US3347313A - Steam drive with viscous volatile buffer - Google Patents

Description

Oct. 17, 1967 s. MATTHEWS ET AL 3,347,313

STEAM DRIVE WITH VISCOUS VOLATILE BUFFER FiledNOV. 13, 1964 .rx-wmi Em INVENTORSI C. S. MATTHEWS R. D. SEBA v THEIR ATTORNEY United States Patent 3,347,313 STEAM DRIVE WITH VISCOUS VOLATILE BUFFER Charles S. Matthews, Houston, and Richard D. Seba, Midland, Tex., assignors to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Nov. 13, 1964, Ser. No. 410,956 6 Claims. (Cl. 166-11) This invention relates to an improved method of recovering petroleum from subterranean reservoir formations using steam injection, and more particularly, to a method involving the use of a steam-distillable buffer to isolate the injected steam for the reservoir petroleum.

It is well known that the primary recovery of petroleum, methods using the natural flow of the petroleum, can recover only a small portion of the petroleum actually in the reservoir. In reservoir formations where natural flow will occur only about 30% or less of the actual petroleum in the reservoir is recovered. To recover additional petroleum from such reservoir formations supplementary techniques must be used which are commonly referred to as secondary recovery methods. Of course, there are also reservoirs where the petroleum is too viscous to flow naturally and supplementary techniques must be employed initially to recover any petroleum, e.g., the tar sands. In both of these situations since the petroleum will not flow naturally it must be displaced from the formation, or the natural conditions modified so the petroleum will flow. Thus, these supplementary techniques usually employ either a displacing fluid which is forced into the reservoir to displace the petroleum or add thermal energy to the reservoir so the petroleum will flow out of the reservoir.

Conventional displacing fluids are water, gases such as hydrocarbons, hydrocaron liquids and steam. Applications of these various displacing fluids have strikingly different results. For example, when using gaseous displacing fluids the ultimate displacement of petroleum from the reservoir is increased approximately 20 of the original petroleum in the reservoir and when water is used as the displacing fluid the petroleum recovery can be increased by 50% of the original petroleum in the reservoir. Further displacement increases can be accomplished with water floods if the Water displacing medium has viscosity-increasing agents contained therein.

These findings support the studies which show that the efliciency of injected fluids in displacing petroleum from reservoir formations is directly related to the relative viscosities, and more properly the mobility ratio of the displacing and the displaced petroleum. This is most noticeable with respect to very viscous crudes, but remains a significant factor in virtually all supplementary recovery methods involving injected fluids to displace the residual petroleum from the reservoir.

This relationship can be expressed mathematically by the following equation which shows the behavior of fluids flowing through porous media, such as petroleum-bearing reservoirs:

wherein M is the mobility of j the petroleum to the reservoir in question M is the mobility of the flooding medium to the reservoir in question 0 is the viscosity of the driven petroleum ,ue is the viscosity of the flooding medium K is the relative permeability of the reservoir toward the flooding medium in the presence of residual petroleum 3,347,313 Patented Oct. 17, 1967 K is the relative permeability of the reservoir toward the petroleum in the presence of connate water.

It is apparent from the equation that when the displacing fluid has the same viscosity as the displaced petroleum, the mobility ratio will approach unit and injected fluid will tend to push the petroleum through the reservoir ahead of it. Thus, the displacing fluid will tend to drive the petroleum from the reservoir ahead of it as the former is injected. If the mobility ratio is approximately equal to unity, a markedly greater volume of petroleum will be displaced by the displacing fluid injected into the formation. Highly viscous displacing fluids give the desired results but they are often expensive and require considerable pressures to force them through the formation which may result in unwanted fractures or thief zones which offset their advantages.

When using liquid displacing fluids, such as water, it is often the practice to add agents to increase the viscosity of the water to obtain a better mobility ratio and a more eflicient displacement. However, there is no ready solution on how to obtain improved mobility ratio when gases are used as the displacing fluid. Steam injection does lower the viscosities of the petroleum but, unless it vaporizes the petroleum, the mobility ratio is still unfavorable, even though additional petroleum is recovered.

The general problem experienced when the mobility ratio is less than one is that the displacing fluid tends to bypass substantial portions of the petroleum in the reservoir since the displacing fluid has a greater mobility in the reservoir and seeks paths or channels where there is the least resistance. Thus, when gaseous displacing fluids are used the mobility ratio is unfavorable since the gases are highly mobile in the reservoir when compared to the petroleum. When petroleum is viscous, displacement via gaseous displacing fluids is generally unsatisfactory.

