US2968350A

US2968350A - Miscible slug followed by gas and water

Info

Publication number: US2968350A
Application number: US462591A
Authority: US
Inventors: Robert L Slobod; Jr Howard A Koch
Original assignee: Atlantic Refining Co
Current assignee: Atlantic Richfield Co
Priority date: 1954-10-15
Filing date: 1954-10-15
Publication date: 1961-01-17
Anticipated expiration: 1978-01-17

Description

Jan. 17, 1961 R, SLOBOD ETAL 2,968,350

MISCIBLE SLUG FOLLOWED BY GAS AND WATER Filed on. 15, 1954 2 Sheets-Sheet 1 5 4 GAS MISCIBLE SLUG RESERVOIR OIL Fig.

Fig. 2

INVENTOR.

Robert L. Slobod BY Howard A. Koch Jr. 7%

Attorney 4 ATTEST R..L. SLOBOD ET AL 2,968,350 MISCIBLE swc FOLLOWED BY GAS AND WATER 2 Sheets-Sheet 2 Jan. 17, 1961 Filed Oct. 15, 1954 INVENTOR. L. Slobod A. Koch Jr.

3.0 D INJECTED) IN PLACE nokthrouqh 2.0 RESERVOIR VOLUMES OF FLUI Rober t Howard Attorney VOLUME INJEGTED(RESERVOIR VOLUMES OF on.

Fig. 5

ATTEST ,zz /aa United States Patent O cc 2,968,350 MISCIBLE SLUG FOLLOWED BY GAS AND WATER Robert L. Slobod and Howard A. Koch, In, Dallas,

Tex., assignors to The Atlantic Refining Company,

Philadelphia, Pa., a corporation of Pennsylvania Filed Get. 15, 1954, Ser. No. 462,591 6 Claims. {CL 166-9) This invention relates to a method for increasing the recovery of oil from reservoirs. More specifically, it relates to an improved method for increasing the recovery of oil with the use of a miscible slug of fluid of a particular size.

In recovery of oil from oil reservoirs, several methods have been proposed for increasing the recovery of the oil in place in the reservoir. Among these proposals has been the injection of Water, commonly referred to as a water flood; the injection of normally gaseous hydrocarbons; the injection of a 100 percent pore volume of propane; and the injection of propane followed by water. Also, in copending application, Serial No. 352,985, filed May 4, 1953, there is disclosed a novel method of increasing the recovery of oil by the injection of a miscible slug of fluid having a volume of between 0.01 percent and'5 percent of a hydrocarbon pore volume, followed by a hydrocarbon gas. In that method, which will here inafter be referred to as the conventional slug process, the miscible slug is miscible with both the reservoir oil and the hydrocarbon gas. Consequently, the gas displaces the miscible slug through the reservoir and the miscible slug in turn displaces substantially 100 percent of the oil from the part of the reservoir it contacts.

From the description of the slug process in copending application, Serial No. 352,985, it should be evident to one skilled in the art, that with the use of the slug process one of the most widely known disadvantages of conventional methods for increasing the recovery of oil has been eliminated, that is, the large percentage of the oil left in place due to surface tension and capillary action. Normally, one would then believe that the slug process previously described would give a marked increase in ultimate oil recovery above that obtained by conventional methods, for example, a water flood. We have determined that with conventional injection patterns, for example, a five spot pattern, this increase in oil recovery resulting from the slug process is not what one would expect. An object of this invention is to provide an improved slug process Which will give a marked increase in oil recovery over that of the conventional slug process.

A further object of this invention is to provide a novel method of increasing the ultimate recovery of oil With the use of a miscible slug of fluid of a particular size followed by a slug of normally gaseous hydrocarbons of a particular size which in turn is followed by water.

More objects of this invention will become apparent upon reading the following specification in conjunction with the drawings wherein:

Figure 1 shows a schematic diagram of the method of this invention applied to a typical reservoir.

Figure 2 shows an injection pattern conventionally used in injection methods.

Figures 3 and 4 illustrate sweepout patterns obtained by the use of the conventional slug process and the method of this invention, respectively.

Figure 5 discloses a chart which illustrates the unexpected increased recovery obtained by the method of this invention as compared with other well known injection processes.

