Phase Equilibria in Hydrocarbon Systems 111. Solubility of a Dry Natural Gas in Crude Oil' W. N. LACEY,B. H. SAGE,AND CHARLESE. KIRCHER, JR., California Institute of Technology, Pasadena, Calif.
G
ASES dissolved in the crude petroleum in underground formations play an important part in the recovery of the oil. The presence of these more volatile hydrocarbons in solution contributes energy to help in forcing the oil through the porous sands to the well and through the tubing to the surface of the ground. It also causes a marked lowering of the surface tension and the viscosity of the oil, thus favoring better drainage from the capillary pores and greater freedom of movement through the sand channels. It is true that, if the gas leaves solution in the form of bubbles suspended in the oil, it may impede flow through the capillaries; but, if formation pressures are maintained] this difficulty may be minimized and benefit obtained from the advantages resulting from the dissolved material.
the latter but involves a mutual interchange of constituents between the two phases, the compositions of both phases changing with increase of pressure. The term "solubility" does however apply satisfactorily in certain special cases. If conditions are such that no appreciable amount of the original liquid passes over to the gas phase during the attainment of equilibrium, the process consists only of the dissolving of a portion of the gas phase in the liquid. This condition would be expected when neither the critical temperature of the solvent nor the critical pressure of the mixture is approached. Another case where the process consists only in the solution of a given gas in the liquid is found in a procedure consisting of a gradual compression of given amounts of gas and liquid until the gas phase just disappears and only liquid is left. The absence of gas phase a t final equilibrium thus AIhli5FVLntS 40 200 prevents loss of volatile constituents by the liquid. It would seem probable that the case of a dry natural gas dissolving in a crude oil a t 100' F. (37.8" C.) might be accompanied by a negligible transfer from liquid to gas. If this were true, the experimental procedure and the calculations to G A S IN E Q U I L I B R I U M determine and describe equilibrium conditions would be much simplified. To determine whether such an assumption would be justified, an experiment was carried out in the density balance unit described in Part 1.1 First, the change in density of a dry natural gas sample upon compression at 100' F. and throughout the pressure range was determined. Then the gas was brought to equilibrium, a t various pressures and the same temperature, with one of the crude oil samples, the variations in the gas density again being determined as a function of equilibrium pressure. A similar pair of runs was made a t 200' F. (93.3' C.). The resulting gas density curves are shown in Figure 1. The results are expressed in terms of 400 000 1200 1600 2000 2400 2800 specific gravity, referred to water a t its maximum density. P R E S S U R E , POUNDS PER S Q U A R E I N C H FIGURE1. EFFECT OF PRESENCE OF OIL UPON GAS DENSITY The composition of the gas was only slightly different from that of the gas used in the rest of the investigation. A knowledge of the solubility of natural gases in crude AT M O S P H E R E 5 40 80 120 160 petroleum and of the changes in properties resulting from I I I I , their presence is of importance in learning the mechanics of the processes taking place in underground formations, in estimating underground reserves, and in arriving a t rational methods of prorating petroleum production within a given pool. The present paper presents values for the solubility of one "dry" natural gas in several different representative crude oils a t 100.Oo F. (37.8' C.) and for pressures from atmospheric to 3000 pounds per square inch (204 atmospheres). In addition, data are presented showing the corresponding changes of the volumes and densities of the liquid phases present at equilibrium. The apparatus and methods used in this investigation have been described in Part I of this series.' The measurements presented here were made prior to certain minor improvements in apparatus and method included in the description. These additions have improved the accuracy of subsequent measurements. 400 800 1200 1600 2000 2400 When dealing with hydrocarbon systems involvhg volatile PRESSURE. P O U N D S PER SQUARE INCH constituents in the original liquid phase, the term "soluSOLUBILITY OF DRY NATURAL GAS IN FIGURE2. bility" in its strict sense cannot be applied. The process CRUDEOILSAT 100' F. whereby equilibrium is attained between the liquid and the The assumption of simple solubility holds reasonably added gas does not consist merely of a partial dissolving of well a t 100" F., but a t 200' F. the divergence of the density 1 Part I appeared on pages 103-8, January, 1934; Part 11, pages 214-17, curves is too great for such an assumption. It is possible that February, 1934. 652
June, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
653
ATMOSPHERES 120
60
40
I
I
160
I
I
I
I 800 PRESSURE,
400
1200 I600 2000 P O U N D S P E R S Q U A R E INCH
2400
GRAVITY OF LIQUID FIGURE 4. CHANGESIN SPECIFIC PHASEDUE TO SOLUTION OF GAS FIGURE3. CHANGES IN LIQUID VOLUMEDUE TO SOLUTION OF GAS AT 100' F. J
40
I
I
ATMOSPHERES 120
60
I
160
I
I
ATMOSPHERES
OF GAS COMPOSITION UPON SOLUFIGURE 5. EFFECT BILITY AT 100' F.
