US3267999A - Solvent flood with expanding oil phase - Google Patents

Description

r 3,267,999 Ice Patented August 23, 1966 3,267,999 SOLI/ENT FLGE WITH EXPANDNG @lL PHASE Ronald L. Reed, Austin, Tex., and .Ioseph 5. Taber, Cheswick, Pa., assignors to Golf Research & Deveiopmcnt Company, Pittsburgh, Pa., a corporation of Delaware Filed Mar. 18, 1963, Ser. No. 265,931 Claims. (Cl. 1613-9) rIhis invention relates to the recovery of petroleum by an improved miscible displacemen-t process.

`In the production of petroleum by primary recovery methods involving the displacement of oil by natural reservoir energy, total recovery is generally significantly less than percent of the oil initially in place. The great volume of presently unrecovered petroleum and the increasing cost of petroleum exploration have motivated a Search for recovery methods characterized by high displacement efliciencies and economic practicability. Of the many recovery schemes proposed, the conventional water hood is often economically attractive. Unfortunately, Water ilood displacement eiciencies are among the lowest for the lavailable secondary recovery methods and result in the bypassing of an appreciable volume of oil, which remains in the :flooded reservoir. Recovery methods involving continuous injection of a solvent would displace the entire Volume of reservoir oil, if injection of the solvent were continued indeiinitely but the cost of such a process is prohibitive because of the large amoun-t of solvent used and left in the reservoir at the end of the ilood.

The miscible slug process, which combines the advantages of Water ilooding 4and solvent ooding, consist of displacing the oil with a predetermined volume of solvent which is driven toward .the producing wells by a Water ilood. yThe recommended solvents, herein referred to as amphipathic solvents, are liquids which are miscible with 'water in substantially .all proportions and are also miscible -with oil in substantially all proportions. Amphipathic solvents which have been suggested and are available at prices allowing their use in a miscible slug process are, however, miscible with mixtures of oil and water only in a narrow range of compositions. Hence, even so-called miscible slug processes involve a two-phase ow of oil and an aqueous phase from reservoirs initially containing an aqueous phase.

The requisite size of the solvent slug is a major economic factor and depends upon the iluid dynamics and phase behavior of the system. Fluid viscosity differences and reservoir heterogeneities cause mixing at the liquid inter- 'faces and the formation of an oil-brine-solvent transition zone `and a solvent-water transition zone at the leading .and trailing edges, respectively, of the solvent slug.

'Man-y methods have been suggested for adjusting the viscosities at the leading edge of the solvent slug to assure a favorable viscosity relationship. One such method proposes injecting a slug of light hydrocarbon, such as LPG, kerosene, or naphtha, ahead of the -solvent to reduce the viscosity of the oil that is contacted by the solvent slug. Although `this device provides a favorable viscosity ratio and in a reservoir devoid oi water would result in a miscible displacement of the oil, it does not cause m-iscible displacement :by the solvent of the iluids from reservoirs containing Water because lthe three components water, oil, and solvent are miscible in only a narrow range of concentrations and the presence of reservoir 'water causes the liquid system to form two phases. Furthermore, the injected hydrocarbon, being substantially completely immiscible with water present `in the reservoir, twill have no effect of displacing connate Water from the reservoir and the net effect of the injection ot the hydrocarbon will be substantially to substitute injected hydrocarbon tor the oil originally in the reservoir at the trailing edge of the stabilized ban-k. Hence, even rwith a favorable viscosity ratio at the dood front, displacement eiiiciency is determined by .the phase behavior of the reservoir liquids with the amphipathic solvent in the transition zone at the leading edge of the solvent slug.

This invention resides in a process yfor the recovery of oil lfrom a reservoir containing an aqueous phase, which comprises injecting a mixture of a solvent, which is miscible with the oil and with water, and a hydrocarbon liquid into a reservoir ahead of a pure solvent slug and following the solvent slug with Water to displace oil and the injected liquids through the reservoir to a production wel-1. `In one embodiment of this invent-ion a slug of a low molecular weight oil such as LPG, gasoline or naphtha is displaced through the reservoir ahead of the mixture of :solvent and hydrocarbon liquid.

