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Energy & Fuels 2006, 20, 1656-1663
Novel Organic Solids Deposition and Control Device for Live-Oils: Design and Applications Mohammed Zougari,* Scott Jacobs, John Ratulowski,† and Ahmed Hammami Schlumberger ReserVoir Fluid Center, Oilphase-DBR, Edmonton, Alberta, Canada T6N 1M9
George Broze, Matthew Flannery,‡ and Artur Stankiewicz§ Shell Global Solutions (US) Inc., 3333 Highway 6 South, Houston, Texas 77082
Kunal Karan Department of Chemical Engineering, Queen’s UniVersity, Kingston, ON, Canada K7L 3N6 ReceiVed December 14, 2005. ReVised Manuscript ReceiVed May 1, 2006
A novel laboratory-scale flow assurance tool termed an “organic solids deposition and control (OSDC)” device has been developed to assess the potential for and severity of organic solids deposition problems from hydrocarbon fluids at realistic production and/or transportation conditions. The OSDC device is a shear cell based on Couette-Taylor flow with a cylindrical geometry. The shear cell geometric parameters are optimized to maximize the Reynolds number. The OSDC device mimics the typical pipeline production conditions of temperature, pressure, composition, turbulence, shear rate, surface type, and roughness. These key parameters can be accurately and independently controlled, allowing the deposition tests to be conducted over a wide range of conditions. Deposition tests were performed on live waxy and asphaltenic oils. Reproducible deposits have been generated under consistent test conditions. Wax deposition rates from OSDC are comparable to those found in pilot-scale flow loops while using orders of magnitude less fluid.
Introduction Organic solids (e.g., waxes, asphaltenes, hydrates, and diamondoids), as well as scales, are encountered in all facets of petroleum production and transportation. An example pressuretemperature phase diagram depicting phase boundaries for wax, asphaltene, and hydrates along with the liquid-vapor phase envelope of typical Gulf of Mexico deepwater black oils has been recently published.1 Long offsets and cold water temperatures present a challenge for subsea developments in deepwater. This is due to the fact that some or all of the solid-fluid phase boundaries may be crossed during production, leading to precipitation and subsequent deposition of organic solids. Experimental techniques to assess the propensity and rate of organic solids deposition under realistic production conditions are relatively limited. A review of open literature on wax and asphaltene deposition revealed that a majority of the experimental studies on wax deposition were carried out mainly in low-pressure systems; only a few mentioned asphaltene deposition. Table 1 summarizes published deposition techniques and corresponding key findings/observations as reported in the open literature. Briefly, the experimental systems for wax deposition could be classified as either batch systems or flow-through systems. The batch systems were essentially variations of * To whom correspondence should be addressed. † Currently with Shell Global Solutions (U.S.) Inc., 3333 Highway 6 South, Houston, TX 77082, USA. ‡ Currently with Shell International Exploration and Production in Houston, TX. § Currently with Shell International Exploration and Production in Rijswijk, The Netherlands. (1) Ratulowski, J.; Amin, A.; Hammami, A.; Muhammad, M.; Riding, M. Flow Assurance and Sub-sea Productivity: Closing the Loop with Connectivity and Measurements. Presented at the 2004 SPE Annual Technical Conference and Exhibition, Houston, Texas, September 26-29, 2004; SPE 90244.
coldfinger or cold spot configurations. One interesting variation was a rotating disk cold spot apparatus. All flow-through systems reported in the literature were pipe loops with the exception of one, which was a plate and frame heat exchanger. Recognizing the limitations of the existing deposition techniques, which include a lack of high-pressure adaptability, the requirement of having a large sample volume, and the inability to simulate intensive turbulent flow regimes, the design and development of a novel laboratory-scale apparatus capable of mimicking the deposition process of wax and/or asphaltene from live reservoir fluids under realistic production and transportation conditions was undertaken. While developing a fundamental understanding of the deposition process was deemed important, major emphasis was placed upon designing and testing an experimental setup that could meet the following key design criteria: (1) small sample volume, to require only small quantities of expensive bottom-hole fluids for testing; (2) accurate temperature control, to ensure testing under welldefined thermal conditions; (3) high-pressure adaptability, to allow testing of fluids at reservoir conditions (up to 105 MPa); (4) broad temperature range operation (-20-200 °C), to allow simulation of seabed to reservoir temperatures; (5) controlled shear at the wall, to enable scalability of deposition results; (6) well-defined hydrodynamics, to allow testing under flow conditions ranging from laminar to fully turbulent regimes; (7) accuracy/precision of detection techniques, to ensure generation of reproducible and reliable data; (8) ability and flexibility to test surface types and roughness, to allow testing of actual pipeline materials as well as assessment of new surface materials and coatings;
