Measurement of Phase Boundaries of Hydrocarbon Mixtures Using

This allows for precise detection of dew and bubble point conditions. ... sample is kept in continuous contact with the fiber optic sensors. ..... rie...
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Measurement of Phase Boundaries of Hydrocarbon Mixtures Using Fiber Optical Detection Techniques Abhijit Y. Dandekar* and Erling H. Stenby Department of Chemical Engineering, Technical University of Denmark, 2800 Lyngby, Denmark

In this paper we present a novel technique for experimental determination of dew and bubble points of oils and gases under reservoir conditions. Extrinsic fiber optic sensors mounted on an 80-mL titanium equilibrium cell are part of the measurement setup. Formation of a new phase in the sample confined within the cell is sharply indicated by the creation of characteristic changes in the fiber optic signal. This allows for precise detection of dew and bubble point conditions. The entire apparatus is transportable and can easily be carried to field locations for quick and accurate studies of bottom hole samples. Phase behavior measurements (dew and bubble points) of three model systems and a real reservoir fluid are presented. Introduction The natural gas and petroleum industries increasingly demand accurate high-pressure phase equilibrium data of petroleum fluids, that is, multicomponent hydrocarbon mixtures. The data are needed for effective reservoir management and enhanced oil recovery processes. The data are part of the saturation properties (dew and bubble conditions) of the petroleum fluids under reservoir conditions. Saturation pressures of petroleum fluids under reservoir conditions are usually determined at a constant temperature in high-pressure stainless steel vessels (equilibrium cells) with sapphire windows. The sample volume/pressure is varied by injection or removal of mercury and the saturation point is observed visually as the formation or disappearance of a phase. Bubble points can sometimes be detected as a break point in a pressure versus volume curve. The relatively large sample volumes (200-2000 mL) used in such cells make the establishment of equilibrium time excessive after a temperature or pressure change, and the visual detection technique is in itself operator-dependent and difficult to automate. Moreover, from a practical point of view, the oil-producing industry cannot always afford to spend expensive time in connection with shipping of samples to conventional laboratories where such analysis is carried out. The apparatus described in this paper attempts to address all the above issues. A precise determination of phase boundaries is achieved from measurements of fiber optic signals. The size of the equilibrium cell is 80 mL, and the apparatus is completely automated. The sample is kept in continuous contact with the fiber optic sensors. The characteristics of the fiber optic signal are in this way monitored while the pressure is reduced along an isotherm or the temperature is reduced along an isobar, beginning with a single-phase sample. The value of the fiber optic signal shows a slight increase, when the pressure is slowly reduced along an isotherm * Corresponding author: Department of Chemical Engineering, Technical University of Denmark, Building 229, 2800 Lyngby, Denmark. Tel.: + 45 45 25 28 80 (direct). Fax: + 45 45 88 22 58. E-mail: [email protected].

until a phase boundary is reached. This value shows a rapid increase (in the case of a bubble point) or decrease (in the case of a dew point) when the saturation point is reached. Apparatus The apparatus was designed and manufactured by the ROP Company of France to measure dew and bubble point conditions for temperatures from 20 to 150 °C and for pressures up to 700 bar. The uncertainties in the cell temperature and pressure are within 0.01 °C and 0.05 bar, respectively. A detailed view of the apparatus is shown in Figure 1. The equilibrium cell is housed in a thermostatic enclosure, in which a high-speed fan provides efficient circulation of air, thus ensuring the preclusion of temperature gradients. The size of the equilibrium cell is 80 mL. The total dead volume is determined to be 17.5 mL. A mechanically driven piston varies the internal volume of the cell. Viton “O” rings placed on Teflon seats mounted in the grooves of the piston permit the maintenance of high pressures in the equilibrium cell. A small magnetic stirrer is placed on the piston to speed up the achievement of a homogeneous sample and a rapid equilibration between the coexisting vapor and liquid phases. The minimum and maximum displacement rates for movement of the piston is 0.001 and 600 mL/h, respectively. Optical detectors are employed to stop the movement of the piston when it reaches either the top or the bottom position in the equilibrium cell. A high-precision Pt-100 thermocouple and a pressure transducer mounted on the cell wall measure the sample temperature and pressure. Standard procedures are used for temperature and pressure calibration. As illustrated in Figure 1, the cell cover contains the fiber optic sensors, which will be explained in detail in the following section. The information on pressure, temperature, and value of the fiber optic signal is gathered by a control box, which communicates with the cell. A personal computer equipped with communication cards records the values of all the parameters from the control box and the cell and displays them in the windows-based program called CYBER. As a self-explanatory example, the screen dump of CYBER is shown in Figure 2. The

