US 3129757 A
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United States Patent 3,129,757 MISCIBLE FLUID DISPLACEMENT METHOD OF PRODUCING AN OIL RESERVOIR Lorld G. Sharp, Irving, Tex., assignor to Socony Mobil Oil Company, Inc, a corporation of New York No Drawing. Filed May 13, 1960, Ser. No. 28,830 8 Claims. (Cl. 16611) This invention relates to a method of producing an oil reservoir. More particularly, this invention relates to a miscible fluid displacement type of secondary recovery of oil from a reservoir.
An oil reservoir normally is initially produced by means of primary recovery techniques which comprise the utilization of native reservoir energy to drive the oil from the reservoir through a well to the surface of the earth. The native energy which is utilized in primary recovery methods exists in the form of water, gas cap, or solution gas drive, either alone or in various combinations thereof. When this native reservoir energy no longer exists in sufiicient quantity to drive the oil from the reservoir, it is necessary that energy be supplied from an outside source to the reservoir in addition to, in some instances, treating the oil in the reservoir in such a manner that it will more readily flow, such as reducing the viscosity of the oil and improving the relationship between the oil-in-place and the surfaces of the formation in which the oil is found. Supplying energy to a reservoir from an outside source is generally referred to as secondary recovery.
Among the methods of secondary recovery which have been suggested and frequently employed is a process commonly known as miscible fluid displacement. This process generally comprises introducing into a reservoir a fluid which is miscible with the oil-in-place in the reservoir and driving the fluid through the reservoir to a production well. The miscible fluid which is introduced into the reservoir may be driven through it by means of another fluid, such as gas, air, or water. The miscible fluid which is driven through the reservoir absorbs the oil-in-place and reduces its viscosity in order that it may more easily be removed through the production well.
Various methods have been suggested for miscible fluid displacement of reservoir oil wherein a mixture of liquefied, normally gaseous hydrocarbons is driven through the formation as the material which is miscible with the reservoir oil. For example, it has been suggested that a high-pressure gas drive be employed which depends primarily on vaporization of condensible light hydrocarbons from the reservoir oil for the development of the miscible condensibles into a miscible bank or slug which may then be driven through the formation to recover the reservoir oil. This process, however, has the disadvantage of requiring injection pressures generally on the order of 4,000 psi. gauge or higher. Another form of miscible fluid displacement process comprises the introduction of a condensing gas drive wherein the hydrocarbon gas injected contains condensible gases which form a miscible slug within the formation. This method, like the first one mentioned, also requires relatively high pressures. Another suggested method of miscible fluid displacement involves injecting into the formation a liquid slug which comprises a mixture having propane or propane-butane as major components with minor amounts of ethane and pentane. These methods, other than the high-pressure 3,129,757 Patented Apr. 21, 1964 gas method, have the disadvantage of requiring local availability of condensible hydrocarbons. In view of the large quantities of condensible hydrocarbons required to effectively carry out a miscible fluid displacement type of secondary recovery operation, it may not be economically feasible in some instances to utilize these methods.
It has been found that the necessary hydrocarbons to constitute a miscible fluid slug may be developed within the reservoir itself from the reservoir oil. This is par ticularly advantageous in those cases where neither is it desired to utilize the high pressures required by a highpressure gas drive nor the economics of providing the required condensible hydrocarbons from outside sources is favorable.
It is an object of the present invention to provide a secondary recovery method of producing oil from a reservoir. It is another object of the invention to provide a secondary recovery method of oil production of the miscible fluid displacement type. It is a further object of the present invention to provide a miscible fluid displacement type of secondary recovery of oil wherein a miscible fluid slug is formed within the reservoir from the constituents of the hydrocarbon fluids naturally existing within the reservoir. It is another object of the present invention to provide a miscible fluid displacement form of secondary recovery of oil which does not require the addition of condensible hydrocarbons from outside sources in order to form a miscible fluid slug within the formation. These and further objects of the invention will be evident from a reading of the following description of the invention.
