Note: Descriptions are shown in the official language in which they were submitted.
CA 02349998 2001-06-05
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REINFORCED CLAY GEL
FIELD OF THE INVENTION
The invention relates to a method for controlling the flow of a fluid through
high
permeability strata, fractures and high permeability channels (wormholes), in
a subterranean
formation. An example is controlling the influx of water during the recovery
of hydrocarbons
from geological formations. In particular, the invention relates to chemical
gel systems
containing a reinforcing agent, (e.g., produced sands) for permeability
modification of such
formations.
BACKGROUND OF THE INVENTION
During the recovery of hydrocarbons from subterranean formations, significant
amounts
of hydrocarbons are left behind because injected or natural drive fluids in
the formation are
produced along with the oil to such an extent that the cost of fluid disposal
makes further oil
recovery uneconomical. In formations with high permeability strata, fractures,
or high
permeability channels (wormholes), natural drive fluids (such as brine or
gaseous
hydrocarbons) in primary recovery processes or flooding fluids (such as brine,
steam or carbon
dioxide) in secondary recovery processes flow through highly permeable zones,
resulting in
progressively less hydrocarbon being recovered per unit volume of fluid
produced. This
increased ratio of drive or flooding fluid to hydrocarbons is usually due
either to early
breakthrough of flooding fluid from injector wells to producer wells, or to
excessive water
encroachment into producer wells. It has adversely affected the economics of
recovery
processes in many parts of the world. For example, it was recently estimated
that in the United
States, 7 barrels of water are produced for each barrel of oil, amounting to
2.1 x 10'° barrels of
water annually. In Alberta, Canada, the ratio of water/oil produced is 6/1,
amounting to 3.0 x
109 barrels of water produced in 1997.
The control of fluid flow in subterranean formations is commonly referred to
as
"conformance control". For the past two decades, research has been directed at
improving the
oil/water ratio during hydrocarbon production by using chemical gel systems to
block water flow
CA 02349998 2001-06-05
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through high permeability zones, fractures and high permeability channels
(referred to herein as
"high permeability regions"). The general approach has been to inject a
mixture of reagents,
initially low in viscosity, into regions of a formation which have high
permeability. Once the
mixture of reagents has reached its destination in the desired region of the
formation, it then
undergoes a chemical reaction to produce a gel which is capable of blocking
the flow of water.
Polymers, chemical gels, silica gels, and other blocking agents have been used
in this way for
conformance control in geological formations.
Ideally, a gel system for conformance control should have the following
properties:
1. The reagents should be easily delivered to the desired location in the
formation. The components therefore should be initially of low viscosity.
No component should be adsorbed out prior to reaching its destination,
and each component should be stable to shear stress encountered during
delivery.
2. The chemical reactions) required for gelation under the conditions found
in the formation.
3. The gel generated should be of high strength under the conditions found
in the formation or its strength reinforced with a readily accessible, low
cost reinforcing agent.
4. The degree of permeability reduction should be high.
5. The system should be of low enough cost to make it economically
feasible.
6. The system should have minimal environmental impact.
All of the chemical gel systems currently available for conformance control
have the
drawback of being so costly that their use is limited. Examples of existing
gel systems are:
1. Polyacrylamide copolymers which are injected together with a cross
linking agent, and a chromium (III) or aluminum (III) compound;
2. Xanthan gum (a natural heteropolysaccharide) which together with a
cross-linking agent, and a chromium (III) compound;
3. Poly (Vinyl alcohol) which is injected together with a cross-linking agent,
and gluteraldehyde; and
4. Acidified sodium silicate, which when neutralized, rapidly undergoes
polymerization to form spherical silica particles.
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The most widely used method of these involves use of polyacrylamide cross-
linked with
chromium ions. Its use is limited by its cost: 1 m3 polyacrylamide costs about
200 to 500 USD,
while typical applications use from about 20 m3 to about 300 m3. It is also
relatively unstable
under the elevated temperature conditions which exist in deep geological
formations or
reservoirs undergoing thermal recovery processes. Furthermore, chromium (VI),
the oxidation
product of chromium (III) is highly toxic, so the use of a chromium (III)
compound as a cross-
linking agent can be an environmental concern.
Another application for a conformance control gel system is to block permeable
regions
to contain a fluid within a certain region. This may be particularly
applicable to the disposal of
fluids in a subterranean formation, i.e. to reduce the flow of a disposal
fluid into other regions.
There is therefore a need to develop further conformance control gel systems
which are
environmentally safe, inexpensive and effective under the conditions
encountered during
hydrocarbon recovery.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method is provided for method for
controlling
the flow of at least one fluid in a subterranean formation having at least a
first region, said first
region having (i) at least Na+, and (ii) a first permeability, K,, with
respect to said fluid, said
method comprising: (a) making an inhibitive electrolyte solution having water
and at least one
inhibitive compound, said inhibitive compound having at least one ration and
anion; (b) making
a clay/reinforcing agent slurry by mixing at least about 15 weight percent of
a swelling clay with
said inhibitive electrolyte solution so that, to the extent clay gel is
produced, if any, the flowability
of said slurry is not substantially inhibited and mixing a reinforcing agent
in the range of from
about 10 weight percent to about 60 weight percent; (c) injecting said
clay/reinforcing agent
slurry into said formation, so that at least a portion of said slurry contacts
said first region; (d)
allowing a reinforced clay gel to form in said first region so that K, is
reduced to produce a lower
permeability, K,~, with respect to said fluid; and (e) controlling the flow of
at least a majority of
said at least one fluid into or from said first region.
