US2380081A - Method of evaluating the content of gas reservoirs - Google Patents

Description

July l0, 1945.

J. P. SLOAN METHOD OF EVALUATING THE CONTENT 0F GAS RESERVOIRS Filed March 2 6, 1942 4 Sheets-Sheet 1 Qby J. P. SLOAN July l0, 1945.

METHOD OF EVALUATING THE CONTENT OF GAS RESERVOIRS Filed March 26, 1942 4 sheets-sheet 2 5 4 4 m 7 Y H 7 sw 5 mZ m 78 W. 7 W5 M j Z A 7% l my 1o, 1945.

J. P. SLOAN METHOD OF EVALUATING THE CONTENT OF GAS RESERVOIRS Filed March 26, 1942 4 Sheets-Sheet 3 K 7 6 5 4 3 Z 0F. 0 l 0 w M A 6 7 6 w 5 w 4 m a m 5 z w P a m N o 7 n A 6 L 5 4 E a m m z m w s m M w P 7 6 s 2 098.765 4 a 2 090765 4 a z 0 Patented July 10, 1945 UNITED'- STATES PAT-ENT OFFICE METHOD OF EVALUATING THE CONTENT` OF GAS RESERVOIRS James Paul Sloan, Houston, Tex., asslgnor to Houston Laboratories, Houston, Tex., a corporation of Texas Application March 26, 1942, Serial No. 436,295

(Cl. i3-'151) 11 Claims.

l The invention relates to a method for evaluating the content of gas reservoirs, gas distillate reservoirs, and the gas cap of oil reservoirs for herein referred to is to determine the volume of gas and distillate or other liquid hydrocarbons that can be ultimately produced therefrom, and

the term behavior as used above refers to the callythere is a definite compound identied as methane containing in addition to hydrogen the one carbon atom, a second identified as ethane containing two carbon atoms; propane' containing three carbon atoms and ,so on up to a definite compound of forty carbon atoms. As the chemical structure becomes, more complex isomers are `forrned such as iso-butane, which is the nquid but highly volatue, the remainder are from relatively stable liquid to' solid. However,

in the natural reservoir, quite a different set of y conditions is present. The temperatures are higher, ranging from 140 1'". to approximately 3005 F. with even higher temperatures in prospect with the advent of' deeper drilling. 'I'his increased temperature tends to vaporize the lighter portions of Ithe normally liquid hydrocarbon mixture described above and to convert the normally solid paraihnes and tarry portions to the liquid state. 'Ihe pressures in these reservoirs range from a few pounds per square inch above atmospheric pressure to approximately six thousand pounds per square inch. These pressures tend to reverse the vaporization effect of the elevated temperatures and to reisomer of normal butane. All natural gases are composed of a mixture of the latter hydrocarbons, and the crude oils'l asphalts and tars are mixtures of the heavier ones. No two of these mixtures is identical in composition and from the enormous number of possible compounds and the Wide variation o`f their properties it is easy to see the impossibility of analyzing each of these mixtures with accuracy.' Both the time andthe cost would be prohibitive. So for convenience the 'lightends are analyzed by cooling and fractional distillation with some degree of accuracy up through compounds containing six or seven carbon atoms. The boiling points of these compounds are relatively far apart and they can be separatedfeasily. The yhigher boiling compounds, whose boiling'points are successively closer together, are then grouped accord-y the gaseous state, the. next three or four are liquefy the ethane to octane group and to force the methane`into solution in the liquid. This stripping of the ethane-plus portionsot the gas by `increasing pressure continues until the pressures reach'approximately nine hundred to one thousand pounds per square inch. In this region the process of squeezing the ethane-plus materials out of the methane comes to a halt, and as the pressure further increases a reversal sets in. From here on, the ethane-plus material begins to reenter the vapor phase, the lighter materials in larger quantities than the heavier. This process continues with successively heavier materials going into the vapor phase as the pressure continues to rise. This phenomenon is known as retrograde vaporization, which means the vaporization is taking place under conditions which are normally expected to produce condensation. Conversely, when the pressure on this vaporized material is reduced, the ilrst of the heavy, then the successively lighter compounds, begin to condense to the liquid phase. This phenomenon is known 4as retrograde condensation.

With the advent of deep drilling in the search for petroleum a number of high pressure, high temperature gas reservoirs have been encountered. Many of these gases contain signincant quantities of the heavier hydrocarbons held in the vapor phase by virtue of the pressure and temperature of the reservoir. In the normal production of these reservoirs, as part of the gas content is withdrawn the pressure upon the remainder is lowered. 'I'his lowering of pressure on the remaining reservoir iluid causes a condensation of some of the heavier hydrocarbons.` Continued withdrawals of material causes a continuing condensation until the pressure in thek reservoir drops to approximately` one thousand pounds per squareinch. The liquid condensate thus precipitated serves to wet the surrounding sand body and is thereby forever lost.