10.1021/ie990834u CCC: $19.00 © 2000 American Chemical Society Published on Web 05/17/2000

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Figure 1. Schematic diagram of the fiber optic cell.

Figure 2. Schematic diagram of CYBER.

apparatus has to be appropriately positioned, depending on the saturation pressure measurement desired, that is, dew or bubble point. The orientation of the apparatus as shown in Figure 1 is capable of measuring the bubble point of a high-pressure oil sample. As the pressure is reduced along an isotherm for an oil sample, for instance, the infinitesimally small amount of vapor evolved will reside at the top of the cell and be hence detected immediately by the fiber optic sensor located in the equilibrium cell cover. Gas condensates under reservoir conditions generally exist as a dense gas, but when the reservoir is subjected to isothermal pressure depletion, a retrograde dew point is encountered. This phenomenon occurs because of the presence of a small amount of heavy hydrocarbon components in gas condensates. Therefore, for determination of the dew point the apparatus has to be positioned accordingly, that is, turned through an angle of 180°. Thus, the fiber optic

sensor located in the cell cover has a bottom position and the first drop of retrograde liquid formed is immediately detected. Detection Principle. Fiber optic sensors have the capability of carrying enormous amounts of information through a relatively small fiber, for example, transporting light over large distances. Basically, fiber optic sensors can be characterized as intrinsic and extrinsic or hybrid type. In intrinsic fibers a single fiber is used for launching the light and also as a carrier for the detected signal. However, in extrinsic- or hybrid-type sensors light from an optical source (normally a lightemitting diode) whose relevant properties remain constant is launched into a fiber via a stable coupling mechanism and guided to the point at which the measurement is to take place. At this point the light can be allowed to leave the fiber and be modulated in a separate zone before being relaunched into a different fiber. Extrinsic- or hybrid-type fiber optic sensors have been extensively used for accurate determination of toxic gas levels, pressure measurements, etc. The apparatus described in this paper uses a sensor consisting of a sapphire roof prism, located in the cell cover, with fiber up (those carrying light to the sapphire roof prism) and fiber down leads (those carrying light or the detected signal from the sapphire roof prism). In such a type of fiber optic sensors, the intensity of the light returned depends on a number of factors, such as source intensity, fiber loss, sensor geometry, bending, etc., and most significant of all, the state of the fluid phase. Because of the complex mathematical relationship between the fiber optic signal and all the above factors, the returned signal mainly serves as a precise indicator of the formation of a second fluid phase. A stable constant value of the fiber optic signal is achieved when the sample is in a single-phase region at a fixed pressure and temperature. The value of the signal starts to increase gradually while the pressure

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Table 1. Compositions of Tested Fluids component

fluid 1 (mol %)

fluid 2 (mol %)

fluid 3 (mol %)

fluid 4 (mol %)

