AU475552B2 - Secondary recovery method - Google Patents

Description

In an oil reservoir which produced to the point en produced to the point

that it is deficient in natural energy; it is often desirable to

institute secondary recovery methods to recover the large

amount of oil still left in the reservoir. Most secondary

recovery operations are generally carried out by injecting an

extraneous fluid into the reservoir which will then migrate to

an output well driving the oil ahead of it. The oil is then

recovered at the output well. Even after most secondary

recovery operations about one-half of the oil is left behind in the

reservoir. The inefficiency of the displacement process in

secondary recovery operations is mainly due to two retentive

forces, viscosity and capillarity.

If the viscosity of the fluid displacing the reservoir oil

to the production wells is lower than the reservoir oil, premature

breakthrough of the driving fluid into the production wells will

occur. The displacing fluid actually fingers through the

reservoir oil and proceeds to the production well before an adequate

portion of the reservoir has been swept. This is referred to in

terms of sweep efficiency. The effects of viscosity on sweep

efficiency may be described in terms of the mobility ratio. The

mobility ratio is defined by the following equation:

52 M where

where

47 5,552

63,200/73 M mobility ratio

u2 u1 viscosity of displacing fluid and displaced fluid

(oil), respectively:

K2 K1 permeability of the formation with respect to the

displacing fluid and the displaced fluid respectively.

At high mobility ratios the phenomenon commonly known as

fingering occurs and the displacing fluid does not display a flat

front to the reservoir oil, but instead, rushes ahead at various

points in finger like protrusions which may prematurely break

through to the production wells. The oil in areas not touched by

the "fingers" of displacing fluid are usually left unrecovered in

pockets in the reservoir. The preceding equation shows that the

mobility ratio and the degree of fingering is directly proportional

to the ratio of the displaced fluid viscosity to the displacing fluid

viscosity, u1 /u 2 Since most displacing fluids are less viscous

than the displaced fluid, oil, the mobility ratio will usually be quite

high, and a poor areal sweep efficiency will occur because of

fingering.

Polymeric compounds which increase the viscosity of

the displacing fluid so as to lower the mobility ratio and increase

the sweep efficiency of the displacing fluid have been developed

and used in recent years. For example, U.S. Patent 3, 039, 529

discloses the use of polyacrylamide solutions to increase the

viscosity of the displacing fluid. Also, U.S. Patent 3, 581, 824

discloses the use of polysaccharides for the same purpose.

Although these polymers are useful for increasing the viscosity of

47 5,552

63,200 73 the driving fluid they are expensive. Also, the displacing fluid

containing these polymers tends to decrease in viscosity as it

travels through the reservoir away from the injection well bore.

This, of course, causes the mobility ratio to rise and increases

the likelihood that fingering will occur.

The method of our invention solves these problems by pro-

viding an injection or displacing fluid which because of its

content of fume silica, will not decrease in viscosity as it

moves away from the injection well bore.

The invention is a method for recovering oil from sub-

terranean oil reservoirs which is penetrated by at least one

injection well and at least one production well wherein a fluid

is injected into the injection well and oil is produced from the

production well. The fluid comprises an intimate mixture of

water and fume silica having a surface area of from 50 to 400

meters2/gram and a particle size from 7 to 15 millimicrons.

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63,200/73

THE FLUID

The preferred injection fluid useful in the process of our invention comprises an intimate mixture of water and fume silica such that the dispersion of the fume silica in the water in stable and not subject to separation into distinct phases. The water may be fresh or mineral ladened as, for example, salt water obtained from a formation in+the vicinity of oil bearing zones. The water selected should be compatible with the formation it is to be injected into so that harmful swelling, for instance, will not occur.