There has been developed in recent years a system of operation known as cycling or recycling which is designed to prevent lthis irretrievable loss of material in these condensate ordistillate reservoirs. In this process, gas is withdrawn from a well in one part of the neld, transmitted by pipe lines to a plant, andl there processed by means of pressure reduction and absorption for the removal of the condensable portions of material, the residual gas being then recompressed to above formation pressure, and reinjeeted into another part of the reservoir. This capable of operation at a constant temperature procedure tends to maintain approximately the original reservoir pressure and thereby preventsv precipitation of the heavier hydrocarbons; also the liquid material composed of gasoline and kerosene fractions is recovered for immediate sale and the excess gas is conserved for future use.

The volume of the recoverable liquid varies widely with different reservoirs. The rates at which it is condensed by pressure reduction is a function of the particular composition of the material in the reservoir. Therefore, in order for the owners to-know whether or not the loss of this liquid by normal withdrawal of the reservoir contents justies recycling, it is necessary that they know the behavior of the material in the reservoir under reservoirtemperature and diminishing pressure. Such information would enable the owners to make a decision as to recycling, absorption, or a simple separation operation as the most profitable method of producing the reservoir. A knowledge of the behavior of the reservoir material at both a reduced pressure and a reduced temperature would enable the' owners to independently plan a program for handling the material after itis brought to the surface, such as separator pressures, absorption pres'- sures, etc. Compresslbility data would enable him to select the proper pipe line size, compressor volumes, or any place in the operation where he might be dealing with pressure-volume-temperature relationships and to more closely estimate his reserve of liquid and gas and the reservoir energy available for the production of the reservoir. s

'I'here are a number of methods in use at the I present time for evaluating distillate reservoirs but these methods are subject to certain objections which are inherent in the particular method. In view of the necessity for information concerning condensate" behavior and in view of the defects of previous methods and apparatus it is' an object of the invention to Drovide a'method for eliminating such defects, and which has been perfected so that reliable information can be secured thereby.

A more specific object of the invention is to, provide a method for evaluating reservoir iuidy which consists in obtaining samples of the liquid and gas of a stabilized well, under conditions of temperature and pressure which exist at the eld separator, recombining the samples of liquid and gas in the same ratio of volume of liquid to volume of gas as that produced by the well during the sampling period, and then making phase equilibrium determinations on the recombined materlal.

'ples are recombined. The combination of liquid and SaS thus produced represents the reservoir nuid entering-*the well tubing. The apparatus is and the pressure-liquid volume behavior of the reservoir fluid may be observed and recorded from separator pressure to reservoir pressure. It is also within the purview of the invention to provide thermostatic control for the apparatus so that any number of isothermal pressure-liquid volume curves may be obtained without disturbing the original charge with respect to composition.

The drawings also disclose a variable volume cell for the purposes above described which incorporates visibility features so that the different phases of the reservoir fluid may be actually observed and questionable readings may be checked for accuracy at any time up to the discarding of the sample.

The present method also requires a fixed volume cell and apparatus whereby the same may be calibrated and operated to determine the pressure-volume relationship of the gas sample taken from the field separator. 'Ihis compressibility data is of value in calculating the equivalent amount of the liquid to be added to a 'dennite volume of the gas to obtain an equivalent mixture to that which entered the well bore' from the formation during sampling. The advantage of this step in the present method resides in the fact that the calculations are made lon the. basis of the actual determined compressibility of the gas. x l

With these and various other objects in view, the invention may consist of certain novel features of construction .and operation, as will be Y more fully described and particularly pointed out in the specification, drawings and claims appended thereto. In the drawings which illustrate an embodiment of the device for carrying out my method, and wherein like referenceeharacters are used to designate like parts- V 't Figure i is an elevational view showing in operative relationship one preferred arrangement of apparatus for carrying out the method of vthe invention; y

Figure 2 is a vertical sectional viewtaken sub- Y stantiallythrou'gh the center of the variable volume cell and showing the movable piston for attaining diiferent pressures and the stirring means for bringing the gas-liquid mixture to equilibrium;

Figure -3 is an enlarged transverse sectional view of the cell shown in Figure 2 taken substantialy along line 3-3 of said gure and lookingin the direction of the arrows;

Figure 4 is an enlarged sectional view illustrating the internal construction of one form of valve 'that may be provided in both the gas and the mercury manifold; Y

Figure 5 is a fragmentary sectional view showing in detail the interior of'the xed volume cell an the manner of connecting the mercury manifold thereto; A

Figure 6 is a graph showing one pressure-volume curve of separator gas at 210 F. v

Figure 7 is a graphshowing twoisothermal .pressure-volume curves; and

Figure 8 is avgraph showing three isothermal liquid phase curves illustrating the condensationl of liquid by reduction of pressure.

The first step in the present method consists in sampling 4a stabilized well, that is, fn collecting samples of the liquid 'and gas `produced by the well under conditions of temperature and pressure whichexist atthe field separator and.'

in measuring the amounts of such production.

