US3035637A - Recovery of petroleum - Google Patents

Description

May 22, 1962 J. c. ALLEN RECOVERY 0F PETROLEUM 2 Sheets-Sheet 1 Filed Sept. 9, 1957 May 22, 1962 J. c. ALLEN RECOVERY 0R PETROLEUM 2 Sheets-Sheet 2 Filed Sept. 9, 1957 United States Patent Office 3,[i35,637 Patented May 22, 1962 3,035,637 RECGVERY F PETROLEUM Joseph C. Allen, Bellaire, Tex., assignor to Texaco Inc., a corporation of' Delaware Filed Sept. 9, 1957, Ser. No. 682,760 4 Claims. (Cl. 166-9) This invention relates to the recovery of petroleum from underground formations.

In the petroleum eld, various so-called secondary recovery processes are employed to recover the petroleum remaining behind in the petroleum reservoir after the primary producing forces (solution gas drive, water drive and the like) have been depleted. Various secondary recovery techniques have been proposed to effect the recovery of this residual petroleum. These techniques have included water flooding wherein water is introduced via an injection Well into the petroleum reservoir to displace petroleum therefrom toward a production well. Gas flooding or gas pressuring wherein gas is introduced into the reservoir via an injection well to drive or displace petroleum toward a production well has also been employed. Gas sweeping wherein a relatively lean gas is passed through the reservoir toward a production well has also been suggested. During this sweeping operation the heavier petroleum components are vaporized into the lean sweeping gas and are produced in gaseous form via the production well. Miscible phase iiooding has also been proposed. In miscible phase liooding a liquefied normally gaseous hydrocarbon, such as LRG., is introduced into the reservoir followed by the introduction of a driving fluid, such as a gas, eg., natural gas, via an injection well to displace petroleum from the reservoir. The resulting displaced petroleum is then produced via a production well.

lt has been recognized that capillary forces play an important role'in the displacement process usually involved in the secondary recovery of petroleum from underground formations. lt has been theorized that if the interfacial tension existing between two separate phases (eg. natural gas and oil) in a petroleum reservoir could be reduced to zero or almost zero substantially all of the petroleum could be displaced therefrom by a suitable driving uid, viz. natural gas. For example, it has been speculated that the addition of suitable surfactant agents to water in a water drive to reduce the interfacial tension between the water and the petroleum to substantially zero would result in the substantially complete recovery or displacement of the petroleum from the reservoir by the water drive or ood. Unfortunately, however, from a practical point of view suitable surfactant agents which so reduce the interfacial tension between the petroleum and water in a water drive process are not available. Accordingly, secondary recovery water flood operations employing this technique have not been completely successful.

Accordingly, it is an object of this invention to provide an improved secondary recovery process for the recovery of petroleum from underground petroleum-containing formations.

It is another object of this invention to provide a process wherein a sweeping gas, having a critical composition with respect tothe petroleum in the reservoir undergoing treatment, is employed in the secondary recovery operation to effect the displacement of petroleum from the reservoir.

Still another object of this invention is to provide an improved secondary recovery operation employing a sweeping gas which has a critical composition with respect to petroleum in the reservoir being swept such that under the formation conditions of temperature and pressure the sweeping gas forms a critical single phase admixture with the petroleum therein.

yStill another object of this invention is to provide a secondary recovery operation wherein the interfacial forces between petroleum and the sweeping uid (gas) is reduced to zero.

Yet another object of this invention is to provide a secondary recovery process wherein it is possible to recover substantially completely all of the petroleum in the reservoir being swept.

How these and other objects of this invention are accomplished will become apparent with reference to the accompanying disclosure and drawings wherein FIG. l schematically illustrates a secondary recovery process in accordance with the practice of this invention, and wherein FlG. 2 graphically illustrates the practice of this invention as applied to the secondary recover] of petroleum from an underground petroleum-containing reservoir wherein a gaseous mixture of hydrocarbons having a special critical composition with respect to the formation petroleum is employed as the driving fluid or displacing medium.

