# Patent

## Title
Preventing flow through subterranean zones

## Abstract

The present invention provides methods of preventing the flow of water or gas or both through a subterranean zone having a high temperature and a depth such that a long pumping time is required to place a sealing composition therein. The methods basically comprise the steps of preparing a polymeric sealing composition comprised of water, a cross-linking agent and a selected water-soluble polymer which reacts with the cross-linking agent and forms a sealing gel which is stable for a desired period of time at the temperature of the zone and has a pumping time before gelation in the presence of the cross-linking agent whereby the composition can be pumped to the depth of the zone and placed therein. Thereafter, the sealing composition is pumped into the zone and allowed to form a sealing gel therein.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a divisional of application Ser. No. 09/697,368 filed on Oct. 26, 2000 now U.S. Pat. No. 6,607,035, patented 19 Aug. 2003.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to improved methods of preventing water and/or gas flow through a subterranean zone, and particularly to such methods wherein the subterranean zone has a high temperature and is at a depth requiring a long pumping time.
2. Description of the Prior Art
The production of water with oil and/or gas from wells constitutes a major problem and expense in the production of oil and gas. While oil and gas wells are usually completed in hydrocarbon producing formations, when there is a water producing zone adjacent to the hydrocarbon producing formation, the higher mobility of the water often allows it to flow into the hydrocarbon producing formation by way of natural fractures and high permeability streaks. In the production of such wells, the ratio of water to hydrocarbons recovered often becomes so high that the cost of producing the water, separating it from the hydrocarbons and disposing of it represents a significant economic loss.
In order to reduce the production of undesired water and/or gas from hydrocarbon producing formations, aqueous polymer solutions containing cross-linking agents have been utilized heretofore. Such aqueous polymer solutions have been pumped into the hydrocarbon producing formations so that they enter water and/or gas zones within and adjacent to the formations and cross-link therein. The cross-linking of the polymer solutions causes them to form stiff gels which stop or reduce the flow of the undesired water and/or gas.
While the use of aqueous polymer solutions for reducing the production of undesired formation water and/or gas has achieved varying degrees of success, the cross-linking agents used to cross-link the polymer have often contained chromium or phenol/formaldehyde. These cross-linking agents are undesirable in that they either have high toxicities or produce cross-linked gels having low gel stabilities at high temperatures and/or at pH values greater than about 8.
In addition, the aqueous polymer compositions utilized heretofore for reducing the production of undesired water and/or gas from subterranean zones having high temperatures and requiring long pumping times have often been unsuccessful. That is, the compositions have not produced gels which remain stable at the high temperatures involved and/or they have not had the required pumping time at the high temperatures to reach and be placed in the subterranean zone.
Thus, there is a continuing need for improved methods of preventing water flow through subterranean zones.
SUMMARY OF THE INVENTION
The present invention provides improved methods of preventing undesired water and/or gas flow through a subterranean zone penetrated by a well bore. The subterranean zone can have a high temperature and be at a depth such that a long pumping time is required to place a sealing composition therein. The methods basically comprise preparing a polymeric sealing composition comprised of water, a cross-linking agent and a selected water soluble polymer or mixture of polymers which reacts with the cross-linking agent and forms a sealing gel which is stable for a desired period of time at the temperature of the subterranean zone and has a sufficient pumping time before gelation in the presence of the cross-linking agent whereby the composition can be pumped to the depth of the zone and placed therein. Thereafter, the sealing composition is pumped into the zone by way of a well bore penetrating it and the sealing composition is allowed to form a cross-linked gel in the zone.
Depending upon the temperature of the subterranean zone to be sealed and the pumping time required, a selected type of water and a selected water soluble polymer are used. The water is selected from natural seawater, synthetic seawater, brine, unsaturated salt water and fresh water. While various water soluble polymers can be utilized, suitable such polymers are polyacrylamide, AMPS®/acrylamide copolymers, sulfonated styrene/maleic anhydride copolymers, vinylpyrrolidone/AMPS®/acrylamide terpolymers and AMPS®/N-N-dimethylacrylamide/acrylamide terpolymers.
The cross-linking agent utilized in accordance with this invention is preferably chitosan which is a biodegradable and non-toxic glucosamine polymer derived from chitin.
In order to reduce the pumping time of a sealing composition of this invention while maintaining the temperature at which a stable gel is formed, a gelation accelerating agent can be included in the sealing composition. The weight ratio of polymer to cross-linker can also be varied to lengthen or shorten the pumping time.
Thus, it is a general object of the present invention to provide improved methods of preventing undesired water and/or gas flow through a subterranean zone.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.


DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention provides methods of preventing undesired water and/or gas flow through a subterranean zone penetrated by a well bore. As mentioned, the subterranean zone can have a high temperature and be at a depth such that a long pumping time is required to place a sealing composition therein. The term “high temperature(s)” is used herein to mean a temperature in the range of from about 150° F. to about 500° F. The term “long pumping time(s)” is used herein to mean a pumping time in the range of from about 2 hours to about 14 days. The “pumping time” is the length of time between when a polymeric sealing composition of this invention is prepared and when the composition forms a gel and can no longer be pumped into or through a porous rock formation without fracturing the formation.
The methods of this invention basically comprise the steps of preparing a polymeric sealing composition comprised of water, a cross-linking agent and a selected water soluble polymer or mixture of polymers which reacts with the cross-linking agent and forms a sealing gel which is stable for a desired period of time at the temperature of the zone. The polymeric sealing composition also has a pumping time before gelation in the presence of the cross-linking agent whereby the composition can be pumped to the depth of the zone and placed therein. After preparation, the sealing composition is pumped into the zone by way of the well bore penetrating it and is allowed to form a cross-linked sealing gel in the zone.
It has been discovered that the temperature at which gelation occurs, the length of time the resulting gel is stable and the pumping time of an aqueous polymer composition containing a cross-linking agent can all be controlled by the proper selection of the water and the polymer or polymers utilized in the composition. The water can be selected from natural seawater, synthetic seawater, brine, unsaturated salt water and fresh water, and the polymer or polymers can be selected from homopolymers, copolymers, terpolymers and mixtures thereof By increasing the level of bulky monomers in the selected polymer or polymers utilized, the temperature at which the polymer compositions form a gel can be raised, the pump times can be increased at a particular temperature and/or the stability of the gel at a particular temperature can be reduced or increased. A relatively short gel stability can be utilized for temporarily sealing a subterranean zone while a gel having long term stability can be used for sealing a subterranean zone for an extended period of time.
In addition to the particular types of water and polymer utilized in the sealing composition, the gel time and gel stability can be varied by varying the polymer to cross-linking agent weight ratio in the sealing composition and/or including a gelation accelerating agent in the composition.
The sealing compositions useful in accordance with the present invention are comprised of water, a cross-linking agent and a selected water soluble polymer or mixture of polymers which reacts with the cross-linking agent and forms a sealing gel that has a desired pumping time before gelation and is stable for a desired period of time at the temperature of the subterranean zone in which it is placed.
The water in the sealing compositions can be natural seawater, synthetic seawater, brine, unsaturated salt water or fresh water. The particular type of water utilized changes the pumping time of the sealing compositions before gelation occurs. Generally, the pumping time increases with increasing salt concentration in the water. When chitosan is utilized as the cross-linking agent, the water can include an organic or inorganic acid to facilitate the dissolution of the chitosan.
The preferred cross-linking agent for use in accordance with this invention is chitosan which is biodegradable and non-toxic. Chitosan is a polymeric glucosamine derivative of chitin which is a naturally occurring glucosamine polysaccharide structurally similar to cellulose. Chitin is the principal constituent of the shells of crabs, lobsters and beetles.
Chitosan polymers have heretofore been cross-linked with cross-linking agents such as glyoxyl, glutaraldehyde, oxo-acids and acid anhydrides and utilized as highly absorbent materials (see U.S. Pat. No. 5,599,916 issued to Dutkiewiez et al. on Feb. 4, 1997), gels having high adsorbing power (see U.S. Pat. No. 4,835,265 issued to Muzzarelli on May 30, 1989), highly enriched magnetized particles (see U.S. Pat. No. 5,864,025 issued to Glasser et al. on Jun. 26, 1999) and other similar materials (as taught by Roberts and Taylor in the Journal of Applied Polymer Science, Vol. 31, pp. 1169-1176 [1986]).
A variety of water soluble polymers can be utilized in accordance with the present invention. Examples of suitable such polymers include, but are not limited to, polyacrylamide, polyvinyl pyrrolidone AMPS®(2-acrylamido-2-methylpropane sulfonic acid)/acrylamide copolymers, sulfonated styrene/maleic anhydride copolymers, vinylpyrrolidone/AMPS®/acrylamide terpolymers, AMPS®/N-N-dimethylacrylamide/acrylamide terpolymers, polyketones and oxidized polyvinyl alcohol. The water soluble polymers which are used in the present invention can themselves be biodegradable and/or non-toxic. Examples of such polymers include alginic acid derivatives, oxidized polyvinyl alcohol and low molecular weight acrylic polymers.
Generally, by increasing the fraction of bulky or less reactive monomers in the polymer, the temperature at which gelation occurs can be increased, the pumping time at a given temperature can be increased and/or the stability of the gel formed can be increased or decreased. For example, when a subterranean zone to be sealed has a temperature in the range of from about 70° F. to about 230° F. and requires a pumping time in the range of from about 2 hours to about 96 hours, the water soluble polymer utilized in the sealing composition can be polyacrylamide or AMPS®/acrylamide copolymer. When the subterranean zone has a temperature in the range of from about 200° F. to about 350° F. and requires a pumping time in the range of from about 2 hours to about 14 days, the water soluble polymer can be a sulfonated styrene/maleic anhydride copolymer, a vinyl pyrrolidone/AMPS®/acrylamide terpolymer, an AMPS®/N-N-dimethylacrylamide/acrylamide terpolymer or a mixture of such polymers.
