US2358920A - Production of distillate - Google Patents

Description

P 1944- A. D. GARRISON PRODUCTION OF DISTILLATE Filed Nov. 27, 1941 ALLEN D. GARRISON W Aw HIS ATTORNEYS OZUDOOwE Patented. Sept. 26, 1944 PRODUCTION OF ms mars Allen D. Garrison, Houston, Tex, assignor to Texaco Development Corporation, New York,

N. vIL, a corporation of Delaware Application November 27, 1941, Serial No..420,675

11 Claims. This invention relates to a process for the production of liqueflable hydrocarbons from fluids flowed from distillate reservoirs,,involving removing a distillate fluid from one or more producing wells, recovering liqueflable hydrocarbons from the fluid and returning residual gases to the same or a different producing formation.

The producing formations of distillate fields are characterized in that-the hydrocarbons are considered to be present iii the sand as a single homogeneous phase. The known distillate formations are usually at depths of 8,000 feet or more and the bottom hole pressures are normally from 3,000 to 5,000 pounds per square inch or above. As deep drilling continues it may be expected that distillate formations will be discovered at increasingly higher pressures.

Distillate fluids are'distinguished from other fluids in producing formations in that they possess the property of undergoing condensation upon an isothermal reduction in pressure; 1. e., they are subject to retrograde condensation. To avoid the operation of this phenomenon in the formation, and to comply with State laws in regard to conservation, it is necessary to maintain formation pressures by returning the residual gases to the formation. As a result of this requirement, the processes which have been proposed have embodied attempts in one way or another to recover the liqueflable hydrocarbons without reducing the pressure any more than necessary. However, because the maximum condensation of hydrocarbons has apparently taken varying from about 1,000 to 2,000 pounds per square inch, depending upon the characteristics of the particular fluid, the processes in actual operation have involved reducing the pressure to some pressure below 2,000 pounds per square inch.

Residual gases, therefore, have been obtained at a low pressure relative to the formation pressure and compression costs have been high.

Perhaps the simplest process that has been employed in the production of distillate has consisted in flowing the distillate fluid from the producing well through a choke to reduce the pressure to the desired-point, cooling the fluid in one or more place at pressures represent an important part of the costs of operating a process of this type.

It is an object of the present invention to provide a simple and eflicient process for the production of a distillate fleld, which process is characterized in that compression costs are reduced, and the amount of processing of the fluid is lessened.

Other objects of the invention in part will be obvious and in part will appear hereinafter.

In accordance with the present invention the production of distillate is accomplished by a process in which a high density liquid is mixed with the distillate fluid in the producing well, preferably at about the depth of the producing formation, and the resulting mixed fluid is permitted to flow from the well due to the pressure of the formation. The mixed fluid is then passed from the well and is treated to separate three fractions:

the high density liquid, the desired liqueflable hydrocarbons, which preferably consist of most of the butanes and substantially all of the pentanes and heavier, and the residual lighter gases. The separation step is preferably carried out at about the pressure at which the mixed fluid is removed from the producing well, a pressure materially below the bottom hole pressure and of course below the pressure to which the residual gases would have to be raised to make possible their re steps, separating liqueflable hydrocarbons which are condensed under these conditions, and compressing and returning the residual gases to the same or a different producing formation. It will be seen that where the formation pressures are in the neighborhood of 4,000 pounds per square inch and the condensation of hydrocarbons is accomplished at a pressure of the order of 1,200 to 1,500 pounds per square inch, the costs of compression turn to a high pressure formation.

-In the case where the residual gases are to be returned to the same producing formation, after the separation of the mixed fluid, the pressure on the residual gases at the surface is raised a relativelysmall amount (e. 8.. 1,000 to 1,200 pounds per square inch) 'above well-head pressure. At

the same time the high density liquid recovered in the separation is raised to the same pressure.

The high densityiiquid and the residual gases are then mixed and passed into the injection well. Atthe bottom of the well, preferably opposite the formation, the residual gases and the high density liquid are permitted to separate. and the gases pass into the formation to maintain the pressure. The high density liquid is removed from the injection well and employed for admixture with disq tillate fluid in the production well in the manner described above.

While the mechanism and manner of operation of the process and the advantages obtained will be more or less obvious as the description proceeds, it may be of advantage to point out here the broad-principle upon which the process operates.