In accordance with the present invention, a body of hydrocarbonaceous material comprising particularly C to C hydrocarbons is created in situ within the formation itself from the native formation fluids. The production of these relatively low molecular weight hydrocarbons containing from 2 to 6 carbon atoms inclusive is accomplished by heating the reservoir oil to a mild cracking temperature in the range of about 500 F. to about 750 F., that is, a vis-breaking temperature, in order to produce a sufficient quantity of the desired low molecular weight hydrocarbons to form a miscible fluid slug. Subsequent to the production of the miscible fluid slug, the slug is driven through the formation toward the production well in a substantially conventional manner by the introduction of a fluid-driving medium into the formation through an injection well. The formation oil and the miscible fluid slug material is then withdrawn from the production well until the fluids flowing from the production well substantially comprise the driving fluid.
The invention may be practiced in any formation which is provided with at least one injection well and at least one production well. Any desired pattern of production and injection walls may be employed. For example, the invention may be practiced with the conventional five-spot pattern of well location. In the five-spot pattern, four production wells are located such that there is one production well at each of the corners of a square, with a single injection well being located at the geometrical center of the square. When utilizing this five-spot pattern of well spacing, the sweep effected by the process ema nates from the center injection well and extends toward each of the corner-positioned production wells. The slug of miscible fluid is developed around the injection well and driven outwardly from the injection well toward each of the production wells.
T he first step in the process of the invention, that is, the cracking or vis-breaking of the reservoir oil to form a miscible fluid slug, may be carried out by various different means. The formation around the injection well is heated to a temperature range of about 500 F. to about 750 F. Preferably, the temperature range to which the formation is heated falls within the range of about 550 F. to about 750 F. This heating may be accomplished by injecting a hot fluid into the formation through the injection well. The fluid may be heated by an electrical heater positioned within the borehole opposite the formation. The formation may be heated by the application of nuclear energy applied at the face of the formation within the injection well. Heating may also be accomplished by passing an electrical current between an electrode placed in the injection well adjacent to the formation to be heated and one or more electrodes placed in the production wells. The preferred form of heating of the formation, however, is effected by the employment of a well-known technique generally referred to as in-situ combustion of the oil in the formation around the injection well. In the in-situ combustion technique, a portion of the oil is actually burned within the formation with the heat produced by the combustion process being employed to raise the temperature of the reservoir oil to the desired cracking or vis-breaking level.
Where step one of the invention, that is, heating an area of the formation around the injection well to a cracking or vis-breaking temperature, is effected by electrical heater means, it is preferred that a fluid be employed as a heat-transfer agent to conduct the heat supplied by the electrical heater into the formation. A heater which may be utilized for this purpose is shown in US. Patent No. 2,670,802, issued on March 2, 1954, to C. S. Ackley. The heater is positioned within the bore of the injection well at a location adjacent to the formation which is to be heated. The fluid acting as the heat-transfer agent is forced into the injection well, through the electrical heater, and into the formation. As the fluid passes through the electrical heater, its temperature is raised to the desired level. The electrical heater may be provided with thermostatic means for maintaining the temperature of the heat-transfer agent within the desired range. An inert gas, such as flue gas, may be used as the heat-transfer agent. Another form of heat-transfer agent which may be employed is superheated steam. In an alternative method of heating the formation, hot flue gas and superheated steam may be injected alternately into the formation through the electrical heater. The temperature of the inert gas injected into the formation must be raised to within a range of about 500 F. to about 750 F., preferably within the range of about 550 F. to about 750 F, to effect cracking of the formation oil to obtain the desired low molecular weight hydrocarbons containing from 2 to 6 carbon atoms inclusive.
As previously stated, the preferred method of obtaining the desired cracking or vis-breaking temperature of the formation is the employment of the in-situ combustion technique of burning the formation oil to obtain the desired low molecular weight hydrocarbons to form a miscible fluid flood within the formation. Initiation of insitu combustion of the formation oil may be effected by an electrical igniter such as disclosed in US. Patent 2,771,140, issued to Harry A. Barclay, et al. on November 20, 1956. The igniter is placed in the injection well adjacent to the portion of the formation in which the combustion process is to be carried out. The temperature of the combustion process may be controlled within the desired range by regulating the oxygen content of an oxygen-containing gas introduced into the formation through the injection well. Control of the temperature of the in-situ combustion process may also be effected by intermittent injection of steam during the carrying out of the combustion process. A particular advantage inherent in the use of steam is the production of steam condensate within the formation which will operate during the second stage of the invention as a simultaneous water flood along with the flood being effected by the miscible fluid slug.
