Measuring CO2 Minimum Miscibility Pressures: Slim-Tube or Rising

Petroleum Engineering Department, Colorado School of Mines, Golden, Colorado 80401 .... Permeability, however, influences the injection rate during fl...
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Energy & Fuels 1996, 10, 443-449

443

Measuring CO2 Minimum Miscibility Pressures: Slim-Tube or Rising-Bubble Method? Adel M. Elsharkawy* College of Engineering and Petroleum, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait

Fred H. Poettmann and Richard L. Christiansen Petroleum Engineering Department, Colorado School of Mines, Golden, Colorado 80401 Received November 21, 1994. Revised Manuscript Received October 4, 1995X

Determinations of carbon dioxide minimum miscibility pressures (MMP) using a slim-tube apparatus were compared with those using a rising-bubble apparatus (RBA). MMPs were determined for 12 different oils, with gravities varying from 34 to 51 °API. The results were found to compare very well when using a specific criterion for the slim-tube MMP. Although the slim-tube method is often referred to as the industry standard, there is no standard design, no standard operating procedure, and no standard criterion for determining MMPs with the slim tube. It is shown that the RBA is faster and more reliable than the slim tube for determining MMP. Bubble behavior is described for both the vaporizing and condensing gas processes.

Introduction Gas injection above the minimum miscibility pressure (MMP) is a widely practiced means for improving oil recovery in many reservoirs. The minimum miscibility pressure is the lowest pressure for which a gas can develop miscibility through a multicontact process with a given reservoir oil at reservoir temperature. The reservoir to which the process is applied must be operated at or above the MMP to develop multicontact miscibility. Reservoir pressures below the MMP result in immiscible displacements and consequently lower oil recoveries. At or above the MMP, miscibility can develop through a vaporizing process, a condensing process, or sometimes a combination of the two processes. In the vaporizing gas process, intermediate molecular weight hydrocarbons from the crude oil are transferred to the leading edge of the gas front enabling it to become miscible with the reservoir crude. In the condensing gas process, the injected gas is enriched with light hydrocarbons, usually LPGs. The reservoir oil left behind the gas front is enriched by net transfer of the light hydrocarbons from the gas phase into the oil. Enrichment of the reservoir oil proceeds until it becomes miscible with the injected rich gas. Miscibility can also develop through a combination of the vaporizing and the condensing processes. As in the condensing process, the light intermediate components in the injected gas condense into the crude oil, while the middle intermediates, C4+, vaporize into the gas phase. With this combination of condensation and vaporization, miscibility may never completely develop. Yet the process can result in low residual oil saturations. Primarily two laboratory methods are used to measure MMPs: the slim-tube method, and the risingbubble method. While there are many more slim-tube * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, December 15, 1995.

0887-0624/96/2510-0443$12.00/0

apparatus in the industry, a large portion of MMPs reported in the literature in recent years were measured with just a few rising-bubble apparatus. Other methods, including a tedious multiple-contact method in phase behavior apparatus, have also been tested for measuring MMPs. There are many correlations for CO2 MMPs in the literature. An adaptation for CO2 MMPs of the Benham-Dowden-Kunzman1 correlation for enriched natural gas MMPs was proposed by Holm and Josendal.2 This adaptation provides for temperature and C5+ molecular weight dependence of MMP. Orr and Silva3 proposed a correlation for CO2 MMPs which requires a more complete compositional description of the crude. Following the method used by Benham et al., Riedel4 developed a CO2 MMP correlation which includes compositional factors. Riedel’s correlation predicted MMPs with an average error of 10%. Riedel's correlation compares very favorably with that of Alston, Kokolis, and James.5 The intent of this paper is to compare the slim-tube method and the rising-bubble method for measurement of CO2 MMPs. First, we will outline design and operation of the slim-tube and the rising-bubble apparatus used in our laboratory, with comparisons to the literature. Next, we will present results of MMP measure(1) Benham, A. L.; Dowden, W. E.; Kunzman, W. J. Miscible Fluid DisplacementsPrediction of Miscibility. Trans. AIME 1960, 219, 229237. (2) Holm, L. W.; Josendal, V. A. Mechanism of Oil Displacement by Carbon Dioxide. J. Pet. Technol. 1974, 1427-1436; Trans. AIME 257. (3) Orr, F. M., Jr.; Silva, M. K. Effect of Oil Composition on Minimum Miscibility PressuresPart 2: Correlation. Paper SPE 14150 presented at the SPE Annual Technical Conference and Exhibition, Las Vegas, September 22-25, 1985. (4) Riedel, K. L. A Correlation for Carbon Dioxide Minimum Miscibility Pressure. M.S. Thesis, Colorado School of Mines, Golden, CO, 1985. (5) Alston, R. B.; Kokolis, G. P.; James, C. F. CO2 Minimum Miscibility Pressure: A Correlation for Impure CO2 Streams and Live Oil. Paper SPE 11959 presented at the SPE Annual Technical Conference and Exhibition, San Francisco, October 5-8, 1983.

