Caustic flooding with stabilized water for enhanced oil recovery

An improved method for enhanced oil recovery utilizing caustic or alkaline water flooding which avoids precipitation of hydroxides in the injection water or plugging of the reservoir. A lignosulfonate material is blended with the injection water before the addition of the alkaline chemical, the amount of lignosulfonate being sufficient to prevent formation of any precipitates.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for the enhanced recovery of oil from a 
reservoir. In one aspect, the invention relates to a method for caustic or 
alkaline waterflooding involving the addition of a lignosulfonate material 
to the injection fluid. In another aspect, the invention relates to the 
addition of a lignosulfonate material to seawater or other injection water 
to prevent the formation of precipitates upon mixing with a caustic 
material. 
2. Discussion of the Art 
Enhanced oil recovery (EOR) is a broad concept which encompasses many 
methods for increasing the recovery of oil remaining in a reservoir after 
the natural pressures are insufficient for economical production. Primary 
recovery from a reservoir often produces only 10 to 30 percent of the oil 
present in the formation. To recover at least a portion of the remaining 
oil, various supplemental methods are available. For example, a widely 
practiced secondary recovery technique involves the injection of a driving 
fluid such as water from one or more injection wells spaced at some 
distance from the producing well in order to force some of the oil towards 
the producing well. Tertiary recovery techniques following the waterflood 
could involve the addition of various amounts and combinations of 
surfactants, solvents, micellar compositions, and polymers. The reference 
to the terms secondary and tertiary is unimportant for the purposes of 
this application, and relates only to the sequence in which the 
supplemental or enhanced recovery methods are carried out. 
Chemical Flooding Methods 
There are several chemical waterflooding methods among the techniques 
proposed for EOR. These involve the addition of one or more chemicals to 
the injection fluid, either together or in sequence. 
Alkaline or caustic flooding uses relatively low concentrations of 
inexpensive chemicals and is believed to decrease interfacial tension 
between the injected fluid and the oil. Surfactant flooding uses surface 
active agents such as petroleum sulfonates, but high cost has prohibited 
their widespread use. The use of micellar emulsions to thicken the 
injection fluid is known in the art. Polymers can also be used for 
mobility control of the front of injected material as it advances through 
the reservoir. 
This invention is concerned only with caustic flooding with stabilized 
water as later described. The use of other chemical flooding methods or 
materials in conjunction with caustic flooding is, of course, possible but 
forms no part of the invention. 
The exact mechanism by which caustic flooding is accomplished is not known. 
According to C. E. Johnson, Jr. ("Status of Caustic and Emulsion Methods," 
Journal of Petroleum Technology, Jan. 1976, 85-92), there are actually 
several proposed mechanisms for caustic waterflooding. These include: 
(1) in-situ emulsification of the crude oil and its entrainment into a 
continuous flowing alkaline water phase; 
(2) wettability reversal of the rock from oil-wet to water-wet; 
(3) wettability reversal of the rock from water-wet to oil-wet; and 
(4) in situ emulsification of the residual oil in a water-wet core followed 
by entrapment in smaller pore throats, causing reduced water mobility. 
All have been demonstrated to be effective under special circumstances. An 
understanding of these theories, however, is not necessary to the practice 
of this invention. 
SUMMARY OF THE INVENTION 
The invention is a process for producing petroleum from oil-bearing 
reservoirs by driving a fluid from at least one injection well to a 
producing well. The process comprises injecting via at least one injection 
well a blend of injection water, lignosulfonate, and an alkaline material.

DETAILED DESCRIPTION OF THE INVENTION 
This invention involves an improved method for caustic flooding whereby a 
lignosulfonate material is mixed with the injection fluid before 
introduction of the fluid into the reservoir. The presence of the 
lignosulfonate appears to prevent the formation of undesirable 
precipitated compounds which can lead to plugging of the well or the pores 
of the reservoir. In a preferred embodiment of the invention, 
lignosulfonate can be combined with mixed with seawater containing 
divalent ions. The resulting mixture shows no evidence of insoluble 
precipitate upon mixing with NaOH. 