The area of the formation around the injection well which is heated to a cracking or vie-breaking temperature comprises at least 10 percent and preferably from about 10 to about 30 percent of the pattern to be swept for recovery of the reservoir oil. Where a conventional fivespot pattern of well location is employed, the heated portion of the formation should extend radially from the injection well to at least 25 percent of the distance from the injection well to at least one of the production wells. The term heated portion, as used herein, means that portion of a formation through which in-situ combustion is propagated. It is desirable that this extent of heating be employed in order to produce in situ the necessary amount of low molecular weight hydrocarbons to provide a miscible fluid slug which will be effective to obtain maximum recovery of the reservoir oil. Not more than about 30 to 50 percent of the oil in the pattern to be swept requires heating in order to obtain the necessary quantity of cracked or vis-broken reservoir oil to provide the needed miscible fluid slug.
After the heating of the formation to the desired temperature has extended to the required distance from the injection well, additional benefit may be obtained from the heat contained within the formation by permitting or effecting back-flow of the reservoir oil toward the injection well. This back-flow of the reservoir oil may be accomplished by discontinuing the combustion of the oil and permitting the formation pressure to force the oil back toward the injection well over the area previously heated. Another form of back-flow of the reservoir oil over the previously heated area may be carried out during the actual combustion process by intermittently pressuring into the formation via the production well or wells an inert gas which is substantially free of uncombined oxygen or by intermittently discontinuing the injection of gas through the injection well and partially evacuating the injection well to a pressure lower than the formation pressure. By either the introduction of an oxygen-free gas into the production wells or the partially evacuation of the injection well, the reservoir oil is caused to flow to ward the injection well over the previously heated area to effect further cracking or vis-breaking of the reservoir oil. By utilizing the back-flow technique, as described, for heating additional reservoir oil, the distance into the formation from the injection well where the in-situ combustion process must be extended may be reduced. Since the back-flow technique results in heating larger quantities of oil, it is thus not necessary that the area over which the in-situ combustion process is carried out be quite as extensive as would be necessary in the absence of the use of the back-flow technique.
It is to be understood that the employment of the insitu combustion technique in the present invention is to be distinguished from the ordinary use of the in-situ combustion technique in that in the present invention its sole objective is the creation of a quantity of low molecular Weight hydrocarbons for the purpose of providing a miscible fluid slug which is subsequently used for the removal of the formation oil through the production wells. No effort is made herein to burn the entire formation which is ordinarily done where the in-situ combustion technique is used.
The length of time required to carry out the first step of the method of the invention will vary, depending upon the temperature employed, together with the size of the area of the formation which is to be heated. For example, the length of the period over which heating of the formation is required may vary from three to about thirty days, and in the instance where a temperature as low as 500 F. is employed, the time of heating must be appreciably extended.
At the completion of the initial step of the invention, a miscible fluid slug comprising intermediate low molecular weight hydrocarbons is established in the formation and the second step of the invention may be carried out by driving the miscible fluid slug through the formation toward the production wells. At this stage the in-situ combustion step, or the provision of the required cracking or vis-breaking temperature by the other means described, is terminated and the established miscible fluid slug, along with the reservoir oil, is driven away from the injection well toward the production well or wells by means of a driving fluid introduced into the formation through the injection well behind the miscible fluid slug. The driving fluid should be substantially free of uncombined oxygen in order that it will not support further combustion. The driving fluid employed preferably comprises an inert gas containing substantially no free oxygen. The inert gas may be a natural gas or a separator gas which is a relatively high methane-containing gas. Other driving fluids which may be utilized are a flue gas containing substantially no free oxygen, a superheated steam, or nitrogen. Each of the driving fluids disclosed herein is, under the reservoir conditions employed, at least partially miscible wtih the in-situ produced miscible slug. As previously stated, the superheated steam has the distinct advantage of providing condensate within the formation which acts as a supplementary water drive along with the driving fluid. If desired, the driving fluid may be heated to a temperature no greater than about 500 F. before injection into the formation. The driving fluid is introduced into the formation at a pressure sufficiently above the formation pressure to drive the miscible slug along with the reservoir oil through the formation toward the production wells.