In accordance with the present invention, there is also provided method for
controlling
3o the flow of at least one fluid in a subterranean formation having at least
a first region, said first
region having (i) at least Na+, and (ii) a first permeability, K,, with
respect to said fluid said
method comprising: (a) making a first inhibitive electrolyte solution having
water and at least one
inhibitive compound, said inhibitive compound having at least one ration and
anion; (b) treating
a swelling clay with said first inhibitive electrolyte solution; (c) making a
clay/reinforcing agent
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slurry having at least about 15 weight percent of the treated clay of step (b)
and a second
inhibitive electrolyte solution having water and at least one inhibitive
compound and mixing a
reinforcing agent in the range of from about 10 weight percent to about 60
weight percent; (d)
injecting said clay/reinforcing agent slurry into said formation, so that at
least a portion of said
slurry contacts said first region; (e) allowing a reinforced clay gel to form
in said first region so
that K, is reduced to produce a lower permeability, K,~, with respect to said
fluid; and (f)
controlling the flow of at least a majority of said at least one fluid into or
from at least said first
region.
DESCRIPTION OF THE DRAWINGS
In drawings which illustrate embodiments of the present invention,
Fig. 1 is a graphical representation of the cumulative weight of effluent
produced over
time with an applied pressure for Test #1 of Example 4;
Fig. 2 is a graphical representation of the cumulative weight of effluent
produced over
time with an applied pressure for the Control sample and Tests #2 and 3 of
Example 4; and
Fig. 3 is a graphical representation of the cumulative weight of effluent
produced over
time with an applied pressure for Test #4 of Example 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In accordance with the invention, a method for controlling fluid flow through
permeable
regions, particularly high permeability regions, in subterranean formations is
provided, using
swelling clay gels comprising a reinforcing agent (hereinafter referred to as
"reinforced clay
gel"), wherein such reinforced clay gel can swell in situ to reduce the
permeability of a region
where the clay gel is introduced. In particular, fluid in a region of a
subterranean formation
having a first permeability K, can be blocked with the reinforced clay gel
such that the
permeability to the fluid is reduced to a lower permeability K,~. Thus, the
flow of fluid in that
region of the subterranean formation can be reduced or blocked.
By "swelling clay" we mean a clay mineral that, when exposed to fresh water or
brine,
can swell from at least two times to many times the volume of the air-dried
clay. The terms clay
or clay minerals as used herein refers to minerals having unique
physicochemical properties,
such as those described below. Generally, clay minerals consist of hydrous
layer silicates that
comprise a large part of the family of phyllosilicates.
To more fully appreciate the types of clay minerals and their physicochemical
attributes,
useful for practicing the claimed invention, we will briefly describe some
common structural and
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chemical characteristics of clay minerals. Broadly described, clay minerals
consist of positively
charged central ions (i.e., rations), such as for example, without limitation,
silicon (Si), aluminum
(AI), iron (Fe) or magnesium (Mg), bound in a coordination polyhedron that is
either a
tetrahedral or an octahedral arrangement with negatively charged ions (i.e.,
anions) of oxygen
(O) and hydroxyl (OH) groups. These tetrahedral and octahedral arrangements
are networked
to form tetrahedral ("T") sheets and octahedral ("O") sheets.
Tetrahedral sheets are networks of tetrahedra typically having Si ration as a
central ion,
although other rations may be present (e.g., without limitation, Fe3+ or
AI3+), and oxygen atoms
forming the four tetrahedral corners. The individual tetrahedra are connected
with adjacent
1o tetrahedra by sharing three corners (i.e., three oxygen atoms). The fourth
tetrahedral corner
points in a direction normal to the T sheet.
Octahedral sheets are networks of octahedra usually, but not always, having a
ration
substantially at the center of each octahedron, for example, without
limitation, AI3+, Mg2+, Fe2+ or
Fe3+, and oxygen atoms and hydroxyl groups at the eight corners. The
individual octahedra are
~ 5 linked laterally with the neighboring octahedra, and vertically with the
tetrahedra, by sharing
oxygen atoms.
The structure arising from the assembly of tetrahedral and octahedral sheets
is called a
structural layer. The two primary types of structural layers recognized by
those skilled in the art
of clay mineralogy are a 1:1 layer or TO layer and 2:1 layer or TOT layer. A
1:1 or TO layer
20 consists of an assembly of one tetrahedral sheet with one octahedral sheet,
while a 2:1 or TOT
layer consists of an assembly of two tetrahedral sheets with an octahedral
sheet sandwiched
therebetween. In the case of a TOT layer, the relative disposition of both
tetrahedral sheets is
inverted so that all apical oxygen atoms point toward the octahedral sheet and
can be shared.
The type of swelling clay minerals useful in practicing the invention belong
to the 2:1 or TOT
25 layer type minerals. For example, the general groups of clay minerals known
as smectites or
vermiculites belong to the 2:1 or TOT layer type.
The intervening space between two successive layers, whether such layers are
TO or
TOT layers, is typically refer-ed to as an interlayer. If the TO or TOT layers
are electrostatically
neutral (i.e., all structural central rations are compensated by OZ' or OH-),
the interlayer will lack
30 any rations or anions for providing an overall charge balance. However,
many clay minerals
possess a negative charge imbalance in their layer structure. For example, the
TOT layers in
smectites typically have a negative charge imbalance of about 0.3 to about 0.6
per O,o(OH)2,
while the TOT layers in vermiculites typically have a negative charge
imbalance of about 0.6 to
about 0.9 per O,o(OH)2. In the case of both smectites and vermiculites, the
TOT layer negative
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charge imbalance is offset by rations, such as, for example, without
limitation K+, Na+, Mg2+ and
Ca2+, found in the interlayer, which are typically, but not always, hydrated.
In turn, the charge imbalance in the layer structure is a good indicator of
the cation-
exchange capacity of the clay mineral. Generally, as the negative charge
imbalance decreases,
the interlayer ration's attraction for the mineral's layer structure
decreases, and hence the
interlayer ration becomes comparatively easier to displace. Accordingly,
smectites, which, on
average, have a lower negative charge imbalance than vermiculites, have a
greater tendency to
swell than vermiculites.
