Note: Descriptions are shown in the official language in which they were submitted.
~57~
PLUGGING HIGH PERMEABILITY E~RTH STRATA
Background of the _v ntion
This invention relates to a process for plugging highly permeable
earth strata encountered in a well bore. In one of its aspects this -lnvention
relates to a method of plugging earth strata by forming therein a foam having ~ ~
1 a condensible gas as its gas phase. In still another aspe~t, it relates to ~ ~ ;
; a method of producing a well by cyclic steam stimulation or direct drive ~
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~i~ steam injection by plugging the high permeability strata with a foam havlng ~ ~ ;
a condensible gas as its gaseous phase. In yet another aspect, it relates
to closely controlling the back pressure of steam injected into a high
` 10 permeability strata by introducing foam into the strata which has as its
gas phase a gas condensible at temperatures and pressures obtained within
I the strata.
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~`~ In oil well production, it is quite common to inject various gases,
; including steam, into oil wells to increase the production of oil from those ~ ~
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wells. One common method of increasing production ls the cyclic steam
stimulation me~hod, wherein production of oil from the well is periodically -
interrupted and steam is injected in the well. The steam serves to melt
~ ; some~of the waxe~ in the~strata, and reduce the viscosity of the oil
i surrounding the well bore so that it will flow more readily into the well
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bore and be produced therefrom. A second me~hod is the diract drive injection
of steam or other gases continuously into one well whereby oils in the various
I earth strata are pushed ahead of the steam or gas being injected and are
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produced from another nearby well. ~ ~`
One of the problems faced in either type of injection of gases or ~ -
steam into the well arises Erom the varying permeability of the different
earth strata. ~here there is a considerable difference in the permeability
` ~, of different strata, the injected gas will flow into the more permeable strata
preferentially, or on occasion almost exclusively. Since the oil to be
produced may be very largely in the less permeable strata, a considerable
~`1, 30 quanti~y of gas or steam will be injected into the well with little success.
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` A method for plugging a highly permeable formation with a foam that
upon condensation of the steam lntroduced as the gaseous phase of the foam
collapses to restore the permeability of the strata plugged as ~set forth in
! U.s. 3,412,793 in which a small amount of a surface-active agent is inJected
dlrectly into the steam line at the surface of the well to be carried in a
solu~ion in the small amount of condensed water in the steam lnto ~he well
where upon reaching the permeable sand or rock strata, the surface-active
agent enters that strata and is foamed by the action of steam pushing through
the surfaces of the strata. The foam produced is relatively stable only so
long as the temperature of the foam remains above the bolling point of water
at the reservoir pressure. When the temperature of the foam drops below the ;
, boiling point the foam automatically collapses and the blocked zones recover
j their original permeability. ~.
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~ I have discovered that by alternating the injection of steam and
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'~'!,' slugs of a surface-active agent into the highly permeable formation a high
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, degree of control of the back pressure of the steam used in producing the
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`' well can be attained. ~,
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`~ It is therefore an object of this invention to provide a process for ~-
producing a well by plugging more permeable strata with a self-destructive
~foam.l It is another object of this invention to provide a method for ~;
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; controlling the steam back pressure within a producing well while temporarily
~, plugging permeable strata.
~, Other aspects, ob~ects and the various advantages of this invention ;~
l~ will become apparent from a reading of this specification and the appended
,l~ claims.
~!l Statement of the Invention
In accordance wieh ehls invention a method is provided for
controlling the back pressure of seeam injected into a highly permeable
formation in thelproduction of an oil bearing formation by steam injection.
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~l~ 30 By this method steam~is injected into the formation and then the steam flow
,~i is interru~ted and a elug of surface-active agent dissolved or dispersed in
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; a suitable solvent or carrier is in~ected into the formation. By repeating
the in~ection of steam, the interruption of this inJection and the in~ection
of surface-active agent a foam is continuously produced and driven into - -
the formation in a sufficient amount to maintain a desired back pressure on
the steam that is in~ected into the formation.
~' In a specific embodiment of the invention surface-active agent
~ can be injected simultaneously with the injection of steam and with the
1 interruption of the steam in~ection the in~ection of surface-active agent
is continued. This embodiment can be stated as in~ecting steam and surface-
active agent simultaneously with the interruption of this injection to ;~
allow the iniection of a slug of surface-active agent.
