Note: Descriptions are shown in the official language in which they were submitted.
D#74, 107
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BACKGROUND OF' THE INVENTION
.
Field_of_the Invention
This invention rela-tes to the recovery of oil from
subterranean reservoirs of varying permeability.
DESCRIPTION OF THE PRIOR ART
Since only a portion oE the oil contained in a
petroleum reservoir can be recovered by primary methods, it
has become conventional practice to employ various secondary
and tertiary recovery techniques to produce additional
quantities of oil not economically recoverable by primary
means. One of the various secondary and tertiary recovery
methods available is to displace the oil through the reservoir
with a driving fluid such as water or steam injected for
that purpose. Normally in carrying out the flooding proc~sses,
a series of input wells placed apart from one or more
producing wells are dxilled into and opened to the oil
producing strata. The injection well locations with
reference to the production well are selected to afford a
desired flood pattern. This selected pattern, depends in
part upon field conditions, the location of existing wells,
and the operator's preference. Aqueous drive fluid such as
steam, water, brine, or a viscous, aqueous polymer solution
is forced into the input wells under pressure and through the
surrounding oil bearing strata toward the producing well or
wells. While water flooding has been rather widely practiced
in recent years, it is not without considerable operating
problems and economic limitations, particularly those
associated with low oil recovery in proportion to the amount
of water injected. Various surfactant and solvent fluids
have been proposed as means for recovering additional
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quantlties of oil over that recoverable by conventional
water flooding. Howevcr these processes face seriou~
operating problems when practiced in heterogeneous
formations containin~ strata o~ channels having permeability
substanti,ally higher than the bulk of the formation. Simi-
lar problems occur with steam flooding.
One of the major problems encountered in a flooding
operation is breakthrough of the flooding medium from the
flood front to the producing well relatively early in the
displacement process ~nd rapidly increasing water/oil
ratio at the production well following -the initial break-
through. These difficulties result from displacing the
medium channeling through a portion of the oil bearing
structllre -to the producing well thus bypassing large zones
of the oil bearing strata. The reason for the channeling
of the flooding medium to the producing well and the resulting
low oil recovery is due in part to the peculiar structure
of the oil bearing strata. Underground oil reservoirs in
most cases consist of layers of sand or rock and since no
resexvoir rock is perfectly uniform in composition and
structure, permeability will vary across the rock face or
strata. Also, fractures, cracks, vugs and other anomallies
can promote channeling of the displacement flu.id.
In the normal flooding operation, maximum oil
recovery is obtai~ed when the driven fluid fills up in a
wide bank in front of the driving fluid which moves uniformly
toward the producing well. To keep this bank of oil intact
and constantly moving toward the producing well, a sub~tan-
tially uniform permeability must exist throughout the strata.
If this uniform permeability does not exist or is not pro-
vided, the flooding fluid will seek the areas of high
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permeability~ and channeling occurs with a consequen-t loss
of some driving fluid energy and the app~arance of excessive
amounts of driving fluid in the producing wells. Moreover,
as the more permeable strata are depleted, the driving fluid
has a tendency to follow channels and further increase con-
sumption of the flooding medium to the point where the
process becomes uneconomicalO The maximum limit in terms
of recovered oil can be as high as 100 barrels of driving
fluid per barrel of oil. It is, of course, desirable to
operate at much lower driving fluid to oil ratios and
preferably not more than 15 and normally a maximum of
5 to 10 barrels of driving fluid per barrel of recovered
oil is considered an acceptable operatlns condition, parti-
cularly where the driving fluid is a low cost agent ~uch
as flood water. While a uniform flood front with reduced
fingering can be obtained in some formations with a drive
fluid rendered more viscous by the addition of various water
soluble polymers or other material capable of impartiny
higher viscosity to the 100d water, viscous water flooding
is often ineffective in formations having severe heterogeniety
as the mobility of the flood water cannot be economically
reduced sufficiently to prevent or substantia].ly restrict
channeling o~ the flooding medium. Also, smaller volumes
of aqueous polymer solution are sometimes injected into a
heterogeneous reservoir to divert subsequently injected
flooding media into the less permeable strata, however,
such treatments are often ineffective or only partially
effecti.ve in highly stratified formations. Where a hetero-
geneous formation is flooded, it is usually found that the
flood water or other flooding medium which is introduced
into th~ injection well will enter -the v~rious strata open
to the well at diEferent flow r~tes depending upon the
permeabllity of the individual strata relative to the
permeabilities of other strat:a in the formation. The water
injection well of this type is said to exhibit a non-uniform
injection profile. Heretofore it has been considered that
a non-uniform injection proîile was symptomatic of channeling
and the resulting poor flood medium conformance. Accordingly,
it was believed that selective plugging or other tr~atment
to improve or correct the injection profile would ameliora-te
channeling and result in improved flood water conformance
and increase oil recovery. However, most commercially
practical selective plugging treatments effect the permeabil-
ity of the strata only at the well face or for a few feet
distance from the well. Since adjacent strata are often in
fluid communic~tion throughout all the rest of the substan-
tial part of the formation subjected to flooding, cro~sflow
between the various strata exists and severe channeling
develops even though a substantially uniform injection
proile is obtained. Thus, in many applications, treatment
of the injection wells to improve the water injection
profile has no demonstrabl~ effect on oil recovery. Hence,
need exists for an improved flooding process or use in
heterogeneous formations that will minimize ehanneling of
the floodins medium or bypas~ing at large areas of the
formation, or for a method for improving -the eff~ctivenes~
of the conventional selective plugging process. Accoxdingly,
a principal object of thi~ invention is to provid~ an improved
flooding process for recovering oil from hetergeno~s petro-
leum reservoir~. Another object of this invention is to
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provide an improved method whereby water flooding processes
for recovering oil from reservoirs having stra-ta of widely
varyins permeability may sweep the ma~imum amoun-t of reservoir
possible and not channel into the highly permeable ~ones~
SU~IMAXY OF THE INV NTION
The invention is a method of reducing the permeability
of a sub-terranean strata comprising contacting in situ an
aqueous slurry of colloidal silica and a surfactant to form
a gel.