methane ethane propane i-butane n-butane i-pentane n-pentane n-decane

70.72 0.00 0.00 0.00 29.28 0.00 0.00 0.00

57.63 0.00 0.00 0.00 28.03 0.00 0.00 14.33

75.80 12.10 6.12 2.99 1.53 0.73 0.73 0.00

composition unknown

is reduced along an isotherm. This is observed because the fluid is becoming less dense until the pressure approaches the phase boundary. Upon reaching the phase boundary, the fiber optic signal rapidly changes because the sample is no longer in a homogeneous single phase. In the case of bubble point measurements the signal increases to a very large value and vice versa in the case of dew point measurements. This particular principle is exploited in this apparatus and all the measurements reported in this paper are performed using this technique. Test Fluids Saturation pressure (dew and bubble points) measurements reported in this paper using the fiber optical detection technique are based on three model systems and a real reservoir fluid (black oil). For the model systems analytical grade chemicals were used; the purity of gaseous and liquid components was better than 99.99% and 99.5%, respectively. Fluid 1 is a binary mixture of methane and n-butane; fluid 2 is a ternary mixture of methane, n-butane, and n-decane. These fluids were prepared gravimetrically in this laboratory. The mixture compositions were accurate to 0.005 wt %. High-pressure sample bottles equipped with a floating piston were used to avoid the mixing of the hydrocarbon phase with the hydraulic fluid (distilled water) because no mercury is used in any of the operations. Stainless steel balls are provided in the hydrocarbon side of highpressure sample bottles to facilitate the achievement of a homogeneous single-phase sample. External displacement pumps were used for pressure maintenance to ensure a homogeneous sample in a single phase. Fluid 3 is a seven-component model mixture of pure hydrocarbons ranging from methane to n-pentane. The composition of this particular mixture resembled a relatively lean gas condensate fluid and was blended and supplied in industrial gas bottles by Hydro Gas, Norway. The real reservoir fluid, fluid 4, was recombined black oil supplied by Preussag Energy, GmbH, Germany. The original compositions of all the tested fluids are given in Table 1. Experimental Procedure All high-pressure samples (fluids 1, 2, and 4) were first maintained at a pressure well above the expected saturation pressure under ambient temperature conditions. External displacement pumps were employed for this purpose. A minor or no change in the volume readings taken from this external pump gave an indication of a homogeneous single-phase sample. The next experimental step involved the connection of the highpressure sample bottle to the fiber optic cell. The entire system is then evacuated with a turbo vacuum pump for more than 48 h to ensure hard vacuum in the

system. The loading of the sample in the fiber optic cell was then accomplished by flashing the sample in the dead volume under ambient temperature conditions and simultaneously maintaining the pressure well above the expected saturation pressure. The magnetic stirrer mounted on the fiber optic cell was kept running continuously to enhance the achievement of a homogeneous single-phase sample. The fiber optic cell was subsequently isolated from the sample bottle after the volume readings on the external pump showed no change. Fluid 3 was supplied in industrial gas bottles and existed as a single-phase fluid at a pressure of 18.0 bar which was well below the normal condensation area on the phase envelope. In this case the gas bottle was directly connected to the fiber optic cell and the entire system evacuated for more than 48 h. The gas was then flashed in the dead volume. However, there was no possibility of encountering the two-phase region because of the reasons mentioned above. The maximum cell volume was then filled with the sample and the gas bottle was isolated from the fiber optic cell. After the above procedures have been carefully followed, the fiber optic cell was ready to perform the desired saturation pressure measurements at various isotherms/isobars. Experimental Results Prior to the carrying out of tests on fluids described in Table 1, the apparatus was tested by measuring the dew points of propane and n-butane at various isotherms. These measurements showed an excellent agreement with the data reported by Sage and Lacey1 and those calculated from the Soave-Redlich-Kwong2 equations of state. The values measured in this work agreed to within 1.5% and 6.6% with those reported by Sage and Lacey1 and calculated by Soave-RedlichKwong2 equations of state for propane and n-butane, respectively. The results were presented at the 5th European Union Hydrocarbons Symposium on The Strategic Importance of Oil & Gas Technology, Edinburgh.3 Fluid 1. The first mixture studied in the fiber optic cell was fluid 1, a binary mixture of methane and n-butane. Sage et al.4 studied the phase behavior of this mixture carefully. The critical point of the mixture (53.9 °C and 132.4 bar) lies within the temperature range of the apparatus, thus permitting dew and bubble point determinations by simply changing the temperature. The measurements were performed at 37.8, 54.4, and 71.1 °C. At 37.8 °C the mixture exhibited bubble point behavior and dew point behavior at the remaining two temperatures. After a stable cell temperature was achieved, the pressure was reduced along the isotherm, beginning with a single-phase sample. The fiber optic signal and the pressure were continuously monitored and plotted by use of CYBER. Figure 3 shows the plot of pressure versus the fiber optic signal for a dew point experiment at 54.4 °C. As the pressure is reduced along the isotherm, the fiber optic signal shows a gradual increase, as the fluid is becoming less dense. Upon reaching the phase boundary, the signal starts to decrease rapidly as the dew point is reached. The dew point at 125.4 bar is easily detectable from the break in the curve. The reported dew point under these conditions by Sage et al.4 is 128.9 bar; this corresponds

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Figure 3. Pressure versus the fiber optic signal for a dew point experiment for fluid 1 at 54.4 °C.

