Selecting salinities in micellar flooding

In the displacement of crude oil through a subterranean reservoir with an aqueous displacement fluid, improved oil recovery is obtained by providing a salinity contrast between the displacement fluid and aqueous formation fluid. The salinity of the displacement fluid is selected such that at least one mixture of the aqueous displacement and the aqueous formation fluid will have the minimum interfacial tension between the displaced crude oil and mixtures of these fluids. The minimum interfacial tension between crude oil and mixtures of aqueous displacement and formation fluids can be determined by mixing these fluids such that some mixtures contain from essentially none to essentially all displacement fluid and determining the interfacial tension between the mixtures and the crude oil. The displacement fluid is a micellar fluid followed by a polymer-containing aqueous fluid. Further improvements in oil recovery are obtained when this salinity contrast is provided and the salinity of the polymer-containing fluid is less than the salinity of the mixture which provides the minimum interfacial tension with the crude oil.

BACKGROUND OF THE INVENTION 
Enhanced oil recovery refers to the displacement of fluids from an 
injection well penetrating a crude-oil containing subterranean reservoir 
toward a production well penetrating the reservoir. Many processes and 
compositions have been developed for improving the percentage of crude oil 
that can be produced by this technique. 
Aqueous micellar fluids have been developed for displacing crude oil 
through subterranean reservoirs. These fluids contain appropriate and 
sufficient amounts of surfactant for micelle formation in the aqueous 
fluid to increase the efficiency which these fluids will displace crude 
oil. Aqueous formation fluids can be connate water, the water remaining in 
the reservoir following a water-flood or water which is injected ahead of 
the micellar fluid to displace water from the reservoir. Sufficient 
micellar fluid is injected to produce a micellar bank of about 3 to about 
25 percent (%) of the total pore volume of the portion of the reservoir 
from which crude oil is to be displaced. Following the micellar bank, an 
aqueous polymer-containing fluid can be injected into the reservoir to 
displace the micellar fluid. The polymer-containing fluid generally has a 
relatively low mobility through the reservoir to aid in the displacement 
of the crude oil. Sufficient polymer-containing fluid is injected to 
provide a polymer bank or mobility buffer bank of about 30 to about 100 
percent of the total pore volume of the portion of the reservoir from 
which crude oil is to be displaced. 
The fluids in the micellar bank and the polymer bank are generally 
formulated to contain inorganic salts. The nature and amount of the 
inorganic salts in the formulation depends in part on the mineral content 
of the reservoir and on the salinity of the aqueous fluids being 
displaced. In the formulation of the micellar fluid, the salinity thereof 
can be selected so that the interfacial tension between the micellar fluid 
and the crude oil is below about 100 millidynes/cm and preferably as low 
as about 10 millidynes/cm or lower. Low interfacial tension aids in the 
displacement of the crude oil through the reservoir. In formulating the 
fluids in the polymer bank, the salinity of the polymer-containing fluid 
can be selected to reduce the loss of surfactant in the micellar bank 
which can occur when the polymer bank mixes with the micellar bank. This 
loss of surfactant in the reservoir can be reduced by formulatng the 
polymer containing fluid at a low salinity. 
SUMMARY OF THE INVENTION 
Crude oil is displaced through an oil-bearing subterranean reservoir 
containing aqueous formation fluid with an aqueous displacement fluid. The 
composition of the displacement fluid is selected such that the 
interfacial tension between the displacement fluid and crude oil changes 
as the displacement fluid mixes with aqueous formation fluid and one 
mixture of the displacement and formation fluid will have the minimum 
interfacial tension with the displaced crude oil. The displacement fluid 
is a micellar fluid followed by a polymer-containing aqueous fluid. The 
displacement fluid is followed by a drive fluid, usually water. The 
minimum interfacial tension between the crude oil and mixtures of aqueous 
displacement and formation fluid is preferably less than about 100 
millidynes/cm and is most preferably about 10 millidynes/cm or less. In an 
embodiment of this invention, a salinity contrast is provided between the 
aqueous displacement and formation fluid. The salinity of the displacement 
fluid is selected such that at least one mixture of the aqueous 
displacement and formation fluids will have the minimum interfacial 
tension with the displaced crude oil. Further improvements in oil recovery 
are obtained when the salinity of the polymer-containing fluid component 
of the displacement fluid is less than the salinity of the mixture which 
provides the minimum interfacial tension with the displaced crude oil.

