 8. Distribution of bubble diameters in foamy oil at 20 C.
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where m is a self-defined parameter, which can be calculated by n and r is the density of the fluid in pipe. the formula m ¼ K8 6nþ2 n (2) Bingham model The constitutive equation of Bingham fluid is shown as follows: du t ¼ t 0 þ hp ð10Þ dr where t 0 is the limiting shear stress, hp is the structural viscosity. Substituting Eq. (10) into the shear stress equation t ¼ D2Lpr, and the velocity distribution equation can be calculated as follows: du Dp ¼ ðr r 0 Þ dr 2Lhp
ð11Þ
7
Integrating Eq. (11), and the velocity distribution equation in the gradient region can be derived as: u¼
i Dp h ðR r0 Þ2 ðr r0 Þ2 4Lhp
ð12Þ
Then the flow rate in the gradient region Q1 can be furtherly obtained as below: Z R u2prdr Q1 ¼ r0 # " pDp R4 2R3 r0 5r0 4 2 2 3 ð13Þ R r0 þ 2Rr0 ¼ 4Lhp 2 3 6
where r0 is the radius of the core region of the flow.
Fig. 9. Foamy oil flow characteristic curves fitted by power law and Bingham models.
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When r ¼ r0 , the velocity in the core region v0 can be expressed as: v0 ¼
Dp ðR r0 Þ2 4Lhp
ð14Þ
And then integrating the above equation, the flow rate in the core region Q 0 can be obtained as follows: Q 0 ¼ pr 0 2 v 0 ¼
pDp 2 2 R r0 2Rr0 3 þ r40 4Lhp
ð15Þ
The total flow rate Q is the sum of the flow rates in the gradient and core region. pR4 Dp 4 r0 1 r40 þ 1 ð16Þ Q ¼ Q0 þ Q1 ¼ 8Lhp 3 R 3 R4 For Dp0 R ¼ Dpr0, Eq. (16) can be furtherly deformed as the following equation: pR4 Dp 4 Dp0 1 Dp0 4 þ Q¼ 1 ð17Þ 8Lhp 3 Dp 3 Dp4 where Dp0 is the initial pressure drop. When the core region of
the
pipe
flow
is
small,R4 r40 ,Dp4 Dp40 , so the simplified equation can be obtained as following by neglecting the higher order terms. pR4 4 Dp Dp0 ð18Þ Q¼ 3 8Lhp Similarly, for the average velocity in the cross-section v ¼ QA , Eq. (18) can be deformed as below:
Dp L
¼
32hp D2
16 t 0 vþ 3 D
ð19Þ
Substituting the tested values of pressure drop Dp and flow rate Q into Eq. (19), the fitted curves can be constructed in the pressure drop gradient DLp versus the average velocity v. The slope of the line 00
is defined as n00 , and the intercept is defined as k . 8 2 00 > > < hp ¼ D n 32 00 > > : t 0 ¼ 3Dk 16
ð20Þ
The generalized Reynolds number for plastic fluid flow Re0 can be defined as below, and 2000 is also taken as the critical value. Re0 ¼
rvD hp 1 þ 6th0pDv
ð21Þ
As shown in Fig. 9, the rheological behavior of the foamy oil in pipe flow can be better described by power law model than Bingham model, all the correlation coefficients of the curves fitted by the former model are more than 0.99, but most of the correlation coefficients of the curves fitted by the latter model basically maintain at about 0.97. At the same pipe diameter, with the increase of temperature, the consistency coefficient K decreases and the flow pattern index n varies irregularly when the prepared foamy oil are treated as power law fluid. Meanwhile, both of the limiting shear stress and structural viscosity reduce when the foamy oil regarded as Bingham fluid. At the same temperature, with increasing the pipe diameter, neither of the consistency coefficient and flow pattern index show any regularity in variation for the power law fluid, but the limiting shear stress decreases and the structural viscosity increases for the Bingham fluid. In addition, all the calculated generalized Reynolds numbers at different experimental conditions are less than 400, which demonstrates a laminar flow. Drag characteristic The friction drag coefficient is a key parameter for the calculation of resistance loss of fluid flow. The Darcy friction coefficient l or the Fanning friction coefficient f (f ¼ l=4) are generally used in engineering and laboratory experiments.
l¼
8t w rv 2
ð22Þ
where t w is the shear stress at the pipe wall. The friction loss is actually the energy loss of the sheared fluid on the macro-performance, thus the shear rate at the pipe wall g_ w is also a significant parameter for measuring the drag characteristics of fluid flows. For power law fluid, the wall-shear rate can be calculated as: 3n þ 1 n 8v n g_ w ¼ K ð23Þ 4n D For Bingham fluid, the wall-shear rate can be expressed as: 4 3
g_ w ¼ t 0 þ hp
8v D
ð24Þ
Fig. 10. Friction drag coefficient versus wall-shear rate.