While biodegradable and non-toxic chitosan is preferred for use as the organic cross-linking agent in accordance with this invention various other organic cross-linking agents can be utilized in accordance with the present invention. Examples of such organic cross-linking agents are polyalkyleneimines, polyalkylenepolyamines and mixtures thereof. In addition, water-soluble polyfunctional aliphatic amines, arylalkylamines and heteroarylalkylamines can be utilized.
Generally, the water soluble polymer utilized is included in a sealing composition of this invention in an amount in the range of from about 0.5% to about 20% by weight of the composition. The cross-linking agent is included in the sealing composition in an amount in the range of from about 0.05% to about 15% by weight of the composition. The weight ratio of the water soluble polymer to a chitosan cross-linking agent is in the range of from about 50:1 to about 1.1:1.
As mentioned above, the weight ratio of water soluble polymer to cross-linking agent can be varied to change the pumping time before gelation and other properties of the sealing composition. That is, the weight ratio of the water soluble polymer used to the cross-linking agent can be varied in the range of from about 10:1 to about 400:1 to achieve desired results.
The gelation accelerating agent mentioned above which can be utilized to reduce pumping time before gelation at a given temperature can be a pH control compound such as an alkali metal carbonate, bicarbonate or hydroxide, a mineral acid such as hydrochloric acid, an organic acid such as acetic acid, a Lewis acid such as boric acid or other compounds such as ammonium chloride, urea and lactose. Of these, boric acid is preferred. When utilized, boric acid is added to the sealing compositions of this invention in a general amount in the range of from about 0.005% to about 0.1% by weight of the composition.
In order to further illustrate the methods and sealing compositions of this invention, the following examples are given.
EXAMPLE 1
Polymeric sealing compositions of the present invention were prepared utilizing fresh water or unsaturated potassium chloride salt water and a polyacrylamide polymer. The polyacrylamide polymer utilized was a solid comprised of 90 mole percent acrylamide and 10 mole percent acrylate salt from acrylamide hydrolysis. The molecular weight of the polymer was 5,000,000. The cross-linker utilized in the sealing composition was polyethyleneimine.
The polymer solutions which were tested were prepared by mixing the solid polymer with the water in a blender for the time period required to completely disperse and dissolve the polymer in the water. The quantities of water and polymer utilized were such that the resulting solution contained polymer in an amount of 0.7% by weight of the solution. If a gelation accelerator was included in the polymer solution, it was added followed by the cross-linking agent with a few minutes additional mixing in the blender. Thereafter, the pumping time of the gel, i.e., the time required for the polymer composition to gel at a specific temperature, was measured using one of three different pumping time measurement procedures as follows.
Method A—Brookfield Viscosity Method
About 300 milliliters of the polymer composition to be tested were placed in a 400 milliliter beaker. The beaker was then inserted into a preheated, thermostat controlled Brookfield viscometer equipped with a No. 2 spindle. The spinning rate of the spindle was set at 10 rpm and the variation in viscosity was monitored at temperature as a function of time. The time required to reach a viscosity of 5,000 centipoises was taken as the pumping time.
Method B—Torque Rheometer Method
In accordance with this method, a torque rheometer capable of heating the polymer solution tested to temperatures in excess of the boiling point of water and high pressures (up to 10,000 psi) was used to monitor the gelation of the polymer composition by measuring changes in torque as a function of time at a given temperature. The viscosity changes were measured in Bearden units (Bc). The polymer composition was considered to be gelled when its consistency reached 50 Bc.
Method C—Sealed Tube Method
A 16 mm×150 mm borosilicate glass tube with a screw cap or a 10.2 centimeter pressure tube with a screw cap was filled to about one-half full with polymer composition to be tested. The tube was purged with nitrogen and the screw cap was sealed with a high temperature silicone cement. In addition, a TEFLON® plug was used inside the cap. The tube was placed in a steel bomb and the bomb was placed in a preheated oven at the test temperature. The tube and sealing composition were heated in the oven and periodically, the steel bomb was removed from the oven and the tube was removed from the bomb and allowed to cool below about 150° F. Thereafter, the tube was inspected by turning it 90° from the upright position and measuring the % gelation. This measurement was accomplished by using the following equation: % gelation=100×[(L−T)/(L−H)] wherein T is the length of the liquid/gel level when the tube is in the horizontal position, L is the length of the tube and H is the height of the initial fluid column when the tube is in the vertical position. The time needed to reach a % gelation of 94% or higher was taken as the pumping time.
If syneresis of a gel took place after it was formed, the syneresis was measured in the same way as described in Method C above except that H in the equation represents the height of the gel column prior to aging the gel at a given temperature.
The results of the pumping time tests as well as the polymer composition components and quantities of components are given in Table I below.







