Miscible displacement of petroleum using carbon disulfide and a hydrocarbon solvent

[57] ABSTRACT

A miscible displacement process for the recovery of petroleum from a petroleum bearing formation is performed in situ by the use of a solvent system miscible with the petroleum, the solvent system comprising carbon disulfide and a hydrocarbon solvent.

Claims, 1 Drawing Figure

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PATENTED NOV 12 197^

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MISCIBLE DISPLACEMENT OF PETROLEUM USING CARBON DISULFIDE AND A HYDROCARBON SOLVENT

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for recovering petroleum by miscible displacement.

2. Description of the Prior Art

Various methods for inducing the recovery of petroleum from underground reservoirs are in existence. These methods include injecting water, steam or some aqueous based mixture to drive the oil from the reservoir. These displacement processes are inefficient. The inefficiency of these displacement processes is partly due to the retentive forces of capillarity and interfacial tension. Miscible flooding provides a method for efficiently displacing the petroleum from a reservoir.

In miscible flooding, solvent for the petroleum is introduced into the reservoir and driven through the reservoir. Dissolution of the petroleum by the solvent permits no two phase system between the solvent and the petroleum to exist at the conditions of temperature and pressure existing in the reservoir. Therefore, the retentive forces of capillarity and interfacial tension are nonexistent. These forces decrease the displacement efficiency of a recovery process where the driving fluid or displacing agent and the petroleum exist as two phases in the reservoir.

In a miscible flood process the solvent has the capability of mixing completely with the petroleum in the reservoir. A transition zone is formed at the leading edge of the solvent between the solvent and the petroleum in which miscibility exists between the solvent and the petroleum. For economic reasons the solvent is normally injected as a slug followed by another fluid such as a gas or an aqueous fluid to drive the solvent slug and the petroleum through the reservoir.

In displacement processes in general, the ideal sought after is piston-like displacement. That is, the displacing fluids should ideally present a flat front to the petroleum in the reservoir and displace it uniformly through the reservoir. Most miscible solvent slugs are followed by an aqueous fluid to drive them through the reservoir. Moreover, most miscible solvents have heretofore been light hydrocarbons with densities less than water. Problems have arisen with such processes, however.

In a vertical miscible flood, for example, using a light hydrocarbon solvent slug followed by water, the water will tend to finger through the less dense solvent, destroying piston-like displacement and resulting in premature breakthrough of the displacing medium water. Further, there are certain petroleum deposits which are only partially soluble in the prior art solvents. One type of petroleum which is only partially soluble in prior art solvents is the tar sand oils.

Throughout the world there are various known locations wherein the earth contains large deposits of tar sands. For example, one of the most extensive and best known deposits of this type occurs in the Athabasca district of Alberta, Canada. In the tar sands in such deposits, the oil typically has a density approaching or even greater than that of water. The Athabasca tar sands extend for many miles and occur in varying thickness of up to more than 200 feet. Although in some places the Athabasca tar sands are disposed practically on the surface of the earth, generally they are located under an overburden which ranges in thickness from a few feet to as much as 1,000 or more feet in depth. The tar sands located at these depths constitute one of the world’s largest presently known petroleum deposits. In these sands, the oil content ranges between about 10 and 20 percent by weight, although sands with lesser or greater amounts of oil content are not unusual. Additionally, the sands generally contain small amounts of water in the range from about 1 to 10 percent by weight.

The oil present in and recoverable from Athabasca tar sands is usually a rather viscous material ranging in specific gravity from slightly below 1.00 to about 1.04 or somewhat greater. At a typical reservoir temperature, e.g., about 48°F., this oil is immobile, having a viscosity exceeding several thousand centipoises. At higher temperatures, such as temperatures above about 200°F. this oil becomes mobile, with viscosities of less than about 343 centipoises, and the tar sands are incompetent. Since this tarry material does not generally command a very high price, particularly when in its crude state, its separation and recovery must involve a minimum of expenditure in order to be economically attractive for commercial practice.

The bitumen in the tar sands is a petroleum material which is not completely soluble in most prior art solvents such as LPG, mixtures of light hydrocarbons having from two to six carbon atoms or propane.