© 1996 American Chemical Society

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Elsharkawy et al. The justification for using long slim tubes is to minimize the effect of transition zone length. Smaller diameter tubing is justified to prevent viscous fingering. There is considerable difference of opinion among researchers on the effect of packing material on MMP. Some say that packing material has no effect, while others maintain that oil recovery depends on the dispersion level caused by the packing material. Slim-tube

Figure 1. Schematic of a slim-tube apparatus. Table 1. Range of Specifications for Slim-Tube Apparatus in the Literature Compared with the CSM Slim-Tube Apparatus internal diameter, in. length, ft packing material porosity, % permeability, darcys displacement velocity, ft/day

literature

CSM

0.12-0.63 5-120 glass beads, sand 50-270 mesh 32-45 2.5-250 30-650

0.12 60 sand 100 mesh 39 3 54

ments for CO2 with 12 different oils. We will show the sensitivity of MMP determination with the slim tube as a function of MMP definition. Finally, we will discuss the temperature dependence of MMP as measured with the rising-bubble method. Slim-Tube Apparatus Design. The slim-tube displacement test is often referred to as the “industry standard” for determining MMPs. Unfortunately, there is neither a standard design, nor a standard operating procedure, nor a standard set of criteria for determining MMPs with a slim tube. The essential elements of a slim-tube apparatus are shown in Figure 1. A range of reported specifications for slim-tube apparatus are compared with our slim-tube apparatus in Table 1. Slim-tube length, diameter, and type of packing, and the permeability and porosity of the packing, have varied greatly in the designs used in industry. There are more than 30 studies in the literature that show the effects of these design variables on MMP determination. Unfortunately, some of the conclusions of these studies are contradictory (see refs 6-38). (6) Orr, F. M.; Silva, M. K.; Lien, C. L. Equilibrium Phase Composition of CO2-Crude Oil Mixtures: Comparison of Continuous Multiple Contact and Slim-Tube Displacement Tests. Soc. Pet. Eng. J. 1983, 281-291. (7) Kossack, C. A.; Hagen, S. The Simulation of Phase behavior and Slim-Tube Displacement with Equation of State. Paper SPE 14151 presented at the SPE Annual Technical Conference and Exhibition, Las Vegas, September 22-25, 1985. (8) Johnson, J. P.; Pollin, J. S. Measurement and Correlation of CO2 Miscibility Pressure. Paper SPE/DOE 9790 presented at the SPE/DOE Joint Symposium on Enhanced Oil Recovery, Tulsa, April 5-8, 1981. (9) Graue, D. J.; Zana, E. Study of a Possible CO2 Flood in the Rangely Field, Colorado. Paper SPE 7060 presented at the SPE Annual Technical Conference and Exhibition, Tulsa, April 16-19, 1978. (10) Kehn, D. M.; Gaske, M. H.; Pyndus, G. T. Laboratory Evaluation of Prospective Enriched Gas-Drive Project. Trans. AIME 1958, 213, 382-385. (11) Stone, H. L.; Crump, J. S. The Effect of Gas Composition upon Oil recovery by Gas Drive. Trans. AIME 1956, 207, 105-110.