TABLE I







Pumping Time Tests Using Acrylamide Polymer and Polyethyleneimine Cross-Linking Agent



















pH of Polymer







Cross-linking Agent


Solution
pH of Polymer



Concentration, % by wt.
Type of Gelationn

Before Adding
Solution After


Gel


Test
of Polymer
Accelerator
Type of Water
Cross-linking
Adding Cross-

Pumping
Stability and/or


No.
Composition
Added-Quantity
Used
Agent
linking Agent
Temperature, ° F.
Time, hrs.
Comments










EFFECT OF GELATION TEMPERATURE















1
0.47
None
2% KCl
—
—
156
21.751
—


2
0.47
None
2% KCl
5.56
10.1
176
11.531
—


3
0.47
None
2% KCl
—
—
166
14.751
—


4
1.0
None
Fresh Water
—
—
200
 32
—


5
1.0
None
Fresh Water
—
—
230
 2.52
Syneresis in 35










days


6
1.0
None
2% KCl
—
—
230
 32
Syneresis in 10










days







EFFECT OF GELATION ACCELERATOR















7
0.47
None
2% KCl
—
—
156
21.751
—


8
0.47
Boric Acid-0.12 g
2% KCl
—
—
158
13.721
—


9
0.47
Sodium
2% KCl
—
10.3
167
14.71
—




Carbonate-2%




by wt.


10
0.47
diluted HCl
2% KCl
—
3.63
167
 7.91
—







EFFECT OF CROSS-LINKING AGENT CONCENTRATION















11
0.47
None
2% KCl
—
—
156
21.751
—


12
0.24
None
2% KCl
—
—
157
171
—





1Method A Used 


2Method B Used 






From Table I it can be seen that the polymer compositions containing polyacrylamide had different pumping times depending upon the temperature, the type of water utilized, the weight ratio of polymer to cross-linker and the pH of the composition.
EXAMPLE 2
The test procedure described in Example 1 was repeated except that the polymer utilized in the tests was a solid AMPS®/acrylamide copolymer containing 25 mole percent AMPS® and 75 mole percent acrylamide and having a molecular weight of 7,000,000. The results of these tests are given in Table II below.







TABLE II







Pumping Time Tests Using AMPS ®/Acrylamide Polymer and Polyethyleneimine Cross-Linking Agent















Cross-linking Agent


pH of Polymer
pH of Polymer





Concentration, % by
Type of Gelation

Solution Before
Solution After


Test
wt. of Polymer
Accelerator Added-
Type of Water
Adding Cross-
Adding Cross-

Pumping Time,


No.
Composition
Quantity
Used
linking Agent
linking Agent
Temperature, ° F.
hrs.










EFFECT OF GELATION TEMPERATURE














1
0.47
None
2% KCl
6.0
10.6
146
20.251


2
0.47
None
2% KCl
—
—
167
8.331


3
0.47
None
2% KCl
—
—
186
3.921







EFFECT OF GELATION ACCELERATOR














4
0.47
Boric Acid-0.28 g
2% KCl
—
9.61
152
0.631


5
0.47
Boric Acid-0.28 g
2% KCl
—
—
123
0.581


6
0.47
Boric Acid-0.04 g
2% KCl
—
—
166
4.881


7
0.47
15% Hydrochloric
2% KCl
2.6
—
160
9.671




Acid


8
0.47
1% Sodium
2% KCl
9.37
10.6
160
15.251




Carbonate







EFFECT OF CROSS-LINKING AGENT CONCENTRATION














9
0.07
Boric Acid-0.03 g
2% KCl
—
—
176
1.71


10
0.7
Boric Acid-0.03 g
2% KCl
—
—
166
4.91







EFFECT OF TYPE OF WATER














11
0.47
None
2% KCl
—
—
186
3.91


12
0.47
None
Seq Water2
8.61
10.2
186
3.251


13
0.47
Boric Acid-0.28 g
2% KCl
—
—
152
0.631


14
0.47
Boric Acid-0.28 g
Sea Water2
—
9.32
185
3.51





1Method A 


2Synthetic Seawater 






EXAMPLE 3
The procedure described in Example 1 was repeated except that the polymer utilized in the tests was a solid vinylpyrrolidone/AMPS®/acrylamide terpolymer containing 30 mole percent vinylpyrrolidone, 60 mole percent AMPS® and 10 mole percent acrylamide and having a molecular weight of 5,000,000. The results of the tests showing the effects of temperature are given in Table III below.