" When flowing a distillate well in the normal way there is, of course, a reduction in the pressure of finely divided form. However, I prefer surface before introducin This reduction in pressure is due to two factors, the pressure of the column of fluid and friction of the tubing. By mixing the distillate fluid in the producing well with a high density liquid the difference in pressure between the formation and the well-head may be adjusted to .any desired value. This is done by controlling the height and/ or density of the mixed fluid and thereby the pansion of distillate fluids to recover liquefiable,

hydrocarbons by a method which stores the energy of expansion in recoverable form. The recoverable energy is used to repressure the reservoir. There are also thermal advantages which result. The heavy liquid injected into the producing well and mixed with the well fluid serves also as a cooling agent. There is added to this the cooling effect of expansion of the gas while doing work in lifting the heavy liquid to the surface, and delivering it at a substantially increased pressure. With certain types of distillates, this cooling together with proper pressure reduction will provide satisfactory hydrocarbon recovery. with other cases where additional cooling or absorption become desirable, their incorporation in .this improved method becomes more economical than would usually be the casesince substantial cooling is already an inherent feature of the process.

In addition to saving in power, a major feature of this invention consists of the elimination of high pressure surface equipment. When using the residual gases for repressuring purposes the introduction of the high density liquid into the gases produces a relatively dense mixed fluid which at a relatively low surface pressure will produce a high bottom hole pressure in the injection well. As a result; the necessity for compressing the residual gases to a high pressure at the surface is eliminated.

It will be understood in view of the variable temperature employed at the well head, which may be 32 F. "or lower. v

The salts which may be employed are any salts that produce a solution of the. desired density, and the selection of a salt solution in a particular case may be made by reference to published information regarding the properties of the solutions. Salts which are of particular interest for use in making up the high density liquids are halides, nitrates and sulfates of various metals which yield water-soluble salts with the acid radical. Specific examples of such salts are calcium chloride, zinc-chloride, zinc bromide, zinc iodide,

barium iodide, and cadmium chloride. Also of conditions encountered in distillate production that various types of high density liquids may be used in the present process. In general it is preferred to employ a liquid in which hydrocarbons are substantially insQuble, although it is considered that in certain cases relatively heavy hydrocarbon oils may be found of advantage. It is also considered that in certain cases it.may be found desirable to employ as the high density liquid water or oil weighted with heavy solids in to employ a water solutionof a heavy material. As suitable heavy materials inorganic salts may be mentioned. In selecting a particular salt one factor which has not yet been referred to should be taken into account. As noted above, on flowing the mixed fluid from the formation to the head of the production well the temperature of the liquid is reduced. The final temperature em- Ployed at the well head may be reduced still further by cooling the high density liquid at the it into the production well. Accordingly, in selecting a salt solution, a solution should be selected which has a freezing point or point of separation of solute below the interest are potassium-chrom-alum, K2Cr2(SO4)4 and soda alum, NazSO4.Al2(SO4)a.24H2O. It will be understood that in order to achieve the desired density and freezing point a solution containing two or more salts may be used. For example, salts such as sodium bromide, sodium idide, sodium nitrate or potassium carbonate may be used in conjunction with soda alum. Sodium silicate solutions are also of interest for use in the present process.

In general, it is preferred to employ a liquid having a density of above 1.2 grams per cc. ordinarily the heaviest liquid that will operate being preferred. Because of ready availability, cheapness, and non-corrosiveness, solutions of calcium chloride are of particular interest. Aqueous solutions of this salt can be prepared to have sufficient density and 'yet be stable at relatively low temperatures. To illustrate the properties of solutions of calcium chloride the following table is given. In this table the minimum temperature is the temperature at which separation of CaCl: from solution takes place.

Specific M iuimum pewent C801 gravity temperature In order that the invention may be understood more fully the process will be described in connection with the accompanying drawing which represents in diagrammatic form a production well, an injection well, and above-ground apparatus elements employed in carrying out the present invention in accordance with one man.- ner of proceeding.

The drawing illustrates a relatively simple apducing well which taps a distillate formation. ,For the purposes of this description the well will be considered as being 8,000 feet in depthwith a bottom hole pressure of about 4,200 pounds per square inch. The well A is provided with a casing l0 which contains perforations I2 opposite the producing sand H. Disposed in the well is a flow tubing it which is provided with an open a single injection well.