In copending applications Ser. No. 357,414 filed May 4, 1973, Ser. No. 357,409 filed May 4, 1973 processes were disclosed which used a solvent miscible with the sand oils. One of those solvents was carbon disulfide. However, carbon disulfide has drawbacks which may limit its exclusive use in some situations. For example, carbon disulfide is highly toxic and very flammable. Carbon disulfide is also expensive at the present time. This invention will alleviate many of these problems while retaining much of the unique beneficial properties of a carbon disulfide miscible flood.

SUMMARY OF THE INVENTION

The invention is a process for the recovery of petroleum from a reservoir by miscible displacement. The process involves introducing a slug of mixed solvents capable of dissolving formation hydrocarbon, the solvent mixture having a density greater than water, into the upper portion of a reservoir and forcing the solvent mixture through the reservoir to some point lower than the point of introduction by means of an aqueous fluid and producing the solvent and extracted fluids from the reservoir at this point. The solvent mixture comprises carbon disulfide or Chlorinated hydrocarbons and aromatic hydrocarbons or aliphatic hydrocarbons.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE depicts the operation of the vertical drive recovery process of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is an improvement in the displacement of heavy oil, such as tar sand oil. The improvement comprises a first downward displacement with a petroleum solvent mixture having a density greater than the driving fluid and preferably a viscosity less than the driving fluid, followed secondly by displacing the sol3,847,221 vent-petroleum solution also downwardly with a driving fluid such as water or gas. The solvent, being heavier than the driving fluid, will substantially eliminate fingering of the driving fluid through the solvent. That is, in a downward drive the lighter driving fluid will ride above the heavier solvent providing a pistonlike displacement of the solvent. Also, if the driving fluid is more viscous than the solvent, the likelihood of fingering of the driving fluid through the solvent is further reduced. Thus, both gravity stabilization and favorable viscosity contrast is provided between the driving fluid and the petroleum solvent.

Although aqueous fluids such as water are the preferred fluids for displacing the solvent through the reservoir, any fluid having favorable properties may be used. The displacing fluid must have a density equal to or less than the solvent and preferably more viscous. Although not absolutely necessary, the fluid should also be substantially unreactive with the solvent.

The types of solvent mixtures useful in the process of our invention are those which are heavier than and chemically inert to water and have solubility characteristics which enable them to dissolve adequate amounts of petroleum. It is preferred that the solvent have a viscosity less than water. Ideally, the solvent should be completely miscible with the petroleum so that the interface between the leading edge of the solvent and the petroleum is removed. The solvent is a mixture of at least two solvents. One solvent is heavier than water. Examples of specific solvents heavier than water include but are not limited to carbon disulfide and chlorinated hydrocarbons such as methylene dichloride and carbon tetrachloride.

The other component of the solvent blend is a hydrocarbon. The hydrocarbon may be aromatic or aliphatic or a blend of each. Benzene, toluene and xylene are typical examples of aromatic hydrocarbons useful in the process of our invention. Aliphatic solvents useful in our invention include light hydrocarbons from two to six carbon atoms. Debutanizer bottoms is a solvent which is an example of blended aromatic and aliphatic hydrocarbons. This brief enumeration is not intended to be exhaustive. The hydrocarbons useful in our invention are far too numerous to mention and will be obvious to those skilled in the art in light of the teachings contained herein.

It is a peculiar property of tar sand oil that only certain components of it are soluble in aliphatic or aromatic hydrocarbon solvents. Consequently, the use of these solvents alone in an oil recovery process from a tar sand deposit will leave many valuable hydrocarbons behind. Also, these solvents are lighter than the most typical driving fluid, water. As a result, solvent displacement processes using aliphatic or aromatic solvents will result in the driving water fingering through the less dense solvent, thus destroying any hope for piston-like displacement of the solvent slug.

Carbon disulfide, however, solves both of these problems. Most tar sand oils or bitumens are essentially completely soluble in carbon disulfide. Also, carbon disulfide has a density greater than water. Thus, carbon disulfide blended with an aromatic or aliphatic solvent to a density equal or greater than water will solve the above enumerated problems. The solvent blend will dissolve all components of the tar sand oil and the driving water will not tend to finger through the equally heavy or heavier solvent because of gravity.