TABLE III







Pumping Time Tests Using Vinylpyrrolidone/AMPS ®/Acrylamide


Terpolymer and Polyethyleneimine Cross-Linking Agent-Effects of Temperature













Cross-linking







Agent



Concentration,



% by wt. of


Test
Polymer
Type of Water
Temperature,


No.
Composition
Used
° F.
Pumping Time1, hrs.
Gel Stability and /or Comments















1
2
2% KCl
230
—
Did not gel in 164 hrs.


2
2
2% KCl
275
336
29% Syneresis in 64 days


3
2
2% KCl
300
72 [90% gelation]
Gel broke in 14 days


4
2
1% KCl
260
120
Stable [>84 days]


5
2
1% KCl
275
24
6% syneresis in 85 days


6
1
2% KCl
230
—
Did not gel in 164 days


7
1
2% KCl
260
336
Stable [>94 days]


8
1
2% KCl
275
96
12% Syneresis in 88 days


9
1
2% KCl
300
24
Slight viscosity in 14 days


10
1
2% KCl
325
4
Gel broke in 4 days


11
1
1% KCl
230
24
Stable [>81 days]


12
1
1% KCl
260
21
Stable [>81 days]


13
1
1% KCl
300
2
Gel broke [chunks] in 23 hrs.


14
1
0.5% KCl
230
28
Stable [>74 days]


15
1
0.5% KCl
275
24
Stable [>81 days]


16
1
Fresh Water
230
32
Stable [>94 days]


17
1
Fresh Water
260
28
Stable [>94 days]


18
0.5
2% KCl
260
168
Stable [>86 days]


19
0.5
2% KCl
275
72
12% Syneresis in 74 days


20
0.5
2% KCl
325
3
Gel broke in 6 days


21
0.5
1% KCl
230
28
Stable [>74 days]


22
0.5
1% KCl
260
48
Stable [>74 days]


23
0.5
1% KCl
275
24
Gel broke [chunks]





1Method A 






The results of tests showing the effects of the type of water utilized are given in Table IV below.







TABLE IV







Pumping Time Tests Using Vinylpyrrolidone/AMPS ®/Acrylamide


Terpolymer and Polyethyleneimine Cross-Linking Agent-Effects of Type of Water













Cross-linking







Agent



Concentration,



% by wt. of


Test
Polymer
Type of Water
Temperature,


No.
Composition
Used
° F.
Pumping Time1, hrs.
Gel Stability and/or Comments















1
1
2% KCl
230
—
Did not gel in 7 days


2
1
0.5% KCl
230
28
Stable [>74 days]


3
1
Fresh Water
230
32
Stable [>94 days]


4
0.5
2% KCl
230
144
Stable [>86 days]


5
0.5
1% KCl
230
28
Stable [>74 days]


6
1
2% KCl
260
336
Stable [>94 days]


7
1
1% KCl
260
21
Stable [>81 days]


8
1
Fresh Water
260
28
Stable [>94 days]


9
0.5
2% KCl
260
168
Stable [>86 days]


10
0.5
1% KCl
260
48
Stable [>74 days]


11
2
2% KCl
275
336
29% Syneresis in 64 days


12
2
1% KCl
275
24
6% Syneresis in 85 days


13
1
2% KCl
275
96
12% Syneresis in 88 days


14
1
1% KCl
275
48
5% Syneresis in 88 days


15
1
0.5% KCl
275
24
Stable [>81 days]


16
1
Fresh Water
275
48
Gel broke [chunks]


17
0.5
2% KCl
275
72
12% Syneresis in 74 days


18
0.5
1% KCl
275
24
Gel broke [chunks]


19
1
2% KCl
300
24
Slight viscosity in 14 days


20
1
1% KCl
300
2
Gel broke [chunks] in 23 hrs.





1Method A 






The results of tests showing the effects of a gelation accelerator and the cross-linking agent ration are given in Table V below.