When steam is used as a displacing fluid, especially for viscous petroleum crudes, much of the petroleum is bypassed even though the heat in steam lowers the viscosity of the petroleum and may distill off portions of it. While some recovery is effected by these factors, considerable petroleum remains in the reservoir, even after several pore volumes of steam have been injected. Since the steam is highly mobile relative to liquids in the reservoir, it will have an unfavorable mobility ratio, hence a low displacement efllciency.

Another problem with steam injection is that the steam contacting the reservoir petroleum tends to drive off the lighter hydrocarbon fractions dissolved in the petroleum leaving a more viscous petroleum residue which is very difficult to displace from the reservoir formation. This is especially true if the reservoir contains viscous crudes.

The purpose of the present invention is to provide a method by which the displacement efliciency of injected steam can be improved, and a greater percentage of pe troleum can be recovered. This is done by injecting, as a slug ahead of the steam, a viscous, steam-distillable liquid. In this way a high displacement efliciency will be achieved by the slug and the slug will be moved through the formation by steam distillation.

In its broadest aspect this invention contemplates the method of achieving efficient petroleum recovery from reservoirs penetrated by at least one injection well and at least one production well by first injecting through the injection well a slug of a viscous, steam-distillable liquid, subsequently injecting steam to force the slug and residual petroleum into the area of the production well, and collecting the petroleum from the production well.

Illustrated in the single accompanying figure is a vertical section of a formation having a petroleum producing reservoir which is penetrated by two spaced well bores and showing the reservoir divided into five zones to diagrammatically show the conditions in the reservoir at an intermediate point during the practice of this invention.

It is through the use of viscous, steam-distillable buffer slug interposed between the reservoir petroleum and the injected steam that this invention can accomplish 'high displacement effi ciencies. This is possible since the mobility of the viscous slug is approximately the same as that of the petroleum, or put another way, their mobility ratio is almost unity. Because the slug is injected ahead of the steam, it isolates the petroleum from the hot steam and the slug efliciently displaces the petroleum as it moves through the reservoir, driven by the steam. The invention requires that the slug be steam-distillable, and it can be appreciated that when the steam is injected behind the slug it will flash-oft (vaporize) the trailing edge of the viscous slug. When the trailing edge of the slug is vaporized, this vaporized slug portion will have high mobility and return to the slug proper releasing its latent heat when it recondenses in the cooler slug. It is through the technique of establishing two zones of where the mobility ratios are favorable for eflicient displacement that this invention is able to accomplish superior displacement of petroleum with steam drives.

A greater understanding of the invention can be had by referring to the accompanying drawing showing a geological formation penetrated by two spaced wells. For convenience, one will be referred to as injection well 10 and the other will be referred to as a production well 11. It should be appreciated that multiple injection wells and production wells could be used but it is not necessary to illustrate additional wells to describe the invention. Normally, the injection well 10 would be circumferentially ringed by production well 11, such as the 5 or 7 spot patterns, or some similar arrangement giving a like effect.

In the .drawing the injection well 10 is identical to production well 11 and they are spaced from one another so that fluids injected through the injection well 10 will pass through the formation to the production well 11 displacing the petroleum in the reservoir formation ahead of the injected fluids.

The borehole of bothwells 10 and 11 havecasing strings 12 which extend downwardly through amultilayered formation 13 and penetrate petroleum-producingreservoir 14.Reservoir 14 is shown sandwiched between overburden andunderburden strata 13 having impermeable layers contiguous to it.Casing strings 12 are sealed in the reservoir at least to the extent of their vertical traverse of thereservoir 14 with asealant 15 such as cement, and the strings and sealant are perforated withports 16 in the area of thereservoir 14 to provide fluid communication between the inside of thecasing strings 12 and the reservoir proper.

Injection well 10 and production well 11 both havepipe strings 17 extending downwardly into thecasing strings 12 to the vicinity ofreservoir 14 and the annulus between the pipe strings and their respective casing strings is sealed near the top of the reservoir withpackers 18. It is preferable that thepipe strings 17 extend to thebottoms 19 of the respective casing strings and this is desirable in the production well 11 so the petroleum can be easily recovered therefrom. Generally, thepipe strings 17 will be secured to the top of thecasing strings 12 at the wellhead with agland 20 or some similar type arrangement.

In the practice of the invention a slug of a viscous, steam-distillable liquid will be injected through injection well 10 and will, as indicated byarrows 21, travel downpipe string 17 into the lower portion ofcasing string 12, from which it moves out into the formation throughports 16. After the buffer slug has been injected it is followed by the injection of steam which follows the same path as the slug.