(12) Holm, L. W.; Josendal, V. A. Effect of Oil Composition on Miscible Type Displacement by Carbon Dioxide. Soc. Pet. Eng. J. 1982, 87-98. (13) Yarborough, L.; Smith, L. R. Solvent and Driving Gas Composition for Miscible Slug Displacement. Soc. Pet. Eng. J. 1970, 298-310. (14) Rutherford, W. M. Miscibility Relationship in Displacement of Oil by Light Hydrocarbons. Soc. Pet. Eng. J. 1962, 340-346. (15) Jacobson, H. A. Acid Gases and their Contribution to Miscibility. J. Can. Pet. Technol. 1972, 11, No. 2, 56-59. (16) Huang, T. S.; Tracht, J. H. The Displacement of Residual Oil by Carbon Dioxide. Paper SPE 4735 presented at the SPE Symposium on Improved Oil Recovery, Tulsa, April 22-24, 1974. (17) Yellig, W. F.; Metcalfe, R. S. Determination and Prediction of CO2 Minimum Miscibility Pressures. J. Pet. Technol. 1980, 160-168. (18) Kuo, S. S. Prediction of Miscibility for the Rich Gas Drive Process. Paper SPE 14152 presented at the SPE Annual Technical Conference and Exhibition, Las Vegas, September 22-25, 1985. (19) Monger, T. G. The Impact of Oil Aromaticity on Carbon Dioxide Flooding. Paper SPE/DOE 12708 presented at the SPE/DOE Joint Symposium on Enhanced Oil recovery, Tulsa, April 15-18, 1984. (20) Glaso, O. Generalized Minimum Miscibility Pressure Correlation. Paper SPE 12893 presented at the SPE Annual Technical Conference and Exhibition, Houston, September 16-19, 1984. (21) Sebastian, H. M.; Wenger, R. S.; Renner, T. A. Correlation of Minimum Miscibility Pressure for Impure CO2 Streams. Paper SPE/ DOE 12648 presented at the SPE/DOE Joint Symposium on Enhanced Oil recovery, Tulsa, April 15-18, 1984. (22) Gardner, J. W.; Orr, F. M.; Patel, P. D. The Effect of Phase Behavior on CO2 Flood Displacement. J. Pet. Technol. 1981, 206781. (23) Cardenas, R. L.; Alston, R. B.; Nute, L.; Kokolis, C. P. Laboratory Design of a Gravity Stable Miscible CO2 Process. J. Pet. Technol. 1984, 111-118. (24) Zick, A. A. A Combined Condensing/Vaporizing Mechanism of the Displacement of Oil by Rich Gases. Paper SPE 15493 presented at the SPE Annual Technical Conference and Exhibition, New Orleans, October 5-8, 1986. (25) Omole, O.; Osoba, J. S. Effect of Column Length on CO2-Crude Oil Miscibility Pressure. J. Can. Pet. Technol. 1989, 28, No. 4, 97102. (26) Cohen, G. S.; Shirer, J. A. Prediction of the Condition Necessary for Multiple Contact Miscibility. Paper SPE 12111 presented at the SPE Annual Technical Conference and Exhibition, San Francisco, October 5-8, 1983. (27) Randall, T. E.; Bennion, D. B. Laboratory Factors Influencing Slim-Tube Test Results. Paper 88-39-119 presented at the Annual Technical Meeting of the Petroleum Society of CIM, Calgary, June 1216, 1988. (28) Flock, D. L.; Nouar, A. Parametric Analysis on the determination of Minimum Miscibility Pressure in Slim-Tube Displacement. J. Can. Pet. Technol. 1984, 23, No. 5, 80-88. (29) Sigmund, J. I.; Aziz, K.; Lee, J. I.; Nghiem, L. X.; Mehra, R. CO2 Floods and their Computer Simulation. Proc. 10th World Pet. Congr., Bucharest 1979, 243-250. (30) Enrick, R. M.; Holder, G. D.; Borsi, B. I. An equation of State Correlation for Minimum Miscibility Pressure in CO2 Floods. Paper SPE 14158 presented at the SPE Annual Technical Conference and Exhibition, Las Vegas, September 22-25, 1985. (31) Boersma, D. M.; Hagoort, J.; Frohlich, P.; Maljarrs, A. E. High Pressure Slim-Tube Experiments with On-Line Sampling Facility. EC Project, Delft University of Technology, 1988. (32) Glaso, O. Miscible Displacement: Recovery Test with Nitrogen. SPE Res. Eng. 1990, 5, No. 1, 61-68. (33) Randall, R. E.; Bennion, D. B. Recent Development in Slim Tube Testing for HCMF Solvent Design. Paper 3 presented at the Annual Meeting of the Petroleum Society of CIM, Regina, October 6-8, 1987. (34) Hudgins, D. A.; Liave, F. M.; Chunge, F. T. Nitrogen Miscible Displacement of Light Crude Oil: A Laboratory Study. SPE Res. Eng. 1990, 5, No. 1, 100-106. (35) Auxiette, G.; Chaperon, I. Linear Gas Drive in High Pressure Oil Reservoir Compositional Simulation and Experimental Analysis. Paper SPE 10271 presented at the SPE Annual Technical Conference and Exhibition, San Antonio, October 5-7, 1981. (36) Frimodig, J. P.; Reese, N. A.; Williams, C. A. Carbon Dioxide Flooding Evaluation of High Pour Point Paraffinic Red Wash Reservoir Oil. Soc. Pet. Eng. J. 1983, 587-594. (37) Pozzi, A. L.; Blackwell, R. J. Design of Laboratory Model for Study of Miscible Displacement. Trans. AIME 1963, 228, 28-47. (38) Henry, R. L.; Metcalfe, R. S. Multiple Phase Generation During CO2 Flooding. Soc. Pet. Eng. J. 1983, 595-601.