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9
Fig. 11. Friction drag coefficient versus generalized Reynolds number. Table 7 Viscosity reduction effect of J7 heavy oil at 30 C. Different oils
Emulsified heavy oil (50 vol.% water) Heavy oil Foamy oil Foamy oil Foamy oil Foamy oil
Viscosity testing device
Rotational rheometer Rotational rheometer Rotational rheometer Simulation installation (D = 8 mm) Simulation installation (D = 5 mm) Simulation installation (D = 4 mm)
The changing regularity that the friction drag coefficient l varies with the wall-shear rate g_ w is similar for the power law and Bingham fluid at the same condition. Therefore, the changing curves for power law fluid are taken as example and shown in Fig. 10. All the friction drag coefficients gradually decrease with the increase of wall shear rates. In addition, as shown in Fig. 10a, the friction drag coefficient reduces with increasing temperature at the same pipe diameter. This can be contributed to that the friction drag coefficient is positively correlated with pressure gradient, and the pressure gradient of foamy oil flow reduces with increasing temperature. Fig. 10b shows the effect of pipe diameter on the friction drag coefficient. At the same temperature and flow velocity, the flow pattern index n deceases with deceasing the internal diameter D, thus the smaller the pipe diameter, the larger the wall shear rate. But at the same temperature and inner diameter, the wall shear rate increases with increasing the flow velocity. What’s more, the l decreases with the increase of internal diameter at the same wall shear rate. This is due to that the larger the pipe diameter, the higher the average velocity in the cross-section, thus the smaller the friction drag coefficient. Numerous studies have been conducted on the prediction of the friction drag coefficient l [45–47]. It is generally believed that the l for the power law and Bingham fluid at laminar flow can be 64 calculated be the theoretical formula l ¼ Re 0 . In this experiment, we found that the fitting equation was in very good agreement with the theoretical formula when the foamy oil was treated as Power law fluid (Fig. 11a), but the goodness of fit decreased obviously when we considered the foamy oil as Bingham fluid (Fig. 11b). Substituting the fitted friction drag coefficient into the Dp equation l ¼ 2D rLv2 , 189 sets of pressure drops of foamy oil flow
under different flow conditions were calculated. Comparatively speaking, the prediction accuracy of the power-law model is higher
Viscosity or apparent viscosity at different shear rates (mPa s) 100 1/s
1000 1/s
3000 1/s
12961.4 4367.4 94.9 46.5 30.4 30.3
9342.3 4367.1 35.4 35.6 23.3 23.0
7991.1 4366.3 22.2 33.2 21.7 20.4
than the Bingham model, the absolute maximum relative errors of the two models can be up to 13.4% and 35.8% respectively, and their respective absolute average relative errors are 4.3% and 6.3%. It is furtherly proved that the properties of the foam oil are more close to that of the power law fluid. Viscosity reduction effect The apparent viscosity of foam fluid is affected by shear rate, foam quality, temperature, pressure, bubble size, etc. In this study, viscosities of J7 heavy oil, and apparent viscosities of the emulsified heavy oil and foamy oil at 30 C were measured or calculated at different shear rates (Table 7). As shown in Table 7, there exist some differences in apparent viscosities of the foamy oil tested by rheometer and small-scale flow loop (D 8 mm), but the apparent viscosity of the foamy oil prepared by Waring Blender method is far lower than that of heavy oil, and the minimum viscosity-reducing rate exceeds 97%. The foamy oil prepared in this experiment contains 50 volume percent of water, and all the apparent viscosities tested by rheometer or flow loop apparatus are less than 100 mPas. Comparing with the emulsified J7 heavy oil with the same water content, the viscosity reduction rates are more than 99.5%. This is probably because the surfactants and bubbles changed the internal structure and greatly improved the flowability of the foamy oil. Conclusions Based on the synergy of foaming agents and foam stabilizers, a kind of stable foamy heavy oil was prepared with 1.5 g/l ABS, 2.5 g/l SF-1, oil–water volume ratio of 1 and nitrogen volume fraction of 63% at 20 C and 0.1 MPa. The foaming volume and half-life of 100 ml foaming fluid can reach 270 ml and 274 min respectively. The foamy oil quality is in the Mitchell quality range of foam fluid. In addition, the bubbles, namely the gas bubbles and water
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droplets in the oil are dispersed in spherical shape and the bubble size distribution is mainly maintained at about 150 mm. Through rheological and flow loop experiments, it is found that the prepared foamy oil can be characterized as non-Newtonian power law fluid. The friction drag coefficient decreases with increasing the shear rate, temperature and pipe diameter, and its fitted empirical formula is in very good agreement with the theoretical equation l ¼ 64=Re0 . The pressure drops predicted by the power-law model and the measured values coincide well with each other, and the absolute relative errors are within 13.4%. What’s more, compared with heavy crudes, the viscosity of the foamy oil decreases significantly, and the viscosity-reducing rates are more than 97%. For plenty of microbubbles existing in the foamy oil, the efficiency of pipeline transportation of heavy oil may be reduced at the same flow rate, but subsequent degassing and dehydration problems still need to be solved. Overall, this study confirms the effectiveness of transportation of heavy crudes as stable foamy oil in small diameter pipes, and the dosage of additive may be greatly reduced compared with the emulsification method. Future investigations will focus on the maintenance of dynamic stability of foamy oil in large diameter pipeline over long distances and its popularization and application in the oil field. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant no. 51074136), Research Fund for the Doctoral Program of Higher Education of China (Grant no. 20115121110004) and Science and Technology Project of Sichuan Province (Grant no. 2015JY0099). References [1] [2] [3] [4]
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Please cite this article in press as: J. Sun, et al., Pipeline transport of heavy crudes as stable foamy oil, J. Ind. Eng. Chem. (2016), http://dx.doi.org/ 10.1016/j.jiec.2016.08.019