Under a given set of conditions of reservoir temperature, pressure and known composition of reservoir petroleum (oil) and gas there exists a composition of injected gas such that when combined with the reservoir petroleum will form a critical mixture of a single phase wherein the reservoir petroleum loses its identity. When this occurs the capillary forces normally associated with a petroleum producing formation (such as are present at a gas-liquid petroleum interface) are reduced to zero, thereby releasing all the petroleum in the formation or reservoir pore space to the critical mixture. Accordingly, in the practice of this invention a gaseous hydrocarbon or mixture of hydrocarbons having a composition capable of forming a critical single phase admixture with the petroleum hydrocarbons under the formation conditions of temperature and pressure is introduced into the formation into contact with the petroleum hydrocarbons therein to form a critical phase therewith. Following the injec` tion of the gaseous hydrocarbons there is introduced into the formation a second gaseous hydrocarbon or mixture of hydrocarbons which has a composition critical with respect to the aforementioned first injected gas so that said critical single phase admixture is maintained. By operating in the above-described manner there is established a critical state between the reservoirv petroleum hydrocarbons and the rst introduced gaseous mixture of hydrocarbons and the critical state established between the rst introduced gaseous mixture of hydrocarbons and the formation petroleum or hydrocarbons is maintained. Accordingly, having the critical state established and maintained within the petroleum bearing formation in the secondary recovery process, the necessary steps for obtaining the optimum in displacement eiciency have been taken.

Referring now to FIG. 1 of the drawing which schematically illustrates a practice of this invention as applied to the displacement of petroleum from an underground reservoir, there is schematically illustrated therein a section of a petroleum-containing reservoir as it is being swept or produced in accordance with this invention. More particularly, as indicated in FlG. l a gaseous mixture of hydrocarbons having a suitable composition is introduced at the left hand end of FIG. l and oil produced from the formation is shown recovered at the right hand end of FIG. l. Intermediate the formation between the introduction of the suitable, ultimate gas and the production of petroleum therefrom there are schematically illustrated various transition zones which exist therein. Proceeding from right to left zone A illustrates a bank of petroleum as displaced from the formation during the treating process. Zone B is a transition zone and comprises petroleum enriched with respect to intermediate hydrocarbons in the range CM. At the trailing edge of Zone B will be the critical mixture, single phase only, corresponding to the formation conditions of temperature and pressure and brought about by the introduction of a hrst gaseous adr'nixture which is critical with respect to the petroleum originally within the formation. Transition zone C is the transition zone between this first introduced critical gaseous mixture and a subsequently introduced second gaseous admixture which is critical with respect to the first introduced gaseous mixture under the formation conditions of temperature and pressure so that a single phase is maintained therein. Zone D has only a single hydrocarbon phase maintained therein and comprises substantially only the subsequently injected second critical gaseous admixture together with some of the first injected driving gas. Transition zones C and D will have varying sizes depending upon the composition of the petroleum being displaced and the makeup of the critical displacing gaseous hydrocarbon mixtures. Following zone D is zone E comprising substantially only the final driving gas which usually comprises substantially only methane. The composition of this driving gas is such to maintain only a single hydrocarbon phase in that portion or Zone of the formation swept by this gas.

In a special embodiment of the practice of this invention as illustrated in FIG. 1 it is preferred to employ a relatively large amount of the first injected gaseous mixture of hydrocarbons which is critical with respect to the petroleum within the reservoir undergoing treatment, such that there exists within the reservoir during the secondary recovery operation a substantial portion thereof which is occupied by substantially only the first introduced critical gaseous admixture.

Referring now to FIG. 2 of the drawing which graphically illustrates the practice of this invention, there is illustrated therein by means of a three component composition graph a complex hydrocarbon system such as is found in petroleum reservoirs and the composition and phase changes which occur in the practice of this invention.

As indicated in FIG. 2, the triangular graph indicates a relatively complex petroleum or oil mixture, such as may be found within a petroleum producing formation, as being made up of three components, (R) methane, (S) hydrocarbons in the range C2 6 and (T) hydrocarbons in the range C7 and higher. Although the designation of a complex petroleum system such as occurs in an underground formation as indicated by FIG. 2 is somewhat arbitrary, this graphical presentation is very useful in qualitatively and thermodynamically explaining what takes place within the petroleum producing formation during the practice of this invention and approaches actual formation conditions. Conditions illustrated therein are for a given temperature and pressure, viz. formation temperature and pressure. In FIG. 2 the formation temperature is 206 F. and the formation pressure is 2,000 p.s.i.a. All compositions noted in FIG. 2 are in mol percent.