Y Samples of the liquid may be taken from the separator by the downward displacement of mercury in a suitable container. Samples of the gas may be taken from a convenient tap in the gas line leading from the separator by purging the container followed with closing of valves in such a manner as to build and maintain the separator pressure. The above samples are taken from the separator usually at pressures. from four hundred to a thousand poun'ds per square inch, and

are maintained at this pressure until used according to Pthe present/invention. 'I'he gas-oil ratio is accurately determined from the orifice meter data showing the gas produced, and from the calibration of the separator by which the actual volume of the liquid produced during the period of test may be computed. These data together with the temperature of the separator and the temperature and pressure of the reservoir are the only fieldv data essential for the method.

The next step in the method is to obtain datal as to the compressibility of the gas sample. For such tests the apparatusincludes a iixed volume cell I0, which is shown in Figure l in associated relation with other parts of the apparatus and in Figure in fragmentary cross section to illustrate the internal construction of the cell. The information as to compressibility ci? the gaseous phase is required for the precise measurement of the` volume of gas to be injected, in accordance with the invention, into the equilibrium cell 20, Figures 1 and v2. 'Ihe latter may be termed' a variable volume cell in order to distinguish from cell I0. Y

The cell I0 is immersed in oil within receptacle II and the electric heating unit I2 is provided for heating the oil to desired temperatures, which are thermostatically controlled by the thermo-l regulator I3. The motor I4 provides a depending armature shaft to which is secured thestirring propellers I5. The cell I0 at its upper end is connected. to a gas manifold, 'to be presently described in detail, by means of `valve IB and conduit I'I. 'I'he bottom of the cell, as best shown in Figure 5, is equipped with a connection i8 which extends through the bottom wall of the receptacle II, being joined by the nipple i9 with a mercury manifold so that mercury may be injected into the-bottom of the fixed volume cell I0.

Following the compressibility determinations a' definite volume of gas, which ,may be accurately calculated by the aid of the compressibility data, is injected into thevariable volume cell along with a definite amount of liquid to produce reservoir material equivalent to that which entered the well bore from the formation during sampling. I

In the variable volume cell a number of phase equilibrium determinations are made on said resultant mixture. The cell 20 is suitably support- `ed within acylindrical receptacle 2l substantially an upper portion 21 and a lower portion 28 for convenience, the said two parts being securely united by bolts 29 and which effect a hermetic seal between thetwo parts by means of the interposed gasket 30. The upper portion 21 containing the oating piston 3| is closed 4by the head 32 and by the retaining.' member 33 which ts over the head, having threaded connection with cylinder 21. Bytightening the bolts 34 the cylinder head may be forced against the interposed gasket 35, thereby makinga mercury-tight closure.' The piston rod 36 passes through the cylinder head, which is provided with a packing gland 31. Said piston rod at its upper end carries an Aindex pointer 38 which moves along the scale 39, thereby indicating to the nearest cubic centimeter the volume of spacenwithin the cell 20 below the piston 3I.

Within the lower portion 28 of cell 20 there is located an electricmotor 40 having an armature vshaft depending therefrom to which is secured the large propeller 4I and the relatively small propeller 42, the latter being located within the bore'43 provided in the lower portion 28 of the cell. Large propeller 4I thoroughly agitates and mixes the gas below piston 3| and the small propeller is located at the extreme lowerend of the shaft so as to project into and stir the liquid within bore 43. By means of this stirring equipment all portions of each phase, that is, the gas and the liquid, may be brought into intimate contact with each other so that equilibrium between them may be established in a short period of time. Bore 43 in portion 28 of the cell is provided with a relatively small slot 44 extending for approximately three-fourths of the length of the bore and which leads tothe exterior so that the bore is thus opened by the slot. In accordance with the invention -the cell 20 is provided with visibility features whereby the liquid phase of the reservoir material Within the cell may be observed. The slot 44 is Atherefore closed by the glass window 45 which is in contact with a slotted gasket 46. On the front side of this glass window there is positioned a similar slotted gasket 41 and said gasket and the glass window are securely held in position by the rectangular metal plate 48 bolted to the portion 2B of the cell by means of bolts 4Q. Adjustment of plate 48 is effected by the set screws 50. The interior of bore 43r is visible through window 45 by means of the longitudinal slot 5I in the said retaining plate 48. A window constructed in accordance with the' foregoing has been4 safely tested to Referring to Figure `2, the variable volume -cell identified by numeral 20 consists of a steel cylinder which may be constructed of two parts,

seven thousand pounds per square inch at 280 F. However, incase oi' breakage of the glass the possibility of injury to the operator is negligible due to the very small unsupported area comprising the slot. It is impossible for an explosion to occur in a lateral direction nor can the gasket blow out toward the top or bottom as the ends are confined within the solid metal of the lower portion 28 of the cell.A This window provides means whereby the operator may observe the liquid-gas interface of the reservoir material within the cell and also the mercury-liquid interface which during operation of the apparatus is positioned at some point\longitudinally of the bore 43. The small cross sectional area of bore 43 has i the effect of increasing the length of the column of liquid and by means of a line scribed on the inside surface of window 45 a zero position 52 is provided for the measurement of both the liquid and gas volumes.l The presence of this index on @the inner face oi the window eliminates the 4 errors due to parallax which would be present if the index were on the outside.