' injection well.

end l3. Disposed about the flow tubing I6 is a concentric tubing 22 of larger diameter. The tubing 22 is joined to the flow tubing with a fluid tight joinder at 24. Leading from the top of tubing 22 is a vent pipe 23 provided with a valve 25. Just'above the joinder at 24 the flow tubing is provided with perforations 26. A valved line 23 leads from the top of flow tubing l6 to a separator 30.

The separator is of such volume and is so constructed as to permit the separation of three phases therein: residual gases, distillate, and the high density liquid. The residual gases may be passed from the separator through a valved-line 32 directly to a compressor 34 and thence through line 36 into an injection well B. However, where the desired percentage 01' liqueflable hydrocarbons is not recovered in the separator, there may be placed between the separator and the compressor a recovery system, indicated generally at II, which system may be an absorption or adsorption system or a system wherein the gases are subjected to additional cooling to remove additional distillate therefrom. The distillate may be removed from this system through valved line 33 leading to line 35 described below, and the residual gases are then passed to the compressor 34.

The high density liquid is removed from the bottom portion of the separator through a valved line 36 leading to a pump 40 and thence through line 42 and at point 43 into line 36 leading to the Distillate is removed from the separator through valved line 44, passes into line 36 and thence into distillate storage vessel 46. A line 46 provided with valve 49 leads from a pump 50 to the interior of tubing 22 in producing well A.

The injection well B is constructed so as to carry out the operation of the present process in the following manner. Similarly to well A, it

is provided with a casing 52 which has perforations 64 opposite the producing formation. Within this casing there is an inner casing or tubing 36 having a diameter only slightly less than the diameter of the outer casing, and having a closed end 58. Disposed within this inner casing is an injection tubing 62 connected by means of valve 64 to line 36. A second tubing 66 is disposed within the tubing 62 and leads to valved line 63 connected with pump 50.

At some distance from the bottom of the well, the inner casing 56 is provided with perforations 12, by means of which gases may pass from the inner easing into the space between the. two casin s and into the formation. It will be noted that the outer and inner casings define an enclosed annulanspace which is open only at the perforations 12 and 54. As will appear from the following description, the perforations I2 may be located in some cases more than half way from the bottom of the well to the surface.

In carrying out the process in an apparatus of the'type illustrated in the drawing, a high density liquid may be selected such that a column of the liquid equalin length tothe depth of the well, or, where the liquid is not introduced into the bottom of the well, equal to the distance from the surface to the point of introduction, will exert a pressure only slightly above the pressure existing in the flow tubing at the point of introduction. Only a suiflcient difierence in pressure is required to cause the high density liquid to flow into the flow tubing.

The process may also be carried out using a liquid of higher density and regulating the height of the column to yield the desired pressure at the-point of introduction. By using a higher density liquid, the pressure of the column of liquid in the injection wellmay be several hundred pounds above the pressure existing in the bottom of the injection well. This latter manner of operating will be particularly described. In the present case the high density liquid used is a 34.5 per cent solution of calcium chloride which has a density of about 1.34 grams per cc.

A column of this liquid extending from the top to the bottom of well B will exert a pressure slightly less than 4700 pounds per square inch.

The process is begun with the tubing 22 filled to a height of about 7200 feet with the calcium chloride solution which is fed in at a constant rate through line 48, trapped gas being released through line 23. The fluid flowing from formation i4 into the end l8 offlow tubing l3 passes the perforations 26 and there comes into intimate contact with the calcium chloride solution. The calcium chloride solution is fed in at a rate controlled by the size of the perforations 26 and the head of about 7200 feet .in tubing 22 such that the solution is mixed with the distillate fluid in the proportion of about equal'parts liquid and fluid by volume. Since the fluid may have an average density of, for example, .20 gram per cc., and the liquid 9. density of 1.34 grams per cc. a mixed fluid may be produced having an average density of about .77 gram per cc. Due to the density of this mixed fluid and the frictional loss the well-head pressure may be of the order of 1200 pounds per square inch. Also, due to the reduction in pressure of the fluid while doing work, and the cooling action of the liquid, which may be increased by cooling the liquid before introducing it into the well, the temperature is at a low point at which effective condensation of liqueiiable hydrocarbons occurs; e. g., at a temperature of about 50 to F.