In any case, a clay mineral's ration-exchange capacity can be used to control
its
swelling ability when exposed to water. For example, Na+, which facilitates
the adsorption of
water in the interlayer, can be used to displace a ration, such as, for
example, without limitation,
Ca2+ or K+, which inhibits the amount of water adsorbed into the interlayer,
and hence clay
swelling, relative to Na+. This is why certain smectites, such as sodium
montmorillonite (also,
commonly referred to as sodium bentonite or Western or Wyoming bentonites)
have notably
higher swelling capacity than calcium montmorillonite (also, commonly referred
to as calcium
bentonite or Southern or subbentonites). Consequently, it is preferable to use
sodium swelling
clays (e.g., sodium smectites or sodium vermiculites) in practicing the
claimed invention,
because of their comparatively greater ration exchange capacity and swelling
capacity.
Suitable swelling clays for use in the claimed invention are smectites and
vermiculites.
Examples of suitable smectites are, without limitation, montmorillonite,
beidellite, nontronite,
hectorite, saponite, sauconite and laponite.
It should be noted that, in earlier literature discussions of clay minerals
and mineralogy,
the term montmorillonite was used for both the group name (now referred to as
smectite) for the
general class of 2:1 or TOT clay minerals that carry a negative layer charge
imbalance and
characteristically expand when solvated with water and/or alcohols, as well as
a particular
species of the group. As used herein, however, the term montmorillonite is
used to refer to a
particular species of the smectite group.
Bentonite is a rock rich in montmorillonite and which may be comprised of
other
smectites as well as other non-clay mineral constituents. Consequently,
montmorillonites or
their mixtures with other smectites are often referred to simply as bentonite.
Mixed-layer
minerals, for example, without limitation, illite-smectite, illite-chlorite-
smectite and illite-smectite-
vermiculite, may also be suitable clay minerals for use in the claimed
invention, provided the
such mixed layer minerals have at least 60 weight percent smectite and/or
vermiculite on a dry
weight basis.
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By "reinforcing agent" we mean non-swelling particles. Examples of suitable
reinforcing
agents include particles of produced sand, silica sand, sandy or silty soil,
crushed rock,
minerals, mine tailings, and combinations thereof. Examples of suitable
minerals include
quartz, feldspar, bauxite, and combinations thereof. Preferably, the
reinforcing agent has an
average largest dimension, e.g. diameter, in the range of from about 20 p,m to
about 3 mm. As
the concentration of reinforcing agent increases, the average fluid layer
thickness between
reinforcing agent articles decreases. Thus, the shear rate within the fluid
layer increases as the
reinforcing agent concentration increases because the reinforcing agent
particles cannot
deform. Accordingly, the viscosity increases and a higher pressure is required
to deform a clay
1o gel as the reinforcing agent concentration increases.
In a preferred embodiment, the reinforcing agent is sand produced, for
example, in cold
production of heavy oil from unconsolidated oil sand. Cold production involves
simultaneous
production of heavy oil and sand, and, under certain reservoir conditions,
production of sand
leads to formation of wormholes, ranging from tens to hundreds of meters in
length, emanating
from the wellbore. Such production of sand causes two major operational costs.
First, the large
quantity of sand must be disposed of. Second, wormholes often break into water
zones
resulting in a significant reduction in oil production with a significant
increase in water
production. Accordingly, a clay gel reinforced with produced sands provides a
means for
disposal of produced sand. As well, the reinforced clay gel can be used to
block the wormholes
created by the produced sand. Produced sands can be "cleaned" to extract any
residual oil
therefrom prior to being added to the clay slurry. Alternatively, the produced
sand can be used
in a naturally produced state.
As discussed above, the swelling clays, in their natural state, are associated
with
rations, such as sodium ions, in the interlayers which provide hydration and
clay swelling. In
the method of the present invention, the swelling clays are treated with an
inhibitive ration which
displaces the naturally-occurring rations in a ration exchange reaction.
Examples of suitable
inhibitive rations include K+, Cs+, Ca2+, Mg2+, Fe2+, AI3' and NR4+, where
each R can
independently be H, CH3, CgHS, or CH2CH3 and combinations thereof. The most
effective
inhibitive ration is believed to be K+. The identity of the anion in the
inhibitive salt is not known
3o to be of any importance. For example, the K+ ration could probably be
provided in the form of
KCI, KN03, K2C03, or another salt containing K+ as the ration. A particularly
suitable source of
K+ ions is KCI.
As used herein, "inhibitive compounds" are compounds having the "inhibitive
rations"
discussed above.
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Swelling clays do not swell significantly when contacted with such inhibitive
rations,
which bind to the clay particles and reduce the extent to which a clay
mineral's interlayer can be
hydrated. Therefore, swelling clays can be dispersed in an aqueous solution
containing an
effective concentration of an inhibitive ration to form a highly concentrated
slurry that is
somewhat viscous. The slurry can be poured or injected into a subterranean
formation.
Sodium ration, Na+, which is present in the brine of many subterranean
formations as
well as in injection fluids, is not an inhibitive ration. Generally, as
discussed above, Na+ tends
to maximize the extent to which a clay mineral's interlayer can be hydrated
and thereby
enhance clay swelling. When swelling clay slurries, which have inhibitive
rations in their
interlayer are contacted with Na+-rich fluids, a ration exchange reaction
occurs, whereby Na+
displaces the inhibitive ration. For example, if the inhibitive ration is K+,
the K+ ions are
replaced by Na+ ions when the slurry is contacted by NaCI brine in the
following ration
exchange reaction;
Na++ K-Smectite -> K+ + Na-Smectite.
The Na' in the interlayer of the clay particles attract water molecules.
Moreover, the interaction
between the Na+ in the clay mineral's interlayer and its adjacent layers is
sufficiently weak to
permit the interlayer to expand as more water molecules are attracted to the
interlayer.
2o Accordingly, the clay swells, causing the slurry to take on a gel-like
consistency.
