CA2191512C - Process for reducing permeability in a subterranean formation - Google Patents

Abstract

A method is provided for blocking high permeability channels in subterranean geological formations, using swelling clay. A slurry is prepared by mixing swelling clay, such as bentonite, into an aqueous solution containing the salt of certain cations, which inhibit clay swelling. The cations K~, Ca2~ and Mg2~, among others, are inhibitive of clay swelling. The clay slurry is introduced into a geological formation, where it preferentialy enters channels of high permeability. There the slurry is contacted by NaCl brine solution present in natural or injected drive fluids, and the inhibitive cations bound to the clay particles are replaced by Na~ ions, which attract water molecules and promote clay swelling. The Na~ - clay swells to up to 10 times its original volume, causing the shiny to acquire a gel- like consistency. The clay gel so formed is capable of blocking the flow of water, and can resist pressure of up to 500 kPa per meter.

Description

,~ -- , -~ _ . . __ IN A SUBTERRANEAN FORMATION
FIELD OF THE INVENTION
The invenfbn relates to a method for controlling the influx of water, which is caused by high permeability strata, fractures and wormholes, during the recovery of hydrocarbons from geological formations. In particular, the invention relates to chemical gel systems far permeability modification.
BACKGROUNa of TH!< II~v~NTtoN
During the recovery of hydrocarbons from geological formations, signlfiaant amounts bf hydrocarbons are left behind because injected or natural drive fluids in the formation are produced along with the oil to such an extent that 16 the cost of fluid disposal makes further oil recovery uneconomical. In formations with high permeability strata, fractures, or 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 prat~sses flow through highly permeable zones, resulting irt progressively less hydrocarbon being recavored 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 fio excessive water encroachment into producer wells. It has adversely affected the ec5onomiCS of recovery processes in many parts of the world. for example, it was recently 25 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, the ratio of waterloil produced is 5/9, amounting to 2.C x 10~ barrels of water produced in 1995.
The art of controliirtg or rnodlfying fluid flows in the recQVery of 34 hydrocarbons from underground formations is commonly referred to as "conformance control". For the past two decades, research has been directed at ___,. _'_ _, __ improving the oihwater ratio during production by using chemical gel systems to blacK water flow through high permeability zones, fractures arid warmhales (referred to herein as "channels"). The general approach has been to inject a mixture of reagents, initially low in viscosity, into regions of a formation which have high permeability channel. 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 !'aen used in this way for canfarmanco central In gealagical farmatians-Ideally, a gel system for conformance control should have the following properties:
'I . The reagents should be easily delivered to the desired lacauon in the farmatlan. The camporrents therefore should be inltiaUy of low viscosity. No component should be ~15 adsorbed out prior to reaching its destination, and each component should ba stable to shear stress encountered during delivery.

2. The chemical reactions) required for gelation under the conditions found in the formation.
0 3. The gel generated should be of high strength the conditions found in the fomnation.
4. The degree of permeability reduction should be high.
5. The system should be of low enough cost to make it economically feasible.
25 6. The system should have minimal environments) 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:
so 1. Polyacrylamide copolymers wh)ch are injected t4gether with a Cross-linking agent, Chromium (111) or Aluminum (II);