~; It is quite common for various strata in a formation to have
~ permeabilities sufficiently clos~ to each other so that some amount of foam
', will be formed in the less permeable strata. When foam enters both types
:~ of strata the permeability of the more permeable stratum is reduced to a
, greater degree than that of a less permeable stratum. One of the factors
causing this is a recogni~ed phenomona that foam will reduce the effective -~
permeability of a high permeability sand to a larger extent than it wlll
affect a low permeability sand. Also, the more ready flow of steam into the -
more permeable stratum causes a larger heat input lnto that stra~um compared
to the heat loss from that stratum than there is in a less permeable sand.
This results in a higher quality steam injected into the higher permeability
stratum, and foam produced by this high quality steam will be longer lasting
because it must lose more heat before the foam will collapse. The high steam ~;
flow rate into the high permeability stratum during the initial injection
of the foaming agent will carry the foaming agent to a greater depth in the
! high permeability zone and, therefore, form a much thicker wall of foam in ~-
`~1 that zone than is formed in the less permeable zones. It can be seen that
~, the invention is workable in strata of any relative permeability.
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Among the advantages of interrupting steam in~ection and then
injecting surfactant so that there is a pattern of alternate steam and
surfactant injections is that the back pressure of the steam in the formation
can be observed and from this back pressure it can be determined lf the
foam is forming as desired from the surfactant as it is being injected. It ~ ;
is apparent that by observing the back pressure the injection of surfactant
can be tailored to the particular formation being produced. This tailoring
of the injections can be carried through the whole period of time within
which the well is being produced by steam injection. After steam and
surfactant have been alternately in~ected for a period of time it will
~ become apparent by the back pressure of the steam whether the foam is
``~ breaking down. If the foam is breaking down, steam injection can be
in~errupted and additional surface-active agent can be injected to increase
the foam in the formation as needed to maintain the back pressure of the
steam at a certain desired level.
Although the invention has been described with respect to the use
of steam as the condensible gas, it should be evident that any gas which is ~ ;~
~i condensible at the temperature and pressure conditions in the particular
~` strata should be equally useful to produce a temporary, self-destructive
`-' 20 foam. Similarly, small amaunts of non-condensible gases can be mixed with
. the condensible gas to vary the length of life of the foam. In general,
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however, because of the low cost and ready availability of steam the invention
would be practiced with steam alone. ~;
Various types of surface-active agents could be used in the process
of my invention, either nonionic, anioAic, or cationic. Commercial surface-
active agents of the alkyl phenoxy polyethoxy ethanol class and commonly
available household cleansers have been tested and found satisfactory in
the practice of my invention. For example, Trend*detergent manufactured by
Puréx Corporation, Ltd., has proved satisfactory, as well as other household
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cleaning compounds, hand and laundry soaps, and rug shampoos. ~-
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~ Nonionic type surface-actlve agents such as Triton X-100 and Igepal
; C0-990 are examples of the alkyl phenoxy polyethoxy ethanol class which are
satisfactory in the practice of my lnvention. Anionic type surface-active
agents such as Alconox and Trend are examples of the alkyl aryl sulfonate
class usable in the practice of my inventlon. ~thomeen 18/60 and Arquad
C-50 are examples of cationic surface-active agents which are usable in
carrying out my invention, and are identified chemically as stearyl amine
polyethylene oxide and n-alkyl trimethyl ammonium chlorlAe, respectively.
To practice this invention it is desirable to in~ect a liquid
` lO solution or dispersion of the surfactant, or mixture of surfactants which
cause the steam to foam. Since a wide range of conditions exist at the
well-head and a wide range of foa~ing surfactants are usable, it is also
within the scope of the invention to have a wide range of carrier liquids.
Liquids which are frequently used in the marketing and transportation of
surfactants are alcohols, oils and water or mixtures thereof. Such fluids
can also be used to advantage in conjunction with the steam foam producing
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surfactants to clean some of the mineral and organic deposlts from the well ;
although the main function in this invention is as a carrier for the
surfactant. It is also desirable to have a liquid which does not become
~, 20 viscous or freeze at winter conditions.
Alcohols containing 3 to 8 carbon atoms can be used with the
surfactants which are injected into the well. Hydrocarbon oils having a
boiling range of 250 to 850F can serve as the liquid carrier for the
surfactants. Water substantially free of salt is used in the steam generators.
However, the water used as a carrier for the surfactants may be salt free or
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contain up to about 5 weight percent of salt. Produced water containing 2-5
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~ ~ weight percent salt can be employed.