the agreement of the curves for 100" F. may be accounted for by a decrease in density of the gas phase due to greater relative solubility of its heavier constituents, which has been compensated for by a transfer of some of the lighter liquid constituents to the gas phase. However, within the accuracy of such measurements, this would be equivalent to the simple solution process. It has been assumed, therefore, for the purposes of this study, that the transfer from the crude oils to the gas phase a t 100" F. is negligible within the pressure range studied. The analysis of the dry natural gas used, in per cent by volume, was as follows: methane 84.4, ethane 8.6, propane 6.6, and heavier constituents 0.4. The sources and properties of the crude oils are given in Table I.
I
400
600 PRESSURE,
1200 POUNDS
1600
PER
2000 S Q U A R E INCH
2400
FIGURE6.
EFFECTOF GAS COMPOSITION UPON CHANQE IN VOLUMEOF LIQUID PHASEAT 100' F.
function of the equilibrium pressure. The solubility is expressed in volumes of gas, measured a t 60" F. (15.6' C.) and 14.73 pounds per square inch (30 inches of mercury, or 1 atmosphere) dissolved in unit volume of original oil, measured a t 60" F. These values may be converted to cubic feet of gas per barrel of oil by multiplying by 5.615. The corresponding increases in volume of the liquid phases due t o solution of the gas are depicted in Figure 3. The increase in volume is expressed as the ratio of the volume of the liquid phase existing at a given equilibrium pressure and temperature to the original liquid volume as measured a t 60" F. before any gas was dissolved. The changes TABLEI. SOURCES AND PROPERTIES OF CRUDEOIL in specific gravity of the liquid phases due to dissolved SAMPLES VISCOSITY gas are shown in Figure 4. The specific gravities are given Av . SP.GR. AT 86' F. as the ratio of the weight of a unit volume of the existing liquid MOL. W T . GRAVITY (100°/400) (30' c.) SOURCE A . P . I. Millipoises phase to the weight of an equal volume of water at its maxi0.781 26.5 Bradford, Pa. 186 45.8 mum density. 0.842 50.1 Santa Fe Springs, Calif. 199 33.4 Lima Ohio 226 38.2 0.818 46.8 Figure 5 gives an indication of the effect of the composition 0.845 70.6 Bartl&ville, Okla. 232 32.8 of the gas upon its solubility in Santa Fe Springs crude oil at 125.6 236 29.5 0.863 Ventura, Calif. 0.861 103.8 Yates, Texas 243 29.9 100" F. The solubility for the dry natural gas is compared 0.676 112.8 Sugarland, Texss 244 27.1 to that for pure methane. The absence of the ethane and Figure 2 shows the relations between solubility of the dry heavier hydrocarbons in the latter case is seen to result natural gas in each of the seven crude oils a t 100.0" F. as a in a much lower solubility. Comparisons of the resulting
INDUSTRIAL AND ENGINEERING CHEMISTRY
654
Vol. 26, No. 6
A T M O S P M ER E S
80
40
0.861
i
120
160
I
I
YI L
0.14
400
800
,200
PRESSURE,
1600
POUNDS P E R S Q U A R E
2000
2400
I
J
INCH
FIGURE8. RELATION BETWEEN SOLUBILITY AND SPECIFIC GRAVITYOF OIL
FIGURE7. EFFECTOF GAS COMPOSITION UPON SPECIFIC GRAVITYOF LIQUIDPHASEAT 100" F.