That portion of FIG. 2 encompassed by the loop or phase boundary curve X delineates the area of compositions wherein two phases exist. Compositions outside of this delineated area exist only in a single phase. A tie line Y (parallel to base composition line RT) is shown cutting across the two phase region encompassed Within line X. The two points Y1 and Y2 on line X represent the saturated oil and the saturated gas, respectively, which would be in equilibrium with each other if placed in contact at the temperature and pressure existing for the given diagram. The limiting tie line is a pointV F representing the critical (only single phase) mixture for the existing temperature and pressure. Mixtures represented by points above and to the left of the saturated vapor portion of curve X are gas (area V) and mixtures represented by points above and to the right of the saturated liquid portion of curve X are oil (area W). Accordingly, mixtures with greater percentages of intermediate (C2 6) or those lying above point F are in the zone that is either a liquid or gas if they can be visibly defined in the presence of one or the other. These are the areas of critical mixtures. Dashed line T passing through critical point F of the phase boundary curve X is the line of minimum richness for the injected critical gas and delineates the areas of critical mixture lying thereabove.

Considering now the application of the practice of this invention as applied to a petroleum reservoir which has a composition such as may be represented by point A within the phase boundary curve X (the reservoir petroleum may have a composition within or without the two phase region delineated by curve X), drawing a line through point A through point F and extending this line to that side RS of the triangular graph to point G, which point represents a composition or gaseous mixture of hydrocarbons containing 65% propane (C2 6) and 35% methane, a gaseous admixture comprising approximately 2/3 propane and 1/3 methane. It is apparent from FIG. 2 that when the gas of composition G is introduced in sucient amount into the formation containing oil having a composition indicated by point A the composition of the oil within the formation, as it is contacted by and is intermingled with the gas of composition A, will change and will move along line AG in the direction indicated by the arrowhead thereon to point F which corresponds to the critical composition under the existing formation conditions of temperature and pressure. When point F is reached all phase boundaries between a gas and oil mixture of that composition within the formation disappears and in effect only one phase exists. From point F a line is extended horizontally, parallel to the base line RT representing components methane and C7 and heavier to point H. Point H represents a gaseous admixture containing approximately 47% propane (C2 6) and 53% methane.

Following the injection of the rst gaseous admixture having a composition indicated by point G, a second gaseous mixture of hydrocarbons having the composition indicated by point H (53% methane and 47% propane) is introduced into the formation. As indicated by FIG. 2, the composition of the petroleum at point F within the formation moves along the line HF in the direction indicated by the arrowhead thereon. Sufficient gaseous mixture having the composition represented by point H is introduced into the formation until the petroleum therein reaches a composition on line HF indicated by point E. When the petroleum in the formation reaches the composition indicated by the point E there is introduced into the formation a relatively dry gas, comprising substantially only methane, such as a gas having a composition represented by point K on the base line RT of the three component triangular graph. The composition of the petroleum within the formation at point E would then move along the line EK in the direction of K as indicated by arrowhead on the line EK.

It is thus seen that when an operation is carried out in the manner described herein the avoidance of the formation of two separate phases within the petroleum formation during the secondary recovery operation is achieved. It is to be noted that line EK does not penetrate or enter into the two phase boundary region delineated by phase boundary curve X but at most passes tangent thereto or outside of phase boundary curve X.

The advantages of carrying out a secondary recovery operation as described hereinabove wherein a portion of the formation petroleum, as it is displaced through the formation, follows the path AFEK would be readily apparent when one considers the operations which would be involved if instead of the selection of a critical gas, such as is represented by point G, or a fluid having a composition represented by a point in the areas V and W of FIG. 2, one were to inject into the formation a substantially pure propane, as would be represented by point S (C2 6) of the three component phase diagram of FIG. 2. Should propane alone be introduced into the formation the formationuid composition would follow the path represented by dashed line AS. It would be necessary to introduce a very substantial amount of propane into the formation, at least sufficient to reach a composition indicated by point C on line AS. Once the composition represented by point C had been reached the introduction of a relatively dry gas comprising substantially only methane (eg. composition K on line RT) might be commenced so as to effect a displacement of the formation hydrocarbons through the formation in the critical phase without entering the two phase region delineated by phase boundary curve X, the area encompassed by curve X depending upon formation temperature and pressure, higher formation temperature and pressure tending to compress the two phase region P.

The phase boundary curve indicated by line X of FIG. 2 is typical of reservoir petroleum fiuids. Further, it has been noted that the critical composition of petroleum fluids usually contains about -13 mol percent intermediate hydrocarbons in the range C2 5.