2. Xanthan gum (a natural heteropolysaccharide) which together with a cross-linking agent, Chramlurn (Ill);

3. Poly (Vinyl alcohol) which is injected together with a cross-linking agent, gluteraldehyde; and s 4. Acidified sodium silicate, which when neutralized, rapidly undergoes polymerization to form spherical silica particles.
The most widely used method of these involves use of polyacrylamide cross-linked with chromium ions. its use is limited by ids cost: approximately $'1000 is required to deliver 1 cubic meter of gel solution into a formation.
It is also relatively unstable under the elevated temperature conditions which exist in geolta~icai formations during thermal recovery processes. Furthermore, Chromium (VI), the oxidation product of Chromium (Ill) is highly toxic, so the use of Chromium (Ill) as a cross-linking agent can be an environmental concern.
w Th~ra is therefore a need to develop further conformance control gel systems which are environmentally safe, inexpensive and effective under the conditions encountered during hydrocarbon recovery.
SUMMARY OF THE INVENTION
zo In accordance with the present invention, a method is provided for plugging hlgh permeability channels during hydrocarbon recovery from geological formations. The method involves using swelling clays as blocking agents. The invention depends on the ability of certain cations to prevEnt clay particles from swelling in aqueous soltrtlon, which allows a highly concentrated clay slurry to be 26 prepared, Such a slurry is made by mixing swelling clays {such as srnectites and vermiaalites) into ~1n aquet~us solution containing a salt whose canons inhibit clay swelling (such as K+, Caz+ or Mg2+). The aqueous solution may also contain a dispersion agent to promote uniform suspension of clay particles in the slurry so if can easily be poured or injected into a formation. Tfte clay slurry is it~jectetl 30 into a geological formation where it preferentially penetrates high permeability channels. There the slung is contacted by Nat ions which era dissolved in the _~ - , -~_ ._.__ natural formation water or in the drive fluid. A spontaneous ration exchange reaction occurs, in which the inhlbitive rations bound to the clay particles are gradually repla~aad by Na+ ions. unlike the inhibitive rations, the Vila'"
ration promotes clay swelling_ The Na-clay becomes hydrated and increases in volume up to 1U-fold. The process of sw$lling transforms the clay slurry into a clay gel which effectively plugs high permeability channels.
Broadly stated, the invention is a method far reducing the permeability of a high permeability channel in a subterranean geological forrnatlon containing Na+
ions, comprising: mixing swelling clay. water and inhibitive rations which inhibit ~a clay swelling, to farm a slurry; and delivering the slurry into the channel sa that it contacts Nat ions, whereby the clay swells in situ to reduce the permeability of the channel.
p~SCRIPTION QF THE DRAWINGS, Figure 1 is a schematic drawing of the laboratory apparatus us8d for carrying out the flow diversion experiments of Example 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In accordance with the invention, a method for blocking water flow through ao high permeability channels in geological formations during hydrocarbon recovery is provided, comprising using swelling clay gels created in sifa to plug high permeability channels.
It is known that swelling days (smectites and vermiculites) becan~
hydrated when exposed to fresh water and swell to many times the volume of the 2s dried clay. However, they can net swell significantly when they are contacted with concentrated solutions of certain salts, whose rations bind to the clay particles and prevent hydration_ Such rations, which include H:~, Ca2~ and Mlga', are referred to herein as "inhibitive rations", and their salts as "'inhibiiive salts".
Swelling clays can ba dispersed in an aqueous snlution containing an effective so concentration of an Inhibitive ration to farm a highly concentrated slurry which Is somewhat viscous. The slurry can be poured or injected into a permeable _ _ , ,__. ___,, ..~,.. . . ..,., ARC-OO1C:A
formation.
Sodium (Nai), a ration which is present in the brine of many geological formations as well as in injection fluids, is not an inhibitive ration.
Instead, it ha;~
the opposite effect of promoting hydration and clay swelling, presumably by binding to the negatively charged surfaces within clay particles and attracting further molecules of water. When concentrated Gay slurries which have been prepared with inhibitive rations are contacted with Na'-rich fluids, a spontaneous ration exchange reaction occurs. The Na* rations in solution exchange with them inhibitive rations bound to the clay particles in the slurry. 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+ ions bound to the slay particles attwact water molecules. The clay then t6 swells, reusing the slurry to take on a gel-like consistency.