10.1021/ef050417w CCC: $33.50 © 2006 American Chemical Society Published on Web 06/07/2006
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Energy & Fuels, Vol. 20, No. 4, 2006 1657
Table 1. Summary of Published Deposition Methods and Key Findings deposition device ref Jessen and Howel14
type
description
flow-through pipe loop
test conditions Wax Deposition Studies laminar/turbulent 0.75/2.0 in. diam + 5 ft long kerosene/wax mixture
Cole and Jessen15
flow-through plate and frame
laminar kerosene/wax mixture
Hunt, Jr.16
batch and
cold spot surface: 0.75 in. × 1.25 in., 20 gauge pipe loop: 1/8 in. diam refined wax-oil mixture + crude oil cold spot surface: 2.0 in. diam × 1/ in. thick 8 refined wax-refined solvent mixture
cold spot
flow-through pipe loop Jorda17
Patton and
batch
Casad18
batch
Bott and Gudmundson19
flow-through
Burger et al.20
flow-through
Newberry21 batch Weingarten and Euchner22 batch and flow-through
Hartley and bin Jadid23
flow-through
Agrawal et al.24
flow-through
Majeed et a.l25
flow-through
Hamouda et al.26
batch
Hsu and Santamaria27
flow-through
Brown et al.28
flow-through
Wu et al.29
batch
Ibrahim et
al.30
de Boer et al.31 Alboudwarej32 Wang et al.33
flow-through
cold spot
main conclusions maximum deposition rate around laminar-turbulent transition deposition controlled by molecular diffusion lower deposition in plastic and coated pipe compared to metal pipes surface wettability important parameter for deposition free surface energy of paraffin and surface responsible for adsorption forces deposit held in place by surface roughness composition not important for deposition surface roughness influenced amount, distribution, wax content of the deposit heat transfer important factor influencing deposition and surface roughness affects heat transfer characteristics
varying surfaces material varying surface roughness cold spot cold spot surface: 2.0 in. diam × 1/8 in. thick surface roughness and coatings do not affect deposition refined wax-refined solvent mixture + plastic coating decrease deposit amount due to insulation effect refined wax-kerosene mixture deposition rate increases with increasing temp differential (Tc - Ts) pipe loop turbulent deposit thickness quickly reaches asymptotic value 13.1 mm i.d., 15 mm o.d., heat flux and shear stress are the main 762 mm entry length + 914 mm factors influencing the asymptotic thickness jacketed length wax-kerosene mixture mechanism of wax deposition is determined by cohesive property of wax particles formed in the boundary layer pipe loop laminar both molecular diffusion and shear dispersion are important deposition mechanism 4.93 mm i.d., 2.9 m long; deposit contains significant amount of oil 10.2 mm i.d., 2.83 m long (83-86%) shear rate 3.5-31 s-1; 2 heat flux 0.9-88 W/m Sadlerochir crude oil rotating disk dimensions not available NA diffusion cylinder cylindrical 6-chamber diffusion cell wax diffusion test provide useful wax diffusion rate and pipe loop pipe loop 0.25 in. diam; sloughing occurs when wall shear stress heat transfer rate 19-5800 W/m2; exceeds strength of wax deposit; this is shear rate 12-4960 s-1 unrelated to laminar-turbulent transition capillary tube dimensions not available none crude oil pipe loop laminar-turbulent maximum wax deposition occurs at laminar-turbulent transition pipe loop: 6 mm i.d., 25 cm long Bombay high crude pipe loop laminar-developing model does not predict asymptotic time-dependent behavior pipe loop: 6, 9, or 12 mm diam, 1.0 m long depletion effects could be significant in laboratory experiments North Sea crude high-pressure cylinder cold spot of 40 mm diam molecular diffusion is the controlling wax coldfinger deposition mechanism North Sea crude high-pressure laminar-turbulent flow turbulence depresses the temperature at which flow loop maximum deposition occurs pipe loop: 0.5 in. i.d., 5 ft long sloughing effect generated under turbulent flow has significant effect on deposition rates stock tank oil and recombined live-oil wax hardness and carbon number increases with aging time pipe loop shear rate: 330-1330 s-1 wax deposition by shear dispersion is not significant stock-tank oil no deposition observed for zero heat flux need for multiphase flow data unknown small sample size (115 mL) cold disk in an agitated vessel flow regime undefined pipe loop unknown wax deposition by shear in a concentric tubes setup; the deposit is produced in the inner tube
flow-through coiled tube (not a loop) flow-through pipe loop flow-through coiled capillary tube
Asphaltene Deposition Studies no details available 1.91 cm o.d., 1.57 cm i.d. 16-32 m long 0.51 mm i.d.
no conclusions available tubular geometry should allow easy scalability. laminar flow regimes only rate of deposition unaffected by flow rate and capillary tube length overthe range studied higher deposition rate with higher molar volume precipitants waxes codeposit with asphaltenes above WAT
1658 Energy & Fuels, Vol. 20, No. 4, 2006
Figure 1. Schematic representation of cylindrical Couette-Taylor device.