TABLE V







Pumping Time Tests Using Vinylpyrrolidone/AMPS ®/Acrylamide Terpolymer and


Polyethyleneimine Cross-Linking Agent-Effects of Gelation Accelerator and Cross-Linking Agent Concentration















Cross-linking









Agent



Concentration,
Gelation
Type
pH of Polymer



% by wt. of
Accelerator
of
Solution After


Test
Polymer
Added-
Water
Adding Cross-
Temperature,
Pumping Time1,
Gel Stability


No.
Composition
Quantity
Used
linking Agent
° F.
hrs.
and/or Comments

















1
0.47
None
2% KCl
10.8
193
34
—


2
0.47
Boric Acid-
2% KCl
9.8
196
13
—




0.28 g


3
0.25
None
2% KCl
—
230
168
Stable [>64 days]


4
0.50
None
2% KCl
—
230
144
Stable [>86 days]


5
1
None
2% KCl
—
230
—
Did not gel in 168 hr.


6
2
None
2% KCl
—
230
—
Did not gel in 168 hr.


7
0.5
None
1% KCl
—
230
28
Stable [>74 days]


8
1
None
1% KCl
—
230
24
Stable [>74 days]


9
1
None
2% KCl
—
260
336
Stable [>94 days]


10
0.5
None
2% KCl
—
260
168
Stable [>86 days]


11
0.25
None
2% KCl
—
260
96
Stable [>64 days]


12
2
None
2% KCl
—
275
336
29% Syneresis









in 64 days


13
1
None
2% KCl
—
275
96
12% Syneresis









in 88 days


14
0.5
None
2% KCl
—
275
72
12% Syneresis









in 74 days


15
1
None
2% KCl
—
300
24
Slight viscosity









in 14 days


16
2
None
2% KCl
—
300
72
Gel broke in 14 days








[90% gelation]


17
0.5
None
2% KCl
—
325
3
Gel broke in 6 days


18
1
None
2% KCl
—
325
4
Gel broke in 4 days


19
2
None
2% KCl
—
325
15
Weak gel





1Method A 






EXAMPLE 4
The test procedure described in Example 1 was repeated except that various polymers were utilized in the polymer compositions which differed from each other in the type and amounts of bulky monomers present in the polymer. The polymers utilized and the results of the tests are shown in Table VI below.







TABLE VI





Pumping Time Tests Using Various Copolymers


and Terpolymers and Polyethyleneimine Cross-Linking Agent1







Terpolymers, % by weight in polymer composition

















AMPS ®/
AMPS ®/

AMPS ®/
AMPS ®/N-N-
AMPS ®/N-N-
AMPS ®/N-N-
VP4/AMPS ®/




AA2 co-
AA co-

AA co-
DMA3/AA
DMA/AA
DMA
AA
VP/AMPS ®/AA



polymer,
polymer,
/AA copolym
polymer,
terpolymer,
terpolymer,
copolymer,
terpolymer,
terpolymer,


Test
30:70
70:30
AMPS ®er, 80:20
90:10
50:30:20
30:60:10
60:40
30:60:10
60:30:10


No.
mole ratio
mole ratio
mole ratio
mole ratio
mole ratio
mole ratio
mole ratio
mole ratio
mole ratio





1
0.7
—
—
—
—
—
—
—
—


2
—
1.4
—
—
—
—
—
—
—


3
—
1.4
—
—
—
—
—
—
—


4
—
1.4
—
—
—
—
—
—
—


5
—
—
1.4
—
—
—
—
—
—


6
—
—
—
1.4
—
—
—
—
—


7
—
—
—
—
—
—
0.7
—
—


8
—
—
—
—
0.7
—
—
—
—


9
—
—
—
—
0.7
—
—
—
—


10 
—
—
—
—
—
1
—
—
—


11 
—
—
—
—
—
2
—
—
—


12 
—
—
—
—
—
—
—
0.7
—


13 
—
—
—
—
—
—
—
—
2

















Cross-








linking Agent



Concentra-


Test
tion, % by wt.
Type of Gelation



Gel Stability and/or


No.
of polymer composition
Accelerator Added
pH
Temperature, ° F.
Pumping Time, hrs.
Comments





1
0.47
—
—
186
3.9
—


2
1
—
10.9
230
18
—


3
1
—
10.9
275
5
—


4
1
—
10.9
325
1.5
Gel broke in 7 days


5
1
—
10.8
325
2
Gel broke in 7 days


6
1
—
10.9
325
18
Gel broke in 2 days


7
0.47
—
—
325
—
No gel in 4 days


8
1.75
—
—
325
12
Gel broke in 4 days


9
1.75
Boric Acid
—
325
4
Gel broke in 4 days


10 
5
—
—
320
5
Stable (>2 days)


11 
13
—
—
320
24
Stable (>2 days)


12 
1
—
—
325
4
—


13 
16
—
—
320
—
No gel in 2 days





1Type of water used was 2% KCl in all tests 


2AA stands for acrylamide 


3N-N-DMA stands for N-N-dimethylacrylamide 


4VP stands for vinylpyrrolidone 






EXAMPLE 5
The procedure described in Example 1 was repeated except that the polymers in the polymer composition were sulfonated styrene/maleic anhydride copolymers having various ratios of sulfonated styrene to maleic anhydride therein. The molecular weight of the polymers was approximately 10,000. In addition, various polymer to cross-linker weight ratios were tested using various types of water with and without a gelation accelerator. The results of these tests are given in Table VII below.