Just above the restricted portion of cell 20 formed by the bere 4s there is provided additional windows so that the behavior of the gaseous phase within the cell may be observed by the operator. These windows are formed by the threaded cups 53 which are threaded into the cell so as to extend completely through the wall of the same. The cups are provided with an axial passage and itwill be understood that the. axial L'passage of one cup is-aligned with that of the other so that a light placed in front of one is visible from the other side of the cell. If necessary the cups may be disposed somewhat eccentric with respect to the axis of the cell in order that the armature shaft of motor 40 will not ob- 'e struct the passage of light. Each cup -is provided with a glass window 54, the same being retained within the cup by means of the threaded closure 55, likewise having an axial passage alignedl with that of the cup. I'he purpose of this set of windows is to provide a means of detecting the iirst formation of fog, that is, dew point of a mixture which has passed entirely through its critical region and has become a single phase system. 'As

pressure on this single phase system is slowly reduced, the material will pass through a very denite point where the formation of minute droplets of liquid takes place. This fogBiDS will cause a sharp decrease in the amount of with suitable' packing and retaining means I8 whereby the pressure on the packing may be adjusted. This is all that is necessary as concerns the piston since the same is never subjected to a differential pressure greater than that necessary to move the piston in the cylinder against its own friction on the cylinder walls. In operation this pressure rarely exceeds fifty pounds per square inch differential and is easily taken care of by the packing on the piston. In addition the diil'erential exists only during'the period of move- .ment of the piston and is not a factor worthy of consideration duringthe equilibrium determinations. The effect of the piston rod is to slightly reduce the area of the top of the piston. Since the mercury above the piston has to support the thrust of the gas below, the resulting pressure of the mercury is always slightly higher than that within the gas chamber. Therefore am] tendency toward leakage around the piston would be for the high pressure mercury to by-pass the piston and enter the chamber below. This would not be detrimental to the reservoir huid the cell as the mercury can be withdrawnple Il. From the nipple the conduit tl connects curate measurement.

with valve l2 and a similar valve Il is located in the mercury manifold. 'I'he connection 55 of cell 2l, which extends through the bottom wall of receptacle 2l is joined to the nipple Il which in turn is threaded to conduit i1, connecting with the valve il. By means of this valve and a substantially similar valve il mercury from manifold il may be admitted or withdrawn from bore I3. 'linee-way valves ofthe conventional or cross-angle type may be used in the apparatus as shown in the drawings. In the type of valve shown in Figure 4 the rotatable element 1I is provided with a diametrically extending passage V1I having a lateralv passage 12. By positioning the element 1l, as shown, the conduit; 13 and 15 are connected andthe conduit 1l is closed. In

other positions of the rotatable element the conduit 'I3 may be connected to 1I, or 'Il may be connected to 15.

The gas manifold i5 is used for the ,transference of gas from the sample cylinder 18 to either of the cells Il, 2l. Conduit "from the manifold delivers the gas to and conducts the same from the cell Il. YThe conduit 11 conducts gas fromthe manifold to cell 2l, and in this case the said conduit also delivers liquid to the cell from the sample cylinder 'Il having connection with both the gas manifold and the mercury manifold. The manometer 'I0 is used to measure accurately the original pressure of the gas in cell Il.

Mercury is supplied to manifold il by the calibrated mercury pump Il and by the Power driven pump Il. The latter is used for the bulk movement of mercury from the tankr into either cell Il or 2l. 'Ihe calibrated pumpr Il iniects accurate quantities into cell Il for the "compressibility measurements. and said pump is also used in measuring the exact quantities of liquid' from the sample container ll'to be delivered to `cell 2l, and inmeasuring the volume of residual liquid in cell 2l. At the end of the mercury manifoldpopposite the pumps a. dial gauge I3 and a dead weight or piston gauge Il are provided for indicating the pressure existing in 'the manifold. The dialgauge indicates the approximate pressure and the piston gauge givesa more ac- After the containers 1I and 1l for the gas and iiquidsamples respectively have been properly connected into the present apparatus, the operation of the same is as follows:

The cell Il and cell 2l are both thoroughly washed with peu-one ether er other mutable solventV under a pressure of from one hundred pounds to four hundred pounds per square inch.