The pressure employed in introducing the driving fluid should be at least as high as 1,000 p.s.i. gauge and preferably is at least 1,500 p.s.i. gauge or higher. Under certain conditions heavy, viscous oils are encountered which have no chance of being displaced by the previously discussed high-pressure gas method wherein miscible material is developed by vaporization from the formation fluids. This is the case where the heavy, viscous oils do not naturally contain large quantities of low molecular weight (C C hydrocarbons which can be vaporized by the high-pressure gas method and which are a necessity to such method. Under such conditions the present invention may be applied with the driving gas being introduced at a pressure range of 3,000 to 4,000 p.s.i. gauge, which approaches the range of the high-pressure gas method.
Introduction of the driving fluid into the formation through the injection well is continued until the eflluent being withdrawn from the production well or wells comprises substantially the driving fluid. That is, injection of the driving fluid extends over a period of time during which substantially all of the reservoir oil along with the in-situ created miscible fluid slug are removed from the formation through the production well or wells.
Application of the invention, as described above, was made in the laboratory in the following manner. A stainless steel coil, 20 feet in length and having an internal diameter of 0.562 inch, was packed with sand and mounted within a high temperature, electrically heated oven with the axis of the stainless steel coil being in a vertical position, while the angle of dip was 2.3. The permeability of the sand pack was 194 millidarcies. No interstitial water was present in the sand pack as prepared for the experiments which are described herein. The temperature of the sand-packed tube was maintained at 270 F. while the pack was saturated with a relatively heavy crude oil having and API gravity of 23. Other properties of the crude oil were: bubble point 270 F. at 155 p.s.i. gauge; gas-oil ratio at atmospheric flash 28 cubic feet per barrel; formation factor 1.058 and a reservoir fluid density of 0.845 at 4,000 p.s.i. gauge and 270 F. The formation volume factor as used herein is defined as the volume of oil at reservoir temperature and pressure, that is, 270 F. and 4,000 p.s.i. gauge, divided by the volume of flashed stock tank oil corrected to 60 F.
and 14.7 p.s.i.a.
Utilizing the above-described apparatus and with the sand-packed stainless steel coil prepared as stated, an experiment was carried out to determine the production of intermediate low molecular weight hydrocarbons, that is C to C hydrocarbons at 710 F. and 4,000 p.s.i. gauge. The pore volume of the sand pack in this particular experiment was 24.7 percent. The temperature of the sand pack was raised from 270 F. to 710 F. during a 24-hour heating-up period. During this heating-up period, 21.65 volume percent of the oil was educted from the stainless steel coil. Following the 24-hour heatingup period, the coil was maintained at a temperature of 710 F. for an additional period of 68 hours, during which time steam was injected into the coil at a flood rate of 10.6 feet per day and a pressure of 4,000 p.s.i.. gauge at 710 F. During the total heating period of 92 hours, the total oil recovered as oil was 85.24 volume percent. A total of 92.46 weight percent of the hydrocarbons was recovered, including gas and oil, the gas accounting for about 9 weight percent. The residue by diflerence was 7.54 percent, the residue being carbonaceous material which was insoluble in kerosene and constituted deposited carbon and tar-like material. No free oil as such was left in the pack.
During the heating of the pack at 710 F., as described in the above experiment, five gas samples were taken and analyzed by mass spectrograph. These samples were taken from the sand pack at the 20th hour, 26th hour, 30th hour, 36th hour, and 41st hour after the initiation of the steam drive. The averages of the analyses of these samples are given in Table I below.
Table I Component M01, percent Weight, percent Hydrogem 5. 20 0.36 002 3. 37 4. 75 C O 2. 18 1. 95 CH4 (Methane) 40. 75 20.71 C2Hfl (Ethane) 16.12 15.34 C3H (Propane) 14. 95 20. 88 Ciiso (Isa-Butane). 3.63 6.68 Gin (n-Butane) 7. 45 13. 74 C iso (Iso-Pentane) 2.09 4.79 0511 (n-Pentane) 2. 16 4. 97 06+ (Hexanes 1. 35 4. 33 Unsaturates 75 1. 49
Inerts (H2 and CH 45. 95 21.07
(C 0, CO2, C2-C 52.70 74. 60 5 1. 35 4. 33
In another experiment utilizing the above-described equipment and 23 API gravity oil, a sand pack having a pore volume of 26.2 percent was saturated with the oil at 270 C. and the saturated pack was then heated to a temperature of 710 F. over a period of six hours. Subsequent to this 6-hour heating period to 710 F., methane Was introduced into the sand pack at a flood rate of 10 feet per day for a period of 48 hours. During the initial 6-hour period of heating, 17.89 volume percent of oil was educted from the sand pack. Over the total period of 54'hours, the recovery of oil was 86.49 volume percent. The total recovery of hydrocarbons, including the gas which was produced, was 94.87 weight percent. During the step of flooding with methane, five samples of gas were taken from the sand pack. The samples were analyzed and adjusted to exclude the injected methane.