TABLE VIII







Pumping Time Tests Using Sulfonated Styrene/Maleic Anhydride Polymers And Polyethyleneimine Cross-Linking Agent
















Polymer:









Cross-

Gelation



Sulfonated
linking

Accelerator



Styrene/Maleic
Agent
Type of
Used-% by


Test
Anhydride Mole
Weight
Water
wt. of Polymer
Temperature,
Pumping
Gel Stability and/or


No.
Ratio
Ratio
Used1
Composition
° F.
Time, hrs.
Comments

















1
1:12
1:0.66
Deionized
—
320
10
—





[DI] water


2
1:12
1:0.66
DI water
Boric Acid-
320
10
Gel broke in 7 days






0.003


3
1:12
1:1
DI water
—
320
10
Gel broke in 3 days


4
3:12
1:1
DI water
—
320
2.5
Gel broke in 3 days


5
3:12
1:0.5
2% KCl
—
320
—
No gelation


6
3:12
1:1
2% KCl
—
320
—
No gelation


7
3:13
1:1.5
2% KCl
—
320
—
No gelation


8
1:14
1:0.5
DI water
—
320
—
No gelation


9
1:14
1:1
DI water
—
320
—
No gelation


10
1:14
1:1.5
DI water
—
320
—
No gelation


11
1:14
1:0.5
2% KCl
—
320
—
No gelation


12
1:14
1:1
2% KCl
—
320
—
No gelation


13
1:14
1:1.5
2% KCl
—
320
—
No gelation


14
3:14
1:0.5
DI water
—
320
—
No gelation


15
3:14
1:1
DI water
—
320
70% gel at
Gel broke in 24 hrs.








5.5 hr.


16
3:14
1:1.5
DI water
—
320
5.5
Viscosity loss in 24 hr.


17
3:14
1:1.5
DI water
Urea-.009
320
1.5
Gel broke in 24 hr.


18
3:14
1:1.5
DI water
Boric Acid-
320
4
Gel broke in 24 hr.






.03


19
1:15
1:1.3
DI water
—
320
1.3
Stable [>17 days]


20
1:15
1:1.3
2% KCl
—
320
24
Stable [>7 days]


21
1:15
1:1.3
DI water
Urea-1%
320
1.5
Stable [>7 days]


22
1:15
1:1.3
DI water
—
320
Stable [>17
—








days]





125% by weight solutions of the polymer and 33% by weight solutions of the cross-linking agent were prepared using the type of water indicated. The solutions were used to form the polymer compositions as noted below. 


210 g of the 25% solution of polymer was mixed with 5 g of the 33% solution of cross-linking agent and the mixture was then diluted with an equal volume of DI water. 


3The 25% solution of polymer was diluted to 5% with DI water and then mixed with the 33% solution of cross-linking agent in a weight ratio of 10 g:5 g. 


4The 25% solution of polymer was diluted to 10% with DI water or 2% KCl as noted and was mixed with the 33% cross-linking agent in a weight ratio of 10 g:5 g. 


5Undiluted solutions of polymer and cross-linking agent were mixed in a weight ratio of 10 g:5 g. 






From Tables II-VII above, it can be seen that the pumping time of an aqueous polymer composition of this invention as well as the gel stability and the time the composition remains in a sealed zone at a high temperature can be controlled by the proper selection of the water used, the polymer or polymers used, the polymer to cross-linking agent weight ratio used and the use or non-use of a gelation accelerator.
EXAMPLE 6
Test portions of a high molecular weight acrylamide polymer with about 10% hydrolysis were dissolved in different types of water. A chitosan cross-linking agent solution prepared by dissolving 0.5 gram of chitosan in 100 grams of a 1% by weight acetic acid solution was combined with each test portion, and the test portions were heated to temperatures in the range of from 150° F. to 190° F. The times required for the test portions to gel and the types of gels formed were determined. The polymer to cross-linker weight ratio, water type, pH and temperature for each test portion along with the test results are given in Table VIII below.