In one aspect of this invention there is provided
a method for reducing the permeability of subterranean strata
comprising
(a) injecting a slug of colloidal silica which is fumed
silica made up of chainlike formations sintered together; which
chains are branched with surface areas of from about 50 to
about 400 meters2/gms and water into the reservoir, and
(b) following the slug of colloidal silica and water
with a surfactant to form a gel on contact with the colloidal
silica slug.
In another aspect of this invention there is pro~ided
a method for reducing the permeability of a subterranean strata
comprising
(a~ injecting a slug of surfactant into the reservoir and
(b~ follo~ing the slug of surEactant with a slug of
colloidal silica which is fumed silica made up to chainlike
formations sintered together, which chains are branched with
surface areas of from about ~0 to about 400 meters2~gms and
water to form a qel on contact with the surfactant slug~
In a further aspect of the invention there is provided
a method of reducing the permeability of a subterranean strata
comprising
contacting in situ an aqueous slurry of colloidal silica
which is f~ed silica made up of chainlike formations sintered
together, which chains are branched with surface areas of f~om
about 50 to about 400 meters2/gms and surfactant to form a
gel.
DESCRIPTION OF T~E PR~E'ERRED EMBODIMENTS
The invention is a method for plugging zones or strata
of very high permeability in heterogeneous reservoirs thus
preventing the flow of fluid into the zones of high permeability
so that fluid may be injected into the zones of lower
permeability. The invention is useful in a situation where a
flooding medium such as water or steam is to be driven from
an injection well to a production well driving oil before it;
where highly permeable zones and heterogeneous reservoirs are
introducing water into the production well at very high rates
thus preventing an economical ratio of oil to water in the
producing well. The method of my invention will plug zones
of very high permeability, thus allowing the fluids to flow into
the zones of lower permeability without competition from the
higher permeability zones.
Briefly, my invention is forming a gel in situ in the
zones of higher permeability thus plugging them and allowing
the fluid to flow in the zones of lower permeability. This
gel may be formed by combining in situ an aqueous slurry
, - 5(a) -
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of colloidal cilica and a suxfactant. When mixed the aqueous
slurry ol colloidal silica and surfactant will form a gel
which encompass as much of the zone of high permeability
as is deslred.
Water and colloidal silica may be mlxed toge-ther
to form a fluid which does not manifest a very high viscosity.
However; upon contact with a surfactant this .~queous colloidal
silica ~ixture will form a gel. Cvlloidal silica useful
in my invention is different from precipitated silica or
silica gel. Colloidal silica useful in my invention is a
fumed silica which is made up of chain-like formations
~e~ter~d together. These chains are branched and have
enormous external surface areas of from about 50 to about
400 meters /gm. Each segment of the chain has many hydroxyl
(OH) groups attached to silicone atoms at the surface. When
the segments come into proximity to each other, these
hydroxy groups will bond to each other, by hydrogen bonding
and form a three-dimensional network. Colloidal silicas
are readily commercially available. One source is the
Cabot Corporation of Boston, Massachusetts under the trade
name CAB-O-SIL. Colloidal silica useful in my invention
is not restricted to this particular product but is defined
generally in U.S. Patent 2,993,809. When colloidal silica
particles are dispcrsed in aqueous media wherein no other
polar substituents are present, viscosity of the solution
will not increase greatly over that of water. However, it
is known that the thickening efficiency of the ~ilica
directly relat~d to the polarity of the liquid to be
thickened. The u~e of selected additives such as surfactants
and/or multi-functional compounds can increase the thickening
efficiency oE the silica many times. It is contemplated
~6315;~77~
in -the method of my invention to use a surfactant which
may broadly be defined as any compound which reduces surface
tension of water. Soap may be used. For example, the
sodium salts of high moleculaLr weight alkyl sulfates or
sulfonates are acceptable. Also very useful are nonionic
surEactants which are usually a reaction production of a
hydrophobic and hydrophilic material such as the reaction
product between mixed monoalkyl phenols and ethylene oxide.