(9) ability to test sour fluids, to allow testing of a wide variety of reservoir fluids. Design of Organic Solids Deposition and Control System The organic solids deposition and control (OSDC) system was designed and developed to study the fundamentals of physical and chemical processes governing wax and asphaltene deposition from petroleum fluids under typical production and transportation conditions. The designated objectives of the OSDC cell operation include deposit formation, qualification, composition and the determination of deposition kinetics as a function of temperature, pressure, shearrate, surface roughness, and surface type. Basic Couette-Taylor Flow and Similitude with Pipe Flow. The OSDC device is based on the flow between rotating concentric cylinders, also called the Couette-Taylor (CT) system. A schematic diagram of the OSDC cell is shown in Figure 1. The OSDC cell comprises a central rotating cylinder and an outer stationary cylinder with the process fluid occupying the annular space. Although, this geometry is clearly different from that of a pipe, there exists similitude between flow in a CT system and a pipe. In analogy with the flow in pipelines, the central rotating cylinder in the CT flow acts as a pump driving the flow in a closed loop and the stationary wall acts as the pipe wall. Lathrop et al.2 and Lewis and Swinney3 performed extensive experimental analyses, which led to the establishment of a preliminary scalability of the two flow processes in terms of a torque-Reynolds number relationship. Dubrulle and Hersant4 and Eckhardt et al.5 characterized a global momentum transport analogy between the two flows using the turbulent model approach and energy dissipation as used in Rayleigh-Be´nard theory. Maynord6 used a CT flow to model the transport of particles (fish eggs) in an open channel and concluded that the CT flow gave a good indication of the shear mortality of eggs in high flow-rate rivers. Van der Berg et al.7 analyzed the effect of surface roughness on dimensionless torque using Grossmann-Lahose theory, which reinforced the analogy between CT and Rayleigh-Be´nard flows. Tan and Swinney,8 Lee and Lueptow,9 (2) Lathrop, D.; Fineberg, J.; Swinney, H. Transition to shear-driven turbulence in Couette-Taylor flow. Phys. ReV. A 1992, 46 (10), 63906405. (3) Lewis, G.; Swinney, H. Velocity structure functions, scaling, and transitions in high-Reynolds-number Couette-Taylor flow. Phys. ReV. E 1999, 59 (5), 5457-5467. (4) Dubrulle, B.; Hersant, F. Momentum transport and torque scaling in Couette-Taylor flow from an analogy with turbulent convection. Eur. Phys. J. B 2002, 26, 379-386. (5) Eckhardt, B.; Grossmann, S.; Lohse, D. Scaling of global momentum transport in Couette-Taylor and pipe flow. Eur. Phys. J. B 2000, 18, 541544. (6) Maynord, S. Concentric Cylinder Experiments of Shear Mortality of Eggs and LarVal Fish; Environmental Report 23, Upper Mississippi RiverIllinois Waterway, 2001. (7) Van der Berg, H.; Doering, C.; Lohse, D.; Lathrop, D. Smooth and rough boundaries in turbulent Couette-Taylor flow. Phys. ReV. E 2003, 68, 445010-445014. (8) Tan, W.; Swinney, H. Mass transport in turbulent Couette-Taylor flow. Phys. ReV. A 1987, 36 (3), 1374-1381.
Zougari et al.
Figure 2. Velocity profile comparison between CT and pipe flows:12 (symbols) CT flow; (line) pipe flow.
and Rosende et al.10 performed theoretical and experimental analyses comparing the turbulent scalar transport process in CT flow to that encountered in pipe flow; evidence of similarities between both flows has been established. Wang et al.11 investigated the effect of shear rate on the aggregation and breakage of particles in CT and pipe flow using both a computational fluid dynamics (CFD) model and some experimentation and found excellent agreement between the two flows. Smith and Townsend12 measured the velocity profile for the CT flow which when compared to pipe flow using the same geometrical dimension (pipe diameter vs flow gap in CT flow) and flow setup produced a similar profile as depicted in Figure 2. The results so obtained correspond to a pipeline i.d. of 0.152 m, a centerline velocity of 4.6 m/s, and a Reynolds number of 700 000. More importantly, Smith and Townsend12 also showed that the CT radial and vertical velocities are very small compared to the rotational velocity and, thus, deemed the centrifugal forces in CT flow negligible. Similarly, Bilson and Bremhorst13 concluded that a virtually identical scalar transport process occurs in both CT and pipe configurations, including average and localized radial velocities and acceleration representative of centrifugal forces. OSDC System Description and Characterization. The OSDC system consists of the following three major components: (1) a pressurized shear deposition cell, including a spindle, deposition surface, and heat exchanger system; (2) a secondary heat exchange system, which includes a cooling/heating bath, heater system, heat exchanger, circulating pump, and controllable valves; (3) instrumentation and control systems, including controllers, data acquisition, and communication interfaces. Figures 3 and 4 show a graphical picture of the whole OSDC setup and a schematic representation of the shear deposition cell, respectively. Table 2 summarizes the key features of the OSDC. Optimization of the OSDC Cell Configuration. One of the key criteria in designing the OSDC cell was the ability to perform experiments in a fully developed turbulent flow regime, a condition difficult to achieve in existing laboratory-scale devices. The geometry of the OSDC cell was optimized to achieve the maximum Reynolds number for a fixed outer cylinder radius allowing the attainment of turbulent flow conditions at reasonable rotational speeds. (9) Lee, S.; Lueptow, R. Mass transfer in rotating reverse Osmosis based on Couette-Taylor flow. Presented at the 13th International Couette Taylor Workshop, Barcelona, Spain, July 3-5, 2003. (10) Rosende, M.; Vieira, P.; Sousa, R.; Giordano, R. L.; Giordano, R. C. Estimation of mass transfer parameters in a Taylor-Couette-Poiseuille Heterogeneous reactor. Braz. J. Chem. Eng. 2004, 21 (02) 175-184. (11) Wang, L.; Vigil, R.; Fox, R. CFD simulation of shear induced aggregation and breakage in turbulent Taylor-Couette flow. J. Colloid Interface Sci. 2004, 285, 167-178. (12) Smith, G. P.; Townsend, A. A. Turbulent Couette flow between concentric cylinders at large Reynolds number. J. Fluid Mech. 1982, 123, 187-217. (13) Bilson, M.; Bremhost, K. Comparison of Turbulent Scalar Transport in a Pipe and a Rotating Cylinder. Presented at the 3rd International Conference on CFD in Minerals and Process Industries, December 10-12, 2003.