Averages of the analyzes of these five samples are set out below in Table II.
Table II Component M01, Weight, percent percent Hydrogen 4. 87 0.33 C O, 2. 66 3. 95 7S 0.07 0.08 CH4 (Methane) 52. 01 28.15 CgHs (Ethane). 11.85 12.01 C 11 (Propane). 11. 67 17. 34 Cdso (Iso-Butane) 2.96 5.80 04h (n-Butane). 5.19 10.16 C iso (Iso-Pentane) 2.02 4. 91 On: (n-Pentane)-.. 2.18 5.30 0 (Hexanes+) 2. 22 7. 49 Unsaturates 2. 4. 48
Inerts (Hz, CH4) 56. 88 28. 48
(CO2, HzS, C2-C5) 4.0. 90 64. 03 O -i- 2. 22 7.49
A further experiment was conducted in which the apparatus consisted of two sand-packed, stainless steel, tubing coils connected in series. Each of the coils was mounted within a separate electrically heated oven, the longitudinal axis of each of the coils being placed in a vertical position. The first coil was 20 feet in length and had an inside diameter of 0.56 inch. Its porosity was 31 percent (336 cc.) and had a permeability of 11.5 darcies. The second coil was 94 feet in length and had an inside diameter of 0.28 inch. The second coil had a porosity of 34 percent (392 cc.) and a permeability of 21.7 darcies.
Each of the sand-packed tubes was saturated with a relatively heavy crude oil having an API gravity of 23 and contained no interstitial water.
The efiiuent or discharge end of the second coil was equipped with a back-pressure regulator which was set at 4,000 p.s.i. gauge and had attached to it a gas/ oil separator. The oil produced from this apparatus was collected in a graduated cylinder, and the gas produced was collected in a gas holder.
The first coil was heated to 710 F. over a 4-hour heating time, and the second coil was heated to a temperature of 270 F., with the pressure in each of the coils being held at 4,000 p.s.i. gauge. After maintaining both the first and second coils at the above conditions of tempera ture and pressure for a period of 20 hours, subsequent to the establishment of these conditions of temperature and pressure, methane was injected into the inlet end of the first coil at a pressure slightly in excess of 4,000 p.s.i. gauge. The frontal advance rate of the methane through the first coil was 11 feet per day which gave a total residence time of the oil within the first coil of 72 hours (4 hours bringing the coil to 710 P., 20 hours soaking time, and 48 hours displacement time).
The total effluent from the first coil was driven by the methane through the second coil at a rate of feet per day. Gas samples, taken prior to breakthrough of the driving gas, were collected at periods of 92, 100, and 116 hours after initial injection of the methane.
A total of 90.76 weight percent of the hydrocarbons was recovered from the first coil, including gas and oil, the gas accounting for 11.62 weight percent. The residue by diflerence was 9.24 percent, such residue being carbonaceous material which was insoluble in kerosene and constituted deposited carbon and tar-like material. No free oil as such was left in the pack.
A total of 98.10 weight percent of the oil was recovered from the second coil.
The over-all recovery from both coils was 94.43 weight percent, oil accounting for 88.00 percent and gas accounting for 6.43 percent. The residue amounted to 5.57 percent.
- Prior to breakthrough of the driving gas, three'gas. samples were taken and analyzed by mass spectrograph. The averages of the analyzes of these samples are given in Table III below.
The above experiments clearly illustrate that temperatures of about 710 F. will produce sufiicient C through C hydrocarbons, even at relatively short soaking periods, to develop a miscible fluid slug within the sand pack. The temperature of the pack to obtain effective cracking of a suflicient amount of hydrocarbons should be at least a minimum of 500 F. and preferably within the range of about 550 F. to about 750 F.