Multi-functional compounds may be used to aid in forming the
gel of my invention. The water portion OL the slug of my
invention which initially carries the colloidal silica into
the reservoir may be fresh water or water containing various
quantities of inorganic salt such as brine.
My invention may be carried out in different ways;
however, the basic aim of my invention is to form in situ
a gel which will completely stop fluid flow wherever the
gel is formed. This objective may be accomplised by
various techniques. Por example, in one technique, an
aqueous slurry of colloidal silica may be pumped into a
heterogenous reservoir. This aqueous slurry of colloidal
silica will not have a large viscosity and will seek
the hiyhly permeable zones and completely saturate them
to any desired depth. The colloidal silica slurry will then
cease to be injected and a surfactant solution will be
injected. This surfactant solution will follow the same
path that the colloidal silica slurry fol]owed before it.
The inter-mlxing of these fluids within the reservoir will
take place almost exclusively within the highly permeable
20nes where most of the colloidal silica has heretofore
gone. Upon contact with the colloidal slurry the
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surfactant and the colloidal silica will form a gel, This
gel will completely plug the matrix of the reservoir in
the highl~ permeable zone and will stop any further fluid
flow from taking place into these ~ones. The fluid flow may
then take place into the les,s permeable zone to obtain a more
uniform sweep of the reservoir~ In another technique, the
colloidal silica slurry and surfactnat solutions may be
contacted in situ by injecting several alternating small
slugs of colloidal silica slurry and surfactant solution.
This intermittent small slugs will selectively seek out
the highly permeable zones and by injecting alternating
slugs of surfactant solution and colloidal silica slurry,
better mixing may be accomplished in some cases. In any
event, a gel will be formed in this case also.
In one embodiment of my invention, it is envisioned
that -this technique will be used where there is to be a
supplemental recovery operation i.e. a water flood or steam
flood. In this situation, it is desired to direct the
driving fluid into the less permeable zones so that the fluid
is not lost in the highly permeable zones, thus bypassing
great amounts of oil in the zones of lower permeabiltiy. It
may be desirable to first plug off the highly permeable zone
with the gel of my invention and then to proceed with the
flooding medium, or alternatively, in some cases it may be
desirable to sweep the highly permeable zones of the oil
and hydrocarbons that they may contain and then to plug them
off with the gel of my invention before proceeding with
the water or steam flood operation in the zones of lower
permeability. In another embodiment of my invention, i~
is contemplated that excessive water production in a
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production well may be stopped or substantially curtailed.
Ln this situation it is usually the case that a zone of high
permeability n~ar the production well is producing water at
an inordinate rate compared with zones of lower permeability
which may contain oil. This is sometimes called bottom
water coning. In this case it may be uneconomical to produce
the high water/oil ratio mlxture. In the method of my
invention, the colloidal silica water slurry may be pumped
into the production well and into the foxmation for as great
a distance as is desired to be followed with a surfactant
solution which upon contact with the colloidal silica
aqueous slurry will form a gel thus stopping the water flow
from the highly permeable zones and allowing oil flow
from the less permeable zones to take place. The effect of
this will be to decrease the water/oil ratio and increase
the economical aspects of the production well. In this
embodiment of my invention, alternating small slugs of
colloidal silica and water and surfactant solution may also
be usedO
Preparation of the Fluids
Colloidal Silica Slurry
To a suitable vessel is added fifty barrels of
water. Two percent by weight colloidal silica is added to
the water with stirring to prepare a slurry. Shear the
slurry to affect deaglomeration using a Cowles high speed
mixer. Store the slurry with low speed mixing to prevent
settling and us~ within 48 hours.
Surfactant Solution
To a suitable vessel is added fifty barrels of
water. One percent by weight of a 9.5 mole propylene oxide
_g_
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adduct of nonyl phenol is added to the water with stlrring.
ctore the surfactant solution with low speed mixing.
Us_ of the Fluids
Using core data and well logying data, determine
the location and approximate pore volume of a highly
permeable thief zone which exists in the same reservoir with
zones of lower permeability. Inject in the thief zone 0.1
thief zone pore volume of the surfactant solution. Then
inject 0.1 thief zone pore volume of the colloidal silica
slurry. Alternate injection of surfactant and colloidal
silica slurry until a fluid entry survey shows that an
improvement in injection profile has been achieved. How the
thief zone is plugged or reduced in permeability and injected
fluid (steam, water, etc.) will enter the zones of lower
permeability.
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