IFIGURE 1 is a ternary phase diagram yfor a system of liquids comprising isopropyl alcohol, isooctane, and two percen-t calcium chloride brine.

`FIGURE 2 is a ternary phase diagram for a system of liquids comprising isopropyl alcohol, isooctane, and two percent calcium chloride brine showing the path followed by the composition of the uids in the reservoir during the process of this invention.

FIGURE 3 shows a comparison of typical brine production histories lfor expanding aqueous phase .and expanding oil phase miscible floods.

It is characteristic of a miscible slug process in reservoirs containing oil and brine that, as the injected liquids advance through the reservoir, `four distinct zones are lformed and move toward the producing wells. The initial ilowing zone consists of pure oil because the connate brine in this region is discontinuous and hence has no mobility. In the event the reservoir has been water flooded to the residual oil saturation, the initial zone is the reservo-ir Water. -In either event, the initial zone is )followed by a stabilized zone of two phase flow in which oil .and brine ow in almost constant proportion. This is followed by the transition zone in which the solvent dissolves in the oil and in the brine, and the proportions of brine phase and oil phase iloyving vary as the solvent concentration increases. Eventually, the three liquids achieve miscibility with the formation of a single flowing phase which constitutes the fourth zone. `With respect to this invention, a description of succeeding fluids in the dood is nt pertinent, and the fourth zone may be treated as one in which `the single phase of miscible liquids behind the transition zone grades into p-ure solvent and subsequently into Water.

The ternary diagrams of FIGURES ll and 2 illustrate the phase behavior of the liquids in the transition zone and in the miscible zone ahead of the pure solvent. Throughout this application, all references to ternary d-iagrams shall be to diagrams showing percent 'water at the lower left angle, 100 percent solvent at the upper angle, an-d 1100 percent oil at the lower right angle of the ternary diagram. rFIGURE 1 represents a conventional miscible slug process in which va solvent slug of isopropyl alcohol is displaced by a water drive through a reservoir containing isooctane and calcium chloride brine. Mixing at the leading edge of the solvent slug will form an oil-brine-solvent transition zone preceding the pure solvent. At a point in the reservoir just ahead of the transition zone, i.e., in the stabilized zone, oil and brine are at concentrations indicated by point A on the oil- 'brine side ofthe diagram. As the transition zone passes this point in the reservoir, the mixture of solvent, oil, and brine follows the composition path of the line AC, and phase relations in the transition zone are indicated by the relation between the composition path line segment AB and the tie-lines such as EF. For example, when liquid concentrations in the transition zone are indicated by point D, the ratio of the volumes of oil phase to brine o phase equals the ratio of the tie-line segments ED to DF. As the liquids approach the miscible region, the tie-line segments representing the proportion of oil phase decrease in length .and vani-sh as miscibility is achieve-d. This occurs because the oil dissolves the brine phase as solvent concentration increases, and in an equilibrium system, as is represented by the ternary diagram, the brine phase continues to grow until the oil phase disappears when the system has the composition B at which line AC crosses the binodal curve. Because of the eventual dominance of the brine phase, such processes are referred to as expanding aqueous phase miscible slug processes.

A significant feature of the expanding aqueous phase process is that, as oil dissolves in the brine phase, the oil volume in the reservoir decreases until the oil phase becomes discontinuous. The oil phase remains discontinuous during the rest of the flooding. When this condition exists, the reservoir permeability to oil vanishes, and the oil phase is present behind the transition zone in isolated ganglia which remain immobile in the reservoir while the continuous brine phase is displaced. Thus the expanding aqueous phase reduces oil recovery because of the volume of oil that is left in the reservoir.

Misc-ible flooding processes exhibit an expanding aqueous phase when `the liquid mixture in the transition zone follows a composition path line that intersects the binodal curve at a point to the left of the plait point as shown in FIGURE l. The compositi-on path line, AB, in FIGURE 2 indicates that, if the liquids lin the transition zone achieve miscibility through a region lying to t-he right of the plait point, the tie-line segments such as DF, representing the proportion of brine phase, eventually vanish. In such a system the brine phase becomes discontinuous, and the oil phase remains continuous and mobile as the liquids achieve m-iscibility. A process with these characteristics is referred to as an expanding oil phase misci-ble slug process.