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Energy & Fuels, Vol. 20, No. 4, 2006 1659 Table 3. Similitude between CT and Pipe Flows parameter Reynolds number34 wall shear stress35,36 Nusselt number37,38 dimensionless6 (∆P, G) friction factor4,39 hydrodynamic boundary layer40,41
Figure 3. Graphical model of the OSDC system.
Figure 4. Schematic diagram of the OSDC deposition cell. Table 2. OSDC System Parameters and Characteristics parameter
value
sample volume heat transfer and deposition area secondary flow rate temperature rating pressure rating rotational speed surface roughness surface type controlled shear flow regime sour fluid handling
∼150 cm3 125.4 cm2 up to 50 L/min -20-200 °C up to 103 MPa (15 000 psi) up to 9000 rpm any any yes any yes
The generalized form of the Reynolds number for CouetteTaylor flow can be written as follows:2 Re )
ΩF Ω r (r - ri) ) ri(ro - ri) µ i o V
(1)
where (ro - ri) is the gap between the inner (rotating) cylinder and the outer (stationary) cylinder, Ω is the rotational speed, and ri is the radius of the rotating device. We see from eq 1 that, for fixed Ω and ro, the Reynolds number is maximized by the choice ri ) ro/2, for which choice: ∂Re ΩF ) (r - 2ri) ) 0 ∂ri µ o
(2)
This optimization allows the OSDC device to achieve the highest possible turbulence regime for a fixed outer diameter. This enhances the wall shear stress and provides for a wider shear rate range. Scalability of OSDC Cell Data to Pipe Flow Conditions. To ensure that the data generated from the laboratory-scale device can
pipe
uDe ν 1 τω ) Ffru2 8
Couette-Taylor
u(ro - ri) ν 1 τω ) FΨu1.8 8
Re )
Re )
Nu ) AReaPrb
Nu ) A′Rea′Prb′
∆P ) C1Re(3/2)+R1 + C2Re2+R2 G ) C1′Re(3/2)+R1′ + C1′Re2+R2′
1
xfr
) C1 log(Rexfr) + C2
ln(Re) δp ) 91De Re
1
xfr
) C1′ log(Rexfr) + C2′ ln(Re)
δCT ) 3.8(ro - ri)
Re0.8
be scaled up reliably to the actual field operating conditions, it is essential that the relevant characteristics of the OSDC device be accurately defined. For scale-up of wax and asphaltene deposition data, in addition to the control of the three thermodynamic variables (temperature, pressure, and composition), the hydrodynamic and heat transfer characteristics of the system must be well defined. Accordingly, an extensive experimental program was undertaken to establish the thermal-hydrodynamic stability of the OSDC system. This paper is not intended to provide a thorough or detailed analysis on CT flow scalability to pipe flow but rather to prove the existence of similitude between the two flows, sufficient to endorse the CT flow approach to model and simulate the deposition process. A detailed and specific fluid mechanics analysis, including extensive experimentation, is ongoing to define and parametrize the scalability process from CT flow to pipe flow, and representative experimental results will be published in a separate paper. Table 3 compares key flow and heat transfer parameters for CT flow and pipe flow processes. The observed similitude is based on established correlations for momentum, heat transfer, and mass transport; hence, the CT flow has been adapted to physically mimic the organic solids deposition process. It is recognized that the OSDC device is a batch system similar to a flow loop; therefore, there are concerns regarding precipitate/deposit depletion effects as well as the possible effects of remnants of hydrodynamic vortices (i.e., CT flow instabilities known to occur below the critical Reynolds number2-13). However, the testing conditions (i.e., duration and flow regime) can be carefully designed so as to avoid both issues or at least minimize their effects. For instance, one can conduct preliminary deposition tests for the candidate waxy fluid as a function of time (i.e., perform a deposition kinetic study). From the plot of the corresponding deposit mass vs time curve combined with high temperature gas chromatograph (HTGC) analytical data of the parent oil and respective deposits (i.e., n-paraffin content and distribution), a critical time during which no significant depletions occur can be determined. Similarly, the initial slope of the deposit mass vs time curve amounts to a deposition rate fairly representative of those encountered in production flow lines. Alternatively, a large oil reservoir and circulation system can be connected to the CT cell so that the total oil volume employed is several times greater than that of the CT cell. Meanwhile, visualization experiments can be conducted to determine the critical rotational speed (or Reynolds number) above which no flow instabilities (vortices) are present. Table 4 shows an example of actual data output from two different pipe configurations and their Couette-Taylor equivalent systems based on the existing and/or new correlations summarized in Table 3. In this comparison, we matched the Reynolds number, wall shear stress, and hydrodynamic boundary layer thickness, likely proven to be among the most important quantities affecting the deposition process, where the input parameters were based on a dynamic viscosity µ ) 1 cP, a density F ) 0.85 g/cm3, fluid temperature Tf ) 25 °C, and a Prandtl number Pr ) 10. The comparison shows that both systems yield similar results provided
1660 Energy & Fuels, Vol. 20, No. 4, 2006
Zougari et al.