Measuring CO2 Minimum Miscibility Pressures porosity in the range of 30-45% does not appear to be a critical factor. Permeability, however, influences the injection rate during flooding. The desired final result from a slim-tube experiment is the oil recovery at a given pressure, so pressure drop across the slim tube should be low. With high permeability, pressure drop is kept low while operating at a high frontal displacement rate. There is a considerable difference of opinion reported in literature on the effect flooding rate has on oil recovery and the MMP. The slim tube used in our studies was made of Hastelloy C tubing, with an 0.25 in. o.d. and an 0.12 in. i.d. Three 20 ft sections of tubing were coupled to give a slim-tube length of 60 ft. The tubing was packed with 100 mesh sand and coiled into an 18 in. diameter coil. The coiled slim tube was mounted horizontally inside an air bath. One end of the slim tube was connected to fluid transfer cylinders as shown schematically in Figure 1. Inside each transfer cylinder, a piston separated mineral oil from either CO2, reservoir oil, or solvent. A positive displacement pump drove mineral oil which in turn drove the pistons in the fluid transfer cylinders. The other end of the slim tube was connected to a visual cell. The visual cell has a dead volume of about 1 cm3 and was also made of Hastelloy C. A back pressure regulator made of Hastelloy C was mounted immediately downstream of the visual cell. The entire assembly has a working pressure of 5000 psi at 300 °F. Injection pressure was measured with a pressure transducer mounted at the slim-tube inlet. Slim-tube fluid temperature was measured by a thermocouple mounted immediately downstream of the visual cell. Temperature of the air inside the air bath was measured by a thermocouple that was connected to the heat controller. A precision pressure gauge was connected to the dome of the back pressure regulator. Fluid coming out of the back pressure regulator is flashed into a gas-liquid separator buret. The liquid was measured with a 0.1 cm3 accuracy. The separator gas flows through a rotameter with an accuracy of 0.5 cm3/h. The produced gas volume was measured by a wet test meter. Operating Procedures. At the start of each displacement test, the slim tube is saturated with oil. Then gas is injected to displace the oil. Most operators inject gas with a positive displacement pump at a constant rate. A back-pressure regulator maintains a fixed pressure at the outlet of the slim tube. Injection rates reported in the literature vary widely. Typically, one displacement test takes one day, with another day or two for cleaning and resaturation of the slim tube. Some operators inject gas at a low rate to establish a mixing zone and then increase the rate in order to complete the run in a reasonable time. Operating the slim tube at a constant injection pressure is also reported. To determine MMP for a gas-oil system with a slim tube requires from 1 to 2 weeks of time. If asphaltene precipitation is a problem, the slim tube can become completely plugged and inoperative. MMP Criteria. To interpret the experiments, oil recovery at gas breakthrough, oil recovery at 1-1.2 pore volumes of injected gas, and ultimate oil recovery are often plotted as a function of operating pressure. In the early literature, MMP was the pressure at which ultimate recovery approached 100%. In later literature, the breakover pressure in these recovery curves was deemed the MMP. If the breakover is not sharp, MMP has been chosen as the pressure for which incremental oil recovery per incremental pressure increase is less than some arbitrary value. Other operators define MMP as the pressure when the oil recovery is 90 or 95%. To determine the minimum enrichment composition for miscible displacement by a condensing process at some limiting reservoir pressure, oil recoveries from several slim-tube tests with increasing levels of enrichment are compared. Oil recoveries at gas breakthrough, or recovery at 1.2 pore volumes, or ultimate recovery are then plotted versus the enrichment level. The minimum required enrichment can be