Sol-vents having the desired miscibility, with oil and als-o with water are in most instances low molecular weight, oxygenated organic liquids. Of these compounds, the monohydroxy alcohols having from 1 to 3 carbon atoms per molecule are preferred amphipathic solvents because of their availability and relatively low cost. Methyl alcohol, and to a lesser extent ethyl alcohol, are miscible only with hydrocarbons of low molecular weight. Their usefulness in the process of this invention is, therelfore, limited to the embodiment of this invention in which the mixture of solvent and hydrocarbon liquid is preceded by a slug of a low molecular weight hydrocarbon which is miscible with those alcohols. Isopropyl alcohol is the preferred alcohol. Because the alcohols listed above -form a system with brine and crude oil in which the plait point is to the right of the binodal peak, the path of the composition of the system as the amphipathic solvent is mixed with the water-oil mixture of the stabilized zone passes to the left of the plait point and, hence, results in an expanding aqueous phase, as shown in FIGURE l. The characteristics of other amphipathic solvents which are miscible with either water or oil such as acetone, dioxane, acetaldehyde, and ethylene oxide .are similar in resulting in an expanding aque-ous phase. Tertiary butyl alcohol also can be used and will result in a system having a plait point to the left of the peak of the binodal curve in systems with a `few reservoir oils and water. However, the high cost of tertiary butyl alcohol will almost always preclude its use in a miscible slug process even in those systems which result in a plait point -to the left of the binodal peak. This invention, in which a liquid hydrocarbon is added to the amphipathic solvent, is useful in miscible slug processes using any yof the solvents mentioned above, which result in the plait point of the system being to the right of the binodal peak.

In this invention, an expanding aqueous phase process is converted into an expanding oil phase process by injecting a slug of solvent mixed with a hydrocarbon ahead of the pure .amphipathic solvent. Reference to pure solvents is to distinguish lthe solvent slug from the solvent to which hydrocarbons are deliberately added according to the method of this invention. Solvents, or mixtures of solvents containing impurities or even incidental amounts of hydrocarbons are referred to as pure solvents in this specication. FIGURE 2 is a ternary diagram for the iso- -octane, `calcium chloride brine, 4and isopropyl alcohol system showing the path of reservoir huid composition through the transition zone when displad by a slug of isopropyl alcohol mixed with isooctane followed by a slug of pure isopropyl alcohol. This diagram indicates that when the transition zone contacts the reservoir liquids at concentrations indicated by point A', the added hydrocarbon gradually increases the oil concentration, and the liquids are forced `to approach miscibility along a composition path that intersects the binodal curve at B' to the right of the plait point. As miscibility is achieved in this process, the tie-line segments such as DF representing the proportion of brine phase in the transition zone, decrease in length and ultimately vanish. This indicates that the brine dissolves in the oil phase as solvent concentration increases. Ultimately, the brine phase volume decreases to the point that the brine phase becomes discontinuous and reservoir permeability to brine does not exist. The brine phase remains behind the flood front in isolated ganglia, and the oil phase expands in volume and constitutes the sole, continuous ilowing phase as miscible displacement is achieved. This results in increased recovery of oil from the reservoir. Furthermore, the higher concentration of brine behind the flood front reduces the reservoir resistance to ow of water, and the scavenging water flood can be conduc-ted at injection pressures lower than those for an expanding aqueous phase process.

A wide variety of hydrocarbon liquids can be mixed with the amphipathic solvent for injection into the reservoir ahead of the solvent slug to achieve the desired expanding oil phase. One suitable liquid hydrocarbon that can be mixed with the solvent is Ithe reservoir oil. In most instances, however, it is preferred to mix hydrocarbon oil of lower molecular weight that the reservoir oil wit-h the solvent. The hydrocarbons of lower molecular weight tend to lower the binodal curve for the resultant 4-cornponent system `and thereby reduce the amount of solvent needed to retain miscibility during the subsequent injection of the pure solvent and lthe following water drive. Suitable hydrocarbons of lower molecular weight than the reservoir oil are kerosene, natural gasoline, and LPG. Any broad or narrow cut of these hydrocarbons is suitable. Highly preferred liquid hydrocarbons for this process are hydrocarbons containing high concentrations of aromatic hydrocarbons. Suitable highly aromatic hydrocarbons are benzene and platformate, a reformed gasoline fraction. The highly aromatic hydrocarbons have advantage over other hydrocarbons of tending to shift the plait point of the resultant 4component system to the left thereby reducing the amount of hydrocarbons required.