Thereservoir 14 between the injection well 10 and production well 11 is shown divided into five zones so that invention can be better understood. It is a representation of conditions after the slug has been injected and short- 1y after the injection of steam has been commenced. Zone A represents a portion of the reservoir which has been traversed by the buffer slug, shown located in Zone C, and which is largely filled with steam, having been stripped of petroleum by the passing of the slug. Zone B represents the steam buffer interface and it is at this interface where the hot steam distills the trailing edge of the buffer, converting it to a vapor which recondenses on the buffer side of the interface, giving up its latent heat to the slug. Since the steam and the vaporized buifer are both vapors they have a mobility ratio approximating unity allowing both to move through the formation with equal ease.

The main portion of the buffer is shown in Zone C and because of its high viscosity and miscibility is able to force the petroleum ahead of it in a bank shown as Zone D. There is an excellent mobility ratio at the buffer petroleum interface as the buffer is pushed through the reservoir by the steam which accounts for efficient displacement of the residual petroleum of this invention. The petroleum bank in Zone D will drain into the production well 11 throughports 16 and be recovered as indicated byarrows 22. Zone E merely represents the reservoir in its natural state or water flood residual.

It should be appreciated that the buffer, which may be injected hot or cold, is actually heated by the steam and the latent heat of vaporized portions of the buffer when it condenses therein. This causes the buffer to act as a heat source for the petroleum that it is displacing, which will lower petroleums viscosity making displacement easier, thereby enhancing recovery.

By using the steam-distillable bufler slug it is possible to achieve the desirable mobility ratios in the reservoir system when using steam so that use of the steams thermal energy in the reservoir accomplished without sacrifice of efficient displacement of the petroleum in the reservoir.

Obviously, the buffer slug acceptable in the practice of this invention must meet certain specific characteristics. The buffer will be a liquid and should have a viscosity close to that of the petroleum to be displaced. It is preferred that its viscosity be equal to or be greater than the petroleum to be displaced. In the case of very viscous crudes this will not be possible. The reason for this requirement is to achieve the optimum displacement of the petroleum by obtaining optimum mobility ratios.

Next, the buffer must be steam-distillable since this is imperative in the practice of this invention. It is immaterial if the viscous buffer is a single compound or a mixture as long as the buffer is more volatile than the reservoir petroleum. Further, the buffer must be miscible with the petroleum in the reservoir. When the viscosity of the buffer is about the same as the petroleum in the reservoir there will be little mixing of the buffer and the petroleum even though they are miscible.

Some specific buffers which are useful in the practice of this invention are alcohols, organic acids, esters, amides, etc., having a viscosity at least substantially equal to the viscosity of the crude oil and it was found that size of the butter slug injected ahead of the steam could vary from 5% to 10% of the petroleum in the reservoir by volume between the injection well and the production well(s). Slugs about 1% of the petroleum to be displaced improved the recovery only slightly while slugs between 5% and 10% demonstrated substantial improvement.

Another advantage of the invention is that the steam injection can be continued until the slug itself is recovered from the reservoir and the buffer can thereafter be recycled in another reservoir. However, it may be necessary to regenerate the buffer before it is recycled.

The steam injected may be either saturated or unsaturated and the only important criteria is that it have sufficient thermal energy to steam distill the trailing edge of the buffer slug.

In the specification a viscous buffer slug has been referred to and this terminology is used relative to the viscosity of the petroleum to be displaced by the slug. Actually, it is desirable that the slug have a viscosity close to that of the petroleum, that is not less than 75% of the viscosity of the petroleum to be displaced, and preferably about the same as the viscosity of the petroleum. A disadvantage of addition of a light volatile petroleum hydrocarbon such as butane, pentane, hexane or a mixture of light hydrocarbons is that these more volatile hydrocarbons will also be much less viscous than the heavy viscous hydrocarbons to be displaced, lowering the displacement efiiciencies. This will lead to fingering of the volatile solvent through the viscous hydrocarbons and the loss of the volatile slug, and its corresponding beneficial eifect on displacement. It should be appreciated that this is a matter of degree and not principle.

The following experiments were conducted to illustrate the invention and are not intended to place any limitation thereon:

Example I Unconsolidated sand packs having a permeability of 25 darcies, a porosity of 36% and an initial water saturation of 22% were prepared. The test sand packs were confined in a container and saturated with normal hexadecane to simulate reservoir conditions. Octyl alcohol was used as the slug material and three dilferent size slugs were used in three separate test packs. Slugs of 1%, 5% and of the hexadecane saturation by volume of octyl alcohol were injected into these sand packs. In each case the buffer slug was followed by steam at 30 p.s.i.g. and a temperature of 274 F.