Figure 5. Pressure versus the fiber optic signal for a bubble point experiment for fluid 2 at 37.8 °C.

Figure 4. Phase envelope of fluid 1. Figure 6. Pressure versus the sample volume for a bubble point experiment for fluid 2 at 37.8 °C.

Table 2. Saturation Pressure Data for Fluid 1 saturation pressure (bar) temperature (°C)

Sage et al.4

this work

37.8 131.4 126.5 54.4 128.9 125.4 71.1 116.5 115.8 average absolute deviation

deviation (%) 3.9 2.8 0.5 2.4

to a deviation of 2.8% from our measurement. This difference is attributed to the fact that a different determination technique was employed by Sage et al.4 Moreover, on re-performing the measurements at 37.8 and 54.4 °C, a repeatability of 0.15% was obtained, thus indicating the accuracy and reliability of our measurements. The results from the experiments at the three isotherms are shown in Figure 4 together with the calculated phase envelope. The results are also presented in Table 2 together with the percentage deviations of our measurements from experimental values reported by Sage et al.4 The calculations were made using the Soave-Redlich-Kwong (SRK) equation of state. There is reasonable agreement between the calculated and the measured saturation points. Fluid 2. A ternary mixture of methane, n-butane, and n-decane followed the study on fluid 1. Bubble point experiments at isotherms ranging between 37.8 and 90.9 °C were performed. The calculated critical point for this system was 197.9 °C and 158.5 bar. Therefore, the mixture exhibited bubble point behavior at all the studied isotherms. Dew point measurements could not be performed because the temperatures were beyond the limits of the apparatus. The plot of the fiber optic signal and the pressure for a bubble point experiment at 37.8 °C is shown in Figure 5. As the pressure is reduced at a slow rate along the isotherm, a gradual increase in the signal is observed. Upon reaching the bubble point the signal starts to increase rapidly and reaches a substantially high value at a pressure of 169.0 bar. A

Figure 7. Phase envelope of fluid 2.

distinct break in the plot is observed, indicating a clear bubble point at 169.0 bar. It is evident from this plot that there is absolutely no ambiguity in the determination of the bubble point. The data on sample volume versus pressure was also plotted simultaneously and is shown in Figure 6. A clear indication of the bubble point can be seen from the PV curve, which also gave a value of 169.0 bar. Reamer et al.5 reported a bubble point of 165.5 bar under these conditions. The deviation of 2.1% from our measurement stems from the fact that a break point in the PV curve, apparently not perfectly sharp, was employed by Reamer et al.5 to determine the bubble point. The bubble point experiment at this particular isotherm was repeated and is also shown in Figure 5. The second measurement gave a value of 168.9 bar, that is, repeatability better than 0.06%, thus confirming the reliability of our measurement. A comparison of the phase envelope of this mixture is given in Figure 7. The results, together with the percentage deviations of our measurements from the values reported by Reamer et al.,5 are also furnished in Table 3.

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Figure 8. Temperature versus the fiber optic signal for a dew point experiment for fluid 3 at 74.7 bar.

Figure 9. Phase envelope of fluid 3.

Table 3. Saturation Pressure Data for Fluid 2 saturation pressure (bar) temperature (°C)

Reamer et al.5

37.8 165.5 55.7 175.6 71.1 182.1 90.9 186.6 average absolute deviation

this work

deviation (%)