Referring to Figure 1, there is shown an injection well 1 anda producing well 2 both of which are in communi- Patented Jan. 17, 1961 cation with the reservoir R containing reservoir oil 3. The wells are constructed and arranged in a conventional manner so that the fluids used to displace the reservoir oil are injected into well 1 in order to displace reservoir oil from well 2. In accordance with this invention a slug of miscible fluid of a predetermined size is first injected into well 1. Thereafter, a slug of normally gaseous hydrocarbons of a predetermined size is injected into Well 1. Thereafter, the slug of hydrocarbon gases is followed by water. The result of this injection is schematically shown in Figure l which shows the miscible slug 4 followed by the slug of normally gaseous hydrocarbons 5 and then water 6. This system provides a mechanism for greatly increasing the recovery of oil from.

a reservoir as will be'explained in more detail hereinafter.

Some of the fluids adapted for use in the method of this invention for displacing the reservoir oil are the same as those fluids contemplated for use in the slug process disclosed in copending application, Serial No. 352,985. Among these is the normally gaseous hydrocarbons which can be natural gas or a similar gas comprising principally methane. It is important that the normally gaseous hy-' each other in all proportions, in the absence of any other fluid which would tend to inhibit said mutual solubility.

The fluids, comprising the miscible slug, which, because of their substantially complete miscibility with typical reservoir 01s, and with natural gas over a wide range of temperatures and pressures, are well adapted to be employed as the miscible slug in accordance with this invention are ethane, propane, butane, sulphur dioxide, and hydrogen sulfide; although many other fluids having similar miscibility characteristics may be employed with varying degrees of success. It will be understood that if desired the miscible slug may comprise two or more of the above mentioned fluids introduced as a mixture or consecutively. For instance, a single slug comprising a mixture of ethane, propane and butane may be employed or three separate slugs each comprising one of these three fluids may be introduced into an injection well consecutively, the butane being introduced first, propane next, and then ethane; that is, in order of their degree of solubility in the reservoir oil.

The size of the miscible slug and the slug of gas is another important aspect of this invention. In this invention the size of the slugs is expressed in percent of a hydrocarbon pore volume, which as used herein is meant the volume occupied by reservoir oil in that portion of the reservoir through which the injected displacing fluids pass in flowing from an injection well to a production well which are both in communication with the reservoir. For example, in cases where the reservoir is relatively large and the injection and production wells are located at only one end or in a portion only of the reservoir, the hydrocarbon pore volume is that volume of the reservoir occupied by the reservoir oil through which the injected fluids actually pass in moving from an injection well to the production well. In the case of the so-called five-spot pattern which is frequently employed 7 in gas sweeping or Water flooding operations and which will be described in more detail hereinafter, fluid injected through an injection well flows toward the surrounding reservoir oil. The hydrocarbon pore volume of a reservoir may be determined sufiiciently close for the purpose of this invention in accordance with procedures well known in the industry from certain information obtained by established investigating methods. Such procedures are so well known in the art that a description thereof is unnecessary.

It is pointed out in copending application, Serial No. 352,985, that the slug of fluid which is miscible with the displaced oil and with the normally gaseous hydrocarbons will maintain its integrity for a considerable distance of travel through a porous medium; however, it will gradually deteriorate and will unless it is sufiiciently large with respect to the hydrocarbon pore volume of the porous medium, ultimately disappear or break down. Therefore, it has been determined by means of laboratory experiments as set forth in copending application, Serial No. 352,985, that benefits will result from the use of a miscible slug ranging in size from 0.01 percent to percent of the hydrocarbon pore volume of the reservoir, as previously defined, there not appearing to be any substan tial advantage in any case in employing a slug smaller than that about 0.01 percent of the hydrocarbon pore volume; and there being no substantial increase of ultimate recovery from the injection of a slug larger than about 5 percent of the hydrocarbon pore volume since a slug of such size is large enough to retain its integrity and serve its purpose as it moves through the entire distance from the injection well to the production well. Therefore, in carrying out the process of this invention, there is injected into the reservoir through an injection well, in an amount between 0.01 percent and 5 percent of a hydrocarbon pore volume, a miscible slug containing fluids which at reservoir temperature and at a convenient pressure to be maintained on the reservoir, are miscible both with the reservoir oil and with the normally gaseous hydrocarbons which are to be employed for displacing the miscible slug through the reservoir. Thereafter, a slug of normally gaseous hydrocarbons whose size is expressed in percent of a hydrocarbon pore volume, as set forth hereinafter, is injected into the same injection well followed by water whereby to force the miscible slug and the slug of normally gaseous hydrocarbons through the reservoir, thus forcing the oil to flow toward a production well through which it is removed to the surface of the earth. Throughout substantially the entire operation there is maintained on the reservoir a pressure sufficiently high to insure that, at reservoir temperature, there will be substantially complete miscibility between the reservoir oil and the fluids of the miscible slug and between the normally gaseous hydrocarbons and the fluids of the miscible slug.