In this operation, while the distillate fluid expands from about 4200 pounds per square inch gage to about 1200 pounds per square inch, the high density liquid is compressed from 0 pounds per square inch to 1200 pounds per-square inch. Under these conditions the mixed fluid is flowed through line 23 to separator 30. The residual gases ar removed from the upper portion of the separator through line 32, and, if no further.

treatment is necessary, are. passed directly to compressor 34 where they are compressed to a pressure of, for example, 2400 pounds per square inch The desired liqueflable hydrocarbons or distillate are removed through line 44 and are collected in distillate storage vessel 46. The high density liquid is removed from the bottom of the separator through line 33 to pump 40, wherein the pressure on theiiquid is raised toabout 2400 pounds per square inch, andpassed through line 42, and, at point 43, into admixture with the residual gases flowing in line 36.- The resulting mixed fluid is then passed through tubing, 62 into the enclosed bottom portion of inner casing 56 which forms a zone 53. Since this mixed fluid may have an average density of the order of .76 gram per cc., the pressure in the inner casing 56 in the neighborhood of the formation may be, for example, 4700 pounds per square inch. Under the quiescent conditions existing in zone 59, the

' gas will separate from the high density liquid and will pass up and through the perforations in inner through line 48, valve and back into tubing 22.

' In certain cases it be found desirable to subject the mixed fluid flowing in line 28 to a limited amount of cooling. This may be done so as to replace the recovery system 3| or in addition to this system. Although the temperature of the mixed fluid at the well head will be below the temperature of ordinary hydrate formation, because of the presence of the calcium chloride or other salt solution no dilliculties due to the formation of hydrates will be experienced.

As previously indicated, in some cases it may be necessary to employ a recovery system in addition to the separator. The general operation of the process in such case will not differ materially from that described. i

It will be noticed that it is necessary to compress the residual gases to only a few hundred pounds above the pressure at the well-head, thus simplifying compressor equipment and reducing power costs for this operation. With regard to the extent of this rise in p'ressure,'this will be dependent upon the density and amount of high density liquid used, the depth of the well, and the formation pressure. In any case. the pressure of the residual gases after compression will be far below the normal well-head pressure of the production well, and of course even farther below the pressure normally required to return. residual gases through an injection well to the formation.

In selecting the high density liquid it is generally preferred to select a liquid such that the hydrostatic head 'of the liquid in the injection well is about equal to the repressuring pressure 1. e., the repressuring pressure less the pressure lost by friction. Such a liquid will flow under easily controlled conditions to the producing well. Also, the flow of the liquid into the distillate fluid in the producing well may be controlled easily by. regulation of the height of the column of liquid and the size of the perforations 26. In this connection, the size of these openings may be made adjustable; for example, by providing means for making tubing 22 rotatable relative to tubing l6 and means operated by the relative rotation to reduce or enlarge the openings.

When using a relatively high density liquid, the amount of the liquid introduced into the distillate fluid is controlled so as to obtainthe desired well-head pressure. For example, using a 50 per cent solution of zinc chloride, which has a density of about 1.57 grams per cc., in the above process, the height of the column of liquid in the producing well would becontrolled to maintain it at about 6700 feet. When using this solution in an operation as described in the drawing, the solution would be mixed with the distillate fluid in the proportions of about 40 per cent of thesolution and about 60 per cent of the distillate fluid by volume.

As indicated above, the drawing herein is merely diagrammatic in character to show the flow of fluids and no attempt has been made to show the elements in the relative sizes that would be used in practice.

From the foregoing it will be seen that the preferred operation of the process involves controlling the density or height or both of the high 'density liquid so as to obtain a pressure at the bottomof thecolumn of liquid in the producing well about equal to the pressure of the distillate fluid. 'The rate of flow of the liquid is best adju'sted by controlling the pump. 50 to yield a mixed fluid of the desired density. The desired density of the mixed fluid is, of course, dependent upon the bottom hole pressure, the optimum pressure at the surface, and the depth of the well. In the normal case, conditions will be adjusted so that the pressure at the head of the producing well will be at about the bottom of the retrograde condensation range for the fluid in question. Thus, conditions may be adjusted so that th well-,-head pressure will be within the range of about 800 to 1600 pounds per square inch.