A highly concentrated clay slurry prepared with Inhibitive rations can be injected Into a geological formation with high permeability channels. The ration exchange reaction which takes place when the slurry is contacted with Na'"-rich iormatian water and injection fluids initiates a galation process. The clay gel so 2o formed has the ability to block Qr reduce tf~e permeability of the high permeability channels and to thereby significantly reduce the early breakthrough of flooding fluid from injector wells ar excessiv~ water production through producer wells.
The swelling clays used in the invention ran be smectites, commonly known as bentonite$, (mantmariilanite, sapanite, nantranite, lapanite, beidellite) and vermiculites. The amount of clay (its dry weight) in the slurry preferably can range from approximately 10 ~- 40 weight percent. If the clay content is too low, the swollen day will not have a g~I-Ilke consistency. If the clay content is too high, the clay slurry will be tea viscous to be poured or injected into a formation.
ideally the clay content should be 25 - 34 weight percent.
To form a clay slurry, the swelling clay is mixed into an ~aqueaus solution containing the salt of an inhibttive ration. The inhibitive rations which are known s ,______ ..___~. _~_ ._.__ to be effective in preventing the swelling of clays are K+, NH~~', N{CH3)4+, Gs', Cap, Mg2~, Fee' and AI3''. The most effective inhibitory cation is believed to be K+. The identity of the anion in the inhibitive salt is not known to be of any importance. For example the K' cation could probably be provided in the form of s KCI, KNOB, KZC03, or another salt containing Kt as the catian. Normally, KCI
is used as the source of K* ions. To be effective, the concentration of KGI
present in the slurry preferably should be between 1-94 weight percent, and ideally bEtween 3-5 weight percent. if the concentration of the inhibitive canon in the slurry is too low, the clay partially swells, reducing the amount of clay that can b~s ~0 mixed into a slurry, and thereby reducing the amount of swelling that can ultimately take place in situ.
As shown in Example 1 below, the strength of a clay gel is increased considerably {up to 90%) if the swelling clay is first pre-saturated with an inhibitive cation before making the stuny. This involves repeated washing of the vs clay in an inhibitive salt solution such as 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 removes all the water-attracting Na'"
ions, leading to maximum swelling of the clay irr sifu.
preferably, a dispersion aggro is included in the slurry to prevent the clay ao particles from sticking together, thereby promoting a uniform suspension of clay particles in the slurry. podium acid pyrophosphate at concentrations ranging from 0.1 -3.0 weight percent has been used as the dispersion agent. However, it is likely that other agents which can uniformly suspend clay particles i~ the slum,.
such as sodium pyrophosphate or lignasulfonates, could also be used. The dispersion agents are thought to act by coating the clay partides with charged molecules which causE the particles to repel each other.
It is anticipated that the viscous clay slurries, when delivered into a geological formation, will preferentially enter channels of relatiuefy higb..
permeability, such as fractures or wormholes, and wilt riot enter and effect so blockage in areas of relatively low permeability in the formation. Results obtained with an experimental model (Example 2 below) showed that injected s --__ ..---~. -J,. .-...
ARC-D01 cA
clay slurry pref~rentially enters and blocks simulated high permeability sand;
it did not enter simulated low permeability sand.
The clay gels formed by the method of the invention have been shown experimentally to be capable of resisting a pressure drawdown in the range of s - 500 k~'alm.
The method of the invention has the further advantage of being inexpensive, because na costly materials are required. The 1096 cost fQr materials to make a cubic meter of clay gel are as follows:
bentonite - $33 - $44, plus Shipping c4sts of $20 - $25;
~o KCI - $7 - $8; and additives - $3 - $4.
The invention can be better understood by reference to the following non-limiting examples which demonstrate the preparation arid tasting of clay slurries for strength, and the use of clay slurries to block high permeability channels in a ~ts model system.
F~cAMP~E ~
This example demonstrates: (1} the ability of a clay slung to remain in the form Of a Clay Slurry with increasing clay content; (2) the theological properties c~f ~o clay gels generated from slurries with increasing percentages of clay; and (3) the increased streng~ of gels generated with K+ pre-saturated clay compared to the day gels of (2).
The swelling clay utilized in the experiments was a commercially avallabl~a Wyoming bentonite (Smectite) (Hydragel 125 from Wyo-Ben Inc., Billings, ~s Montana). Clay slurries were prepared by adding varying amounts of bentonite into an aqueous solution containing ~ weight percent of dissolved KCt (a salt of the inhibitive canon K+), and 0.1 weight percent of sodium acid pyrophosphate (~~
dispersion agent). The clay was gradually stirred into the solution. (The clay should not be stirred into water, because it will immediately swell and start to so form a gel before the KCt can be added.) The slurries were aged for at Isast four hours before testing.
T

