Energy & Fuels 2001, 15, 1033-1037
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Transitiometric Investigation of Asphaltenic Fluids Under In-Well Pressure and Temperature Conditions Ch. Stachowiak,† J.-P. E. Grolier,*,† and S. L. Randzio‡ Laboratoire de Thermodynamique des Solutions et des Polyme` res, Universite´ Blaise Pascal, 63177 Aubie` re, France, and Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Poland Received February 28, 2001. Revised Manuscript Received June 19, 2001
Scanning transitiometry, a newly developed version of pVT-controlled scanning calorimetry, has been used to investigate the thermodynamic behavior of asphaltenic fluids under in-well pressure and temperature conditions. The use of the technique, which allows measurements over extended T and P ranges, is shown to be suitable to induce phase changes in aliphatic systems during various scans of the three variables p, V, and T. Extension of the technique to asphaltenic fluids shows its potential to investigate such complex systems, as already observed with polymer materials. Successive compression/decompression cycles performed on an asphaltenic fluid confirm that both flocculation and solubilization in such systems are governed by slow kinetic phenomena.
Introduction Flocculation of asphaltenes is a major concern in the petroleum industry in such activities as production, extraction, and transport. With the aim of characterizing flocculation phenomena and primarily the flocculation threshold, titration calorimetry has already been used to study the effect of solvents on asphaltenic fluids;1 the precipitation of asphaltenes is in that case induced by the addition of a solvent (usually n-alkane, according to the definition of asphaltenes). We have recently developed a new experimental technique, scanning transitiometry,2,3 which appears typically suitable to investigate phase changes in asphaltenic fluids. This technique which makes use of a calorimetric detector allows scanning one of the three independent variables p, V, or T while one of the others is held constant. From the recording of the variation of the dependent variable and of the associated heat effect, thermomechanical coefficients of the bulk phase can be determined and phase changes detected very accurately. The scanning rates as well as the operating ranges of T and p permit rigorous monitoring of the thermodynamic behavior of the system loaded in the transitiometric cell. Moreover, a full thermodynamic study is possible over the extended p, V, T surface, and fluids under high T and high p can be treated. This means also that the possible reversibility of phase transitions can be investigated with this technique. *
[email protected] fax: (33) 473 407 185. † Universite ´ Blaise Pascal. ‡ Polish Academy of Sciences. (1) Stachowiak Ch.; Grolier J.-P. E.; Randzio S. L..; Achard, C.; Rogalski, M.; Voguie´, J. R. An Investigation of Heat Effects in n-Alkanes + Asphaltenes Systems; Oral presentation, Second International Conference on Petroleum Phase Behavior and Fouling, Copenhagen, 2000 (to be submitted for publication). (2) Randzio S. L. Chem. Soc. Rev. 1996, 25, 383. (3) Randzio S. L.; Grolier, J.-P. E. Anal. Chem. 1998, 70, 2327.
Figure 1. Schematic view of the high-pressure calorimetric vessel (actually a pVT calorimetric cell). The capillary tubing is connected through high-pressure valves (not shown) to the high-pressure fluid reservoir containing the asphaltenic fluid.
Figure 2. Thermodynamic scheme of scanning transitiometry. Thermal and mechanical output derivatives lead respectively to the different thermomechanical coefficients, i.e, isobaric thermal expansion Rp, isothermal compressibility κT, isochoric thermal pressure coefficient βV, and heat capacities cp and cV at constant pressure and constant volume.4
We report here a preliminary investigation on a longchain paraffin, n-tetracosane, and real petroleum fluids
10.1021/ef0100505 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/02/2001
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Figure 3. Examples of transitiometric thermograms obtained with pure tetracosane: (a) T ) 447.45 K, while scanning pressure p ) f(t). (b) T ) 447.45 K, while scanning volume V ) f(t). (c) p ) 150 MPa, while scanning temperature T ) f(t).