Table 4. OSDC vs Pipe Parameters Benchmark parameter diameter/gap (m) flow velocity (max) (m/s) production rate/ rotational speed Reynolds no. wall shear stress (Pa) wall velocity (m/s) wall Reynolds no. Reδ ) uδ/V boundary layer thickness (µm) Nusselt no.
pipe-1
couette-1
pipe-2
couette-2
0.1524 4.10
0.0711 8.85
0.0889 3.0
0.0432 6.1
40 000 Bpd
1200 rpm
10 000 Bpd
1355 rpm
615 000 4.5
615 000 4.5
263 567 2.80
263 567 2.80
0.082 1240
0.082 2800
0.065 1300
0.065 1150
31
34
38
37
1050
880
527
564
a careful selection of testing parameters is ensured. As illustrated in Table 4, a CT design can be developed to closely correspond to a specified pipe configuration. Therefore, one can design the OSDC cell to mimic any pressure, temperature, surface type and roughness (through carefully designed and machined cylindrical inserts), and either Reynolds number or wall shear stress but not always both. OSDC Thermal Characteristics. Two sets of thermal experiments were undertaken. The first set was to establish how accurately the OSDC could be maintained at constant uniform temperature. The second set was to define the heat transfer coefficient of the shear cell. The response of thermocouples placed in the test fluid (on the primary side) and those located inside the outer cylinder near the inner wall surface were measured to be within (0.3 °C at any given time during isothermal test runs, confirming that uniform and constant temperatures of up to 200 °C can be achieved and maintained. Figure 5 shows the temperature profiles from the bulk
Figure 5. Temperature distribution during a heat transfer run.
of the fluid to the coolant side at different vertical locations as a function of time, which reveals remarkable thermal stability and control of the implemented cooling system. OSDC Pressure Stability Tests. Precipitation of asphaltenes as well as waxes can be effected by changes in fluid pressure. As such, the maintenance of stable pressure at a specified level during deposition testing of high-pressure live (saturated or undersaturated) petroleum fluid is important to ensure high quality and reliable experimental data. The pressure profile during an isobaric run was measured over ∼6 h. The OSDC cell is able to maintain a test pressure of up to 103 MPa with a maximum deviation of 0.1 MPa or 1 atm.
Case Studies Wax Deposition Tests. Detailed OSDC wax deposition experiments completed for two petroleum fluids of different origins, namely Gulf of Mexico (GoM) and Thailand, have been reported elsewhere.42 In this study, we briefly discuss some of the key findings obtained for the GoM fluid. The tested samples consisted of 36 American Petroleum Institute (API) stock-tank oil (STO) and corresponding recombined live-oil with 321 m3/
m3 Gas-to-oil ratio (GOR). It is important to note that the deposition tests were carried out at a fixed pressure, temperature differential between bulk fluid and deposition wall, fluid composition, and constant spindle speed. Hence, the deposition results for each OSDC test correspond to one specific point/ location (of similar conditions) along a production pipeline. A minimum of three deposition tests (covering the temperature range and fluid residence time) would be required to mimic a typical deposition profile expected along production and/or transportation flow lines. The STO sample was also tested for wax deposition using a Shell pilot-scale flow loop and a Shell coldfinger apparatus. Table 5 summarizes the specifications and characteristics of