This liquid is caused to enter the gas manifold' BI and by opening'valves Il and II the liquid is introduced into the respective cells. thoroughly washing all residual oily material which might be left from a previous determination and which would be a source of error. The washing solvent is withdrawn' through valves l2 and il, being through thel bottom connection Il, havln threaded connection with the lower end of-the bore Il. y

The iixed volume cell Il and the variable vol- Vume-celll are connected to the mercury manifold Il by somewhat similar structure. lThe expelled through the outlet conduit 8i provided for each ot said valves. After closing the said outlet -vnlves the cells II and 2|.are dried-by -tboromhly evacuating them through the gas manifold. The cylinder I lis no' filled with gas from sample. container 1l until the pressure is approximately ten pounds per square inch above ahnosphericLpressure. This pressure is accurately measured by means of the mercury ma Ynuuietcr 1l which connects with'tbe gas mani- -told through the three-way valve '81. Also the' y temperature of the oil surrounding cell I sure, and the composition of the gas.

adjusted to approximately room temperature is recorded to within plus orminus .2 F. by means of an ordinary thermometer suspended in the oil bath. By means of the application of the gas laws to the known volume of gas under the above described conditions of temperature and pressure, the actual volume of the sample within the cell I0 at standard conditions of 60 F. and 14.7 pounds per square inch absolute can readily vbe found. 'I'he object now is to determine the compressibility of this particular gas.

For the compressibility determinations an accurately measured volume of mercury is injected into cell I0 by means of the calibrated mercury pump 80 which requires that valve 88 be opened to said pump and that valve 63 be opened to connect the cell with the mercury manifold. The resultant rise in pressure in cellA l0 is indicated approximately by dial gauge 83 and the same may be accurately measured by the piston gauge 84. The reduced volume and increased pressure are recorded on a suitable form by the operator. An additional measured volme of mercury is now forced into cell l0 by pump 80 and this further reduced gas volume and further increased pressure are recorded in tabulated form with the first reading. This procedure is repeated until the gas pressure in cell I0 is of the order of one thousand to fteen hundred pounds per square inch. During this procedure the cell l0 is maintained at a constant temperature, controlled by the thermo-regulator I3. The tabulated information is corrected as to volume by the application of the known behavior of mercury to temperature changes and to pressure changes. An adjustment is likewise made to correct the original calibrated volume of cell I 0 to its actual vol ume at the temperature of determination. The differences in these corrected volume of the cell and the injected mercury give the true volumes of the known amount of the gas sample at the respective elevated pressures. This information is thenplotted and a graph is secured for the particular gas sample similar to that shown in Figure` 6,\whch indicates the pressure-volume relationship of separator gas at 210 F. By draining the mercury from cell I0 back into the tank 82, requiring actuation of valve 89 to admit the mercury, tothe vertical conduit 90 leading to the tank, the'process can be repeated at any numberv of elevated temperatures up to the capacity of the heating unit l2.

' By the plotting of one or more of these l isothermal pressure-volume curves, the engineer can solve many of the problems of gas reserves,

Vcompressor types, and capacities of transmission pipe lines in the gas industry. The value'of these 'data lies in the fact that in engineering work there are no perfect gases, that is, those which obey the laws of Charles and Boyle. They do this with fair accuracy fora relatively few poundspressure above atmospheric, but then the y 'gases begin to deviate, and the magnitude of this deviation varies with the temperature, pres- It is for the determination of this deviation factor or compressibility factor that the method `above described has been developed. By continuing the* method from plus ten pounds to onethousand pounds .per square inch, then through 'another cycle starting with -cell I0 filled to bsix hun- 'dred or eight hundred pounds per square inch and running up to approximately ve thousand pounds, the actual volume occupied by one cubicfoot of gas at 60 and 14.7 pounds pressure absolute can be accurately determined at any temperature and pressure up to and including the formation temperature and pressure.

Having determined the compressibility factor A' or the specific volume of the gas, it is now possible to calculate bya simple ratio from the production data of the well during sampling, the

equivalent amount of liquid to be added to a definite volume of the gas to give an exact equivalent mixture to that which entered the well bore from the formation during sampling. In other words, said mixture mayV be considered the'reservoir fluid produced .by the particular well during the sampling period. The phase equilibrium determinations are carried out in cell 20. Gas from the sample con# tainer 'I6 is admitted to saidpreviously washed and evacuated cell, valve 85 being open for the purpose. The mercury manifold is open to allow mercury in back of the pistn 3| to return to the reservoir tank .82. The piston will therefore rise under the pressure of the'gas and the withdrawal of mercury is continued until thev piston is very close to the top of its chamber when valve 92 in the mercury manifold and valve 85 in the gas manifold are closed. The quantity of gas in cell 20 is then allowed to come to the through the calibrated mercury pump 80. The

exact volume of the gas within cell 20 Vis now noted from the position of the index pointer 38 with respect to the scale 39. The pointer is actuatedr by the piston rod 36 which moves with the piston 3| From the data obtained during the compressibility determinations, this volume-pressure-temperature information is now translated into cubic feet at 60 F. and 14.7 pounds per square inch absolute. With this gas volume known, it -is now possible to calculate from the well test data, giving the number of cubic feet of gas -at 60 F. and 14.7 pounds per square inch absolute per barrel of separator liquid under separator pressure and temperature, the exact equivalent volume of liquid contained in the cylinder 18, which must be added to the gas within the gas chamber of cell 20 so that the resultant mixture is equivalent to the reservoir fluid which entered the bottom of the well from the formation during the sampling period. The injection within the cell 20 of this calculated volume of liquid is done as follows:

With valves 85 and 69 closed as regards cell 20, and with valve 92 closed to prevent further withdrawal of mercury from the upper end of cell 20 through conduit 93, and with valve 88A open, pressure is built up in the mercury manifold by pump to approximately the pressurev 20. It is possible to' obtain an indication of themercury entering the zas chamber iw mmm-a .of the window 45. The pressure on the mercury manifold is then noted and valve 94 is closed. Valves 98 and 99 are now opened and the pressure in the cylinder 18 is adjusted by means of pump 80 until it is equal to the gas pressure in cell 29. In order to be sure that this is true, valve 94 is opened, with valves 98 and 99 remaining open, and any slight difference in pressure is equalized throughout the entire system consisting of cell 20, the liquid sample container 18, mercury manifold 69, gauges 83 and 84, and the calibrated mercury pump-89.

Valve 91 remains closed during the above procedure. The exact volume as indicated on the scale and dial of mercury pump 80 is ynow read to .01 cc. and recorded. With valve 94 closed and valves 98 and 99 open, the upper valve 91 also being open, a path is provided for the mercury to enter the bottom of container 18 to displace an equal amount of hydrocarbon liquid from said container through valve 91 into the conduit 11 and which is eventually injected Vinto cell 20. The volume of liquid calculated as above to be added tothe gas in cell 2li` is displaced by the mercury and when the exact amount has been forced from cylinder 18 the valves are closed. To drive out the llast remaining quantity of liquid from conduit 11 valve 94 is opened and mercury is forced by pump 80 through the by-pass conduit 96 and into said conduit 11 so that all the liquid is displaced by the mercury, which forces the same into cell 20. .When the dropping of mercury from the inner end of conduit 11 is visible from window 45, the valve 94 is closed and the cell 28 is ready for the phasev equilibrium determinations.

The said determinations are usually run at three temperatures, one at approximately atmospheric,l one at approximately 125 F., and one at the temperature of the reservoir. The oil bath surrounding cell 20 is brought up to the desired pumped into the upper end of cell 20 above the piston 3| to force the piston downward, thereby compressing the gas-liquid mixture within the gas' chamber. At intervals the pump 8| may be stopped and the pressure on the manifold noted so as to secure an indication as to the approximate pressure rise within the gas chamber. When the pressure has been raised from fifty to two hundred pounds per square inch above the pre-r vious equilibrium point the necessary valves are closed and the mercury is confined above piston 3| so as to maintain the desired pressure on the temperature by means of the heating unit 22, and

the same is maintained constant by the thermoregulator `23. As soon as the desired temperature is reached stirring motor 40 within cell 2|J is started.` With one propeller located in the gas chamber and the other located within bore 43 so as to contact the liquid, both phases, of the reservoir material are agitated to bring each into intimate contact with the other. In the course of approximately two to three hours complete equilibrium "between them is achieved. The motor 40 is then stopped,l its heat is allowed to dissipate, and the mercury level is adjusted until the lowest point of the liquid-gas interface is exactly level with the zero mark 52. This is done by opening valves 8.9 and 88 and manipulating the piston of pump .80.' Thev exact pressure exerted by the mixturel i within cell 29 is determined by means of gauges 83 and 84. The volume of the gas phase is def termined by noting the position of the poinr 38 on scale 39. Then the reading of the mercury pump 89 is recorded. Mercury is now injected by said pump until the topmost position of the liquid-mercury interfaceis exactly opposite the zero index, and the reading of pump 80 is again recorded.

The difference between this last reading and previous ones, after'the applicationof a correction. due to the capillary displacement of the menisci, is recorded as the volume of equilibrium liquid at the observed temperature, pressure, and equilibrium gas volume. The power driven mercury pump 8l is now'started and byclosing certain valves and opening others mercury can be vphase curves.

reservoir fluid. The rise in pressure has disturbed the equilibrium of the fluid, and since a new equilibrium must be established the motor 4i) is started and allowed to run until equilibrium has been established. After its heat has been `dissipated another set of readings of gas-phase volume, liquid-phase volume, and pressure is made. This procedure of raising the pressure, attaining equilibrium, and reading the pressure-volume relationship and liquid-gas relationship is continued until the maximum pressure desired by the operator is obtained. This procedure in the method has been referred to as the phase equilibrium determinations.

The tabulated data with all necessary corrections is finally plotted on a phase-equilibrium graph, see Figure 8, in which the vertical scale represents barrels of equilibrium liquid per one million cubic feet of gas at F. and 14.7 pounds per square inch absolute, and the horizontal scale represents absolute pressures in pounds per square inch. 'Ihe tabulated data then forms an isothermal line of the reservoir iiuid in equilibrium. Additional isotherms are determined in the same manner as described on the identical sample. Operation of the apparatus additional isotherms is as follows:

Valves 92 and 89 are opened to permit the mercury contained in the upper portion of cell 20 in back of piston 3| to return to tank sz, which takes place automatically as the gas pressure within the chamber will force the piston to the top of the cell. This lowering of the pressure will cause a precipitation of liquid from the gas phase. The liquid is allowed to drain down the sides of the gas chamber While the temperature of the oil bath is raised to the next temperature level. Following this, the second set of pressure-volume data for the gas and liquid is accumulated. This data may be plotted on the same graph as the iirst. The resulting graph will appear very much as in Figure 8, showing three isothermal liquid A graph such as shown inl Figure may also be plotted from the data obtained as above described, which indicates the pressurevolume relationship of reservoir fluid.