The fume silica useful in our invention is different from precipitated silica or silica gel. It is made up of chain-like formations sintered together. These chains are branched and have enormous external surface areas of from 50 to 400 meters2/gram. Each segment in the chain has many hydroxyl (OH) groups attached to silicon atoms at the surface. When the segments come into proximity to each other, these hydroxy groups will bond to each other by hydrogen bonding to form a three dimensional network. Fume silicas are readily available from manufacturers. One source is the Cabot Corporation of Boston, Massachusetts under the trade name CAB-O-SIL R. Fume silica is also available from other commercial sources and the reference to one source is not intended to limit the scope of our invention. When the silica particles are dispersed in a liquid medium, the network structure formed by the silica particles restricts the movement of the molecules of the liquid medium. This results in an increase in the viscosity of the liquid. The fume silica acceptable for use in the method of this invention has a particle size ranging from 7 to 15 millimicrons

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63, 200 size In this range the fume silica will pass through even

reservoirs with very small pore size. For example, a

reservoir having very low permeability of say 0.016

millidarcies (md) has a correspondingly small pore size of

to 100 mu. Thus, the fume silica suitable for use

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63,200 /7 in the process of this invention will pass through even the

smallest pores encountered in hydrocarbon reservoirs and will maintain a constant viscosity in the driving fluid.

The thickening efficiency of the silica is directly related to the polarity of the liquid to be thickened. The use of selected additives (surfactants and/or multifunctional compounds)

can increase the thickening efficiency of the silica. In the case of thickening a hydrocarbon solvent, these additives react with*

the interface between the silica and the solvent and increase the

degree to which the silica particles form the three dimensional

network. This allows less silica to be used to achieve

equivalent thickening of the solvent. Usually less than

of the additive based on the weight of total liquid to be thickened

will achieve marked increases in viscosity.

For liquids of high polarity such as water, aldehydes, ketones, etc., cationic and nonionic surfactants oleoyl trimethylene diamine and nonyl polyethoxy ethanols can cause

dramatic increases in viscosity. Low polarity or non-polar

liquids, such as hydrocarbons, are thickened by the use of anionic

surfactants such as sodium linear allylate sulfonate and multi-

functional compounds such as ethylene glycol.

Depending on the system, dramatic changes in viscosity can occur by using two additives such as a nonionic surfactant,

and a cationic type.

The multifunctional compounds mentioned above fall into the general class of compounds with a plurality of groups

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63.200/73 available for hydrogen bonding. Examples of such compounds

are amines, ethylene glycol, glycerine, and propylene glycol.

The surfactant which may be used in the injection fluid

of our invention may broadly be any compound which reduces

tension surface tension of the water, thus reducing the surface

between the water and the reservoir oil. Soap may be used, for

alkyl sulfates instance, the sodium salts of high molecular weight

or sulfonates. Also very useful are nonionic surfactants which

a hydrophilic are usually a reaction product of a hydrophobic and

material, such as the reaction product between mono-alkyl

phenols and ethylene oxide.

Other ingredients may be used in the fluid of this

from the invention to give it additional benefits without detracting

scope and benefits already described.

Also, a solvent may be added to the fluid of this invention

which is miscible with the reservoir oil. Examples of suitable

toluene and hydrocarbons are aromatics such as benzene and

aliphatics such as LPG, propane, butane, isobutane, pentane,

isopentane, and hexane. Also, any mixture of suitable hydro-

miscibility carbon solvents which when mixed retain their

characteristics with the reservoir oil are acceptable.

Other additional ingredients may also be envisioned

which would be acceptable. For example, an inert gas could be

added to the fluid. Such a gas could be nitrogen or methane for

example.

viscosity The fluid of our invention should be adjusted in

475,552

63.3L0jS so that the mobility ratio is/t-ss than abo 0.1 nor

than abot 10. At the higher mobility ratios fingering will have

more of a tendency to occur and at the lower mobility ratios the

fluid will become progressively more viscous and difficult to

pump. It is especially preferred that the mobility ratio range

from about 0.9 to aboet 3.

Of course, once the desired mobility ratio is known,

the necessary viscosity of the fluid may easily be calculated.

The viscosity of the fluid described in our invention may be tailored

to fit the needs of the user by variations of ingredients. Due

to the number of ingredients, a detailed explanation of methods of

varying viscosity is impractical to give. However, it will be

evident to one skilled in the art what effect each ingredient has on

the viscosity so that an infinite number of fluids may be made

which will fall within the scope of our invention.