TABLE VIII







Cross-Linking Reactions Using Chitosan












Polymer to Cross-
Water Type
Composition





Linker Weight Ratio
Used
pH
Temp., ° F.
Gel Time, Hrs.
Gel Type





14:1
Seawater
5.6
150
42
Lipping


14:1
Fresh Water
5.9
150
No Gelation
—



Fresh Water


14:1
w/2% KCl
5.6
150
No Gelation
—


14:1
Seawater
5.6
170
 8.3
Lipping


14:1
Seawater
5.6
190
 3
Lipping


28
Seawater
5.6
190
No Gelation
—


 7
Seawater
5.6
190
 3.3
Lipping









From Table VIII, it can be seen that chitosan functions well and is useful as a cross-linker in accordance with this invention.
EXAMPLE 7
Test portions of a t-butylacrylate/acrylamide copolymer solution or an AMPS®/acrylamide copolymer solution in synthetic seawater was combined with the chitosan cross-linking agent solution described in Example 6 above. The cross-linked test portions were heated to temperatures in the range of from 170° F. to 200° F. The times required to gel and the types of gel formed were noted. The polymer to cross-linker ratio, water type and temperature for each test portion along with the test results are given in Table IX below.







TABLE IX







Cross-Linking Reactions With Chitosan












Base
Polymer/Cross-Linker
Mix
Temperature
Gel Time
Gel


Polymer
Weight Ratio
Water
(° F.)
(hr)
Type





t-butylacrylate/
14
seawater
170
no gelation
—


acrylamide



in 23 hrs.


t-butylacrylate/
14
seawater
190
5.5
ringing


acrylamide


AMPS ®/
14
seawater
250
48
lipping


acrylamide









Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned as well as those which are inherent therein. While numerous changes may be made by those skilled in the art, such changes are encompassed within the spirit of this invention as defined by the appended claims.

## Claims

1. A composition for preventing the flow of water or gas or both through a subterranean zone penetrated by a well bore comprising:
water; 
a biodegradable and non-toxic cross-linking agent comprised of chitosan; and 
a selected water soluble polymer which reacts with said cross-linking agent and forms a sealing gel which is stable for a desired period of time at the temperature of said zone and has a pumping time before gelation in the presence of said cross-linking agent whereby said composition can be pumped to be depth of said zone and placed therein.

2. The composition of claim 1 wherein said water soluble polymer is selected from the group consisting of polyacrylamide, 2-acrylamido-2-methylpropane sulfonic acid/acrylamide copolymers, sulfonated styrene/maleic anhydride copolymers, vinylpyrrolidone/2-acrylamido-2-methylpropane sulfonic acid/acrylamide terpolymers, 2-acrylamido-2-methylpropane sulfonic acid/N-N-dimethylacrylamide/acrylamide terpolymers and mixtures thereof.

3. The composition of claim 1 wherein said water is selected from the group consisting of natural seawater, synthetic seawater, brine, unsaturated salt water and fresh water.

4. The composition of claim 1 wherein said subterranean zone has a temperature in the range of from about 70° F. to about 230° F., requires a pumping time in the range of from about 2 hours to about 96 hours and said water soluble polymer is polyacrylamide.

5. The composition of claim 1 wherein said subterranean zone has a temperature in the range of from about 70° F. to about 230° F., requires a pumping time in the range of from about 2 hours to about 96 hours and said water soluble polymer is an 2-acrylamido-2-methylpropane sulfonic acid/acrylamide copolymer.

6. The composition of claim 1 wherein said subterranean zone has a temperature in the range of from about 200° F. to about 350° F., requires a pumping time in the range of from about 2 hours to about 14 days and said water soluble polymer is selected from the group of vinylpyrrolidone/2-acrylamido-2-methylpropane sulfonic acid/acrylamide terpolymers, 2-acrylamido-2-methylpropane sulfonic acid/N-N-dimethylacrylamide/acrylamide terpolymers, sulfonated styrene/maleic anhydride terpolymers and mixtures thereof.

7. The composition of claim 1 wherein the weight ratio of said water soluble polymer to said cross-linking agent is in the range of from about 50:1 to about 1.1:1.

8. The composition of claim 1 wherein said water soluble polymer is present in said sealing composition in an amount in the range of from about 0.5% to about 20% by weight of said composition.

9. The composition of claim 1 wherein said cross-linking agent is present in said composition in an amount in the range of from about 0.05% to about 15% by weight of said composition.

10. A biodegradable and non-toxic composition, for sealing subterranean zones comprising water, chitosan and a biodegradable and/or non-toxic cross-linkable polymer.

11. The composition of claim 10 wherein said biodegradable and non-toxic polymer is an alginic acid derivative.