DETAILED DESCRIPTION 
It has been discovered that the mixing of aqueous formation fluid with 
displacement fluid, comprising aqueous micellar and polymer-containing 
fluids, can be used to improve the percentage of crude oil that can be 
recovered in an enhanced oil recovery project. By this method, the 
displacing fluid is selected such that as it mixes with increasing volumes 
of the formation fluid, the interfacial tension between the mixtures and 
the crude oil with change. Improved oil recovery occurs when the 
interfacial tension changes, such as from a high to a low interfacial 
tension or from a low to a high interfacial tension or from a high to a 
low and back to a high interfacial tension. 
The displacement of crude oil with a micellar bank is enhanced when the 
interfacial tension between the micellar bank and the displaced crude oil 
is at a low value. Micellar fluids can be designed that have interfacial 
tensions as low as 1 millidyne per centimeter (cm) or lower. At these low 
interfacial tensions, the micellar fluid is substantially miscible with 
the displaced crude oil and very efficiently displaces the crude oil from 
the reservoir. The crude oil recovery efficiency of the micellar bank is 
decreased at higher interfacial tensions; however, micellar fluids have 
considerable crude oil displacement efficiency at interfacial tensions as 
high as about 100 millidynes per cm and are even more efficient at 
interfacial tensions of about 10 millidynes per cm. 
It is known that changes in the salinity of a micellar bank can alter the 
interfacial tension between a micellar bank and crude oil and that the 
interfacial tension between the micellar bank and crude oil can be 
minimized by formulating the micellar fluid at the appropriate salinity. 
In one embodiment of this invention, the salinity of the micellar fluid is 
selected such that on mixing of the micellar fluid with aqueous formation 
fluid there will be a change in the salinity of the micellar fluid. This 
change in salinity will cause the interfacial tension between the crude 
oil and the micellar fluid to increase or decrease. If the micellar fluid 
is at an appropriate salinity for minimizing the interfacial tension 
between the micellar fluid and crude oil, mixtures of the micellar fluid 
with aqueous formation fluid of either higher or lower salinity will 
increase the interfacial tension. If the micellar fluid is at a lower 
salinity than required for minimizing the interfacial tension between the 
micellar fluid and the crude oil, mixtures of the micellar fluid with 
aqueous formation fluid of higher salinity will reduce the interfacial 
tension. If the salinity contrast between the aqueous formation and 
micellar fluids is appropriate, the interfacial tension between the 
micellar fluid and the crude oil will be reduced to a minimum and 
thereafter increased. 
During the movement of the micellar fluid and polymer fluid through the 
reservoir, there can be mixing of the two fluids. Because of this mixing, 
these fluids will be referred to in this description as the displacing 
fluid or displacing bank. When the two fluids have different salinities, 
there will be changes in the salinity distribution within the displacement 
bank. In one embodiment of this invention, the salinity of the 
displacement bank is selected such that mixtures of the displacement bank 
and the aqueous formation fluid will have salinities different from the 
salinity of the displacement bank. The salinity of the displacement bank 
is selected such that one mixture of the bank with aqueous formation 
fluids will be at the minimum interfacial tension between the displacement 
bank and the crude oil. The minimum interfacial tension is the lowest 
interfacial tension between the displacement bank and the crude oil that 
can be achieved by adjusting the salinity of the displacement bank. 