The tendency to form a gel-like consistency is inhibited using inhibitive
rations. It is
therefore possible to inject a highly concentrated reinforced clay slurry
prepared with inhibitive
rations into a subterranean formation having a permeable region, because the
clay slurry is not
transformed into a gel. The ration exchange reaction which takes place when
the slurry is
contacted with Na+-rich formation water and/or injection fluids once in the
subterranean
formation initiates a gelation process. The reinforced clay gel so formed has
the ability to block
or reduce the permeability of the permeable region and to thereby
significantly reduce the early
breakthrough of flooding fluid from injector wells or excessive water
production through
producer wells. The reinforcing agent in the reinforced clay gel provides even
greater gel
strength and therefore, an increased ability to block or reduce the
permeability of the permeable
region of a subterranean formation.
The amount of clay in the reinforced clay slurry is preferably in a range from
about 15 to
about 35 weight percent (dry weight). If the clay content is too low, the
swollen reinforced clay
slurry will not have the desired gel-like consistency. If the clay content is
too high, the
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CA 02349998 2001-06-05
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reinforced clay slurry will be too viscous to be poured or injected into a
formation. More
preferably, the clay content should be in the range from about 25 to about 35
weight percent.
The amount of reinforcing agent in the reinforced clay slurry is preferably in
a range of
from about 10 to about 60 weight percent, more preferably in a range of from
about 30 to about
50 weight percent, and most preferably in a range of from about 40 to about 50
weight percent.
If the amount of reinforcing agent content is too high, i.e., greater than
about 60 wt.%, then the
required injection pressure will be too high. If the amount of reinforcing
agent content is too low,
i.e., less than about 10 wt.%, then the reinforcing agent may settle out
within the high
permeability region.
The concentration of the inhibitive compound in the slurry preferably should
be in the
range of from about 0.5 to about 5 weight percent. Preferably, the inhibitive
compound
concentration should be in the range of from about 1 to about 3 weight
percent. If the
concentration of the inhibitive ration in the slurry is too low, the clay may
swell partially,
reducing the amount of clay that can be mixed into a slurry, for injection
into the subterranean
formation.
To form a reinforced clay slurry, the swelling clay is mixed into an aqueous
solution
containing an inhibitive compound to form a clay slurry.
In one embodiment, the swelling clay is first pre-saturated with an inhibitive
ration before
making the slurry. This involves repeated washing of the clay in an inhibitive
compound solution
such, as a KCI solution. The clay is then dried before use. This step is
thought to be effective
because saturation of the clay particles with an inhibitive ration displaces a
sufficient number of
Na+ ions, to provide a slurry which is capable of being injected into a
subterranean formation.
Preferably, a dispersion agent is added to the clay slurry to reduce
agglomeration of clay
particles , thereby promoting a uniform suspension of clay particles in the
slurry. Suitable
dispersion agents include sodium acid pyrophosphate, sodium pyrophosphate and
lignosulfates.
The dispersion agent may be added at a concentration ranging from about 0.1 to
about 3.0
weight percent.
The clay slurry is then mixed with a reinforcing agent to produce a reinforced
clay slurry.
It is anticipated that the viscous reinforced clay slurries, when delivered
into a
subterranean formation, will preferentially enter channels of relatively high
permeability, such as
fractures or wormholes, and will not enter and effect blockage in areas of
relatively low
permeability in the formation. Experimental model results show that injected
clay slurry
preferentially enters and blocks simulated high permeability sand and does not
enter simulated
low permeability sand.
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The reinforced clay gels of the present invention are useful for controlling
the influx of
water into the hydrocarbon producing region of the subterranean formation.
Also, the reinforced
clay gels may be used to contain fluids, for example contaminated fluids, in a
certain region of a
subterranean formation by blocking permeability channels in the region,
thereby controlling the
flow of contaminated fluids.
As discussed above, in a preferred embodiment, the reinforcing agent is
produced sand.
In this way, the reinforced clay slurry provides both a means for disposal of
produced sands and
a means for controlling the flow of fluids to and from a region of a
subterranean formation.
Also, the reinforced clay gel can be used as a means for disposal of
contaminated
solids. The contaminated solids act as a reinforcing agent in a reinforced
clay slurry. The clay
particles coat the contaminated solids and when the reinforced clay slurry is
injected into a
subterranean formation, a reinforced clay gel is formed. The clay gel
encapsulates the
contaminated solids. Accordingly, fluid in the region of the subterranean
formation occupied by
the reinforced clay gel substantially reduces that rate at which such
contaminants are leached
from the region containing such a reinforced clay gel.
The following non-limiting examples of embodiments of the present invention
are
provided for illustrative purposes only.
EXAMPLE 1
Preparation of Clay Slurry
A 30 wt.% clay slurry was prepared by mixing 30 g of bentonite (NATURAL GELTM
from
Canamara United, Edmonton, Alberta, Canada in 70 g of an aqueous solution of 5
wt.% KCI
and 0.2 wt.% sodium acid pyrophosphate. Sodium acid pyrophosphate was used as
a
dispersion agent.
A 35 wt.% clay slurry was prepared in the same manner as above, using 35 g of
bentonite in 65 g of solution.
EXAMPLE 2
Preparation of Reinforced Clay Slurry
100 g of the 30 wt.% clay slurry produced in Example 1 was mixed with 100 g
produced
sand, added stepwise, by shaking in a 1-L NALGENETM bottle. The produced sand,
having a
residual oil content of about 2 wt.%, was obtained from Husky Oil. The
reinforced clay slurry
CA 02349998 2001-06-05
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had a volume of about %2 L.
In most of the samples tested in Examples 3 and 4, the natural produced sand
was first
"cleaned" by extracting the residual oil with toluene. However, produced sand
in its natural
state with residual hydrocarbons intact (hereinafter, "natural" sand) was also
tested in Test #3 of
Example 4. Following toluene extraction, "cleaned" sand was dried at
110°C. Both the
"cleaned" and "natural" sand samples were sieved through an 18 mesh screen to
homogenize
the sample, break up large lumps and to remove small pebbles and other
extraneous material.
The resultant reinforced clay slurry contained 15 wt.% clay, 50 wt.% produced
sand, and
2.5 wt.% KCI.