Energy & Fuels, Vol. 10, No. 2, 1996 445 identified by criteria similar to those used for MMP assessment: the enrichment at which recovery approaches 100%, the enrichment at which the recovery curves breaks over, etc. Visual observations of effluents moving through the sight cell downstream from the slim tube are also helpful for interpreting experiments. The visual data may be collected on video tape or with optical density measurements. Typically, the visual data indicate whether or not interfaces still exist in the effluent from the slim tube. The size of a methane bank in the effluent of the slim-tube apparatus has been discussed as an indicator of minimum miscibility pressure by some researchers.39 Below the MMP, a distinct methane bank is observed. As pressure nears the MMP, the size of the bank shrinks toward zero. Because this method requires frequent measurement of effluent composition throughout a slim-tube displacement experiment, its use is the exception and not the rule.

Rising-Bubble Apparatus Design. The rising-bubble apparatus, or RBA, was developed in the early 1980s.40-43 For later applications, see refs 39 and 44-48. A flowsheet for the RBA appears in Figure 2. The most essential feature of the apparatus is a flat glass tube mounted vertically in a high-pressure sight gauge in a temperature controlled bath. The glass tube is flat for better viewing of bubbles rising in opaque oils. The rectangular internal cross section of the glass tube is 0.04 by 0.20 in. (1 × 5 mm). The visual portion of the tube is about 8 in. (20 cm) long (Figure 2). The tube is back lighted for visual observation of the tube contents. A hollow needle is mounted at the bottom of the sight gage and protrudes into the rounded portion of the glass tube. The tip of the needle is kept about 1-2 in. (2.5-5 cm) below the flat portion of the tube. The needle diameter can be varied to control the bubble size. To obtain a permanent record of the shape evolution of a rising bubble, the apparatus includes a video camera mounted on a rack parallel to the path of the bubble in the sight gauge.49 This permits a magnified view of the bubble on a monitor and the rising bubble is recorded on a video cassette for subsequent reviewing. Operating Procedure. Initially, the sight gauge with the glass tube and the hollow needle are all filled with distilled (39) Novosad, A.; Costain, T. G. New Interpretation of Recovery Mechanisms in Enriched Gas Drives. J. Can. Pet. Technol. 1988, 27, No. 2, 54-60. (40) Christiansen, R. L. Method of Determining the Minimum Level of Enrichment for a Miscible Gas Flood. U.S. Patent 4,610,160 (September 9, 1986). (41) Christiansen, R. L.; Kim, H. Method of Determining Minimum Level of Gas Enrichment for a Miscible Gas. U.S. Patent 4,621,522 (November 11, 1986). (42) Christiansen, R. L.; Kim, H. Apparatus and Method for Determining Minimum Miscibility Pressure of a Gas in a Liquid. U.S. Patent 4,627,273 (December 9, 1986). (43) Christiansen, R. L.; Haines, H. K. Rapid Measurement of Minimum Miscibility Pressure with the Rising-Bubble Apparatus. SPE Res. Eng. 1987, 2, No. 4, 523-527. (44) Sibbold, L. R.; Novosad, Z.; Costain, T. G. Methodology for the Specification of Solvent Blends for Miscible Enriched Gas Drives. SPE Res. Eng. 1991, 6, No. 3, 373-378. (45) Novosad, Z.; Costain, T. G. Mechanisms of Miscibility Development in Hydrocarbon Gas Drives: New Interpretation. SPE Res. Eng. 1989, 4, 341-347. (46) Sibbold, L. R.; Novosad, Z.; Costain, T. G. Analysis of One Dimensional Rich Gas Displacements with an Equation of State Simulator. J. Can. Pet. Technol. 