In addition to changing interfacial tension for improving oil recovery, 
additional improvements in oil recovery can be obtained by selecting the 
salinities of the displacement fluids to minimize surfactant loss. Except 
at very low salinities, such as at salinities of 0.1 normal (N) or less, 
surfactant loss increases in proportion to salinity. It has been found 
that surfactant loss resulting from the mixing of micellar fluid with 
aqueous polymer-containing fluid having a higher salinity than the 
micellar fluid is greater than the surfactant loss resulting from the 
mixing of micellar fluid with aqueous polymer-containing fluid having a 
lower salinity. The most advantageous selection of salinities for the 
enhanced oil recovery of this invention is exemplified by the use of a 
displacement bank having an appropriate salinity contrast with the aqueous 
formation fluid and the polymer fluid component of the displacement bank 
having a lower salinity than the salinity of the micellar fluid component. 
Surfactant loss is even further reduced when the salinity of the polymer 
fluid is less than the salinity of the mixture of displacement fluid and 
aqueous formation fluid which exhibits the minimum interfacial tension 
between the mixture and crude oil. 
As the micellar bank moves through the reservoir, the volume of surfactant 
in the micellar bank diminishes. The mechanisms which cause the surfactant 
to diminish are not completely understood, but are thought to include 
adsorption of surfactant on the reservoir rock and loss of surfactant due 
to mixing of the micellar fluid with the crude oil. Sufficient volume of 
micellar bank is needed for displacing crude oil from an injection well to 
a production well. This would require the use of about 3 to about 25 
percent of the total pore volume of the portion of the reservoir from 
which crude oil is to be displaced. The loss of surfactant is determined 
experimentally by simulating the flow of a micellar bank through a 
reservoir. In making this experimental determination, it is preferred to 
flow the micellar fluid through a core taken from the reservoir and to 
utilize the same fluids that would be involved in the enhanced oil 
recovery project. 
The micellar bank is displaced through the reservoir with a bank of lower 
mobility fluid which is formulated with sufficient polymer to increase the 
viscosity of the fluid. The movement of the polymer fluid through the 
reservoir is adjusted by the viscosity of the fluid. At an appropriate 
viscosity, the polymer fluid will move through the reservoir at the same 
rate as the crude oil. With the crude oil and lower mobility fluid moving 
through the reservoir at the same rate, the risk of the displacement fluid 
flowing through a portion of the crude oil is diminished. The bank of 
lower mobility fluid is commonly referred to as a mobility buffer bank or 
a polymer bank. 
The micellar bank is displaced with a sufficient volume of polymer bank to 
provide a separation between the micellar bank and drive water. Drive 
water can be surface or formation water and is generally the water that is 
most convenient to the enhanced oil recovery site. Drive water is used to 
displace the micellar fluid and polymer bank from an injection well toward 
a production well. In some enhanced oil recovery projects, the drive water 
has a high salinity and is not compatible with the micellar bank. 
Therefore, the polymer bank can provide a separation between the micellar 
bank and the drive water. It can require about 30 to about 100 percent of 
the total pore volume of the portion of the reservoir from which crude oil 
is to be displaced to provide the separation between the micellar fluid 
and the drive water. The volume and appropriate viscosity of the polymer 
bank can be determined experimentally by the previously described 
procedures for determining the appropriate volume of the micellar bank. 
Additionally, reservoir simulation techniques that are well known to those 
skilled in the art of enhanced oil recovery can be valuable tools for use 
in the selection of the volume and appropriate viscosity of the polymer 
bank. 
The formulation of displacement fluid at a salinity to minimize the 
interfacial tension between the displacement fluid and the displaced crude 
oil is illustrated in FIG. 1. These interfacial tension measurements were 
made on samples prepared by mixing about equal volumes of normal decane 
and an aqueous micellar fluid formulated by homogeneously mixing about 90 
parts by volume potable water with about 5 parts by volume isopropyl 
alcohol, about 5 parts by volume petroleum mahogany sulfonate and the 
quantity of sodium chloride shown on FIG. 1. An interface was permitted to 
form between the normal decane and the micellar fluid, and the interfacial 
tension was measured with a spring drop interfacial tensiometer. It is 
illustrated by FIG. 1 that the selection of salinity is important for 
minimizing interfacial tension between oil and micellar fluid. 