Caustic Flooding 
Caustic flooding, also known as alkaline flooding, is a process in which 
the pH value of the injected flood water is increased by adding a caustic 
or alkaline chemical. The most common choice of chemical is sodium 
hydroxide because of its ready availability and low cost, and it will be 
used in the following discussion to illustrate the invention. Other 
chemicals which accomplish the same result, including but not limited to 
sodium carbonate, barium hydroxide, trisodium phosphate, polyethylenimine, 
and ammonia, are contemplated equivalents for purposes of this invention. 
Sodium hydroxide (NaOH) is mixed with the injection water to form a 
solution before introduction into the reservoir. The solutions can range 
from about 0.1 to 5 percent or more NaOH by weight, but typically are less 
than 1 percent in actual use. The actual amount of NaOH to be used depends 
upon the peculiar characteristics of both the rock and the oil in a 
reservoir. Further directions are available to those skilled in the art 
from a number of published materials, including a monthly series of 
articles on EOR techniques by N. Mungan published in World Oil beginning 
February, 1981. Conventional caustic flowing techniques known in the art 
can be used in conjunction with the inventive process. 
Injection Water 
The injection water used in this invention can come from any convenient 
source. Previously, the choice of a fluid for caustic flooding was 
restricted because of the necessity to prevent significant premature 
plugging of the reservoir. Plugging often resulted when the hydroxide 
(OH.sup.-) ions from the NaOH contacted the divalent ions (typically 
calcium, magnesium, and barium) naturally present in the injection water 
or in the connate water, forming insoluble precipitates. 
To address this problem, suggestions have been made in the prior art to 
soften the injection water through ion exchange to remove the divalent 
ions. Another approach involved removing the precipitated hydroxides by 
filtration before the mixture was injected into the reservoir. Still 
another involves injection of fresh or treated "soft" water into the 
reservoir in order to condition the reservoir and prevent serious plugging 
near the well site. The present invention obviates the need for such 
procedures. 
The water for injection can therefore be withdrawn from any suitable source 
with minimal concern for its salinity, particularly the presence of 
divalent cations. A preferred embodiment of the invention permits the use 
of ordinary seawater. This aspect of the invention can be especially 
advantageous if the well is located in a coastal area or off-shore where 
fresh water is valued more highly for alternate uses. 
Lignosulfonate 
Lignosulfonate is a general term applied to materials that are most 
commonly derived from the wood pulping process, and obtained by 
sulfonating alkali lignins. Lignin is a polymer which is the second only 
to cellulose as the principal constituent of woody plants. Lignins and 
their derivatives are described more fully in the Kirk-Othmer Encyclopedia 
of Chemical Technology, Third Edition, Vol. 14, pp 294-312, the disclosure 
of which is incorporated by reference. 
Uses of lignosulfonates in other EOR techniques have been proposed, 
including its use as a sacrificial material to be mixed with a surfactant 
as described in U.S. Pat. No. 4,157,115 to Kalfoglou. An aqueous solution 
of the lignosulfonate can also precede the lignosulfonate-surfactant 
mixture into the injection well. The description of lignosulfonate 
material in the above patent is generally useful for further information 
and it is hereby incorporated by reference. 
Any suitable lignosulfonate material as described above can be used in the 
practice of this invention. Particularly preferred, however, are 
lignosulfonates which are capable of preventing significant precipitation 
of insoluble hydroxides at the conditions found in the reservoir. 
Preferred sources and types of lignosulfonates can be readily identified 
for a particular injection water by adding small known amounts (usually 15 
wt % or less) of the lignosulfonate to a beaker of the injection water, 
followed by the addition of NaOH. If no precipitate forms as NaOH is 
added, the lignosulfonate is suitable for use with that source of 
injection water. Minimal effective amounts can be determined by repeating 
the procedure above using lesser amounts of the lignosulfonate, until 
precipitation occurs. 