Other reinforced clay slurries were produced using the 35 wt.% clay slurry of
Example 1
with 67 g, 97 g and 100 g produced sand, resulting in reinforced clay slurries
containing,
respectively:
21 wt.% clay, 40 wt.% sand, and 3 wt.% KCI
18 wt.% clay, 49 wt.% sand, and 2.5 wt.% KCI
17.5 wt.% clay, 50 wt.% sand, and 2.5 wt.% KCI
EXAMPLE 3
Addition of a reinforcing agent increases the viscosity of a clay slurry up to
20 times.
Surprisingly, however, estimated injection pressures calculated below
demonstrate that the
reinforced clay slurry can be injected into a formation at a reasonable rate.
Viscosity of Reinforced CIa~Slurries
The viscosity of reinforced clay slurries produced in Example 2 was measured
in order
to estimate the pressure required to inject the slurries into a subterranean
formation. Because
the slurries are shear thinning (i.e., the viscosity decreases with increasing
shear rate), the
measurements were performed at approximately the same shear rates as would be
encountered in the field.
Measurements were made by flowing slurries at different flow rates through
capillary
tubes and measuring the pressure drop.
The stainless steel capillary tubes used for measurement were (1 ) 0.775 cm
inside
diameter x 30 cm long and (2) 0.214 cm inside diameter x 30 cm long. Another
length of tubing
(30 cm) having the same diameter as the capillary tube was placed at each end
of the capillary
tube to minimize turbulence in the capillary tubing used for measurement. This
was verified by
calibrating the capillary tube with a Newtonian fluid.
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Also, the viscosity measurements were not corrected for viscosity differences
arising
from various shear rates, typically known as slip or slippage. The two
different diameter capillary
tubes were used to determine the amount of slip at the wall of the capillary
tube. Differences in
viscosity for a given shear rate indicate slip. Because the viscosity
measurements for a given
shear rate were within experimental error, it appeared that slippage could be
neglected in the
viscosity calculations.
The pressure drop was measured with four differential pressure transmitters
(Rosemount) with ranges of 4 kPa to 1000 kPa.
Reinforced clay slurries prepared in Example 2 were placed in a stainless
steel
accumulator. A clay slurry, produced in Example 1, without reinforcing agent,
was used for the
Control sample.
A Jeffri pump was used to move the slurry from the accumulator to the
capillary tube.
The slurry was collected in a stainless steel cylinder, maintained at a
pressure of 1 MPa (145
psi), to reduce the effect of trapped air on the measurements. Slurry was
injected into the
capillary tube at flow rates of 5, 20, 80, 320, 500, 1000 and 1982 cm3/hr.
The viscosity, rl, of the slurries was approximated using equation (I),
according to Bird et
al ("Transport Phenomena" John Wiley & Sons, Toronto; 1960):
r1 = T,~Y~r~ = k(Y~on)~' (I)
where:
rl = viscosity,
~W = shear stress at wall = (OP",/L)R/2
where OPm is the measured pressure drop across the capillary tube
L = length of capillary tube
R = radius of capillary tube
y~~ = shear rate corrected for shear thinning = (3n+1 )/(4n)(8Um/D)
where Um = average velocity across section of capillary tube
= Q/(~R2)
where Q = volumetric flow rate
3o k = power law constant; and
n = power law index
The shear stress at the wall of the tube can be written as:
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T = k' Y"aPP ( I I )
where k' = k((3n+1 )/(4n))"; and
YaPP - $ U,t,/D.
The shear stress, ~, and apparent shear rate, YaPP, were calculated according
to
equations (I) and (II) and plotted on a log-log plot. The data was fitted to a
straight line. The
slope of the line is the power law index, n, and the y-intercept is equal to
log k'. The corrected
shear rate was then calculated using equation (III):
Y~rt = (3n+1 )/(4n)(8U",/D) (III)
Accordingly, the viscosity was calculated using equation (IV):
r1 = ~/Ycorr (IV)
and the power law constant was calculated from equation (V):
k = k'(4n)"/(3n + 1 )" (V)
The viscosity measurement was not corrected for yield stress because, at
higher shear
rates, the slurry yields completely within the capillary tube. It was
difficult to accurately measure
the yield stress. Accordingly, the influence of yield stress on the viscosity
calculation was tested
at the highest shear rate. As an upper limit for the yield stress, the wall
shear stress at the
lowest shear rate was used.
The viscosity, power law constant (k), power law index (n) and corrected shear
rate are
tabulated in Table 1 for the 0.214 cm diameter capillary tube and in Table 2
for the 0.775 cm
diameter capillary tube. For ease of comparison between clay slurries
containing no reinforcing
agent and reinforced clay slurries, the amount of clay is the amount of clay
in the clay slurry
prior to addition of reinforcing agent. For example, a 100 g sample containing
35 g clay, added
to 100 g sand is reported in Tables 1 and 2 as 35 wt.% clay plus 50 wt.% sand.