ARC-OOiCA
The theological properti~s {yield stress and plastic viscosity) were determined using aliquots of each slurry.
The slurries were then subjected to a gelation test as follows.
Approximately 10 m! of each slurry was poured into a fritted glass tube (which Is.
permeable to water) capped with a rubber stopper at one end.. The ether end of the tube was then capped with a rubber stopper, and the tube was immersed in a sc~lu4on bf D.3M {1.73 weight percent) NaCI solution in order for the catlon exchange process to take place and gelation to occur. After 45 hours, the glass tube was taken out of the NaCI solution, and the rubber stoppers removed. To to test the strength of the gel, sufficient pressure (loading) was applied to one end of the clay gel to extrude it from the glass tube. From the measurement ofthe loading or extruding pressure, the shear stress between the clay gel and the glass wall of the tube was e$timated using the equation_ T=rPP2t_, where T is the shear stress, r is the inside radius of the glass tube {0.65 cm), P is the extruding pressure, and L. is the tube length (6.0) crn.
Clay slurries were prepared using either (A) 2U, (B) 25, or (C) 30 weight percent of bentanite, or (D) 30 weight percent of bentonite which had been previously saturated with K*' ions and dried. Far sample 0, pre-saturation of the clay with K* was carried out by washing it twice in a 1 M solution Qf KCI, foflowinc~
2o by a wash in Hzla to remove excess KCI, and then drying ft. A Control sample was prepared using 20 weight percent of bentonite in fresh water. The theological properties of the clay slurries and clay gels which they formed after they were immersed in NaCI brine are shown in Tables 1 and 2. Irt each of samples A-0, clay slurries were fomved at clay contents ranging from 20-3090 25 (wt.y. Before getation, elf tour of the clay slurries of Samples A, B, C
and D were of a thin enough consistency that they could be poured. Both the yield stress and plastic viscosity of the ~arnples A-f~ clay slurries increased with the clay concentration in the slurry. In contrast, the Control sample of ~0% (wt) clay in fre$h water formed a gel instantly, i.e. a pourable slurry was not farmed.
a0 Accordingly, yield stress and plastic viscosity could not be measured for the Control sample, even at a clay content equivalent to that of the lowest clay a "~ , _~_ ._.__ ARC-001 ~A
content Sample A. After gelatiort, when the rubber stoppers were removed, all four of the clay gels of Samples A, B, C and D were thick enough to be retained in the tul~. The extruding pressure required to remove the clay gels from the tubes increased with the clay content= 3.2 kPa (3~,t~00 dyr7e/cm2) for the 20%
gef, 11 kC'a (110,000 dynelcm2) for the 25% gel, and A-0 kPa (400,000 dynelcm2) for the 30°I° gel. The corresponding shear stress between the glass wall and the play gel for samples A, B and C was 0.2 kPa (2,000 dynelcm'), 0.6 kPa (fi,000 dynelcm2) and 2.2 kPa (22,000 dynelcrn2). Thus the gel strength increased with the clay content of tha gels aver the range ~0 -30 weight percent. If placed in a 1o fracture with an opening of 1.0 cm, the 30°r6 clay gel could be sxpecteci to resist a pressure gradient of 435 kPa per meter. Thi$ estimate was made by using the Shear stress of the 30% gel from the experimentally obtained value for shear stress (~.~ kPa. 22,000 dynelcm2), and using the ecfuation PIL ~ 2TlS where P!L
is the pressure gradient, T is the shear stress, and ~ is the opening of the ~ts fracture.
The extruding pressure and shear stress of the Cantral sample were 44 kPa 040,000 dynelcm2) and 2.4 kPa (24,000 dynelan2), respectively. H4wever, as indicated in Table 1, a clay gel was farmed instantly in the Control sample.
This demonstrates the advantage of the claimed invention, in whioh clay slurries can be formed with clay contents equal to and greater than the Control sample.
TA~~~ 't :5 RHEOLQGICAL PROPERTIES of cLAY sLURRnS
Clay GanientKGI s~URRY