In summary, in accordance with the practice of this invention a secondary recovery operation is carried out in the following manner. There is introduced into the formation a gaseous admixture having a composition (such as G) such that it forms with the formation hydrocarbons a critical single phase therewith. When a sufcient amount of this first injected gaseous mixture of hydrocarbons has been introduced into the formation there is introduced into the formation a second gaseous mixture of hydrocarbons equivalent to a gaseous mixture preferably having the composition H or lying along the line HG of FIG. 2. A suicient amount of this second injected mix-ture of gaseous hydrocarbons is introduced to form within the formation undergoing treatment a composition indicated by point E. Thereupon there is introduced into the formation a relatively lean gas, such as a gas comprising substantially only methane, as indicated by a gas having the composition indicated by point K or a point along the line RK or in the area L of the three component diagram of FIG. 2. By operating in the above-indicated manner a completely critical single phase displacement of petroleum hydrocarbons can be effected and the benefits derivable therefrom are obtained.

It is mentioned that those driving gas compositions set forth hereinabove with respect to the operation of the subject invention as illustrated by FIG. 2 are not limitive of this invention but are merely illustrative thereof. Any suitable gaseous hydrocarbon mixtures such as a gaseous mixture having a composition within the area V of FIG. 2 might be employed. It is mentioned, however, that once a critical single phase composition has been obtained only a single phase should be maintained throughout the formation being swept extending from the injection well to the foremost edge or boundary of the established critical phase in the direction of the production well.

The embodiment of this invention described with respect to FIG. 2 is the preferred embodiment wherein the composition of the first injected gas is selected so that the reservoir petroleum changes composition in a straight line directly to its critical composition as illustrated by point F. Likewise, it is preferred that the second injected gas have a composition such as indicated by point H so that the formed critical single phase, as its composition is changed by contact with a gas of composition H, does not penetrate the two phase region P delineated by phase boundary curve X, but passes at most only tangent thereto as indicated by line EF. Accordingly, a gaseous hydrocarbon mixture having a composition in the area L would not be suitable in accordance with the practice of this invention since the initially formed single phase of composition F would revert to a two phase composition in the region P upon the continued injection of such a gas. Accordingly, the limiting composition of the second injected mixture of hydrocarbons is within the areas V and W, and delineated by that portion above dashed line T.

In a particularly preferred embodiment it is preferred to carry out the foregoing operations such that the shortest possible path is taken from a point, such as point A which is representative of the composition of the reservoir petroleum, through its critical point F, to point R or K or an area surrounding the same which corresponds to a gas consisting essentially of substantially only methane without said path penetrating or entering the two phase region P delineated by phase boundary curve X. As is apparent from FIG. 2 only one such path exists for a given composition of reservoir petroleum and reservoir conditions of temperature and pressure but, on the other hand, it is realized that there are an infinite number of paths, other than the preferred and shortest path, which might be taken by an operator and still practice this invention and enjoy and attain the benefits and advantages derivable therefrom with respect to the efficiency and completeness of a single phase displacement or fiooding operation.

Although the description of this invention has been made hereinabove with respect to an injection well and a production well it is realized that one or more injection wells may be involved and one or more production wells may be involved.

As will be apparent to those skilled in the art many substitutions, changes and alterations are possible in the practice of this invention without departing from the spirit or scope thereof.

I claim:

l. In a secondary recovery operation of petroleum from an underground petroleum bearing formation wherein a fluid is introduced via an injection well into said petroleum bearing formation to drive petroleum from said formation toward a production well, the improvement which comprises employing as said fluid a first gaseous hydrocarbon mixture having a composition with respect to said petroleum in said formation such that said first gaseous hydrocarbon mixture is capable upon admixture with said petroleum within said formation of forming therewith a first single phase admixture in the critical state having the composition indicated at the critical point of said petroleum within said formation under the formation conditions of temperature and pressure as indicated on a three component methane -C2 6-C7+ phase diagram for said petroleum under formation conditions of temperature and pressure, forming the aforesaid first single phase admixture within said formation by injecting said rst gaseous hydrocarbon mixture thereinto, subsequently injecting into said formation via said injection well a second gaseous hydrocarbon mixture having a composition with respect to said first single phase admixture such that said second gaseous hydrocarbon mixture is capable upon admixture therewith of maintaining the resulting admixture in single phase condition under formation conditions of temperature and pressure, said second gaseous hydrocarbon mixture comprising a normally gaseous hydrocarbon and a hydrocarbon in the molecular weight range of C22-C6 of lesser proportion than contained in said first gaseous hydrocarbon mixture, and thereafter introducing into said formation a relatively lean gas having a composition comprising substantially only methane chosen from outside the 2-phase region of said three component phase diagram capable only of mixing with and maintaining the aforesaid resulting admixture in single phase condition thereby to effect a substantially complete single phase displacement of petroleum from said formation.