The information about' the behavoir of a reservoir material obtained in this manner eliminates many of the errors inherent in other methods. First and foremost of these is the integrity of the sample. A high pressure, wet gas contains in its reservoir condition an appreciable quantity of heavy hydrocarbons. These are inthe vapor phase due to the temperature and pressure under which they exist in the reservoir. As these reservoirs are produced, both the temperature and pressure are reduced as the material comes up the well bore. This upsets the equilibrium of the material and causes a precipitation of a large part of the heavy hydrocarbons in the form of liquid. This liquid may exist in the well bore in the form of liquid slugs, or as a liquid illm on the walls of the flow string, or as a mist or fog in the gas for determining these material.

.l volume at 60 F. and 14.7# absolute.

stream, or a combination of all three. As a rel at which the precipitation of liquid occurs, the

materials are relatively stable and. can be handled in the laboratory with considerable ease.

With respect to the present method, it should be noted that any number of phase equilibrium determinations can be made on one sample of These determinations are therefore comparative because they are made on the same sample. Also as a result of the visibility features oi the cell 20 the phases of the reservoir material may be actually seen, and questionable readings can be checked for accuracy at any time up to the discarding of the sample.

Another advantage of the present method is that after all the required graphical data have been obtained, it isA entirely feasible to secure equilibrium at any point of temperature and pressure, and withdraw for analysis a sample of the liquid or gas, lor both, thus answering the specific question of composition at one critical point of equilibrium. Additional analyses can be made by preparing additional mixtures of liquid and gas in the cell.

A further advantage of the method resides in the fact that the measurement of the volume of separator gas within cell 2li is made on the basis of actual determined compressibility of that gas and not upon a value calculated by means of critical data 'from the analysis. There is reason to believe that calculated compressibilities are subject to errors. By using the actually determined compressibility such errors are circumvented. The behavior of the material under temperature and pressure is determined instead of attempting to predict its behavior from necessarily incomplete data.

In this connection, an alternate method is disclosed which may be used to determine compressibility of gas at elevated temperature and pressure where only one set of data is required. In

cases Where the entire compressibility curve data are not needed but instead only the one datum; namely, the true volume of the gas under the predetermined pressure and temperature at which it is injected into the Variable volume cell for the phase equilibrium determinatioman alternate and much simplified method of securing this compressibility datum is followed. It consists of measuring the gas sample into the 'compressibility cylinder I0 at approximately 10 pounds above atmospheric and at approximately room temperature. This volume is converted to 'I'he oil bath is then heated to the desired temperature after which mercury is pumped-into the cylinder I0 by power pump until the resultant pressure as indicated by piston gage is at the desired A point. As soon as the entire cylinder, mercury, and tank assembly are uniformly at the desired temperature as indicated by a stable pressure, the true volume occupied by the known quantity of gas at the new conditions of temperature and pressure is determined. 'I'his is possible by displacement of the compressed gas with a measured quantity of mercury from volumetric pump,

cury used to displace the gas due to the increaseA Yin temperature from room to the oil bath. This method is much more rapid as only at one point mustthe compressibility be determined; the major part of the pumping is done by power means,

and only one volume measurement need be made; this one volume corrected for but one change in condition, namely, temperature.

The invention as hereinabove set forth may be variously embodied within the scope of the following claims.

What is claimed is:

l. A method for evaluating the content of gas reservoirs and the like, which consists in obtaining samples of the liquid and gas produced by a stabilized well under conditions of temperature and pressure existing at the field separator, recombining the said liquid and gas in the same ratio of volume of liquid to volume of gas as that produced by the well during the sampling periodto provide 'a mixture substantially the equivalent of the reservoir fluid that entered the well bore during said period, andY in making a plurality of phase equilibrium determinations on said mixture while maintaining the mixture at a denite constant temperature.

2. A method for evaluating the content of gas reservoirs and the like, which consists in obtaining samples of the liquid and gas' produced by a stabilized Well under conditions of temperature and pressure existing at the eld separator, recombining the said liquid and gas in the same ratio of volume of liquid to volume of gas as that produced by the well during the sampling period to provide a mixture substantially the equivalent of the reservoir fluid that entered the well bore Atemperature, and in repeating said phase equilibrium determinations on said mixture -at different constant temperatures.

3. A method for evaluating the content of gas reservoirs andthe like, which consists in obtaining samples of the liquid and gas produced by a stabilized well under conditions of temperature and pressure existing at the field separator, recombining'the said liquid and gas in the same ratio of volume of liquid to volume of gas as that produced by the well during the sampling period -to provide a mixture substantially the equivalent of the reservoir fluid that entered the Well bore during the period, and in obtaining data indicating the isothermal behavior of the said mixture 4 in equilibrium for a range of pressures.