In certain applications carbon disulfide is preferred over the other heavier than water solvents because of its unique properties or ease of manufacture and recovery. Where the recovered crude is to be catalytically treated in a refinery, for example, carbon disulfide is preferred. It is a characteristic of covalently bonded halogens such as those found in halogenated hydrocarbons that they tend to poison some refinery catalysts. Carbon disulfide does not and in addition is quite easily removed from recovered crude by physical separation processes to be reused again, leaving the crude substantially free of carbon disulfide. Carbon disulfide may also have a great economic advantage over halogenated hydrocarbons since it may be manufactured by the reaction between coke (carbon) and sulfur. Coke and sulfur are often found in excess near prolific tar sand deposits such as the Athabasca tar sands of Canada. The use of these materials would be an aid to conservation of the environment.

The process of our invention may be carried out by a variety of techniques. In one technique, for example, at least two wells are needed, one for injection and another for production. The solvent mixture may proceed through the formation horizontally from the injection well to the production well, but the benefits of this invention become greater as the angle the solvent proceeds through the formation approaches 90° from the horizontal. Ideally, the interface between the solvent mixture and aqueous fluid driving it should be horizontal. This configuration allows gravity stabilization to have its maximum effect on the system.

The process of this invention is operable in a variety of petroleum reservoirs containing petroleum of widely differing gravities. One preferred embodiment, however, is to recover tar sand oil using a solvent blend of carbon disulfide and/or chlorinated hydrocarbons and an aromatic and/or aliphatic hydrocarbon. As pointed out previously, it is a characteristic of the bitumen constituents of tar sand oil or petroleum that they are soluble in carbon disulfide and less Soluble in most other solvents.

A very important advantage of using carbon disulfide is the lack of an emulsification of the separate driving water and carbon disulfide phases. The phases separate into distinct layers easily separable from each other. This feature is advantageous for many reasons. For example, emulsification within the formation could lead to a reduction in permeability due to what is commonly known as “emulsion blockage.” The lack of emulsification when carbon disulfide is used prevents this problem from occurring. Also, emulsification could destroy piston-like displacement. Another advantage of the lack of emulsion forming tendency between carbon disulfide and water occurs when the solvent, bitumen and water are produced and separation of the carbon disulfide is desired. Emulsion formation would distinctly hamper these operations.

The size of sol vent slug to be used will depend on the solvent chosen and the degree of recovery desired. The degree of recovery desired is a matter of economics and may be determined by those skilled in the art without engaging in inventive effort. As an aid in determining the size of slug needed the following procedure may be used but is not intended to limit the scope of our invention or tie it to any routine calculation procedure. The size of a slug of solvent mixture, for example, may be calculated by a formula such as:

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Solubility of bitumen amount of bitumen in solvent mixture x per acre-foot of formation acre-fcet degree of depletion desired x in formation x (decimal) = amount of solvent mixture required

Routine laboratory experimentation may be used to determine the solubility of a given bitumen in a particular solvent mixture and core analysis will yield information on the amount of bitumen per acre foot of formation. Thus, the size of solvent slug for any field may be determined.

The temperature of the solvent slug should be low enough to avoid having the carbon disulfide react with water. Normally the slug will be introduced at ambient temperature and will take on the reservoir temperature in a short period of time. In Canadian tar sands, for example, the reservoir temperature is about 45°F. In no case when carbon disulfide is used should the temperature of the solvent be above that at which carbon disulfide reacts with water, about 400°F.

The temperature of the displacing or driving water should be so low that, in combination with dissolved salt content, its density exceeds that of the solvent blend.

A fairly thick reservoir is preferred in the process of this invention to allow as near to a vertical miscible flood as possible. The placement of the injection and production wells is related. They should be situated so that the injection of the solvent and an aqueous drive fluid takes place at a point in the reservoir above the point where the production is taken from the reservoir. The lateral as well as vertical spacing of the production and injection points should be such that a blanket of solvent followed by water will cover the largest area of the reservoir consistent with economics. The above factors should be understood as given to explain how to maximize the effectiveness of this invention. However, the invention should not be construed as limited to any particular well configuration or reservoir type.