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~ To illustrate the benefit of in~ecting the surfactant solution from
; which the foam is produced instead of injecting a substantial amount of steam
with the sur~actant solution, consider the two cases presented in the follow-
ing example. Using a readily available surfactant such as sodium linear
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alkalate sulEonate (Stepan Agent Bio Soft D-40) which i8 a detergent base,
requires about 10,500 pounds or two zones each fLve feet thick with the well
spacing being 5-acre~ The concentration would be about 3 percent for the
first 2500 pounds oE surfactant in about 240 barrels of water, and 1 percent
for an additional 8000 pounds in 2300 barrels of water. In fresh water such
compositions are known to produce foam readily upon agitation and the surfactantis known to be thermally stable at temperatures above 325F, a temperature
used in many steam applications in oil recovery in Californla.
EXA~LE
Consider two zones of equal pore volumes having permeabillties of
225 millidarcys and 4000 millidarcys. Using the laboratory data presented on
; pages 268 and 269 of "Effect of Foam on Permeability of Porous Media to Gas"
~-^ by George C. Bernard and L. W. Holm in Soc. of Pet. Engr. Jour., Sept., 1964,
pp. 267-274, the following two cases were calculated.
Case A: Sufficient surfactant solution and steam are injected to
fill 10 percent of the pore volume of both of the zones. Before the injection
o the surfactant soIution and steam, because of the permeability difference, ~`
about 95 percent of the steam injected will go into the more permeable zone
and only about 5 percent will enter the less permeable zone. After injection
~1 20 of the surfactant solution with the steam, because of the increased back
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~ pressure resulting from the presence of the foam, subsequent steam injectlon
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-` will distribute about 35 percent into the high permeability (4000 md) zcne
~ and 65 percent into the lower permeability (225 md) zone.
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, Case B: The same amount of surfactant solution as in Case A is
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injected to fill 10 percent of the pore volume of both zones with foam being
produced upon the subseqoent injection of steam. Before the surfactant
solutlon was injected the injected steam would distribute about 95 percent
into the more permeable (4000 md) zone and about 5 percent into the less
' l ~ permeable (225 md) zone. Since the surfactant solution would distribute in
' 30 a similar manner, about 95 percent of the solution will enter the most
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permeable ~one and about 5 percent will enter the less permeable zone. After
the lnjection of steam to create a foam, the back pressure will increase and
the continued steam injection will be distributed 15 percent into the high
permeability (4000 md) zone and 85 percent into the less permeable (225 md)
zone.
It can thus be seen that when the surfactant solution is in~ected
without the accompanying steam, the less permeable zone takes 85 percent of
the subsequent steam injection; whereas, when the surfactant solution and
steam a~e injected simultaneously, the less permeable zone takes only 65
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10 percent of the subsequen~ injection of steam. This improvement results ~
because the presence of foam decreases the permeability of higher permeability ~ -
porous media to a greater extent than lower permeability porous medla.
In Case A the volume of foam which is generated in the more
permeable zone is limited by the generation of foam before the in~ection of
the surfactant solution is completed. The use of the injection procedure
of Case B results when this invention is applied. The stea~ back pressure -
resulting from the injection sequence of Case B would be expected to exceed `
the reservoir pressure by an amount about 2 to 3 times that obtained in Case A.
For example, if the back pressure in Case A exceeded the reservoir pressure
by 200 psi, then in Case B the steam back pressure would be expected to
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' exceed the reservoir pressure by 400 to 600 psi. ;"~ ' ~
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SUPPLEMEiNTARY DISCLOSURE
Among the various types of sur~ace-active agents that can be used -~
in the process of this invention a preferred type includes the nonlonic
surface-active agents having cloud points above 200F (93C~ - as set out
in Nonionic Surf~ctants, 1967, edited by Martin J. Shlck, Also preferred for
use in this invention are commercial nonionic surface-active agents of the
alkyl phenoxy polyethoxy ethanol class containing 10 to 100 mol ethylene- :
oxide per mol of surfactant. Some specific surface-active agents useful in
: the invention are Igepal C0-880*, an example of the alkyl phenoxy polyethoxy
ethanol class, and such anionic surfactants as Biosoft B-40* manufactured by
, the Stepan Chemical Company, Sulframin 45* manufactured by Witco Chemical
Company, and Siponate DS-4* manufactured by Aloolac~ Inc.
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