Obviously many modifications and variations of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

I claim:

1. In the production of a distillate field wherein a distillate fluid is flowed from a producing well tapping the producing formation, liqueflable hydrocarbons are recovered from the distillate fluid. and residual gases are returned through an injection well to a formation to maintain pressures. the process which comprises mixing the distillate fluid in the producing well with a high density liquid, permitting the resulting mixture to flow to the surface, whereby the temperature and pressure of the distillate fluid are reduced and liqueflable hydrocarbons are condensed, separating from said mixture liqueflable hydrocarbons, residual gases, and said high density liquid, passing a mixture of said residual gases and said high density liquid into the injection well at a pressure above the pressure atthe head of the producing well, separating said last-mentioned mixture in the injection well into said residual gases and said high density liquid, passing said residual gases into the formation to maintain pressures, and returning the high density liquid to the producing well foradmixture with the distillate fluid.

2. A process in accordance with claim 1 in which the high density liquid is a liquid in which thehydrocarbons of the distillate fluid are substantially insoluble.

- 3. A process in accordance with claim 1 in which the high density liquid has a density of at least 1.2 grams per cc. and comprises an aqueous high density liquid into the producing well out I of contact with the distillate fluid so as, to maintain a moving column of high density liquid therein, mixing the high density liquid with the distillate fluid at substantially the depth of the formation, controlling the pressure at which the high density liquid is introduced into the distillate fluid to a pressure adapted to cause the flow of the high density liquid but being. not substantially above the pressure of the formation, permitting the resulting mixture to flow to the surface, whereby the temperature and pressure of "the distillate fluid are reduced and liqueflable hydrocarbons are condensed, separating from said mixture liqueflable hydrocarbons, residual gases and said high density liquid, raising the pressure on said residual gases and on said high density liquid, mixing said residual gases and said high density liquid at the raised pressure, passing the resulting mixture into the injection well, separating said last-mentioned mixture in the injection well into said residual gases and said high density liquid, passing said residual gases into the formation to maintain pressures, and returning the high density liquid to the producing well for admixture with the distillate fluid.

6. In the production of a distillate field where-' in a distillate-fluid is flowed from a producing well tapping the producing formation, liqueflable hydrocarbons are recovered from the distillate fluid, and residual gases are returned through an injection well to the formation tomaintain pressures, the process which comprises flowing a high density liquidinto the producing well out of contact with the distillate fluid so as to maintain a moving column of high density-liquid therein, mixing the high density liquid with the distillate fluid in the producing well, the density of the high density liquid being such that. the pressure of the column of liquid at the point of intro,- duction into the distillate fluid is slightly greater than the pressure on the fluid, and the propor f tions in which the distillate fluid and the high? returning the high density liquid to the producing 7 well for admixture with the distillate fluid.

8. In the production of a distillate field wherein a distillate fluid is flowed from a producing well tapping the producing formation, liqueflable hydrocarbons are recovered from the distillate fluid, and residual gases are returned through an injection well to the formation to maintain pressures, th process which comprises flowing. a high density liquid in which the hydrocarbons of the distillate fluid are substantially insoluble into density liquid are mixed being such that the re- 5 suiting mixture will flow from the producing well at the surface at a selected reduced pressure withinthe retrograde condensation range of the distillate fluid, permitting the resulting mixture to flow to the surface, whereby the temperature and pressure of the distillate fluid are reduced and liqueflable hydrocarbons are condensed, separating from said mixture liqueflable hydrocarbons, residual gases and said high density liquid, raising the pressure on said residual gases and on said high density liquid, mixing said residual gases and said high density liquid at the raised pres-- sure, passing the resulting mixture into the injection well, separating said last mentioned mixture in the injection well into said residual gases and said high density liquid, passing said residual the producing well out of contact with the distillate fluid so'as to maintain a moving column of high density liquid therein, mixing the high density liquid with the distillate fluid at substantially the depth of the formation, controlling the height of the column of liquid to obtain a pressure of the column at the point of introduction into the distillate fluid slightly greater than the pressure on the fluid, permitting the resulting mixture to flow to the surface, whereby the temperature and pressure of the distillate fluid are reduced and liqueflable hydrocarbons are condensed, separating from said mixtureuliquefiable hydrocarbons, resid-.