1990, 29, No. 1, 43-49. (47) Novosad, Z.; Sibbold, L. R.; Costain, T. G. Design of Miscible Solvents for a Rich Gas DrivesComparison of Slim Tube Tests with Rising Bubble Test. J. Can. Pet. Technol. 1990, 29, No. 1, 37-42. (48) Eakin, B. E.; Mitch, F. J. Measurement and Correlation of Miscibility Pressures of Reservoir Oils. Paper SPE 18065 presented at the SPE Annual Technical Conference and Exhibition, Houston, October 2-5, 1988. (49) Mihcakan, M.; Poettmann, F. H. A Motion Tracking Optical System for the Rising Bubble Apparatus to Determine MMP. Proc. 8th Pet. Congr. Turkey, Ankara (April 16-20) 1990, Paper 61, p 181.

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Figure 2. Schematic of a rising-bubble apparatus. and deionized water. Then, oil is injected downward into the flat glass tube, displacing the water. At the end of this step, the lower circular portion of the glass tube is filled with water, while the remainder of the tube contains oil. The pressure inside of the sight gauge can be adjusted to the desired level by addition or removal of water. After the pressure is set, a bubble of gas is formed at the tip of the hollow needle in the water phase. When the buoyant force lifting the bubble exceeds the adhesive forces holding the bubble to the needle, the bubble rises through the water, through the water-oil interface, and up through the column of oil. The behavior of the rising bubble is recorded on video tape. After one or more bubbles have risen through the oil, the contaminated oil can be replaced with fresh oil. MMP Criteria. The minimum miscibility pressure, or MMP, is indicated by the evolution of shape of bubbles rising through the oil in the RBA. For a vaporizing gas process, the evolution of bubble shapes was described by Christiansen and Haines.43 At pressures below the MMP, a gas bubble will have a shape similar to that in Figure 3A as it rises through the oil column. As shown in Figure 3A, the bubble usually shrinks as it rises because of the solubility of the gas in the surrounding oil. At the MMP, Figure 3B, the upper surface of the bubble retains its bullet shape, however, as it moves up the column the bottom of the bubble degrades and the bubble quickly disperses into the oil. At pressures well above the MMP, the gas bubble may disperse immediately upon contact with the oil without any indication of interfaces. The RBA can be used for measuring MMPs in a condensing gas process as well as in a vaporizing gas process; however, the method for measurement is different from that for a vaporizing gas process. For a condensing gas process, 5 to 10 bubbles of enriched gas are sequentially injected at each pressure. Far below the MMP, the evolution of bubble shape is similar for each of the sequentially injected bubbles. Specifically, the shape indicates low interfacial tension when the bubble first contacts the oil; however, after rising a short distance through the oil, the bubble shape evolves to indicate high interfacial tension (Figure 4A). At or above the MMP, the evolution of bubble shape changes with each successive bubble injected into the column of oil. The first bubble evolves from a shape indicating low interfacial tension to a shape indicating high interfacial tension, just as described above. But, with each additional bubble injected, the size of the bubble, which emerges from the swirl of gas and oil at low interfacial tension, shrinks. After four or more bubbles are injected, the pattern shown in Figure 4C is seen.