The slug of normally gaseous hydrocarbons contemplated for use in this invention consists of an amount between 20 percent and 40 percent of the hydrocarbon pore volume. A size of gas slug within these limits is necessary when employing this invention in order to give the increased recovery made possible by this invention, although the specific pore volume within these limits depends upon the specific reservoir upon which the method of this invention is being employed. Considering all reservoirs we have found that a gas slug having a size which is at least 20 percent of a hydrocarbon pore volume is necessary for all reservoirs in order to prevent the water from entirely bypassing the slug of gas and the miscible slug, thus eliminating the effect of the slug as previously set forth.

We have discovered that for a specific reservoir the size of the slug of gas should not be less than the volume of residual gas which is estimated will be left behind the advancing front of the water. Accordingly, for a specific reservoir the percent residual gas, that is the amount of gas remaining in the reservoir after the water was employed to displace gas, has to be first determined. Such percent residual gas for any one specific reservoir can be determined in accordance with procedures well known in the industry from information obtained by established investigating methods. Such procedures are so well known in the art that a description thereof is not deemed necessary. After determining the percent residual gas for a specific reservoir and estimating the percent of the V reservoir upon which the miscible slug and gas slug combination is effective, one can readily calculate the smallest size slug of gas which should be employed in the method of this invention. For example, if the percent residual gas has been estimated to be 30 percent and the volume of the reservoir expected to be contacted by the miscible slug and gas, while that combination is still effective, is estimated to be 75 percent, the smallest size of gas slug employed in the method of this invention should be not less than 30 percent of the volume of reservoir expected to be contacted by the miscible slug which is approximately equal to 22.5 percent of the hydrocarbon pore volume of the entire reservoir. The method for estimating the volume of the reservoir upon which the combination of a slug of intermediates and gas is effective is accomplished in a well known manner utilizing the X-ray shadowgraph technique described hereinafter.

In accordance with this invention a volume of gas in an amount less than 40 percent of a hydrocarbon pore volume should be injected immediately after the miscible slug and immediately previous to the injection of water. This size of gas slug is necessary to give the increase in recovery contemplated by the method of this invention in that employing a larger size gas slug would result in breakthrough of the miscible slug and the gas slug before the water was injected, thus preventing the increase in sweepout efficiency and ultimate recovery made possible by the method of this invention. The maximum size of the slug of gas for any one specific reservoir on which the method of this invention is contemplated for use, can be determined by so-called X-ray shadowgraph techniques, described in more detail hereinafter. We have determined that as applied to all reservoirs the size of the gas slug should not exceed 40 percent of a hydro carbon pore volume.

Figure 2 shows a typical arrangement of injection wells 1 and production wells 2 which is commonly used in conventional injection processes. The particular arrangement shown in Figure 2 comprises what is commonly referred to as five-spot patterns in that each pattern includes one injection well surrounded by four production wells. In this arrangement of wells it is apparent that the injection and production wells are arranged in such a manner that each production well is a part of several five-spot patterns such as that shown by production well 2a in Figure 2. Accordingly, the flow of the displaced oil is not radial but has a variety of paths such as paths 6 and 7 shown in a quarter of one of the fivespot patterns. Consequently, it is well known that the displacing medium usually breaks through and is produced out of the production wells before the portion of the reservoir represented by the five-spot pattern is entirely swept out by the displacing media.

Figures 3 and 4 schematically show typical sweepout patterns produced by different displacing media in onequarter of a five-spot pattern such as the quarter indicated at Q in Figure 2. Similar to the operation described in relation to Figure 2, the displacing media are injected into injection well 1 and the oil is produced out of pro duction well 2. Figure 3 illustrates a sweepout pattern obtained when employing a miscible slug followed only by gas. Figure 4 shows a sweepout pattern obtained when employing the method of this invention wherein a miscible slug is displaced through the reservoir by gas, in an amount equal to between 20 percent and 40 percent of a hydrocarbon pore volume, followed by Water.