ual gases and said high density liquid, raising the pressure on said residual. gases and on said high density liquid, mixing said residual gases and said high density liquid at the raised pressure, passing 'arating said last mentioned mixture in the injec- =the resulting mixture into the injection well, seption well into said residual gases and said high gases into the formation to maintain pressures,

density liquid into the producing well out of contact with the distillate fluid so as to maintain a moving column of high density liquid therein, 'mixing the high density liquid with the distillate fluid at substantially the depth of the formation, controlling the height of the column of liquid to obtain a pressure of the column at the point of introduction into the distillate fluid slightly greater than the pressure on the fluid,

permitting the resulting mixture to flow to the surface, whereby the temperature and pressure of the distillate fluid are reduced and liqueflable density liquid, passing said residual gases into the formation to maintain pressures, and returning the high density liquid to the producing well for admixture with the distillate fluid, the density' of the high density liquid being such that the pressure of a column of the liquid substantially equal in height to the depth of the injection well exerts a pressure substantially equal to the pressure required for passing said residual gases into the formation to maintain pressures, whereby the formation is repressured and the high density liquid flows from the head of the injection well.

9. In the production of a distillate field wherein a distillate fluid is flowed from a producing well tapping the producing formation, liqueflable hydrocarbons are recovered from the distillate fluid, and residual gases are returned through an injection well to the formation to maintain pressures, the process which comprises flowing a, high density liquid in which the hydrocarbons of the distillate fluid are substantially insoluble into the producing well out of contact with the distillate fluid so as to maintain a moving column of high density liquid therein, mixing the high density liquid with the distillate fluid in the producing well, permitting the resulting mixture to flow to the surface, whereby. the temperature and pressure of the distillate fluid are reduced and liqueflable hydrocarbons are condensed, separating from said mixture liqueflable hydrocarbons, residual gases and said high density liquid, Passing a mixture of said residual gases and said high density liquid at a raised pressure into the injection well, separating said last-mentioned mixture in the injection well at about the depth of but out of contact with the formation into said residual gases and said high density liquid, passing said residual gases to the formation to maintain pressures, and returning the high density liquid to the producing well for admixture with the distillate fluid.

10. In the production of a distillate field where- I density liquid in which the hydrocarbons of the distillate fluid are substantially insoluble into the producing well out of contact with the distillate fluid so as to maintain a moving column of high density liquid therein, mixing the high density liquid with the distillate fluid in the producing well, permittingthe resulting mixture to flow to the surface, whereby the temperature and pressure of the distillate fluid are reduced and liqueflable hydrocarbons are condensed, separating from said mixture liqueflable hydrocarbons, re-

sidual gases and said high density liquid, passing a mixture 01' said residual gases and said high density liquid at a raised pressure into the injection well, separating said last-mentioned mixture in the injection well at about the depth of but out of contact with the formation into said residual ases and said high density liquid, passing said residual gases to the formation to maintain pressures, and returning the high density liquid to the producing well .for admixture with the distillate fluid, the density of the high density liquid bein such that the pressure of a column of the liquid substantially equal in height to the depth of the injection well exerts a pressure substantially equal to the pressure required for passing said residual gases into the to tion to maintain pressures, whereby the formation is repressured and the high density liquid flows from the head of the'injection ,well.

tact with the distillate fluid so as to maintain a moving column or high density liquid therein, mixing the high' density liquid with the distillate fluid in the producing well, permitting the result- ,ing mixture to flow to thessurface, whereby the temperature and Pressure of the distillate fluid are reduced and liqueflabl hydrocarbons are condensed, separating from said mixture liqueflable hydrocarbons, residual gases and said high density liquid, passing a mixture of said residual gases and said high density liquid at a raised pressure into the injection well, separating said last mentioned mixture in the injection well at about a depth of the formation into said residual gases and said high density liquid, passing said residual gases into the formation to maintain pressures,

and returning the high density liquid to the producing well for admixture with the distillate fluid, the density of said high density liquid being such that the pressure of a column of the liquid substantially equal in height to the depth of the in- Jection well exerts a pressure substantially equal to the pressure required for passing said residual gases into the formation to maintain pressures,

whereby the formation is repressur'ed and the high density liquid flows from the head of the injection well at substantially atmospheric pressure.

ALLEN D. GARRISON.

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