USE OF THE FLUID

A firm gel of the fluid to be used may be prepared by

mixing water, oloidasilica and a surfactant and/or a multi-

functional compound if needed. If a less firm gel is desired the

surfactant and multifunctional compound may be reduced or

eliminated. This gel may then be reduced to the proper

viscosity with more water, which thickens the fluid to a point

due to hydrogen bonding, above which additional water thins it.

The amounts of water to be used will depend on the amounts of

the other ingredients and the ingredients themselves. The gel

may also be reduced in viscosity by adding hydrocarbon solvent.

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63,200/73 The fluid of this invention is then injected into the

reservoir in order to displace the oil in the reservoir to

production wells. The fluid of our invention may conceivably

be the only fluid injected into the formation but considering its

relatively expensive character and the vast quantities which

would be needed to flood an entire oil reservoir, it is preferred

to use the fluid of our invention as a slug. A slug of the fluid

of our invention would be injected into the reservoir followed by

another fluid. The trailing fluid may be water, gas or some

treated fluid.

The thickened slug of our invention may be used according

to standard methods of fluid displacement. The lower mobility

of the slug of our invention will remove problems of fingering and

overriding encountered with conventional fluids such as water. It

is within the skill of the art to determine the proper slug size and

rate of displacement to be used.

The fluid of this invention provides an improvement over

the polymer thickened fluids in shear resistance. When polymer

thickened fluids are subjected to the tremendous shear forces

present as they are pushed through the reservoir rock they lose

viscosity, particularly in the vicinity of the well bore, and the

mobility ratio rise results in possible fingering effects. However,

the fluid of our invention does not lose appreciable viscosity due to

shear forces.

Injection of the fluid of this invention may be in a secondary

recovery operation or in a tertiary recovery operation. For

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63,200/73m example, after a conventional water flood or polymer flood or

any other secondary recovery operation the fluid of this invention

may be injected to remove additional hydrocarbons.

Experimental

I.

Preparation of a Typical Fluid

of Our Invention

Three-hundred (300) milliliters (ml) of water and

grams Of-eoda silica were mixed at 15, 000 revolutions

per minute (rpm) in a waring blender. A thickened liquid of

8 centipoise (cp) resulted. This fluid would be acceptable for

injection.

Ii.

Effect of Additives

To investigate the effect of additives, 5 drops of a

nonionic surfactant were added to the mixture of o~o4-siica and

water described in I. Immediately upon mixing a firm gel formed

too viscous to measure. The addition of 100 ml of water and 10 g.

of sodium chloride did not effect the gel.

III.

Preparation of a Fluid of Our Invention

Containing Various Ingredients

Two-hundred (200) ml of n-hexane was measured into a

waring blender with 4 grams of cGollodl/iica. After blending

for 1 minute at 16, 000 rpm, a soft gel formed. While blending

3 ml of a nonionic surfactant, 3 ml of glycerine, and 20 mjl of

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63,200/73 water were added. A firm gel formed. One-thousand (1000)

ml of n-hexane and 210 ml of water were added with blending.

The viscosity of the mixture was about 160 cp.

A fluid made as described above has remained stable for

about a year.

475,552

Claims (4)

63,200 /73 THE CLAIMS DEFINING THE INVEN'TION ARE AS FOLLOWS:

1. A method for recovering oil from subterranean oil bear-

ing reservoirs comprising:

penetrating the reservoir with at least one in- jection well and one production well which are in fluid

communication with the reservoir, and

injecting into the injection well a thickened fluid comprising an intimate mixture of water and fume silica having

a surface area of from 50 to 400 meters 2/gram and a particle

size from 7 to 15 millimicrons, and

producing oil from the production well.

2. A method as claimed in Claim I, wherein the injected

fluid is injected as a slug.

3. A method as claimed in Claim 2, wherein the injected fluid comprises an intimate mixture of water, a surfactant and

the fume silica.

4. A method as claimed in Claim 1, wherein the injected fluid and the reservoir oil have a mobility ratio, as hereinbef ore defined, from 0.1 to

A method as claimed in Claim 4, wherein the mobility ratio

is from 0.9 to 3.

Dated this 4th day of May, 1976.

TEXACO DEVELOPMNT CORPORATION By their Patent Attorney:

~II ~of G.,IFFITII, HASSEAL -IrFZALE.R.

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