169.0 179.1 185.3 190.1

-2.1 -2.0 -1.7 -1.9 1.9

Fluid 3. As mentioned earlier in the section on test fluids, fluid 3 resembled a relatively lean gas condensate. To our knowledge there is no previously reported experimental data on this system. The calculated critical conditions were -12.3 °C and 101.5 bar. All the measurements reported on this system are dew points because the mixture exhibited bubble point behavior under cryogenic conditions and they are outside the limits of this apparatus. The calculated cricondentherm for this mixture was at 28.7 °C and 65.0 bar, thus allowing the dew point measurements under isobaric conditions by simply lowering the temperature from a value sufficiently above the cricondentherm. The pressure was maintained at a constant value by using the option of pressure stabilization available on CYBER. The plot of the fiber optic signal and temperature for a dew point experiment at 74.7 bar is shown in Figure 8. As the temperature is reduced along the isobar, a gradual decrease in the signal is observed, as the fluid becomes denser. A rapid decrease in the fiber optic signal, to a significantly low value, is observed at 25.0 °C. A clear break in the curve thus gives a precise indication of the dew point. Such accurate determination of the dew point in conventional, windowed cells is not always reliable, especially for lean gas condensates. This is because no color changes take place in such systems under dew point conditions and visual detection can thus introduce large uncertainties. The phase envelope of this mixture together with the calculated values is shown in Figure 9. Fluid 4. This fluid was received as a recombined sample at 120.0 bar and ambient temperature. Because the critical conditions were considered to be too high for this reservoir fluid, all experiments performed covered only bubble points. The tests were carried out at isotherms ranging from 28.0 to 93.8 °C. Figure 10 shows the pressure versus the fiber optic signal for a bubble point experiment at 93.8 °C. As the pressure is reduced along the isotherm, the fiber optic signal remains at a constant value until the bubble point is reached. A significant increase in this value is observed under bubble point conditions. The bubble point at 82.6

Figure 10. Pressure versus the fiber optic signal for a bubble point experiment for fluid 4 at 93.8 °C.

Figure 11. Pressure versus the sample volume for a bubble point experiment for fluid 4 at 93.8 °C.

bar is easily detectable from the sharp break point in Figure 10. A pressure-volume curve for a bubble-point experiment at 93.8 °C is shown in Figure 11. The bubble point at 82.1 bar is determined from this curve. The experiments at other isotherms showed a similarly clear indication of the bubble point. A comparison of the bubble points is shown in Figure 12. Conclusions An apparatus that uses fiber optic sensors for the detection of phase transitions in high-pressure petroleum mixtures has been described. It has enabled a considerable reduction of the sample volume compared to conventional methods, and a high degree of automation has been incorporated. It has also been demonstrated that the determination of saturation pressure by the described apparatus is much more accurate and reliable, to within 0.1 bar and at least 2 times faster, compared to the conventional methods of determination.

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Acknowledgment Financial support from the Danish Technical Research Council (STVF) is gratefully acknowledged. Literature Cited

Figure 12. Bubble points of fluid 4.

A clear break is generally seen in the pressure versus the fiber optic signal curves, thus leaving no ambiguity in the interpretation of the saturation pressures. Moreover, it has an added advantage of being a transportable PVT unnecessary equipment, which can be easily carried to field locations. This allows the oil companies to save time in shipping samples to PVT laboratories, especially during the exploration phase. The superiority of the fiber optical detection technique is demonstrated by results from experiments with three model mixtures and a real reservoir fluid.

(1) Sage, B. H.; Lacey, W. N. Thermodynamic Properties of the Lighter Paraffin Hydrocarbons and Nitrogen; American Petroleum Institute: New York, 1950. (2) Soave, G. Equilibrium Constants from a Modified RedlichKwong Equations of State. Chem. Eng. Sci. 1972, 27, 1197. (3) Dandekar, A. Y.; Stenby, E. H. Field Instrument to Determine Phase Behavior for Hydrocarbon Resources Using Microwave and Fiber Optical Detection Techniques. Poster presented at the 5th European Union Hydrocarbons Symposium on The Strategic Importance of Oil & Gas Technology, Nov 1996. (4) Sage, B. H.; Budenholzer, R. A.; Lacey, W. N. Phase Equilibria in Hydrocarbon Systems, Methane-n-Butane System in the Gaseous and Liquid Regions. Ind. Eng. Chem. 1940, 32, 1262. (5) Reamer, H. H.; Sage, B. H.; Lacey, W. N. Phase Equilibria in Hydrocarbon Systems, Methane-n-Butane-Decane System. Ind. Eng. Chem. 1947, 39, 77.

Received for review November 16, 1999 Revised manuscript received March 13, 2000 Accepted March 20, 2000 IE990834U