In the pack having a 1% buffer slug no appreciable improvement over the use of steam alone was observed. Using the 5% buffer slug it was found that substantial improvement in the recovery of hexadecane was observed while recovery of the hexadecane in the pack having the 10% buffer slug was not noticeably better than that of the pack with the 5% buffer slug.

Example 11 A sand pack identical to those described above was saturated with normal hexadecane also and a mineral oil having a viscosity approximately that of the octyl alcohol was used for the slug. This slug was a 5% slug and was not steam-distillable. It was followed by steam injected at the same temperature and pressure as in Example I.

Data showed this slug breakthrough into the recovery zone after about 70% of the hexadecane had been recovered. When steam-distillable octyl alcohol was used under identical conditions (Example I) breakthrough of the slug did not occur until almost 95% of the hexadecane was swept from the sand pack.

We claim as our invention:

1. A method of recovering petroleum from subterranean reservoirs using the injection of steam in which high displacement elficiencies of the petroleum are achieved comprising:

(a). penetrating a petroleum-producing reservoir with at least one injection well and at least one production well spaced therefrom;

(b) establishing fluid communication between each of said wells and said petroleum reservoir;

(c) injecting through said injection well a slug of a steam-distillable buffer which is a viscous liquid miscible with and more volatile than the petroleum in the reservoir having a viscosity of not less than of the petroleum in the reservoir;

((1) subsequently injecting through said injection well steam at a temperature suflicient to steam distill at least the trailing edge of said steam-distillable buffer; and

(e) recovering displaced petroleum from said reservoir through said production well as it is flowed into said well by said injection of steam.

2. A method according toclaim 1 in which the slug of the steam-distillable butter is at least equal to 5% by volume of the petroleum to be displaced from the reservoir.

3. A method according toclaim 1 in which the steamdistillable buifer has a viscosity essentially the same as the viscosity of the petroleum to be displaced and a boiling point below that of the petroleum.

4. A method according toclaim 1 in which the steamdistillable buffer is a constant boiling azeotrope having a viscosity essentially the same as the petroleum in the reservoir and a boiling point below the boiling point of the petroleum in the reservoir.

5. A method according toclaim 1 in which the injection of steam is continued until a substantial portion of the slug of the steam-distillable viscous liquid buffer is recovered through the production well.

6. A method of recovering petroleum from subterranean reservoirs using injected steam in which high displacement efficiencies are obtained comprising:

(a) penetrating a petroleum producing reservoir with at least one injection well and one production well spaced apart;

(b) establishing fluid communication between each of said wells and said reservoir;

(c) injecting through said injection well a slug of liquid, steam-'distillable butler having a viscosity range of from at least 75% to essentially that of the viscosity of the petroleum to be displaced, said slug being equal to from 5 to 10% by volume of the petroleum to be displaced;

(d) subsequently injecting through said injection well steam at a temperature sufiicient to effect in situ distillation of at least a part of said slug in said reservoir; and

(e) recovering displaced petroleum at said production well which is displaced by said slug and said steam injections until said injected steam breaks into said production well.

References Cited UNITED STATES PATENTS 2,867,277 1/1959 Weinaug et al 1669 3,085,063 4/1963 Turbak 166-9 FOREIGN PATENTS 511,768 8/1939 Great Britain.

CHARLES E. OCONNELL, Primary Examiner. STEPHEN I. NOVOSAD, Examiner.

Claims (1)

1. A METHOD OF RECOVERING PETROLEUM FROM SUBTERRAMEAN RESERVOIRS USING THE INJECTION OF STEAM IN WHICH HIGH DISPLACEMENT EFFICIENCIES OF THE PETROLEUM ARE ACHIEVED COMPRISING: (A) PENETRATING A PETROLEUM-PRODUCING RESERVOIR WITH AT LEAST ONE INJECTION WELL AND AT LEAST ONE PRODUCTION WELL SPACED THEREFROM; (B) ESTABLISHING FLUID COMMUNICATION BETWEEN EACH OF SAID WELLS AND SAID PETROLEUM RESERVOIR; (C) INJECTING THROUGH SAID INJECTION WELL AS A SLUG OF A STEAM-DISTILLABLE BUFFER WHICH IS A VISCOUS LIQUID MISCIBLE WITH AND MORE VOLATILE THAN THE PETROLEUM IN THE RESERVOIR HAVING A VISCOSITY OF NOT LESS THAN 75% OF THE PETROLEUM IN THE RESERVOIR;

Images