Shell pilot-scale flow loop. The Shell coldfinger apparatus is a 15.9 mm (5/8 in.) o.d. tube approximately 7.6 cm (3 in.) long, which is closed at the bottom. Coolant is injected through a tube near the inside bottom of the finger and exits through a tube at the top. The finger is immersed in 50 mL of oil in a sample container. The test oil is mixed using a magnetic stirrer bar at speeds ranging from 50 to 200 rpm. The sample container is heated by a water bath. The coolant temperature, oil temperature, and stirrer speed are controlled and recorded. At the end of the experiment, deposits are removed with a solvent. The recovered deposit-solution mixture is topped, weighed, and analyzed to determine the wax and oil content.43 (14) Jessen, P. W.; Howell, J. N. Effect of Flow Rate on Paraffin Accumulation in Plastic, Steel and Coated Pipe. Pet. Trans., AIME 1958, 213, 80-84. (15) Cole, R. J.; Jessen, P. W. Paraffin Deposition. Oil Gas J. 1960, 58 (38), 87-91. (16) Hunt, E. B., Jr. Laboratory Study of Paraffin Deposition. JPT, J. Pet. Technol. 1962, (November), 1259-1269. (17) Jorda, R. M., Paraffin Deposition and Prevention in Oil Wells. JPT, J. Pet. Technol. 1966, (December), 1605-1612. (18) Patton, C. C.; Casad, B. M. Paraffin Deposition from Refined WaxSolvent Systems. SPE J. 1970, (March), 17-24. (19) Bott, T. R.; Gudmundsson, J. S. Deposition of Paraffin Wax from Kerosene in Cooled Heat Exchanger Tubes. Can. J. Chem. Eng. 1977, 55, 381-385. (20) Burger, E. D.; Perkins, T. K.; Striegler, J. H. Studies of Wax Deposition in the Trans Alaska Pipeline. JPT, J. Pet. Technol. 1981, (June), 1075-1086. (21) Newberry, M. E. Chemical Effects on Crude Oil Pipeline Pressure Problems. JPT, J. Pet. Technol. 1984, (May), 779-786. (22) Weingarten, J. S.; Euchner, J. A. Methods for Predicting Wax Precipitation and Deposition. SPE Prod. Eng. 1988, (February), 121-126. (23) Hartley, R.; bin Jadid, M. Use of Laboratory and Field Testing to Identify Potential Production Problems in the Troll Field. SPE Prod. Eng. 1989, (February), 34-40. (24) Agrawal, K. M.; Khan, H. U.; Surianarayanan, M.; Joshi, G. C. Wax Deposition of Bombay-High Crude under Flowing Conditions. Fuel 1990, 69 (June), 794-796. (25) Majeed, A.; Bringedal, B.; Overa S. Model Calculates Wax Deposition for N. Sea Oils. Oil Gas J. 1990, 18, 63-69. (26) Hamouda, A. A.; Viken, B. K. Wax Deposition Mechanism Under High-Pressure and in Presence of Light Hydrocarbons. Presented at the SPE International Symposium on Oilfield Chemistry, New Orleans, LA, March 2-5,1993; SPE Paper 25189. (27) Hsu, J. J. C.; Santamaria, M. M. Wax Deposition of Waxy Live Crudes Under Turbulent Flow Conditions. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, LA, September 2528, 1994; SPE Paper 28480. (28) Brown, T. S.; Niesen, V. G.; Erickson, D. D. Measurement and Prediction of the Kinetics of Paraffin Deposition. Presented at the SPE Annual Technical Conference and Exhibition, Houston, TX, October 3-6, 1993; SPE Paper 26548. (29) Wu, C.-H.; Wang, K.-S.; Shuler, P. J.; Tang, Y.; Creek, J. L.; Carlson, R. M.; Cheung, S. Measurement of Wax Deposition in Paraffin Solutions. AIChE J. 2002, 48 (9), 2107. (30) Ibrahim, J.; Toma, P.; Ivory, J.; Korpany, G.; de Rocco, M.; Holloway, L. Direct Observations on Paraffin Deposition Mechanism under Laminar and Turbulent Flow for Direct Applications to Paraffin Oil Transportation. Presented at the 5th International Conference on Petroleum Phase Behaviour and Fouling, Banff, Canada, June 14-17, 2004.