Because of the variations among commercial pulping and sulfonation 
processes, the product lignosulfonates may be more or less suitable for 
use with this invention. Optimization of the amounts used and the 
particular selection of lignosulfonate material is within the skill of 
workers in the art from the directions given herein and in published 
literature. 
Commercially available lignosulfonates are typically furnished as a powder 
or in a pre-solubilized form. Either form is suitable for use in the 
inventive process. If poorer grades of material containing minute 
particles of cellulose are used, it is advantageous to filter the 
cellulose from the injection water to avoid any plugging of the reservoir. 
The Process 
The process of the invention is accomplished by adding lignosulfonate to 
the injection water in an amount sufficient to prevent the precipitation 
of insoluble compounds such as the hydroxides of divalent cations. The 
order of addition is crucial to the success of the process. The 
lignosulfonate and injection water must be mixed together before any 
alkaline material is added. Otherwise, precipitation would occur 
immediately and subsequent addition of lignosulfonate would have little if 
any effect. 
Although the relative proportions of alkaline material and lignosulfonate 
in the water can vary widely, the preferred proportions are at or near the 
minimum amounts necessary to accomplish the intended technical effect. In 
EOR flooding operations, the cost of the chemicals used is usually the 
limiting factor. 
As one example, a powdered lignosulfonate could be added to a tank of water 
in a weight ratio of about 1 to 50. This solution is then withdrawn after 
removing any solids (e.g. cellulose) which may be present by filtration or 
settling. Sodium hydroxide can then be added in any desired proportion, 
such as 1 to 100. 
The resulting blend of injection water, lignosulfonate, and sodium 
hydroxide is then injected into one or more injection wells, by methods 
well known in the art. This fluid blend is then driven through the 
reservoir by injecting additional amounts of fluid into the injection 
well, forcing petroleum towards the producing well. 
SPECIFIC EMBODIMENTS 
Example 1 
A solution of lignosulfonate in seawater was prepared by the following 
method, with percentages expressed by weight. The seawater included about 
700 ppm magnesium and 250 ppm calcium, by weight. A commercially available 
solubilized lignosulfonate, ERA-4 (a lignosulfonate product containing 
about 30-40% solid material and manufactured by American Can Company, 
Greenwich, Connecticut) was added to the seawater and mixed well. The 
mixture was translucent and had no precipitate. 
Sodium hydroxide pellets were added to the above mixture and the solution 
was stirred. The mixture consisting of 96% seawater, 3% lignosulfonate, 
and 1% sodium hydroxide showed no formation of insoluble hydroxides. 
Example 2 
A second solution was prepared by the method described in Example 1, except 
that the solution consisted of 90% seawater, 5% lignosulfonate and 5% 
sodium hydroxide. No precipitate formed. 
Example 3 
A third solution was prepared using 84% seawater, 6% lignosulfonate, and 
10% sodium hydroxide. No precipitate formed. 
The solution of Example 3 was used in a core test using a conventional 
Berea sandstone core. Good recovery was obtained from this system. 
Comparative Example A 
A fourth solution was prepared using 98% seawater, 1% lignosulfonate, and 
1% sodium hydroxide. A greenish-brown precipitate formed immediately upon 
addition of the sodium hydroxide pellets. 
While not intending to be bound by theory, observations suggest that the 
lignosulfonate is stabilizing the seawater rather than neutralizing the 
added sodium hydroxide. As noted in the above examples, a minimal amount 
(above 1%) of the lignosulfonate tested was essential to prevent 
precipitation of sodium hydroxide added even in trace amounts. The test 
solutions also retained a high pH value (above 12), confirming their 
potential effectiveness when used in EOR processes. 
Although only a few embodiments of the invention have been described above, 
many modifications can be made without departing from the spirit and scope 
of the invention. All such modifications are intended to be included 
within the scope of the invention, which is to be limited only by the 
following claims.