Figure 3. Bubble behavior for vaporizing gas process. Figure 4B shows shape evolution for a bubble just below MMP for a condensing gas process. The RBA permits the direct visual observation of miscibility and requires less than 2 h to determine MMP. Asphaltene precipitation does not appear to be a problem. Specks of asphalt can be seen precipitating out on the walls of the tube; however, bubble behavior can still be observed. The RBA also requires considerably less fluids to determine the MMP.

Comparison of MMPs from the RBA and the Slim Tube MMPs were measured on the RBA and the slim tube for 12 different systems: CO2 and decane, CO2 and a mixture of 43% n-C5 and 57% n-C16, and CO2 with 10 crude oils.50 The results are compared in Table 2. The method described above for measuring MMPs for vaporizing gas processes with the RBA was used for all the data of Table 2. Three criteria from the literature were

Measuring CO2 Minimum Miscibility Pressures

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Figure 4. Bubble behavior for condensing gas process.

used to determine the MMP from slim-tube tests. As expected, the MMP varied depending on the criterion used. During CO2 tests of oil D in the RBA, specks of precipitate (probably asphaltenes) appeared on the surface of the glass tube; however, these specks did not interfere with the determination of the MMP. When the same crude was tested in the slim tube, precipitation (probably asphaltenes) during CO2 injection completely plugged the slim tube, an MMP could not be measured. (50) Elsharkawy, A. M. CO2 Minimum Miscibility Pressure Measurements: A Comparison between Slim Tube and Rising Bubble. Ph.D. Dissertation, Colorado School of Mines, Golden, CO, 1991. (51) Thomas, F. B.; Zhou, X. L.; Bennion, D. B.; Bennion, D. W. A Comparative Study of RBA, P-x, Multicontact and Slim Tube Results. J. Can. Pet. Technol. 1994, 33, No. 2, 17-26.

In their comparison of the RBA with other methods for measuring MMP, Thomas et al.51 indicated that they were not able to see the rising bubble for a system that exhibited excessive solid precipitation. They resorted to a slim tube for the measurement. Oil recoveries at 1.2 PV of CO2 injected into the slim tube were plotted against pressure for MMP interpretation. Three sets of MMPs were recorded: pressure at 90% oil recovery, pressure at 95% oil recovery, and the breakover pressure in the oil recovery plot (or if curvature of the plot was large, the MMP was taken as the pressure for which oil recovery did not increase more than 1% per 100 psi pressure increasesin the remainder of the paper, we call this the “1%/100 psi” criterion).

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Table 2. A Comparison of CO2 MMP Measurements with Slim-Tube and Rising-Bubble Apparatus50 minimum miscibility pressure (MMP), psi oil n-C10 43/57n-C5/C16 X Y Z Md Nd A B Ce De E

gravity, temp, °API °F

41.4 40.7 38.0 51.0 35.0 38.0 39.0 36.0 34.0 43.0

100 122 140 161 100 190 184 121 180 140 100 140

slim-tube apparatus 95%b BOPc 90%a 1200 1500 1840 1990 2150 2380 1100 1185 2020 2140 2760 2920 1880 2080 2420 2470 3000 3450 plugged slim 1760 1880

1250 1550 1910 2380 1260 2200 3000 2280 2600 3600 tube 1960

RBA 1280 1550 1980 2370 1200 2210 3000 2360 2610 3650 3410 2010

Figure 5. Concave downward relationship for MMP and temperature for oil Y.

a MMP defined as pressure at 90% oil recovery. b MMP defined as pressure at 95% oil recovery. c MMP defined as pressure at breakover point or where oil recovery does not increase more than 1% per 100 psi. d Live oil. e Precipitation (probably asphaltenes) observed in RBA.