changes in volume and specific gravity of the liquid phases are shown in Figures 6 and 7. If the solubilities are expressed in terms of volumes of dry natural gas dissolved in a unit volume of original oil per unit of pressure [lo0 pounds per square inch (6.8 atmospheres) is taken here as a convenient unit of pressure], they may be roughly correlated with the specific gravities of the original oils as shown in Figure 8. The extent to which the change in volume of the liquid phase is a function of the amount of dry natural gas dissolved is illustrated in Figure 9. The points shown are taken from the individual measurements and not from the smooth curves drawn through them in Figures 2 and 3. Adjacent points from different oils in Figure 9 may correspond to rather widely different equilibrium pressures. That such a correlation applies, however, to only one gas composition is shown by the lower dashed curve for pure methane in Santa Fe Springs crude oil. The experimental points for this latter curve are omitted to avoid confusion, but they fall close upon the line drawn. If the assumption is made that the volume of the crude oil itself remains constant and that changes of volume are due to dissolved gas, the apparent specific gravity of dissolved gas can be calculated. Figure 10 shows the results of such calculations for each of the individual points of Figures 2 , 3,5,and 6, plotted as a f u n c t i o n of equilibrium p r e s sure. The apparent specific gravity appears t o be n e a r 1y independent of pressure, although a slight increase is indicated at the higher pressures. The divergence of the points i s somewhat i n creased b y t h e l I I 20 40 60 80 sensitiveness of the S O L U B I L I T Y VOL. O F G A S PER UNIT V O L O F O I L apparent specific FIGURE 9. RELATIONBETWEEN CHANGES IN VOLUME OF LIQUID PHASE gravity values to small errors in the AND AMOUNT OF GAS DISSOLVED AT 100' F. m e a s u r e m e n t of the volume of the liquid phase. The apparent specific gravity of methane, as would be expected, is found to be somewhat lower than that for the dry natural gas. CONCLUSIONS The assumption that a simple dissolving process occurs when a gas is brought to equilibrium with a liquid in complex
hydrocarbon systems is valid only when the system is far enough below the critical temperature of the solvent and the critical pressure of the mixture that there is no appreciable transfer of the components of the original liquid phase into the gas phase. The solubility of a dry natural gas in a given crude oil at 100' F. is almost directly proportional to the saturation pressure. The ratio of solubility to equilibrium pressure has in general a roughly linear relation to the specific gravity of the crude oil. Change in gas composition has a marked effect on solubility as is shown by the large difference in the solubility of methane and of the dry natural gas in the same crude oil. ATMOSPHERES ",
~
0.4 8
120
8
u
FIGURE 10. RELATION BETWEEN APPARENTSPECIFIC GRAVITY OF DISSOLVED GAS AND EQUILIBRIUM PRESSURE AT 100' F.
The change in volume of the oil for any definite amount of a given gas dissolved is almost the same for all the crudes studied. However, the pressure a t which this solubility would be obtained would vary for each crude oil. The apparent specific gravity of a given dry natural gas dissolved in crude oil a t 100" F. is nearly independent of the pressure and of the solvent. ACKNOWLEDGMENT This investigation was carried out as part of the work of Research Project 37 of the American Petroleum Institute and was made possible by funds allotted by that organization. The following agencies kindly furnished the authors with the samples of crude oil used: Forest Oil Company, Shell Oil Company, Ohio Oil Company, Bartlesville Experiment Station of the Bureau of Mines, and the Humble Oil and Refining Company. The authors are also indebted to the Standard Oil Company of California for the dry natural gas used. The assistance of E. S. Hill in measuring the viscosities of the crude oils and of J. G. Schaafsma in the preparation of the figures is gratefully acknowledged. RJXEIVEDFebruary 3. 1934.
GERMAN 1933 DYETRADE.The receding trend in German dye exports in 1932 was arrested in 1933. Total exports of all types of dyestuffs amounted t o 26,818 metric tons valued at 133,738,000 marks in 1933, compared with 29,142 tons valued at 133,662,000 marks in 1932.