2. An operation in the process of claim l wherein said first gaseous hydrocarbon mixture comprises substantially methane and a hydrocarbon in the molecular Weight range CZ-CG.

3. An operation in the process of claim l wherein said ananas? first gaseous hydrocarbon mixture comprises substantially methane and propane.

4. A petroleum recovery process which comprises introducing into a petroleum bearing formation a rst gaseous hydrocarbon mixture, said rst gaseous hydrocarbon mixture having a composition with respect to said etroleum within said formation such that under the formation conditions of temperature and pressure said rst gaseous hydrocarbon mixture forms a single phase admixture in the critical state with said petroleum Within said formation, the composition of said single phase admixture being substantially the same as the composition indicated at the critical point of said petroleum Within said formation under the formation conditions of temperature and pressure as indicated on a three component methane-C2 6-Cf,+ phase diagram for said petroleum under formation conditions of temperature and pressure, forming said single phase admixture having the aforesaid composition within said formation by introducing said first gaseous hydrocarbon mixture thereinto, subsequently introducing into said formation a second gaseous hydrocarbon mixture, said second gaseous hydrocarbon mixture having a composition with respect to the aforesaid critical state single phase admixture formed Within said formation such that said second gaseous hydrocarbon 25 mixture under formation conditions of temperature and pressure is capable only of mixing with the aforesaid critical state single phase admixture within said formation and maintaining the resulting mixture in single phase condition, said second gaseous hydrocarbon mixture comprising a normally gaseous hydrocarbon and a hydrocarbon in the molecular Weight range C2-C6 of lesser proportion than contained in said rst gaseous hydrocarbon mixture, and thereafter introducing into said formation a relatively lean gas having a composition comprising substantially only methane chosen from outside the Z-phase region of said three component phase diagram capable only of mixing with and maintaining the aforesaid resulting mixture in single phase condition thereby to effect a substantially complete single phase displacement of petroleum from said formation.

References Cited in the le of this patent UNITED STATES PATENTS 2,718,262 Binder Sept. 20, 1955 2,742,089 Morse et al. Apr. 17, 1956 2,822,872 Rzasa et al. Feb. l1, 1958 2,880,801 Crump Apr. 7, 1959 FOREIGN PATENTS 696,524 Great Britain Sept. 2, 1953

Claims (1)

1. IN A SECONDARY RECOVERY OPERATION OF PETROLEUM FROM AN UNDERGROUND PETROLEUM BEARING FORMATION WHEREIN A FLUID IS INTRODUCED VIA AN INJECTION WELL INTO SAID PETROLEUM BEARING FORMATION TO DRIVE PETROLEUM FROM SAID FORMATION TOWARD A PRODUCTION WELL, THE IMPROVEMENT WHICH COMPRISES EMPLOYING AS SAID FLUID A FIRST GASEOUS HYDROCARBON MIXTURE HAVING A COMPOSITION WITH RESPECT TO SAID PETROLEUM IN SAID FORMATION SUCH THAT SAID FIRST GASEOUS HYDROCARBON MIXTURE IS CAPABLE UPON ADMIXTURE WITH SAID PETROLEUM WITHIN SAID FORMATION OF FORMING THEREWITH A FIRST SINGLE PHASE ADMIXTURE IN THE CRITICAL STATE HAVING THE COMPOSITION INDICATED AT THE CRITICAL POINT OF SAID PETROLEUM WITHIN SAID FORMATION UNDER THE FORMATION CONDITIONS OF TEMPERATURE AND PRESSURE AS INDICATED ON A THREE COMPONENT METHANE -C2-6-C7+ PHASE DIAGRAM FOR SAID PETROLEUM UNDER FORMATION CONDITIONS OF TEMPERATURE AND PRESSURE, FORMING THE AFORESAID FIRST SINGLE PHASE ADMIXTURE WITHIN SAID FORMATION BY INJECTING SAID FIRST GASEOUS HYDROCARBON MIXTURE THEREINTO, SUBSEQUENTLY INJECTING INTO SAID FORMATION VIA SAID INJECTION WELL A SECOND GASEOUS HYDROCARBON MIXTURE HAVING A COMPOSITION WITH RESPECT TO SAID FIRST

Images