Referring to Figure 3, one skilled in the art will rec- 5. ognize that the areas A and A+B, represent various sweepout patterns. In Figure 3 the area A represents the area swept out by a miscible slug in a conventional slug process previous to breakthrough of the miscible slug and where only gas was employed to drive the miscible slug through the reservoir. Area B represents the area swept out by the gas and the miscible slug after breakthrough of the miscible slug at the production well 2. It should be understood that the oil in the portion of the reservoir represented by area A is substantially all displaced by the miscible slug while a large percentage of the oil will be left in place in the fraction of the reservoir represented by area B. This oil left in place is because the miscible slug as it breaks through at producing well 2 starts to lose its efliciency and eventually it has no efiect in displacing oil. However, the gas will continue to displace some of the oil and therefore gas injection is usually continued until the oil produced decreases to such an extent that further gas injection will not be economically justified. In other words, when producing well 2 produces injected gas at such a high gasoil ratio that very little reservoir oil is produced, the process is usually discontinued because the further injection of gas is not economically feasible. In Figure 3 the fraction of the specific reservoir under consideration into which the gas is injected before the above economical limit is reached is represented by area A-j-B which approximates 65 percent. Therefore in that specific reservoir less than 65 percent of the oil in place would be recovered employing a conventional slug process.

Figure 4 illustrates the unexpected sweepout pattern obtained by the method of this invention. Similar to Figure 3 area A represents the portion of the reservoir swept out by a miscible slug previous to the breakthrough of the miscible slug. It should be apparent from comparing Figures 3 and 4 that A is a greater area than A, consequently more of the reservoir is swept out by the miscible slug when employed in the method of this invention as compared to the use of a conventional slug process. This increase in the area swept out by the slug results in a marked increase in recovery in that the slug displaces substantially all the oil in area A Furthermore, it should be apparent from Figure 4 that the economic limit for injecting fluid in a conventional slug process is exceeded by employing the method of the present invention. The water following the slug of gas continues to push the gas and the miscible slug through the reservoir and, consequently more of the reservoir is contacted by the miscible slug before the entire miscible slug has been produced. Furthermore, the water, after the effect of the miscible slug has been completely lost, will flow through the reservoir to the producing well at a slower rate than if gas were used, and will invade a much greater portion of the reservoir before the wateroil ratio becomes so great that the further injection of water is not economically feasible. The water as it flows through the reservoir also has a better displacement efliciency than gas. Accordingly, an increase in recovery of oil in place in the reservoir is obtained not only by the increase in the sweepout area by the miscible sing but also the water itself as a displacing medium contributes to the increase in recovery. For the particular porous medium which was used in comparing expected sweepout patterns obtained when employing a conventional slugprocess and the method of this invention, it has been calculated that before breakthrough the method of this invention will cause the miscible slug to sweep through 50 percent of the reservoir as compared to 40 percent when employing a conventional slug process. After breakthrough the gas in a conventional slug process will only contact 15 percent more of the reservoir before it becomes economically unfeasible to inject any more gas, while in the method of this invention the water will contact substantially all the rest of the reservoir and displace substantially .50 percent of the oil in place.-

The laboratory experiments used in this determination? involved what is commonly known as X-ray shadowgraph studies of areal sweepout efficiencies and artificial were sealed with a ceramic type glaze fused onto the- Alundum surface. The metal holders had metal fittings through which fluid could be injected and produced.

Thus an arrangement was provided whereby fluid could be injected into one fitting and displaced through the plates out of anotherfitting.

Inithe specific experiment to study the sweepout efii-' ciency of the method of this invention, the plate was first saturated with a fluid simulating formation fluids and containing X-ray absorbing material. Thereafter, fluids simulating propane and gas in an amount less than 40 percent of a hydrocarbon pore volume represented by the model was injected into the plate followed by water. As the fluids simulating the propane and gas, and the water displaced the oil, a uniform field of X-rays wasdirected against one face of the plate, and a photographic film was mounted behind the plate to record the transmitted X-ray. This was done periodically so that the developed pictures produced by the film showed the relative position of the oil as it was being displaced. An example of such relative positions shown at a time after breakthrough of the gas is shown in Figures 3 and 4.