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TABLE 1
Shear Rate and Viscosity for 0.214 cm diameter capillary tube
Slurry Type Flow RateOP Shear Viscosityk n
(cm'Ihr) Pa Rate (s')(mPa-s) (mPa-s)
30 wt.% clay5 1,800 1.88 1,710 2,600 0.46
(no sand) 20 3,800 7.51 903 2,600 0.46
80 7,500 30.0 445 2,600 0.46
320 14,200 120 210 2,600 0.46
1,000 19,300 375 91.7 2,600 0.46
35 wt.% clay5 8,000 2.09 6,830 10,000 0.36
(no sand) 20 12,300 8.35 2,625 10,000 0.36
50 14,800 20.9 1,264 10,000 0.36
80 18,000 33.4 961 10,000 0.36
200 21,300 83.5 83.54 10,000 0.36
320 48,000 134 640 10,000 0.36
800 36,700 334 196 10,000 0.36
1,000 65,000 418 278 10,000 0.36
1,982 57,000 828 123 10,000 0.36
30 wt.% clay5 22,000 2.00 19,565 22,700 0.39
+
40 wt.% sand20 25,000 8.02 5,560 22,700 0.39
80 37,000 32.1 2,055 22,700 0.39
320 74,000 128.3 1,030 22,700 0.39
1,000 180,000 400 800 22,700 0.39
30 wt. % 5 58,000 1.95 53,102 51,100 0.42
clay +
50 wt.% sand20 73,000 7.79 16,710 51,100 0.42
80 45,000 31.2 2,575 51,100 0.42
320 200,000 124.7 2,860 51,100 0.42
1,000 600,000 389 2,750 51,100 0.42
35 wt.% clay5 70,000 2.08 60,045 66,400 0.36
+
40 wt.% sand20 72,000 8.32 15,440 66,400 0.36
80 77,000 33.26 4,130 66,400 0.36
320 195,000 133 2,614 66,400 0.36
1,000 498,000 416 2,136 66,400 0.36
35 wt.% clay20 112,000 6.87 29,070 52,200 0.57
+
50 wt.% sand80 140,000 27.5 9,085 52,200 0.57
320 370,000 109.9 6,000 52,200 0.57
1,000 1,000,000344 5,190 52,200 0.57
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TABLE 2
Shear Rate and Viscosity for 0.775 cm diameter capillary tube
Slurry Type Flow Rate0P Shear Viscosityk n
(cm3/hr) kPa Rate (mPa-s) (mPa-s)
(s')
30 wt.% clay500 1,800 4.31 2,695 7,075 0.37
(no sand) 1,000 2,710 8.63 2,028 7,075 0.37
1,982 3,010 17.1 1,136 7,075 0.37
35 wt.% clay500 2,870 3.31 5,603 7,600 0.74
(no sand) 1,000 4,720 6.62 4,607 7,600 0.74
1,982 7,950 13.1 3,915 7,600 0.74
30 wt.% clay500 6,350 3.42 11,985 17,350 0.67
+
40 wt.% sand1,000 8,900 6.84 8,400 17,350 0.67
1,982 15,900 13.6 7,570 17,350 0.67
30 wt.% clay500 10,800 3.45 20,200 30,250 0.65
+
50 wt.% sand1,000 15,400 6.90 14,400 30,250 0.65
1,982 26,400 13.7 12,460 30,250 0.65
35 wt.% clay500 15,500 3.40 29,440 42,600 0.68
+
40 wt.% sand1,000 23,100 6.80 21,940 42,600 0.68
1,982 39,500 13.5 18,930 42,600 0.68
35 wt.% clay500 31,300 3.89 51,930 106,5000.47
+
50 wt.% sand1,000 43,400 7.78 36,000 106,5000.47
The data in Table 1 shows that for a flow rate of 1000 cm3/hr, the viscosity
increased by
about 8 times when the reinforcing agent content increased from 0 wt.% sand to
40 wt.% sand,
for both the 30 wt.% and 35 wt.% clay slurries.
For the 30 wt.% clay slurry, with the exception of the viscosity measured at
80 cm3/hr,
the viscosity increased by about 3 times when the sand content was increased
from 40 wt.% to
50 wt.%. For the 35 wt.% clay slurry, the viscosity increased by about 2 times
when the sand
content was increased from 40 wt.% to 50 wt.%.
When the clay content was increased from 30 wt.% to 35 wt.%, the viscosity
increased
by about 2 times, for both the 40 wt.% and 50 wt.% reinforcing agent samples.
The data in Table 2 shows that for a flow rate of 1000 cm3/hr, the viscosity
increased by
about 4 times when the reinforcing agent content increased from 0 wt.% sand to
40 wt.% sand,
for both the 30 wt.% and 35 wt.% clay slurries.
For the 30 wt.% clay slurry, the viscosity increased by about 1.7 times when
the sand
content was increased from 40 wt.% to 50 wt.%. For the 35 wt.% clay slurry,
the viscosity
increased by about 2.3 times when the sand content was increased from 40 wt.%
to 50 wt.%.
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When the clay content was increased from 30 wt.% to 35 wt.%, the viscosity
increased
by about 2.6 times and 3.6 times, for the 40 wt.% and 50 wt.% reinforcing
agent samples,
respectively.
Estimated Injection Pressure
The pressure P required to inject a clay slurry into a formation is a function
of the
pressure drops OPS across a surface injection pipe, OP~ down a vertical pipe
and ~P, across
pipe fittings and the bottom hole pressure Pb.
The OPS across a surface injection pipe is estimated by equation (VI):
OPs/L = k(Ycorr)~(2/R) (VI)
where:
k = power law constant
n = power law index
R = injection pipe radius
L = injection pipe length
Y~rc = corrected shear rate = (3n+1 )/(4n) Yepp
where Yapp = apparent shear rate = 4Q/(~R3)
and Q = flow rate through pipe.
Using equation VI, the OP~/L required to inject a 35 wt.% clay + 40 wt.% sand
slurry
through a 2" (5.08 cm) pipe at a flow rate of 0.5 m3/min was estimated to be
60 kPa/m.
The OP~ down a vertical pipe was calculated from equation (VII), according to
Shook et
al ("Slurry Flow: Principles & Practice" Butterworths; pg. 112, 1991 ):
OP~/L = k(Ycorr)"(yR) - P9 (VII)
where:
p = fluid density
g = gravitational constant
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Using a tubing diameter of 89 mm in a vertical well, the OP~/L required to
inject the same
slurry down a vertical pipe was estimated to be 3.05 kPa/m.
The pressure drop across fittings (OPf) was estimated using equation (VIII):
OPr = KPUm2 (VIII)
where:
K = resistance coefficient
Um = average superficial velocity = Q/(~RZ)
1 o Q = volumetric flow rate
R = pipe radius
The resistance coefficient relationships were determined by Ma ("Stability,
Rheology and
Flow in Pipes, Bends, Fittings, Valves and Venturi Meters of Concentrated Non-
Newtonian
Suspensions" Ph.D. Thesis, University of Illinois, Chicago, 1987) for flow of
laterite (an iron rich
clay) and gypsum slurries and were therefore considered to be reasonable
estimates for the 35
wt.% clay + 40 wt.% sand slurry. In order to select the appropriate resistance
coefficient
relationship, the Reynolds number (Re) was first calculated to determine
whether flow was
laminar or turbulent, using equation (IX):
Re = U",D/('n/p) (IX)
The Reynolds number for injection of a 35 wt.% clay + 40 wt.% sand slurry
through a 2"
(5.08 cm) diameter pipe (D) at a flow rate of 0.5 m3/min was calculated to be
411, where rl = k
(Y~,~)"~', n = 0.36 (Table 1 ), yin = 936 s' and p = 1,640 kg/m3. Accordingly,
because the
Reynolds number was less than 2000, flow was determined to be laminar.