(lNt..~o) Solud4n Yield PlBStic Sample (wt.96) Stress viscosity (dyr<elcrn2)c Control209~b 09'0 ~~1 Form~sd Instantly A 2090 3% 59 11 -s 2s~ ~~ t or 1 s p $0/N, ~ 3/ 1,468 1,315 pre-serrated with Ki RH>:O1.OGICAL PROPERTIES OF CLAY GELS
Prevursor Com sitionGEI-Cla NUater Precur~or_ _ ~

Clay KCI blurry Extrudin Shear Pressure Stress Gbntent solutionFlowable?(kpg)(dynelcmz)_-~kP~)..
-.

Sample (wt.96) (wt.96) (YesINo) Control20% 0% Nn slurry44 440,000 2.4 2d,000 formed, gel formed instant) A 2096 3% Yes 3.2 82,D00 0.2 2,000 B 26% 3% Yes 71 110,00D O.t3 8,000 G 30% 3% YBS 40 400,000 2.2 22.000 D 30r6. 39'o Yes 77 770,OOCI 4.2 42,000 pre-satursted With K+

Ths extruding pressure and shear stress for the sample t~ gel (l7 kPa {77fl,000 dynelcmZ) and 4.2 kPa (42,000 dyneJcm2y, respectively) was greater 1o than for the sample C gel {40 kPa (400,000 dynelcma) and 2.2 kPa {22,004 dynelcm2), respectively). This indicates that a stronger clay gel can be generated if the clay is first saturated with an inhibitive ration. which has the effect of removing the 1V4+ ions which were originally bound to the clay.
E)CAMPLE ~
A flow diversion test was utilized to demonstrate that a clay slurry, when delivered in situ, blocked flooding through a simulated high permeability channel.
The experimental apparatus, illustrated schematically in Figure 1.
consisted of a 2-dimensional plexiglass visualization chamber 1 of dimensions 19.8 cm x 17.3 crn x 2.2 cm housed in a steel frame 2_ Rerforatidns 3 were uniformly spaced along each side to allow the passage of fluid into and out of the visualization chamber. A fluid space 4 on ontr side of the visualization chamber received brine which was pumped from an Injector cylinder 5 by means of a ARC~Op~ øA
Rusks pump 6. The fluid space 7 on the other side of the visualization chamber was connect$d to a drain S. The visualization chamber was packed with two layers of glass beads: the bottom layer 9 with one glass beads (20 Darcy), to simulate a low permeability zone, and the tap layer 10 with coarse glass beads s (20Qg party) to simulate a high permeability channel, such as a fracture or a wormhole in a geological formation. The apparatus also provided far clay slurry to be pumped into the high permeability layer through an Injector cylinder 11 by means of the Rusks pump 6.
The layers in the visualization c~rnber were first equilibrated with a 0.3 M
to NaCI brine solution. This solution was designed to simulate the, brine commonly found in a geological formation. Before the injection of clay slurry, a blue dye was added to the same brine solution, which was then pumped into the visualization chamber. The colored solution quickly spread through the high permeability layer at the top of the pack, but bypassed the low permeability layer ~s below. In a second run a clay slurry, which had been prepared by mixing 30 weight percent Wyoming bentonite into a 3 weight percent KCI solution containing (~.1 weight percent of sodiufrt acid pyropho$phate, was injected into the high permeability layer. The visualization chamber was then shut in to allow the ration exchange process to occur and a gel to form. After 3 days, the dye-~>o colored NaCI brine solution was agefn injected into the visualization chamber.
This time the bottom, low permeability layer became uniformly colored with blue dye, whereas the upper, high permeability layer remained completely uncolored.
This demonstrated that the clay g~I had been effective In blocking the flow of brine through the upper. high permeability layer. It also demonstrated that the ze clay slurry stopped at the boundary between the high perrneabUity channel and the low permeability zone, and did not invade the low permeability none.
The scope of the invention is defined in th6 ~Glallris now following.

Claims (22)

1. A method for controlling the flow of at least one fluid in a subterranean formation having et least a first region, said first region having (i) at least Na+, and (ii) a first permeability, K1, 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 inhibitive cation and anion;

(b) making a clay slurry by mixing at least about 20 weight percent of clay with said inhibitive electrolyte solution so that, to the extent clay gel is produced, if any, the flowability of said day slurry is not substantially inhibited;

(c) injecting said clay slurry into said formation, so that at least a portion of said clay slurry contacts said first region having at least Na+;

(d) allowing a clay gel to form in said first region so that K, is reduced to produce a lower permeability, K1L, with respect to said fluid; and (e) controlling the flow of majority of said at least one fluid into or from said first region.