Figure 5 shows a comparison of the oil recoveries obtained by the method of this invention as compared topreviously known methods. This graph obtained from calculations based on laboratory experiments shows recovery as the ordinate and the volume of fluid injected as the abscissa, therefore eventually indicating the ulti mate recoveries. Curve D shows the recovery due to a water flood, while curve B shows the recovery expected from a conventional slug process. Curve F discloses the increase in recovery contemplated to be obtained by the use of the method of this invention. Reference numeral I on curve F denotes the beginning of the water injection in the method of this invention. Breakthrough of the gas in the processes represented by curves E and F are shown on such curves at the points marked Breakthrough and Breakthrough respectively. It should be apparent from the graph that the breakthrough of gas and intermediates in a conventional slug process is previous to that of the present invention and consequently after Breakthrough the production of oil per volume of injected fluid declines while in the method of this invention the production per barrel of injected fluid continues at a constant rate until breakthrough at Breakthrough After Breakthrough greater recoveries than those obtained in a conventional slug process are obtained with a smaller volume of fluid than is necessary in the conventional slug process. The graph of Figure 5 also shows that the method of the present invention results in an ultimate recovery greater than that expected from a we.

ter flood or a conventional slug process.

While the present invention for the purpose of simplicity and clarity has been explained and described in conjunction with a recovery method applied to a fivespot pattern, it should be understood that the method is not restricted to such use. Similar results will be obtained with the use of the method of this invention in a nine-spot pattern or any other pattern. It should be understood that this invention is limited only by the following claims:

We claim:

I. A method of increasing the recovery of oil from a subterranean oil reservoir having in communication therewith at least one injection well and one production Well comprising the steps of injecting into said reservoir through said injection well, in an amount equal to be tween 0.01 percent and percent of a hydrocarbon pore volume, a slug of a first fluid which is miscible with the reservoir oil at reservoir temperature and at a predetermined pressure, thereafter injecting into said reservoir through said injection well in an amount equal to between 20 percent and 40 percent of a hydrocarbon pore volume, a slug of a second fluid comprising normally gaseous hydrocarbons which is miscible with said first fluid at reservoir temperature and at said predetermined pressure, and thereafter injecting water into said reservoir through said injection well whereby to force said first fluid and said second fluid through said reservoir, said first fluid being maintained in contact with the reservoir oil, simultaneously with said injecting steps maintaining said predetermined pressure on the reservoir, said predetermined pressure being at least as high as the pressure necessary to maintain miscibility between the first fluid and the reservoir oil and between the first fluid and the second fluid, and withdrawing reservoir oil from said reservoir through said production well.

2. A method of increasing the recovery of oil from a subterranean oil reservoir having in communication therewith at least one injection well and one production well comprising the steps of injecting into said reservoir through said injection well, in an amount equal to between 0.01 percent and 5 percent of a hydrocarbon pore volume, a slug of a first fluid, comprising principally hydrocarbons higher than methane and lower than pentane, which is miscible with the reservoir oil at reservoir temperature and at a predetermined pressure, thereafter inecting into said reservoir through said injection well, in an amount equal to between 20 percent and 40 percent of a hydrocarbon pore volume, a slug of a second fluid comprising normally gaseous hydrocarbons which is miscible with said first fluid at reservoir temperature and at said predetermined pressure, and thereafter injecting water into said reservoir through said injection well whereby to force said first fluid and said second fluid through said reservoir, said first fluid being maintained in contact with said reservoir oil, simultaneously with said injecting steps maintaining said predetermined pressure on the reservoir, said predetermined pressure being at least as high as the pressure necessary to maintain miscibility between the first fluid and the reservoir oil and between the first fluid and the second fluid, and withdrawing reservoir oil from said reservoir through said production well.