For a laminar flow regime, the resistance coefficient for 2" (5.08 cm)
fittings can be
estimated from equation (X) for a 90° elbow:
K = 36.81 (Re)'°~~' (X)
The resistance coefficient for a globe valve is 6.65.
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Using the resistance coefficient relationships, the OPf for a globe valve was
calculated as
184 kPa and the OPf for a 90° elbow was calculated to be 72 kPa. For an
installation with 1
globe valve and three 90° elbows, the total pressure drop across the
fittings was calculated to
be OPf = 400 kPa.
The bottom hole pressure is typically in the range of from about 1000 to 4000
kPa for
cold production wells in Alberta. For this discussion, we will assume a bottom
hole pressure Pb
of 2000 kPa.
Accordingly, the pressure P required to inject a 35 wt.% clay + 40 wt.% sand
slurry at a
flow rate of 0.5 m3/min through 30 m of surface injection pipe, three
90° elbows, one globe
valve, 500 m of vertical pipe into a formation with a bottom hole pressure of
2000 kPa is:
P = OP~/L (30 m) + OP"/L (500 m) + OP, + Pb
P = 60 kPa/m (30 m) + 3.05 kPa/m (500 m) + 400 kPa + 2000 kPa
P = 5725 kPa
This estimated injection pressure of 5725 kPa is a reasonable injection
pressure for field
applications. Accordingly, the reinforced clay slurry can be injected into a
formation. This is a
surprising result because even a 20 wt.% clay/water composition, without an
inhibitive ration,
would form a non-pumpable clay gel, instead of a clay slurry. The inhibitive
ration, however,
surprisingly maintains a pumpable clay slurry consistency at 35 wt.% clay.
Moreover, a
commercially reasonable injection pressure (e.g., 5725 kPa) for injecting the
clay slurry
composition can still be maintained even with 40 wt.% sand added to that
composition.
EXAMPLE 4
This example shows that the strength of clay gel, i.e., its ability to prevent
fluid
breakthrough in otherwise high permeability regions, such as wormholes, is
significantly
increased by the addition of a reinforcing agent. The clay gel strength was
calculated from the
maximum pressure that the gel could resist without yielding. The strength of a
reinforcing clay
gel was 3 to 6 times greater than a similar clay gel without a reinforcing
agent.
Wormhole Blocking Simulation
The diameter of wormholes in a subterranean formation can vary from
approximately 10
cm near the wellbore to 5 cm up to a hundred meters away. In order to simulate
the efficacy in
blocking of a wellbore, the reinforced clay gels were tested in a concentric
cell flow system. A
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cylindrical cell (16.51 cm long, 7.62 inside diameter) has a central channel
(2.67 cm diameter,
13 cm long), defined by a 32 mesh stainless steel cylindrical support screen
covered with a 200
mesh screen, and an annulus surrounding the channel. The annulus was filled
with 70/140
mesh Ottawa sand. The mesh sizes are US Standard Sieve sizes.
The cell was closed and mounted in the flow system. The cell was purged with
carbon
dioxide, evacuated and the sand was saturated with a sodium chloride exchange
solution, to
simulate brine in a subterranean formation. The pressure drop across the
central channel was
measured with a differential pressure transducer attached on endcaps of the
cylindrical cell.
A reinforced clay slurry produced in Example 2 was injected into the central
channel at
1124 cm3/hour. Slurry was injected into the channel over a 15 to 20 minute
period during which
the pressure drop reached steady state. The injected volume of slurry was
several times
greater than the channel volume.
The cell was then shut in for a minimum of four days to allow ion exchange
between the
reinforced clay slurry and the NaCI brine in the annular pore space. The ion
exchange between
the potassium ions of the reinforced clay slurry and the NaCI brine caused the
clay to hydrate
and swell, forming a reinforced clay gel. The strength of the reinforced clay
gel was determined
by a displacement fluid applied to the inlet of the central channel at a
constant pressure. The
displacement fluid used was either 1.75 wt.% NaCI brine or a 30 wt.% clay
slurry. The inlet
pressure was increased periodically until the gel inside the central channel
yielded to the fluid
pressure. Effluent was collected to determine how and when the gel yielded.
Five tests were conducted with different slurry compositions, exchange brine
compositions and displacement fluids. The experimental conditions are
summarized in Table 3.
The results of the tests are presented in Table 4 and Fig. 2 to 5. The Control
sample was a clay
slurry produced in Example 1, without the reinforcing agent added in Example
2.
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TABLE 3
Test Clay Clay contentSand contentExchange Displacement
#
content in reinforcedin reinforcedsolution Fluid
in in
clay slurryclay slurry clay slurryFlow System
wt.% wt.% wt.% wt.% NaCI
Control35 N/A 0 1.75 1.75 wt.%
NaCI brine
1 35 21 40 1.75 Brine initially,
(cleaned then 30 wt.%
sand cla slur
2 30 15 50 1.75 30 wt.% clay
(cleaned slurry
sand
3 30 15 50 1.75 30 wt.% clay
natural slur
sand
4 35 17.5 50 1.75 30 wt.% clay
(cleaned slurry
sand
The injection pressure was increased in fairly large increments during the
test until the
gel yielded completely inside the channel. Complete yielding of the slurry was
demonstrated by
a sharp increase in the effluent produced, as illustrated in Figs. 1 to 3. The
injection pressure at
yield of the gel inside the central channel is presented in Table 4. It will
be understood that the
actual yield pressure may fall within the range of the highest pressure
without yield and the
pressure at which yield was detected because the pressure was incrementally
changed.