The liquid hydrocarbon is admixed with the solvent in a predetermined amount designed to induce a path of the reservoir fluids composition which passes to the right of the plait point. Referring to FIGURE 2 of the drawings, the injected mixture should have a concentration of oil slightly in excess of that indicated by the intersection of a line from A through the plait point with the side of the ternary diagram connecting the percent solvent and 100 percent oil vertices. A line constructed to indicate the proper oil concentration would be one such as line A'G'. The concentration of the hydrocarbon required to the injcated-solvent-hydrocarbon mixture can be determined by a series of bottle tests in which samples of reservoir oil and water typical of stabilized zone composition are mixed with hydrocarbon-solvent solutions of increasing hydrocarbon concentration until the concentration of hydrocarbon required to cause an expanded oil phase is determined. 'Ihe sizes of the slug of the mix- .ture of hydrocarbon oil and solvent and the following slug of pure solvent are designed to avoid complete breakdown o-f the pure solvent slug until it traverses a surfiicient distance in the reservoir to obtain the benefits of this invention through the entire reservoir volume swept by the flooding process. A slu-g having -a volume of 2 to 20 percent of the reservoir pore volume ordinarily should lbe used. The size of the slug of mixed solvent and hydrocarbon liquid, and of solvent, is determined for each reservoir oil by preliminary core tests.

The improvement in displacement efficiency resulting from an expanding oil phase process is illustrated in FIG- URE 3, which presents a comparison of a brine production histories as a function of cumulative liquid production from the reservoir for the expanding aqueous phase and the expanding oil phase processes. The upper curve, shown as a solid line, applies to the expanding equeous phase process and shows the fraction of brine, fw, in the flowing liquids -for each stage of the recovery process. In the zone of pure oil flow, fw is zero, but in the stabilized zone the fractional ow of brine increases rapidly to approximately 70 percent, with oil comprising the remaining 30 percent of the owing liquids. The transition zone is characterized by Ithe appearance of solvent dissolved in the ilowing oil and bine phases and by marked changes in the conditions of flow. From this point onward the fw curve represents not the fractional flow of :brine in the volume of oil and brine flowing but rather the -fractional ilow of the brine solution in the total flowing volume. For this expanding aqueous phase proce-ss, the increase of the value of fw to 100 percent when only the brine phase is iiowing in the miscible region reects the shrinkage of the oil phase with the consequent loss of continuity and oil mobility.

The lower curve in FIGURE 3, shown as a broken line, illust-rates the brine production history of an expanding oil phase process and the improvement in displacement of efficiency resulting from this process. The fw curve did not become `greater than zero until a greater cumulative production of oil was attained, indicating Va larger volume of the pure oil zone. In addition, fractional ow of brine in the stabilized zone was `only approximately 58 percent and the flow of oil, approximately 42 percent. In the transition zone, fractional flow of the brine phase decreased to zero, indicating Ithat the oil phase has expanded as the continuous, mobile phase while the brine phase remains immobile behind the flood front. Sustaining a mobile oil phase causes increased flow of oil in the transition zone with a high oil concentration at the leading edge, and part .of Ithe reservoir o-il `ahead of that zone forms a bank that increases in length and results in a larger volume of oil ilowing in the stabilized and pure oil zones. In this manne-r a larger Volume of oil is displaced from the reservoir volume contacted by the solvent slug.

The improvement in oil recovery resulting from this invention is indicated in Table I. The two runs reported in Table I where made in a Berea sandstone core that was 35 feet long and satura-ted with isooctane and two percent calcium chloride brine. Initial saturation of the cores was performed by evacuating land `admitting deaerated water into the core until full. High oil saturations (.i.e., irreducible Water to an oil flood) were achieved by flowing three to ve pore Volumes of oil through the Water-saturated core until water product-ion ceased. Residual oil saturations were obtained by Water ooding the core at this stage. Run 1 represents a conventional miscible slug process in w'hich a slug of pure isopropyl alcohol is followed by a Water drive. It is customary to measure slug size -by the length of the slug in the core. In Run 1, the slug of solvent was 5.2 feet long, corresponding to approximately percent of the pore volume. This slug traveled a distance of 23 feet, or lapproximately 66 percent of the total length of the core before mixing caused the entire solvent slug to be consumed in the formation of two phases. Oil recovery by this process was 79 percent of the original oil volume.