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Energy & Fuels, Vol. 20, No. 4, 2006 1661
Table 5. Shell Pilot-Scale Flow Loop Characteristics and Specifications storage tank volume flow loop specifications
flow conditions heating and controls
up to 10 bbl 5.08 cm diameter 60.96 m overall jacketed length 12.19 m preheat section for precision controlled tests 6.10 m deposition test section with countercurrent coolant flow in jacket 1.22 m spool piece for wax deposit removal maximum 2000 BOPD turbulent or laminar flow 16 Chromalox heaters in eight 3.05 m (10 ft) zones 14 matched calibrated RTDs for process measurement and control Farex SR-Mini PID controller with 7 control zones control oil and coolant temperatures in test section to within +0.2 °C
Figure 6 presents photographs depicting the deposit formed only on the inside surface of the outer OSDC stationary cylinder (i.e., deposit wall) from the GoM samples.42 The spindle surface appears coated with only residual oil as expected. The deposit wall and spindle surfaces were photographed before and after being subjected to rinsing with cold dichloromethane (DCM), which is used to remove the oil (dark) film. Evidently, the post rinse wax deposit is essentially white (like candle wax) with a tint of yellow indicative of slight oil entrapment in the n-paraffin rich material. The post rinse spindle surface is virtually clean (i.e., deposit-free). One of the remarkable features that can be observed from these photographs is the uniformity of the deposit thickness. In other words, there is negligible variation (if any) in deposit characteristics over the entire deposition surface. This qualitative evidence suggests well-controlled hydrodynamics within the OSDC cell. The relevant analyses of the total deposit, such as composition, can be averaged without introducing any significant error.42 Hot toluene (∼70 °C) is used to dissolve the postrinse deposit (i.e., the wax deposit) under high rotational spindle speed for at least 20 min. The resulting solution is then collected and subjected to roto-evaporation under a fume hood to dryness (i.e., constant mass). The resulting wax is weighed and subsequently (31) de Boer, R. B.; Leerlooyer, K.; Eigner, M. R. P.; van Bergen, A. R. D. SPE Prod. Facil. 1995, 2, 55-61. (32) Alboudwarej, H. Asphaltene Deposition in Flowing Systems. PhD Dissertation, University of Calgary, Calgary, Alberta, Canada, 2002. (33) Wang, J.; Buckley, J. S.; Creek, J. L. Asphaltene Deposition on Metallic Surface. J. Dispersion Sci. Technol. 2004, 25 (3) 287-298. (34) Bird, B.; Stewart, W.; Lightfoot, E. Transport Phenomena, second ed.; John Wiley & Sons: New York, 2002. (35) Necati, O. In Heat Transfer: A basic Approach; McGraw-Hill: New York, 1985 (36) Wendt, F. Ing. Arch. 1933, 4, 577. (37) Reohsenow, W. M. In Handbook of Heat Transfer, third ed.; McGraw-Hill: New York, 1998. (38) Gardone, M.; Astarita, T.; Carlomagno, M. Infrared Heat Transfer Measurements on a Rotating Disk. Opt. Diagn. Eng. 1996, 1 (2) 1-7. (39) Colebrook C. F. Turbulent flow in Pipes with Particular Reference to the Transition Region Between the Smooth and Rough Pipes Laws. J. Inst. CiVil Eng. 1939, 11, 133-156. (40) Schlichting, H. In Boundary Layer Theory, 8th revised and enlarged ed.; Springer: Berlin, 2000. (41) Pope, S. B. Turbulent Flows, 1st ed.; Cambridge University Press: Cambridge, UK, 2001. (42) Zougari, M.; Hammami, A.; Broze, G.; Fuex, N. Live-Oils Novel Organic Solid Deposition and Control Device: Wax Deposition Validation. Presented at the 14th SPE Middle East Oil & Gas Show and Conference, Manama, Bahrain, March 12-15, 2005; SPE 93558. (43) Ratulowski, J.; Westrich, J.; Leitko, A. Thermodynamic Model For Wax Precipitation in Live-Oil Systems. Presented at the ACS Spring National Meeting, San Francisco, CA, March 26-30, 2000.
Figure 6. Photographs showing GoM oil wax buildup formed on OSDC deposition wall and spindle.
analyzed for n-paraffin composition and entrapped oil content using HTGC and simulated distillation (SIMDIST), respectively. The deposition test parameters and results so obtained are summarized in Table 6. Figure 7 shows a graphical comparison of wax deposition rates measured for the same STO using the OSDC, Shell pilot-scale flow loop, and Shell coldfinger. Under similar test conditions, the OSDC cell produced results that are fairly representative of those obtained using the flow loop in terms of deposit oil content and deposition rate. However, the OSDC uses only a fraction (∼0.01%) of the oil volume (10 bbl) required for the flow loop. Meanwhile, the coldfinger yielded wax deposits with very high entrapped-oil contents (minimum 70%) and, in turn, the corresponding wax deposition rates are significantly lower than those generated using OSDC. These results are plausible since oil content is an inverse function of wall shear stress. In the case of the coldfinger, the wall shear stress is significantly lower than those attained in the OSDC and flow loop. More importantly, only the OSDC could be used to conduct wax deposition measurements for typical reservoir fluid compositions at realistic production conditions. It is useful to mention that uncertainty in empirical scaling procedures often results in conservative predictions for wax deposition. The OSDC cell deposition data can be scaled on the basis of fundamental and relevant fluid mechanics parameters such as Re, wall shear stress, thermal gradient, etc. Accordingly, the modeling of the flow-line conditions results in less conservative estimates.1 Asphaltene Deposition Tests. Asphaltene deposition tests were completed for a recombined South American live-oil at the reservoir temperature and ∼1 MPa above the corresponding saturation pressure. Bottom hole fluids from the same field are known to precipitate asphaltenes upon pressure depletion at the reservoir temperature.44-46 Field observations indicate the reservoir fluid is problematic; slow asphaltene deposition has
1662 Energy & Fuels, Vol. 20, No. 4, 2006
Zougari et al.