2. The method of claim 1 wherein said method is used in the production of hydrocarbons from said formation.

3. The method of claim 1 wherein said method is used to contain a majority of said at least one fluid from said first region into a second region, said second region having at least a portion contiguous with said first region.

4. The method of claim 1 wherein the amount of clay in said clay slurry is in the range from about 20 weight percent to about 40 weight percent.

5. The method of claim 1 wherein the amount of clay in said clay slurry is in the range from about 20 weight percent to about 35 weight percent.

6. The method of claim 1 wherein the amount of clay in said clay slurry is an the range from about 25 weight percent to about 35 weight percent.

7. The method of claim 1 wherein the concentration of the inhibitive compound in said inhibitive electrolyte solution is in the range from about 3 weight percent to about 70 weight percent.

8. The method of claim 1 wherein said cation is selected from the group consisting of Al3+, Cs+, Ca2+, Fe2+, K+, NH4+, N(CH3)4+, Mg2+ and combinations thereof.

9. The method of claim 1 wherein said formation further comprises a second region having a second permeability, K2, with respect to said fluid that is less than K1, said first and second regions being contiguous to each other, wherein, (a) when said clay gel is formed in step (d), said K1L, is less than K2 and, (b) the flow of a majority of said at least one fluid is controlled into or from said first region.

10. A 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, K1, 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 inhibitive cation and anion;

(b) treating a clay with said first inhibitive electrolyte solution;

(c) making a clay slurry having at least about 20 weight percent of the treated clay of step (b) and a second inhibitive electrolyte solution having water and at least one inhibitive compound;

(d) injecting said clay slurry into said formation, so that at least a portion of said clay slurry contacts said first region having at least Na+;

(e) allowing a clay gel to form in said first region so that K1 is reduced to produce a lower permeability, K1L, with respect to said fluid; and (f) controlling the flow of a majority of said at least one fluid into or from at least said first region.

11. The method of claim 10 wherein the treated clay of step (b) has a reduced ability to form a clay gel, if any, when said treated clay is mixed with distilled water.

12. The method of claim 10 wherein the treated clay of step (b) is treated by washing said clay at least once with said first inhibitive electrolyte solution and drying said washed clay.

13. The method of claim 12 further comprising washing said clay with water at least once after said clay is washed at least once with said first inhibitive electrolyte solution but before said clay is dried.

14.The method of claim 12 wherein the concentration of the inhibitive compound in said first inhibitive electrolyte solution is in the range from about 1 weight percent to about 20 weight percent.

15.The method of claim 10 wherein the concentration of the inhibitive compound in said second inhibitive electrolyte solution is in the range from about 1 weight percent to about 10 weight percent.

16.The method of claim 10 wherein the control of said fluid is used in the production of hydrocarbons from said formation.

17.The method of claim 10 wherein the control of said fluid is used to inhibit a rate of flow of at least said fluid from said first region into a second region, said second region having at least a portion contiguous with said first region.

18. The method of claim 10 wherein the amount of clay in said clay slurry is in the range from about 20 weight percent to about 40 weight percent.

19. The method of claim 10 wherein the amount of clay in said clay slurry is in the range from about 24 weight percent to about 35 weight percent.

20. The method of claim 10 wherein the amount of clay in said clay slurry is in the range from about 25 weight percent to about 35 weight percent.

21.The method of claim 10 wherein said cation is selected from the group consisting of AI3+, Cs+, Ca2+, Fe2+,K+, NH4+, N(CH3)4+, Mg2+ and combinations thereof.

22. The method of claim 10 wherein said formation further comprises a second region having a second permeability, K2, with respect to said fluid that is less than K1, said first and second regions being contiguous to each other, wherein (a) when said clay gel is formed in step (e), said K1L. is less than K2 and, (b) the flow of a majority of the said at least one fluid is controlled into or from at least said first region.