4. A method for evaluating the content orf gas reservoirs and the like, which consists in obtaining samples of the liquid and .gas produced by a uring the volume oi' the liquid phase of the mixture at different pressures from separator pressure to reservoir pressure while maintaining the mixture at a definite constant temperature, and in obtaining similar data at diierent constant temperatures without changing the mixture with respect to composition.

5. A method of evaluating the content of gas reservoirs and the like, which consists in obtaining samples or liquid and gasproduced by a stabilized well under conditions of temperature and pressure existing at the iield separator, subjecting a portion of the gas sample to compressibility tests to secure data concerning the pressurevolume behavior o! the gas at constant temperatures, recombining a portion of the gas calculated as to volume by the aid of the said compressibility data with a volume oi liquid having a certain ratio to said volume of 'gas to provide a mixture substantially the equivalent oi the reservoir fluid that entered the well bore during the sampling period, and in subjecting said mixture to tests for obtaining data indicating the isothermal behavior of the mixture in equilibrium from separator pressure to reservoir pressure.

6. A method of evaluating the content of gas reservoirs and the like, which consists in obtaining samples of liquid and gas produced by a stabilized well under conditions of temperature and pressure existing at the'eld separator, subjecting a portion of the gas sample to compressibility tests to secure data concerning the pressurevolume behavior of the gas at constant temperatures, recombining a portion of the gas calculated as to volume by the aid of the said compressibility data with a volume of liquid having a certain ratio to said volume of gas to provide a mixture substantially the equivalent of the reservoir iluid Y that entered the well bore during the sampling period, and subjecting said mixture to different pressures rangingr from separator pressure to reservoir pressure while maintaining the mixture at a constant predetermined temperature for obtaining data indicating the isothermal behavior of the mixture for said pressures.

'7. A method of evaluating the content of gas reservoirs and the like, which consists in obtaining samples of the liquid and gas produced by a stabilized well under conditions of temperature and pressure existing at the eld separator, separately injecting into a pressure cell a portion of the gas and a portion of the liquid, said gas being calculated as to volume by theaid of actual determined compressibility data, and the volume of said liquid bearing the same ratio to said volume of gas as that produced by the well during the sampling period to provide a mixture substantially the equivalent of the reservoir iluid that entered the well bore during the rpericd,

and in making a plurality of phase equilibrium determinations on said mixture while maintaining the mixture at a definite constant temperature.

8. A method of evaluating the content of sas reservoirs and the like, which consists in obtaining samples of the liquid andgas produced by a stabilized well under conditions of temperature and pressure existing at the field separator, sepa-- rately injecting into a pressure cell a portion of Vthe gas and a portion of the liquid, said gas being calculated as to volume by the aid of actual determined compressibility ldata, and the volume of said liquid bearing the same ratio to said volume of lgas as that produced by the well during the sampling period to rprovide a mixture sub stantially the equivalent of the reservoir fluid that entered the well bore during the period, observing and measuring the volume of the liquid phase of the mixture at diilerent pressures from separator pressure to reservoir pressure while maintaining the mixture at a definite constant temperature, and in obtaining similar data at diierent constant temperatures without changing the mixture with respect to composition.

9. A method of evaluating the content of gas reservoirs and the like, which consists in obtaining samples of the liquid and gas produced by a stabilized well under conditions of temperature and pressure existing at the. field separator. separately injecting into a pressure cell a portion of the gas and a portion of the liquid, said gas being calculated as to volume by the aid of actual determined compressibility data, and the volume of said liquid bearing the same ratio to said volume of gas as that produced by the well during the sampling period to provide a mixture substantially the equivalent of the reservoir fluid that entered the well bore during the period, subjecting the mixture within said cell to a. plurality of diilerent pressures, agitating the mixture until equilibrium is attained between the gas and liquid for each pressure, and in measuring for each pressure the gas volume and the liquid volume while maintaining the mixture at a denite constant temperature.

10. A method of evaluating the content of gas reservoirs and the like, which consists in obtaining samples of the liquid and gas -produced by a stabilized well under conditions of temperature Y said liquid bearing the same ratio to said volume of gas as that produced by the well during the sampling period to provide a mixture substantially the equivalent of the reservoir fluid that entered the well bore during the period, subjecting the mixture within said cell to a plurality of diierent pressures, agitating the mixture until equilibrium is attained between the gas and liquid for each pressure, measuring for each pressure the gas volume and the liquid volume while maintaining the mixture at a definite constant tem- .perature,'and in repeating said operations tov secure similar data at different constant temperatures without changing the material with respect to composition.

11. In a method of evaluating the content of gas reservoirs and the like,. the step which consists in obtaining samples of the liquid and gas at the eld separator of the reservoir under conditions of stabilized now, subjecting a portion ot the gas sample to compressibility tests to secure amunsnoss.

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