3. A method of increasing the recovery of oil from a subterranean oil reservoir having in communication therewith at least one injection well and one production well comprising the steps of injecting into said reservoir through said injection well, in an amount equal to between 0.01 percent and 5 percent of a hydrocarbon pore volume, a slug of a first fluid comprising principally hydrocarbons higher than methane and lower than pentane, which is miscible with the reservoir oil at reservoir temperature and at a predetermined pressure, thereafter injecting into said reservoir through said injection well in an amount equal to between 20 percent and 40 percent of a hydrocarbon pore volume, a slug of gas comprising principally methane which is miscible with said first fluid at reservoir temperature and at said predetermined pressure, and thereafter injecting water into said reservoir through said injection well whereby to force said first fluid and said gas through said reservoir, said first fluid being maintained in contact with the reservoir oil, simultaneously with said injecting steps maintaining said predetermined pressure on the reservoir, said predetermined pressure being at least as high as the pressure necessary to maintain miscibility between the first fluid and the reservoir oil and between the first fluid and the gas, and withdrawing reservoir oil from said reservoir through said production well.

4. A method of increasing the recovery of oil from a subterranean oil reservoir having in communication therewith 'at least one injection well and one production well comprising the steps of injecting into said reservoir through said injection well, in an amount equal to between 0.01 percent and 5 percent of a hydrocarbon pore volume, a slug of a firstfluid comprising principally propane, thereafter injecting said said reservoir through said injection well in an amount equal to between 20 percent and 40 percent of a hydrocarbon pore volume, a slug of fluid comprising normally gaseous hydrocarbons which is miscible with said first fluid at reservoir temperature and at a predetermined pressure, and thereafter injecting water into said reservoir through said injection well whereby to force said first fluid and said gas through said reservoir, said first fluid being maintained in contact with the reservoir oil, simultaneously with said injecting steps maintaining said predetermined pressure on the reservoir, said predetermined pressure being at least as high as the pressure necessary to maintain miscibility between the first fluid and the reservoir oil and between the first fluid and the gas, and withdrawing reservoir oil from said reservoir through said production well.

5. A method of increasing the recovery of oil in a subterranean oil reservoir having in communication therewith at least one injection well and one production well comprising the steps of injecting into said reservoir through said injection well, in an amount equal to between 0.01 percent and 5 percent of a hydrocarbon vpore volume, a slug of a first fluid comprising propane, thereafter injecting into said reservoir through said injection well in an amount equal to between 20 percent and 40 'percent of a hydrocarbon pore volume, a slug of gas comprising principally methane, and thereafter injecting water into said reservoir through said injection well whereby to force said propane and said gas through said reservoir, said propane being maintained in contact with the reservoir oil, simultaneously with said injection steps maintaming a pressure on the reservoir at least as high as the pressure necessary to maintain miscibility between the propane and the reservoir oil and between the propane and the gas, and withdrawing reservoir oil from said reservoir through said production well.

6. A method of increasing the recovery of oil from a subterranean oil reservoir having in communication therewith at least one injection well and one production we ll comprising the steps of injecting into said reservoir through said injection well, successively, and in the order named, a slug of butane, a slug of propane, and a slug of ethane, the total volume of said slugs being equal to between 0.01 percent and 5 percent of a hydrocarbon pore voliime, thereafter injecting into said reservoir through said injection well in an amount equal to between 20 percent and 40 percent of a hydrocarbon pore volume. a slug of gas comprising principally methane, and thereafter injecting water into said reservoir through said injection well whereby to force said slugs of butane, propane and ethane and said slug of gas through said reservoir, simultaneously with said injection steps maintaining pressure on the reservoir at least as high as the pressure necessary to maintain miscibility between the reservoir oil and butane, butane and propane, propane and ethane, and ethane and said gas, and withdrawing reservoir oil from said reservoir through said production well.

References Cited in the file of this patent UNITED STATES PATENTS 1,101,605 Wright June 30, 1914 2,609,051 Brownscombe Sept. 2, 1952 2,669,306 Teter et al Feb. 16, l954 2,742,089 Morse et al Apr. 17, 1956 FOREIGN PATENTS 1,047,772 France July 29, 1953 696,524 Great Britain Sept. 2, 1953 UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No. 2,968,350 January 17, 1961 Robert L. Slobod et a1,

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 34, for "015" read oils line 64, for "the", first occurrence, read a column 8, line 8, for -"said", first occurrence, read into Signed and sealed this 1st day of August 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No. 2,968,350 January 17, 1961 Robert L. Slobod et a1,

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below'.

CoIumn 2,- line 34 for "01s" read oils line 64,- for "the", first occurrence, read a column 8, line 8 for said first occurrence, read into Signeddand sealedthis 1st day of August 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attcsting Officer Commissioner of Patents