The results in Table 2 show that a slim-tube oil recovery criterion of 90% will usually yield MMPs lower than those from other criteria for the slim tube. An oil recovery criterion of 95% will yield MMPs which may or may not agree with those obtained from the oil recovery breakover criterion (which is replaced by the 1%/100 psi criterion in the absence of a sharp breakover). However, the oil recovery breakover (or 1%/100 psi) criterion produced an MMP from the slim tube that was in excellent agreement with the MMP from the RBA. With this criterion, the largest difference between MMP values measured by the slim tube and RBA was 80 psi. Although our comparison has shown that the risingbubble method and the slim-tube method give nearly comparable results, we recognize that the rising-bubble method requires a judgment that can vary from person to person. Good understanding of the mass-transfer process and fluid mechanics of the rising bubble will minimize the subjective error. And because the MMP measured with a slim tube may depend on its design, the 1%/100 psi criterion may not produce good agreement with the RBA for a slim tube that is different from the one used in our experiments. Our experience with the two methods for measuring MMPs demonstrated two other features of the rising bubble method. First, the rising-bubble method does not consume as much oil and gas as the slim-tube method. And, second, the rising-bubble method has the advantage of visually demonstrating the pressure at which miscibility can develop. Temperature Dependence of MMPs as Measured on the RBA With the RBA, the temperature dependence of MMP was measured for each oil listed in Table 2 up to 200 °F. Three types of temperature dependence are shown in Figures 5-7. For some oils, MMP increases less than linearly with temperature (Figure 5), while for other oils, MMP increases more than linearly with temperature (Figure 7). Some oils show a linear increase in MMP with temperature over the range tested (Figure 6). Our data show that MMP increases 10-20 psi/°F with an average of approximately 15 psi/°F.

Figure 6. Linear relationship of MMP and temperature for oil X.

Figure 7. Concave upward relationship of MMP and temperature for oil B.

Finding the temperature dependence of MMP with the RBA usually required 3 or 4 days; the majority of this elapsed time was needed for equilibration of the apparatus at each temperature. With the slim tube, we measured just one MMP for each oil sample because of the time needed to make a measurement. To find the temperature dependence of MMP with a slim tube would probably take 6-8 weeks. Why measure the temperature dependence of MMP for one oil? The temperature dependence of MMP is an essential feature of MMP correlations. For all the effort applied to correlating MMPs, very little data in the literature details the temperature dependence of MMP for any one oil. Most MMP correlations were built by regression on a set of MMPs, each representing one oil at one temperature. This approach to correlating MMPs with temperature was necessitated by the high cost of slim-tube measurements.

Measuring CO2 Minimum Miscibility Pressures

Conclusions 1. The MMP measured with the slim-tube method is very dependent on the criteria used to interpret slimtube performance. The slim-tube MMP may also depend on packing material, slim-tube dimensions, and operation. 2. Rising-bubble MMPs were in good agreement with those of the slim tube when slim-tube MMPs were determined by the oil recovery breakover point, or by the point where oil recovery increased less than 1% per 100 psi incremental pressure change. 3. The RBA is considerably faster means of measuring the MMP for both vaporizing or condensing systems than the slim tube. The RBA takes 1-2 h per MMP determination (excluding preparation time), while the slim tube takes 1-2 weeks per MMP determination.

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4. Asphaltene precipitation can completely plug slim tubes, whereas such precipitation is only a minor hindrance in the RBA. 5. The construction of a rising-bubble apparatus is considerably less costly than that of a slim-tube apparatus. Excluding the cost of licensing and the cost of labor, our RBA cost was about $15 000 including video equipment. Excluding the cost of labor, the cost of building our slim-tube apparatus exceeded $40 000. Acknowledgment. We express our appreciation to Marathon Oil Co., Shell Oil Co., and R. C. Earlougher, Sr. for financial support for the construction of the rising-bubble apparatus and the slim-tube apparatus. The rising-bubble apparatus was built and operated under a license from Marathon Oil Co. We thank Marathon Oil Co., Surtek, and Tiorco Inc. for providing the crude oils used in this study. EF940212F