under in-well conditions of temperature and pressure. Fluids containing asphaltenes have been used to il-
lustrate the advantages of scanning transitiometry to investigate such systems. Of particular importance is
Asphaltenic Fluids
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the transferring of the fluid system into the measuring cell under isobaric conditions. Experimental Section Scanning transitiometry is a relatively new technique which is based on inducing a change in the thermodynamic state of a sample under study by scanning at a low rate (or stepwise) one of the independent variables (p, V, or T) and keeping automatically constant the other independent variables. Figure 1 shows a schematic view of the high-pressure calorimetric vessel used in this study. From the output signals recorded simultaneously (rate of heat exchange and variation of the mechanical variable, volume, or pressure), a respective pair of thermodynamic derivatives is obtained simultaneously as a function of the scanned variable. A schematic presentation of four thermodynamic situations, possible to realize in such a pVT-controlled scanning calorimeter, is given in Figure 2.4 The operating instrument performs over wide ranges of pressures and temperatures. The principles of the scanning transitiometer used here have been described in details elsewhere;2 the transitiometer allows measurements up to 570 K under pressures up to 400 MPa. Its main characteristic is the possibility to precisely control the three variables p, V, or T to induce and monitor thermodynamic changes and transitions while simultaneously measuring the energy (i.e., the calorimetrically measured heat flux) associated with such modifications. To this end, the scanning of the inducing variable must be sufficiently slow in order to keep the system under study close to equilibrium and make sure then that thermodynamic relations are valid. In this way, a transitiometer is perfectly suitable to investigate the reproducibility of possibly reversible phenomena induced by temperature, pressure, or volume. Three examples of transitiometric thermograms obtained with tetracosane are given in Figure 3a-c. n-Tetracosane was supplied by Acros Organics and had a purity of 99%. Figure 3a shows the thermogram obtained when pure tetracosane is studied under isothermal conditions, at 447.45 ( 0.05 K. Thermodynamic changes are induced by a linear scanning of pressure p (as a function of time t); the associated calorimetric signal and volumetric response are recorded, which allow the determination of (∂S/∂P)T and (∂V/∂P)T, respectively. In the case of Figure 3b, pure tetracosane is studied at T ) 447. 45 K ( 0.05 K, and the thermodynamic changes are induced by a linear scanning of volume V. Figure 3c shows an example of the thermogram obtained by scanning temperature T, while p is maintained constant at 150 MPa. In Figure 3 a-c, the linear scans of the variables are shown with the corresponding recording of the dependent variables together with the associated calorimetric changes for fusion of n-tetracosane, illustrating then three of the four possible situations shown in Figure 2. To study real fluids, the main challenge was the introduction of the asphaltenic fluid into the transitiometric cell under isobaric conditions from the highpressure vessel where the fluid is being kept above (4) Randzio S. L. Thermochim. Acta 1997, 300, 29.
Figure 4. Detailed view of the capillary tubing connecting the high-pressure vessel to the detection zone where the asphaltenic fluid has been transferred.
mercury (after having been introduced under in-well conditions). Such an isobaric procedure should protect from an accidental flocculation of asphaltenes from the high-pressure reservoir fluid. A schematic presentation of the experimental setup connecting the fluid container with the calorimetric vessel is shown in Figure 4. The connection between the high-pressure fluid reservoir and the inside of the calorimetric vessel is made through a thin capillary tubing which minimizes any pressure drop during the fluid transfer; the calibrated length of this capillary also permits estimating the volume of asphaltenic fluid introduced into the calorimetric vessel. Mercury is used as an hydraulic fluid to pressurize the whole system and push the fluid during the transfer operation into the calorimetric vessel. Figure 4 shows a detailed view of the capillary tubing together with the surrounding mercury. The detection zone is the upper part of the calorimetric vessel, where the asphaltenic fluid is transferred through the capillary tubing. The transfer of the asphaltenic fluid from the high-pressure fluid reservoir into the calorimetric vessel is made in four steps. A set of five high-pressure valves (non shown in Figures 1 and 4) helps to connect or isolate the calorimetric vessel, the high-pressure fluid reservoir, and/or the connecting capillary tubing. The first step consists of establishing complete vacuum in the calorimetric vessel and in the connecting lines. Then, in the second step, mercury is introduced in the calorimetric vessel and in the connecting lines. In the third step, the high-pressure fluid reservoir is connected to the calorimetric vessel, and pressure in the whole assembly is equalized. When this equilibrium pressure is set, the last valve connecting the cell to the capillary tubing from the high-pressure fluid reservoir is open; then, by virtue of gravity, mercury from the detection zone “falls” down to the high-pressure fluid reservoir, pushing up, through the central capillary tubing, the asphaltenic fluid into the calorimetric detection zone. In this manner, the petroleum fluid can be transferred isobarically to the measuring vessel. To work under in-well conditions of temperature and pressure, the temperature of the calorimetric vessel and of the high-pressure fluid reservoir is brought up to the desired value. After the asphaltenic fluid is loaded into the calorimetric vessel, series of decompression/compression can be performed. An example of typical results is shown in Figure 5 in the case of a compression run performed
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Figure 5. Typical results obtained with an asphaltenic fluid during a compression by reducing the volume at a rate of (5.60 ( 0.01) × 10-5 cm3 s-1 at 430.7 K.