What is claimed is:
1. In a method of producing oil from a reservoir penetrated by at least one injection well and one production well the steps which comprise initiating in-situ combustion of a portion of said oil within said reservoir around said injection well, injecting a gas having a controlled oxygen content into said reservoir through said injection well to maintain the temperature of said combustion within the range of about 550 F. to about 750 F., terminating said in-situ combustion after a maximum of about 30 percent to 50 percent of said oil has been heated for a time sufiicient to produce a miscible fluid slug comprising C to C hydrocarbons, injecting into said reservoir through said injection well a driving fluid at least partially miscible with said fluid slug and substantially free of uncombined oxygen, forcing said miscible slug and said oil through said reservoir toward said production well by means of said driving fluid, and producing said oil and said miscible slug through said production well.
2. In a method of producing oil from a reservoir penetrated by at least one injection well and one production well the steps which comprise heating a portion of said oil within said reservoir around said injection well to a temperature within the range of about 500 F. to about 750 F. and for a time sulficient to crack said oil to provide in-situ a miscible fluid slug comprising intermediate low molecular weight hydrocarbons, introducing into said injection well a driving fluid at least partially miscible with said fluid slug and substantially free of uncombined oxygen, driving said miscible slug through said reservoir to said production well by means of said driving fluid, and producing fluids from said production well including said oil and said miscible slug.
3. A method as defined by claim 2 wherein the step of heating a portion of said reservoir is carried out until a maximum of about 30 percent to 50 percent of the oil in the pattern to be swept by said method is heated.
4. A method as defined by claim 2 wherein the heating of said oil Within the said reservoir is efiected by injecting a heated fluid into said reservoir through said injection well, said fluid being heated by being passed through electric heating means positioned within said injection well.
5. A method as defined by claim 2 wherein the heating of said oil in said reservoir is eflected by initiating in-situ 9 combustion of said oil and continuing said combustion within said controlled temperature range for a period of time suflicient to produce said miscible fluid slug compris ing intermediate low molecular weight hydrocarbons.
6. In a method of producing oil from a reservoir penetrated by at least one injection well and one production well, the steps which comprise initiating in-situ combustion of a portion of said oil within said reservoir around said injection well, injecting gas having a controlled oxygen content into said reservoir through said injection well to maintain the temperature of said combustion within a range of about 500 F. to about 750 F., terminating said in-situ combustion after the desired portion of said oil has been heated for a time sufficient to produce a miscible fluid slug comprising intermediate low molecular weight hydrocarbons, injecting into said reservoir through said injection well a driving fluid at least partially miscible with said fluid slug and substantially free of uncombined oxygen, forcing said miscible slug and said oil through said reservoir towards said production well by means of said driving fluid, and producing said oil and said miscible slug through said production well.
7. A method according to claim 6 wherein the temperature of said combustion is maintained within a range of 10 about 550 F. to about 750 F. and said in-situ combustion is terminated after a maximum of about percent to percent of said oil in the pattern to be swept by said method is heated.
8. A method according to claim 6 wherein the step of controlling the temperature of said in-situ combustion is effected by alternately injecting a gas having a controlled oxygen content and superheated steam into said reservoir through said injection Well.
References Cited in the file of this patent UNITED STATES PATENTS 2,771,951 Simm Nov. 27, 1956 2,788,071 Pelzer Apr. 9, 1957 2,795,279 Sarapuu June 11, 1957 3,036,632 Koch et al May 29, 1962 OTHER REFERENCES Hackhs Chemical Dictionary, by Grant, Third Edition, 1944, published by The Blakiston Company, Philadelphia Pa.
McNiel, J. 8., Jr. and Nelson, T. W., Thermal Methods Provide 3 Ways to Improve Oil Recovery," Oil and Gas Journal, January 19, 1959, pages 86-98.
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 129,757 April 21, 1964 Lorld G. Sharp It is hereby certified that error appears in the above numbered pat.- ent requiring correction and that the said Letters Patent should read as corrected below.
Column 5, line 73, for "and" read an column 6, line 63, for "270 C." read 270 F. column 8, line 3, for "analyzes" read analyses Q Signed and sealed this 15th day of September 1964 (SEAL) Attest:
ERNEST W. SWIDER EDWARD J. BRENNER Aitesting Officer Commissioner of Patents