Accordingly, the yield pressure in Table 4 is presented as a pressure range.
A yield stress range was calculated from the yield pressure range using the
equation
(XI):
T = F/A = ~cR2P/(2~RL) = RP/(2L) (XI)
where ~ is shear stress, F is the force applied to the gel, A is the contact
area, R is the radius of
the central channel, P is the pressure applied to the gel and L is the length
of the channel. The
yield stress range is also presented in Table 4.
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TABLE 4
Test Yield PressureYield Stress
# kPa kPa
Control14-25 0.6 - 1.2
1 47-65 2.4 - 3.3
2 51-75 2.6-3.8
3 65-95 3.3 - 4.9
r 4 100-160 5.1 - 8.2
~
Control
The Control test was conducted with a 35 wt.% clay slurry. After 3 days of
shut in for ion
exchange with 1.75 wt.% NaCI brine, the endcaps and the flowlines to and from
the central
channel were filled with 1.75 wt.% NaCI brine. Brine flow through the channel
was blocked by
the 35 wt.% clay gel at 14 kPa (2.0 psi) but the clay gel yielded at 25 kPa
(3.6 psi), as shown in
Fig. 2.
Test #1
Test #1 was conducted with a 35 wt.% clay slurry. The clay slurry was mixed
with 40
wt.% "cleaned" sand.
After four days of shut in for ion exchange with 1.75 wt.% NaCI brine, the
endcaps and
the flowlines to and from the central channel were filled with 1.75 wt.% NaCI
brine. The inlet of
the central channel was subjected to a loading of about 15 kPa (2.2 psi)
pressure for 1 hour.
The pressure was increased to 27 kPa (3.9 psi) for 1 hour and then to 47 kPa
(6.8 psi),
representing a pressure gradient of 360 kPa/m, for 1 hour. The reinforced clay
gel yielded at 65
kPa (9.4 psi), as shown in Fig. 1.
Test #2
Test #2 was conducted with a 30 wt.% clay slurry, which was mixed with 50 wt.%
"cleaned" sand.
After four days of shut in for ion exchange with 1.75 wt.% NaCI brine, the
endcaps and
the flowlines to and from the central channel were filled with 1.75 wt.% NaCI
brine.
When the brine was loaded at the inlet end of the central channel at a
pressure of 34
kPa (4.9 psi), brine was produced slowly from the outlet end of the channel.
It was determined
that brine had channeled through the sand in the cell annulus. At this point,
brine was removed
from the central channel inlet and was replaced with 30 wt.% clay slurry. Clay
slurry was
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applied to the inlet of the central channel at a pressure of 28 kPa (4.1 psi)
for 0.8 hours. The
clay slurry did not pass through the 200 mesh screen into the cell annulus. As
a thin filter cake
was built around the screen, bypass flow through the annulus was blocked.
Loading of the clay
slurry to the inlet of the central channel was increased to 51 kPa (7.4 psi),
representing a
pressure gradient of 390 kPa/m, for 0.5 hour without further yielding. The gel
yielded when
loading was increased to 75 kPa (10.9 psi), as shown in Fig. 2.
Test #3
Test #3 was conducted in the same manner as Test #2, using 50 wt.% "natural"
sand to
1 o produce the reinforced clay slurry.
After seven days of shut in for ion exchange with 1.75 wt.% NaCI brine, the
endcaps and
the flowlines to and from the central channel were filled with 30 wt.% clay
slurry. Clay slurry
was applied to the inlet of the central channel at a pressure of 18 kPa (2.6
psi) for 1 hour.
Loading of the clay slurry to the inlet of the central channel was increased
to 28 kPa (4.1 psi) for
2.2 hours, 46 kPa (6.7 psi) for 1.5 hours, and 6.5 kPa (0.9 psi), representing
a pressure gradient
of 500 kPa/m, for 1.1 hours without further yielding. The gel yielded when
loading was
increased to 95 kPa (13.8 psi), as shown in Fig. 2.
Test #4
2o The clay slurry concentration was increased to 35 wt.% clay. The clay mixed
with 50
wt.% "cleaned" sand.
After six days of shut in for ion exchange with 1.75 wt.% NaCI brine, the
endcaps and
the flowlines to and from the central channel were filled with 30 wt.% clay
slurry. Clay slurry
was applied to the inlet of the central channel at a pressure of 25 kPa (3.6
psi) for 0.5 hour.
Loading of the clay slurry to the inlet of the central channel was increased
incrementally to 51
kPa (7.4 psi) for about 0.8 hour, and 100 kPa (14.5 psi), representing a
pressure gradient of 770
kPa/m, for 1.1 hours without further yielding. The gel yielded when loading
was increased to
160 kPa (23.2 psi), as shown in Fig. 3.
Results
The yield stress of the reinforced clay gels of Test #1 (40% sand) and Test #4
(50%
sand) was about 3 and about 7 times greater, respectively, than the yield
stress for the Control
clay gel. These results indicate that a reinforcing agent increases the yield
stress of a clay gel
and that the yield stress increases with increasing content of reinforcing
agent.
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Tests #2 and #3 were conducted to determine the influence of the reinforcing
agent
purity on clay gel yield stress. The yield stress was slightly greater for the
reinforced clay gel of
Test #3, using the "natural" sand. This indicates that "natural" produced
sands could be used
as a reinforcing agent in clay gel, without adversely affecting the strength
of the clay gel. This
provides a useful solution for disposal of produced sands.
The same reinforcing agent content was used in Tests #2 and #4. However, the
clay
content was lower in Test #2. The results demonstrate that the yield stress is
higher at higher
clay contents.
Preferred embodiments of the present invention have been described. It will be
understood that the foregoing is provided for illustrative purposes only and
that other
embodiments and applications can be employed without departing from the true
scope of the
invention described in the following claims.
23
