Use of peroxide in waterflood oil recovery

For enhanced recovery of petroleum, particularly for recovery of residual oil during conventional water-flooding, an aqueous solution of peroxide is injected into an oil-bearing formation. The concentration of peroxide in the aqueous solution ranges from about 0.1 to about 3.0 weight percent, and injection of peroxide need not be followed or accompanied by injection of alkaline solution. The aqueous peroxide solution may then be displaced toward the producing well or wells to sweep out residual oil.

BACKGROUND OF THE INVENTION 
1. Setting of the Invention 
In the practice of secondary recovery of petroleum from subterranean 
formations, waterflooding is commonly employed. In such operations water 
is injected down an input well and into the formation to displace oil from 
the formation towards a producing well through which the oil is recovered. 
Additives such as sodium carbonate or sodium hydroxide in waterfloods have 
been used to improve oil recovery. Additionally, surfactants have also 
been utilized to improve oil recovery. However, both techniques, although 
effective, are expensive and are applicable only to limited types of 
formations bearing special varieties of petroleum. 
2. Pertinent Art 
Pertinent art in the area is illustrated by U.S. Pat. No. 3,757,861 issued 
to Willis G. Routson on Sept. 11, 1973 and entitled "Oil Recovery 
Employing Peroxides and Alkalis". The process employed by Routson requires 
injection of an aqueous solution containing at least about 5% by weight of 
an inorganic peroxide in an amount of at least about 0.04 pore volume of 
the formation followed by or injected with an aqueous solution containing 
at least about 0.05% by weight of alkaline agent in an amount at least 
equal to that of the peroxide solution. This peroxide/alkaline agent pad 
was in turn forced through the formation by solutions of water containing 
mobility control agents such as xanthan gum. Peroxides and and alkalines 
in Routson are allowed to co-act and the products of reaction of the 
peroxide and the copolymer are maintained in the bank which is then 
displaced into the formation toward one or more producing walls. 
Additional pertinent art is illustrated by U.S. Pat. Nos. 3,298,436, 
3,330,347 and 3,344,858 which discloses production of surfactant compounds 
in situ in an oil bearing formation by the injection of an acid compound 
followed by a caustic solution. Such operations, however, are, as 
mentioned above, inherently expensive. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an improvement in recovery of 
fluid hydrocarbons such as petroleum from subterranean petroleum bearing 
formations may be realized by introducing into the formation an aqueous 
solution of peroxide, typically hydrogen peroxide, as an additive to the 
waterflood agent. In general, the invention is carried out in accordance 
with the usual practice of waterflooding whereby the waterflood agent 
containing the peroxide is introduced into the injection well and 
displaced through the formation toward one or more of the producing wells 
from which the petroleum is recovered. 
In an additional method, the additive may be injected as a slug or pad 
(making up a small percentage of the pore volume) and subsequently 
followed by injection of a drive fluid. 
DETAILED DESCRIPTION OF THE INVENTION 
The method of the present invention will normally be practiced in oil 
fields which have been engineered for secondary recovery by conventional 
waterflooding. The present method may be employed as the secondary 
recovery method of choice or employed after conventional waterflooding has 
been carried out to the point where yields are no longer economically 
feasible although substantial amounts of petroleum are known to remain in 
the formation. In conventional waterflood operations, it is the standard 
practice to drill a number of wells to penetrate the oil bearing strata. 
At least one such well is employed for injecting fluids into the oil 
bearing strata and one or more wells spaced apart from each injection well 
are employed for producing oil displaced by the injected fluid. Since a 
plurality of wells are normally involved in any given oil field, it is 
usually possible from the study of core logs, production logs and the like 
to estimate the average porosity of the oil bearing strata and the amount 
of residual oil in place. 
In carrying out the invention, an aqueous solution of between about 0.1 and 
about 3.0 weight percent peroxide is pumped into one or more injection 
wells and out into the oil-bearing formation. Weight percent of peroxide 
used throughout specification and claims is defined as the weight of 
[-HO.sub.2 ] divided by weight of the solution. The number of pore volumes 
of solution injected is controlled primarily by the economics. However, it 
has been found that once free gas generated during peroxide displacement 
breaks through at the production well, the recovery of oil is 
substantially reduced. Therefore, the injection of solution should not 
continue beyond gas breakthrough at the production well. The amount of 
solution injected can amount to up to about 2 or more pore volumes of the 
formation being treated. 
Alternatively, an aqueous solution of peroxide may be injected into the 
formation through an injection well until a sufficient pressure is 
attained to force the solution into the outer regions of the formation 
surrounding the wellbore. The solution is then maintained therein for a 
sufficient period to allow reaction of the peroxide with the formation. 
Thereafter the pressure is relieved on the injection well and oil is 
produced back through the injection well from the formation. 
One of the major benefits of this invention is since the peroxide is 
inexpensive and is used in limited concentrations, use in a continuous 
manner is economical. Furthermore, injection of the peroxide solution need 
not be accompanied with or followed by injection of an alkali solution, 
thus making the overall process even more economical. 
In carrying out the slug method of this invention, an aqueous solution 
containing at least 0.1 weight percent of peroxide is pumped into one or 
more injection wells and out into the oil bearing formation until an 
amount of the solution has been injected corresponding to about 0.1 to 
about 0.5 pore volume and preferably about 0.25 pore volume of the 
formation to be treated. Thereafter, sufficient pressure may be applied to 
the injection well to push the solutions into and through the formation to 
displace the oil toward the production well or wells. Alternatively, after 
introduction of the peroxide, sufficient pressure may be applied to the 
injection well to push the respective solution into the formation and 
maintain same therein for a period sufficient for reaction to occur and 
thereafter the pressure is relieved and oil is produced through the 
injection well. 
The waterflood agent containing peroxide in the slug form may be followed 
by suitable mobility control agents contained in an aqueous drive fluid. 
Preferred mobility control agents, solutions in water or brine of high 
molecular weight water soluble polymers having resistant properties, are 
defined in U.S. Pat. No. 3,282,337 which is hereby incorporated by 
reference. In this manner, optimal efficiency in recovery of residual oil 
is realized. 
In preparing the solution of peroxide for the present method, it is 
desirable to test the source of water to be employed to assure that the 
water does not contain impurities which might decompose peroxide before it 
reaches the oil in place in the formation. In general, the water employed 
for preparing a peroxide solution should be relatively free of readily 
oxidizable ingredients such as emulsified petroleum and other organic 
matter, sulfide or sulfite ions, ferrous ions or the like. In some cases, 
it may be desirable to incorporate a neutral buffer or other preservative 
in the solution to assure that the peroxide reaches the oil bearing 
formation without decomposing. The water used may be fresh water or brine. 
The pH of the water is not critical. It is preferred, however, that the 
water be neutral or slightly alkaline. The polymer solution used in the 
slug method of the invention should also be unreactive with the peroxide 
additive. 
It is believed that it is necessary that the peroxide reach the oil-bearing 
formation without decomposing since it is necessary for the peroxide to 
react with the formation rock to create a free gas as opposed to a 
dissolved gas in place, that is, in the formation. 
The type of peroxides utilized in the method of the invention should 
liberate oxygen freely. The preferred peroxides include hydrogen peroxide 
and inorganic peroxides such as zinc and calcium peroxide since these 
peroxides liberate oxygen more freely. More preferably, hydrogen peroxide 
is used in this method. The exact amount of peroxides to be employed in a 
given solution will vary depending upon the nature of the formation to be 
treated, the nature of the petroleum contained therein, and most 
importantly the bottomhole pressure of the given formation. Good results 
are obtained when employing from about 0.1 to 3.0 weight percent of 
peroxide in a waterflood aqueous solution. The weight percent of peroxide 
is directly proportional to the pressure of the formation, that is, the 
higher the formation pressure, the higher the required peroxide 
concentration. 
Performance of this peroxide containing solution may be increased by 
reduction of interfacial tension between the solution and the formation 
oil. Therefore, addition of effective amounts of surfactant to the 
solution will increase the effectiveness of the oil recovery. This 
invention is generally useful in shallow wells where the bottomhole 
pressure does not exceed 200 psi. This typically means wells having depths 
less than 500 ft. This limitation is due to economic considerations of 
concentration of peroxide since greater depth requires greater peroxide 
concentration. 
The process of this invention, although useful on both oil-wet and 
water-wet formations, is more useful on water-wet formations.