As the micellar fluid is being displaced through the reservoir, surfactant 
can be lost to the formation. Loss of surfactant in enhanced oil recovery 
is illustrated in FIG. 2. The surfactant loss, as obtained by material 
balance, is shown for core tests made by flowing four pore volumes of 
normal decane through approximately 1 foot (0.3 meter) long sections of 
approximately 2 inch (5 centimeters) diameter Berea sandstone cores, 
preflushing the cores with about 1 core volume of a preflush brine, 
displacing the oil and preflush with about 0.1 pore volume of a micellar 
slug and displacing the micellar slug with about 2 pore volumes of a 
polymer bank. In these tests, the preflush, micellar bank and polymer bank 
each contained the same concentration of sodium chloride. It is 
illustrated by FIG. 2 that surfactant loss increases in proportion to the 
salinity of the micellar fluid for salinities above about 0.1 N sodium 
chloride. 
The micellar slugs illustrated in FIGS. 1 and 2 were formulated with the 
same constituents and in the same proportions. The polymer slug 
illustrated in FIG. 2 was formulated by homogeneously mixing 1000 parts 
per million Xanflood brand polysaccharide marketed by Kelco Company, 
potable water and the amount of sodium chloride shown on FIG. 2. 
Oil recovery tests were also conducted with the same test procedure and 
sequence of fluids used to obtain the surfactant loss data illustrated on 
FIG. 2. The results of these oil recovery tests are shown in the following 
table. Also listed on this table are the salinities, as normality (N), of 
each of the fluids used in these tests. 
______________________________________ 
Salinity Percent of 
Micellar Polymer Pore Space 
Test Preflush Bank Bank Occupied by Oil 
No. (N NaCl) (N NaCl) (N NaCl) 
at End of Test* 
______________________________________ 
1 0.03 0.03 0.03 29.8 
2 0.03 0.2 0.2 10.0 
3 0.03 0.37 0.37 25.7 
4 0.03 0.03 0.37 14.3 
5 0.03 0.37 0.03 6.8 
6 0.2 0.2 0.2 12.3 
7 0.2 0.03 0.2 11.0 
8 0.2 0.37 0.2 11.3 
9 0.2 0.2 0.37 11.0 
10 0.2 0.2 0.03 3.0 
11 0.37 0.37 0.37 25.3 
12 0.37 0.2 0.2 12.2 
13 0.37 0.03 0.03 8.0 
14 0.37 0.03 0.2 10.8 
15 0.37 0.03 0.37 13.1 
16 0.37 0.37 0.03 2.3 
______________________________________ 
*35 percent of the pore space of the cores used for these tests was 
occupied by oil prior to flowing preflush through the cores. 
A comparison of each test shown on the above table illustrates that 
salinity contrast between the preflush and the displacement fluid provides 
improved oil recovery. The displacement fluid is a combination of the 
micellar fluid and the polymer containing fluid. In each test having a 
salinity contrast between the displacement fluid and the preflush, there 
can be a mixture of the preflush with the displacement fluid which would 
have a salinity of about 0.2 normal sodium chloride. This is the salinity 
that is shown on FIG. 1 to provide the lowest interfacial tension between 
this micellar fluid and oil. In addition to the salinity contrast, it is 
illustrated by Tests 5, 10, 13, and 16, that further improvement in oil 
recovery is obtained by formulating the polymer bank at a salinity below 
0.2 normal sodium chloride, the salinity which exhibits the lowest 
interfacial tension between this oil and micellar fluid. Below this 
salinity for minimizing interfacial tension, it is shown on FIG. 2 that 
surfactant loss is also minimized. 
While certain embodiments of the invention have been described for 
illustrative purposes, the invention is not limited thereto. Various other 
modifications or embodiments of the invention will be apparent to those 
skilled in the art in view of this disclosure. Such modifications or 
embodiments are within this scope of the disclosure.