Table 6. Wax Deposition Test Parameters and Results (Run Time ) 3 h) oil sample
∆Ta (°C)
pressure (MPa)
Re
wall shear stress (Pa)
total deposit (mg)
wax deposit (mg)
oil content (mass %)
wax deposition rate (mg/(hr cm2))
STO live-oil
2.6 3.3
0.1 13.8
2.5 × 104 1.3 × 105
32 19
183.4 261.1
137.8 221.1
25 15
0.49 0.70
a
∆T ) bulk oil temperature - deposition wall temperature. Bulk oil temperature ) cloud point or wax appearance temperature in this study. Table 7. Sampling Information and Measured Properties for South American Live-Oil reservoir conditions
a
pressure (MPa)
temp (°C)
single stage flash GOR (m3/m3)
asphaltene contenta (mass %)
°API
saturation pressure (MPa)
asphaltene onset pressure (MPa)
60
138
123
5.8
30
19
28
For topped STO, asphaltene content ) 6.7 mass %.
case of wax deposition, the asphaltene deposits appear quite uniform in thickness and texture; however, they are qualitatively much thinner than the aforementioned wax deposits.
Figure 7. Comparison of shear cell (OSDC) wax deposits to those obtained using flow loops (FL) and coldfinger (CF): (a) deposition rate, (b) entrapped oil.
wells.45-46
been reported in few producing Table 7 summarizes the sampling information and measured/reported properties of the recombined oil. Figure 8 shows typical photographs of the spindle smooth surface coated with oil film and the OSDC stationary wall covered with asphaltenelike deposit. As in the (44) Karan, K.; Hammami, A.; Flannery, M.; Stankiewicz, A. Systematic Evaluation of Asphaltene Instability and Control During Production of LiveOils: A Flow Assurance Study. Pet. Sci. Technol. 2003, 21 (3 and 4), 629645. (45) Stankiewicz, A. B.; Flannery, M. D.; Fuex, N. A.; Broze, G.; Couch, J. L.; Dubey, S. T.; Iyer, S. D.; Ratulowski, J.; Westrich, J. T. Prediction of Asphaltene Deposition Risk in E&P Operations. In Proceedings of the 3rd International Symposium on Mechanisms and Mitigation of Fouling in Petroleum and Natural Gas Production, AIChE 2002 Spring National Meeting, New Orleans, LA, March 10-14, 2002; Paper 47C, pp 410416. (46) Flannery, M.; Cornelisse, P.; Zougari, M.; Hammami, A. Asphaltene precipitation and deposition under live-oil conditions in a novel experimental device. Presented at the 4th International Conference on Petroleum Phase Behavior and Fouling, Trondheim, Norway, June 23-26, 2003.
DCM was used to simultaneously dissolve and recover the oil film and asphaltene deposit from the cell deposition wall. The resulting solution was collected and subjected to rotoevaporation under a fume hood to dryness (i.e., constant mass). The resulting topped residue (denoted as the total deposit) is weighed and subsequently subjected to asphaltene content using a modified IP-143 method.47 Table 8 summarizes the deposition test parameters and corresponding results. Note the test matrix includes one deposition measurement (test #1) at 17 MPa above the measured asphaltene onset pressure; thereby, no measurable asphaltene deposition is expected. The results of this test yield the asphaltene deposit baseline (i.e., lowest detectable deposit mass) of the OSDC cell. The corresponding total deposit (i.e., topped residue of the DCM rinse solution) and total asphaltenes (i.e., precipitated + deposited) amount to 54.5 mg and 3.7 mg, respectively. The difference between these two quantities yields the mass of maltenes (i.e., de-asphalted, topped residue of the DCM rinse solution). This mass combined with the topped STO asphaltene content (i.e., 6.7 mass %) is then used to calculate the amount of asphaltenes precipitated from the oil phase in the total deposit. Finally, the asphaltene deposit mass is determined using a mass balance (i.e., total asphaltenes precipitated asphaltenes). For test #1, the asphaltene deposit mass amounts to 0.1 mg, which is within the margin of error of measurement. As such, an asphaltene deposit mass less than 1 mg is deemed negligible. Tests #2-4 were designed to evaluate the effect of wall shear stress (by varying the spindle speed) on the amount of asphaltene deposit. From the results listed in Table 8, three main observa-
Figure 8. Photographs of OSDC deposition wall and spindle following the isothermal and isobaric asphaltene deposition experiment for South American live-oil.
NoVel Organic Solids Deposition and Control DeVice
Energy & Fuels, Vol. 20, No. 4, 2006 1663
Table 8. Asphaltene Deposition Test Parameters and Results (Run Time ) 1 h at 138 °C)
c
test #
spindle speed (Hz)
pressure (MPa)
Re × 10-4
wall shear stress (Pa)
total deposita (mg)
total asphaltenesb (mg)
maltenesc (mg)
asphaltene deposit (mg)
asphaltene deposition rate × 102 (mg/hr/cm2)
1 2 3 4
40 100 60 40
45 20 20 20
5.5 13.7 8.2 5.5
15.2 72.4 30.4 15.2
54.5 87.0 132.0 300.0
3.7 9.5 13.6 36.5
50.8 77.5 118.4 263.5
tracesd 3.9 5.1 17.6
0.0 3.1 4.1 14.0
a Total deposit ) topped DCM wall rinse solution (i.e.,maltenes + total asphaltenes). b Total asphaltenes ) deposited asphaltenes + precipitated asphaltenes. Maltenes ) topped de-asphalted DCM wall rinse solution. d Traces ) asphaltene mass is