MINING TECHNIQUES

In one embodiment of the invention a “dump” flood may be performed. This involves drilling a large diameter hole into the crestal portion of a tar sand formation, for example. A solvent mixture heavier than water as described heretofore, is dumped into the cavity. The solvent will gravitate into the formation and be imbibed by the formation displacing the oil toward producing wells completed lower in the formation. An aqueous fluid such as water is introduced into the cavity to maintain a layer over the solvent. This prevents evaporation of the solvent. Dump flooding is particularly useful where the formation is near to the surface of the earth where high injection pressures could result in breakthrough of injected fluids through the overburden to the surface. Near to the surface could include, for example, depths of 200 to 300 feet or more.

A similar mining technique could also be used where tar sands outcrop at the surface. A dam of earth, for example, could be constructed surrounding the outcrop providing a recepticle for the injection of solvent and water. Production wells drilled down dip from the outcrop would withdraw the combined solvent-tar extract.

As a rule of thumb so-called mining techniques are normally considered to be feasible where the ratio of the distance from the surface of the earth to the thickness of the tar sand reservoir or pay is one or less.

IN SITU RECOVERY TECHNIQUES

For deeper formations other embodiments of our invention involve having at least one production well and one injection well. The preferred configuration is that the point of injection be far enough above the point of production to allow a somewhat vertical traverse for fluids entering the reservoir through the injection well and being produced through the production well. A slug of solvent mixture heretofore described is introduced through the injection well followed by an aqueous fluid to push the solvent through the reservoir to the production well.

A variation of this includes using one well having upper and lower perforation and which is internally equipped so as to avoid fluid communication in the well between the upper and lower perforations. The heavy solvent is injected through the upper perforations and produced through the lower perforations along with petroleum driven ahead of the solvent. If desired, the solvent may be followed by an aqueous fluid.

The process of our invention may be illustrated by reference to the accompanying FIGURE which depicts one embodiment of our invention. Other embodiments will, of course, occur to those having had the benefit of the teachings contained herein.

A reservoir containing a very viscous petroleum 10 is penetrated by an injection well 11 and production wells 12 and 13, the injection well having communication with the reservoir through perforations 14 above the perforations 15 in the injection wells. The FIGURE represents a point in time well into the recovery program where a slug of solvent mixture comprising carbon disulfide and toluene 16 having a density greater than water has been injected into the reservoir through the injection well followed by an aqueous fluid 17 which is presently being injected into the injection well. The petroleum 18 is moving towards the production well perforations where it is produced. The interface shown between the solvent blend slug 16 and the petroleum 18 is, of course, not as distinct as shown in this illustration.

EXPERIMENTAL

The superior performance of carbon disulfide followed by water in removing tar sand oil has been demonstrated in laboratory experiments. A weighed quantity of tar sand was placed in a glass tube 1.5 inches in diameter above a glass wool filter to retain the sand. Sufficient carbon disulfide was introduced into the tube to completely saturate and cover the tar sand. Water was then placed above the carbon disulfide-tar sand layer. The water formed a distinct layer of water formed above the carbon disulfide-tar sand system. The assembly was closed and shut-in over night. The next day a stopcock at the bottom of the assembly was opened and all fluid allowed to drain out into a graduate cylinder. A two phase system formed in the graduate cylinder: a lower phase consisting of carbon disulfide and dissolved tar sand oil, and an upper clear water phase. The water passed easily through the tar sand and was drawn off easily since the phase boundary was distinct. No emulsion formation was noted at the interface

3,847,221 between the water and the carbon disulfide-tar sand oil mixture.

Virtually all of the carbon disulfide was recovered from the original tube. The carbon disulfide-tar sand oil mixture was vacuum-distilled. No frothing or foaming occurred during the operation even though the solution actually boiled. All of the carbon disulfide was removed and only pure, heavy, viscous tar sand oil remained.