Figure 6. Stationary state heat flow curves showing exothermic effects and their disappearance during successive decompressions of an asphaltenic in-well fluid performed by volume expansions at a rate (5.60 ( 0.01) × 10-5 cm3 s-1 at 430.7 K (after recompressions to the high pressure as illustrated in Figure 5 as an example).
by reducing the volume at a rate (5.60 ( 0.01) × 10-5 cm3 s-1 at 430.7 K. In the heat flow curve (calorimetric thermogram), one can see a shallow endothermic effect, caused most probably by the dissolution of flocculated asphaltenes. To investigate the complete thermodynamic behavior of a given petroleum fluid containing asphaltenes, cycles of compression/decompression were operated. Figure 6 reports the results obtained at 430.7 K, with three successive decompression runs performed on a petroleum in-well fluid with a low asphaltenic percentage (=2.5%) (provided by the Petroleum Company TOTAL in a high-pressure reservoir). A sample of the petroleum in-well fluid was transferred from the reservoir to the calorimetric vessel at 61 MPa when both the fluid reservoir and the calorimeter were at 430.7 K. Then the sample placed in the calorimetric vessel was compressed to 94.3 MPa. After a few hours left for thermal, mechanical, and chemical equilibrium to be achieved, a slow decompression started with a volume expansion rate of (5.60 ( 0.01) × 10-5 cm3 s-1. The shallow
exothermal maximum shown by the first expansion would correspond to flocculation/precipitation of asphaltenes; the second decompression performed 12 h after recompression reflects a rather small effect of such flocculation/precipitation. The fourth decompression at the same conditions does not show any more such phenomenon, indicating that re-compression has not reformed asphaltenes. It is worth noting that during isothermal volume expansions, the heat flow of such a process is proportional to the (∂p/∂T)V of the sample under investigation (see Figure 2). When inspecting carefully the heat flow signals presented in Figure 6, one can see that after successive decompression and recompressions cycle, the shape of the thermograms is changing toward a form similar to the (∂p/∂T)V versus pressure relationship known for simple liquids, such as n-hexane.5 Thus, after three decompression/re-compression cycles, the original asphaltenic in-well fluid has become a simple fluid, at least with respect to its thermophysical properties, because all the asphaltenes have been already flocculated and could not be redissolved by successive recompression over the time frame of the experiment. Discussion and Concluding Remarks Preliminary results obtained with scanning transitiometry confirm what was already observed with polymer materials,3 that this technique is well adapted also to investigate thoroughly phase transitions in petroleum fluids. However, both solubilization and flocculation/ precipitation of asphaltenes are known6 to be very slow processes. The preliminary results obtained so far which do not show clear phase separation during successive compression/decompression cycles would confirm that longer periods of time should be allotted between cycles for permitting slow kinetic phenomena to take place. Systematic studies are currently underway with as(5) Randzio, S. L.; Grolier, J.-P. E.; Quint, J. R.; Eatough, D. J.; Lewis, E. A.; Hansen, L. D. Int. J. Thermophys. 1994, 15, 415. (6) Savvidis, T. G.; Fenistein, D.; Barre´, L.; Be´har, E. AIChE J. 2001, 47 (1), 206.
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phaltenic fluids of different compositions to check whether flocculation/precipitation is a reversible phenomenon. In addition, tentatively transitiometric investigations will be combined with in situ spectroscopic analysis.
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Acknowledgment. Financial support for this work came through the ECOCEV-CNRS Concerted Research Action “New Methods in Petroleum Thermodynamics”. EF0100505