In the expanding oil phase process Iof Run 2, a 1.3 foot slug of p-ure isopropyl alcohol was preceded by a mixed slug of isooctane (the reservo-ir oil) and isopropyl alcohol, in Which the net alcohol slug was 3.9 feet long. Thus the total slug of alcohol used in Run 2 was 5.2 feet and equal to that in Run 1. In this process, the slug traveled 30 feet, or approximately 86 percent of the total core length, before degeneration of the pure solvent slug, and the oil produced represented 86 percent of the total volume of original oil plus the volume of oil injected in the slug.

A fundamental principle of this invention is that the system is forced to achieve miscibility at an oil concentration higher than that of the plait point. In practically all reservoirs the concentration of oil in the stabilized zone will be less than 50 percent; hence, any line connecting the composition of the stabilized bank with the 100 percent solvent vertex of the ternary diagram will intersect the binodal curve to the left of the plait point in any system in which the plait point is to the right of the peak of the binodal curve. It is imperative, therefore, that a ypredetermined volume of additional hydrocarbon be added to the solvent prior to injection to induce a transition zone composition path that crosses the binodal curve to the right of the plait point.

The preceding description defines our invention of an improved miscible `slug process comprising the injection into a petroleum reservoir of a mixed slug of solvent mixed with -a hydrocarbon, followed by a slug of solvent displaced by a scavenging Water flood. This order of steps assures an expanding oil phase as the system achieves miscibility, thereby improving the relationship between slug size and distance traveled to slug degeneration, increasing the recovery of reservoir oil, reducing the total volume of solvent required, and decreasing the required injection pressure for the water flood portion of the process.

Therefore, we claim as our invention:

1. In a method for recovery of oil from a petroleum reservoir containing brine and oil 'by introducing into said reservoir through an injection well a slug of amphipathic solvent and displacing said solvent nl warilnlllrullillgww x...

wellrhynhejnjectionnoawateLiuto the reservoir at the injection well, the improvement comprising injecting into the reservoir ahead of said solvent slug a mixture of said amphipathic solvent and a hydrocarbon that is miscible with the reservoir oil, the amounts `of solvent and hydrocarbon in said mixture being such that said mixture adjusts the liquid concentrations in a transition zone ahead of said solvent slug in a manner such that as said solvent forms brineand toil-solvent phases in `said transition Zone said oil-solvent phase will increase in volume.

2. A method for recovery of oil from a petroleum reservoir containing oil and brine comprising introducing into said reservoir through an injection well a mixed slug of an amphipathic solvent mixed with a hydrocarbon that is miscible with the reservoir oil, following said mixture of solvent and hydrocarbon with a slug of amphipathic solvent introduced into said reservoir through said injection well, and displacing said amphipathic solvent slug through said reservoir toward a producing well by injection of water into said reservoir whereby said mixed slug forms a transition zone with said oil and brine ahead of said amphipathic solvent slug, the amounts of solvent and hydrocarbon in said mixed slug being such that said mixed slug adjusts the liquid concentrations in said transition zone in a manner such that, as said liquids achieve miscibility, an oil-solvent phase is formed which increases in volume and is displaced toward said producing well by said amphipathic solvent slug.

3. A method according to claim 2 wherein said hydrocarbon that is mixed with said amphipathic solvent is crude oil.

4. A method according to claim 2 wherein said hydrocarbon that -is mixed with said amphipathic solvent is platformate.

5. A method according to claim 2 wherein said hydrocarbon that is mixed with said amphipathic solvent is LPG.

6. A method according to claim 2 wherein said hydrocarbon that is mixed with said amphipathic solvent is selected from the group consisting of gasoline, kerosene, naphtha, LPG, benzene and platformate.

7. A method according to claim 2 wherein said hydrocarbon that is mixed with said amphipathic solvent is a rened aliphatic hydrocarbon having from three to twelve carbon atoms per mo1ecule.