The following Example illustrates the invention but is not to be construed 
as limiting same. 
EXAMPLE 
Cylindrical core samples about 2 inches in diameter by 12 inches in length 
cut from Berea sandstone of known porosity are mounted in Hassler core 
holders equipped with pressure fittings on opposite faces of the cores so 
that fluids can be forced lengthwise therethrough. The holders are 
maintained at 500 psig on the annulus. The cores are initially evacuated, 
flooded with carbon dioxide, re-evacuated, and saturated with a 2 N sodium 
chloride brine. The pore volume is then determined and the cores are 
injected with 2 pore volumes of the 2 N sodium chloride brine. 
Subsequently, the cores are equilibrated with 3000 ppm sodium chloride 
brine at 12 cc per hour for 12 hours. The cores are then oil flooded with 
a crude oil obtained from the Torchlight field near Basin, Wyoming, and 
the connate water saturation is determined. Cores are then waterflooded 
with a 3000 ppm brine test solution to residual oil saturation. A back 
pressure of 30 psig is maintained on the core and effluents are directed 
into a fraction collector or a gas-liquid separator. During these tests, 
the cores are continuously injected at 12 cc per hour with waterflood test 
solution. Test solutions consist of 5 ml of 30% H.sub.2 O.sub.2 per liter 
of 3000 ppm brine (0.208% weight percent (--HO.sub.2)). As shown in Table 
I, the recoveries at 2 pore volumes of produced fluid range from 17.9 to 
28.6% of tertiary oil for an average of 24.2%. 
TABLE I 
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Connate Residual Final 
Water Oil Oil 
Run No. Saturation Saturation 
Saturation 
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1 38.9 35.1 28.8 
2 44.3 29.4 21.7 
3 43.9 28.0 20.0 
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Similar improvements can be obtained by using a solution having higher 
peroxide concentrations of 2.5 weight percent.