8. A method according to claim 2 wherein said amphipathic solvent is a monohydroxy alcohol having from one to four carbon atoms per molecule.

9. A method according to claim 2 wherein said amphipathic solvent is tertiary butyl alcohol.

10. A method according to claim 2 wherein said amphipathic solvent is isopropyl alcohol.

11. A method according to claim 2 wherein said amphipathic solvent is ethyl alcohol.

12. A method according to claim 2 wherein said amphipathic solvent is selected from the group consisting of acetone, acetaldehyde, dioxane, ethylene oxide, alcohols having one to three carbon atoms per molecule, and tertiary butyl alcohol.

13. A method for recovery of oil from an oil-bearing formation containing oil and 'brine comprising displacing a low molecular Weight hydrocarbon liquid selected from the group consisting of kerosene and hydrocarbon liquids of lower average molecular weight than kerosene down an injection well penetrating said formation and through said formation from the injection well to a production well, following said low molecular weight hydrocarbon with a mixed slug of a mixture of an amphipathic solvent and a hydrocarbon that is miscible with the formation oil and with said preceding low molecular weight hydrocarbon, following said mixed slug with a slug of said amphipathic solvent, and following said slug of amphipathic solvent with an aqueous liquid to displace oil in the formation through said formation to the production well the amounts of solvent and hydrocarbon in said mixed slug being such that said mixed slug adjusts the liquid concentrations in a transition zone ahead of said amphipathic solvent slug in a manner such `that as said solvent forms brineand oil-solvent phases in said transition zone said oil-solvent phase increases in volume.

14. A method according to claim 13 wherein said low molecular weight hydrocarbon liquid that is displaced down `the injection well and 4into the formation ahead of said mixed slug is selected from the group consisting of gasoline, naphtha, LPG, benzene and platformate.

15. A method according to claim 13 wherein the amphipathic solvent is selected from the group consisting of methyl alcohol and ethyl alcohol.

References Cited by the Examiner UNITED STATES PATENTS 2,742,089 4/1956 Morse 166-9 2,867,277 1/1959 Weinaug 166-9 3,101,781 8/1963 Connally 166-9 FOREIGN PATENTS 696,524 9/-1953` Great Britain. 726,712 `3/ 1955 Great Britain.

OTHER REFERENCES Taber, J. I. et al.: Mechanism of Alcohol Displacement of Oil from Porous Media. In Society of Petroleum Engineers Journal, vol. No. 3, September 1961, pp. to 207.

Burcik, E. J.: The Ternary Phase Diagram for Water, Isopropyl Alcohol, and Liquid Propane. The Pennsylvania State Univ. Mineral Industries Experiment Station, circular No. 61, Oct. 23-25, 1961, pp. 156 to 163.

CHARLES E. OCONNELL, Primary Examiner. C. H. GOLD, T. A. ZALENSKI, Assistant Examiners.

Claims (1)

1. IN A METHOD FOR RECOVERY OF OIL FROM A PETROLEUM RESERVOIR CONTAINING BRINE AND OIL BY INTRODUCING INTO SAID RESERVOIR THROUGH AN INJECTION WELL A SLUG OF AMPHIPATHIC SOLVENT AND DISPLACING SAID SOLVENT TOWARD A PRODUCING WELL BY THE INJECTON OF WATER INTO THE RESERVOIR AT THE INJECTION WELL, THE IMPROVEMENT COMPRISING INJECTING INTO THE RESERVOIR AHEAD OF SAID SOLVENT SLUG A MIXTURE OF SAID AMPHIPATHIC SOLVENT AND A HYDROCARBON THAT IS MISCIBLE WITH THE RESERVOIR OIL, THE AMOUNTS OF SOLVENT AND HYDROCARBON IN SAID MIXTURE BEING SUCH THAT SAID MIXTURE ADJUSTS THE LIQUID CONCENTRATIONS IN A TRANSITION ZONE AHEAD OF SAID SOLVENT SLUG IN A MANNER SUCH THAT AS SAID SOLVENT FORMS BRINE- AND OIL-SOLVENT PHASES IN SAID TRANSITION ZONE SAID OIL-SOLVENT PHASE WILL INCREASE IN VOLUME.

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