Plugging an abandoned well with a polymer gel

An abandoned well penetrating a subterranean formation is plugged using a gel comprising a high molecular weight, water-soluble carboxylate-containing polymer and a chromic carboxylate complex crosslinking agent. The gel components are combined at the surface and injected into the wellbore to form a continuous single-phase gel therein.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The invention relates to a process for plugging an abandoned well and more 
particularly to a process for plugging an abandoned well with a polymer 
gel wherein the wellbore penetrates a subterranean hydrocarbon-bearing 
formation. 
2. Description of Related Art 
Wells employed in the production of oil and gas are abandoned inter alia 
because they reach the end of their useful life or they become damaged 
beyond repair. State and federal regulations require that abandoned wells 
be plugged to protect safety and environmental interests. The well is 
commonly plugged by pumping Portland cement into the wellbore and curing 
the cement in situ. 
Formulation of Portland cement in the field is largely a product of trial 
and error by field personnel to meet irregularities in the cementing 
composition and the downhole environment. Cement quality control is 
difficult to achieve under such conditions. As a result, Portland cement 
cured in situ can exhibit cracking, shrinking, or poor adhesion to 
wellbore tubulars. An imperfect Portland cement plug can enable 
undesirable fluid flow through the wellbore via leaks along or through the 
cement. 
A process is needed which employs a plugging material having a broad range 
of highly controllable and predictable set-up times providing ease of 
operation and design at a relatively low cost. A process is needed 
employing a substitute material for Portland cement in conventional 
plugging processes which forms a more effective plug and seals the 
wellbore indefinitely. 
SUMMARY OF THE INVENTION 
The present invention provides a process for plugging an abandoned well. 
The well is plugged by means of a plugging material comprising a 
tailor-made crosslinked polymer gel. The gel contains a high molecular 
weight, water-soluble carboxylate-containing polymer and a chromic 
carboxylate complex crosslinking agent. The gel is prepared by forming a 
uniform gelation solution above ground containing the polymer and 
crosslinking agent, pumping the solution into a wellbore penetrating a 
hydrocarbon-bearing formation and curing the solution to a gel in the 
wellbore. The gelation solution may be advantageously designed to be at 
least partially gelled by the time it reaches the wellbore to inhibit or 
prevent its propagation into a less permeable subterranean material, such 
as a formation matrix, which may adjoin the wellbore face where no 
plugging is required. The gelation solution sets up in the wellbore 
without requiring the further injection of any additional components. The 
gel is a continuous single-phase material which substantially plugs the 
well. 
The plugging material of the present invention generally outperforms 
Portland cement at a comparatively lower cost. The gelation solution, as 
initially injected into the wellbore, is a uniform solid-free solution 
which avoids bridging and reduces fluid loss. Biocides or other special 
chemicals can be readily incorporated into the gelation solution if 
desired. 
The mature gel resulting from the solution forms a tenacious chemical bond 
with the rock of the wellbore face and the wellbore tubulars to provide a 
tight seal. The gel is substantially impermeable to subterranean fluids 
and permanent. It is resistant to in situ degradation and corrosion. The 
gel does not shrink or crack, yet is sufficiently deformable to fill 
microvoids in the treatment region. The gel is sufficiently strong to 
resist displacement from the plugged well by natural forces, but is 
sufficiently elastic to accommodate minor shifts in the earth's surface 
without cracking or faulting. 
If it is desired to return an abandoned well to operation after plugging, 
the present process can be reversed by dissolving the gel with a suitable 
solvent and pumping the dissolved gel to the surface. Alternatively, the 
gel can be drilled out or mechanically removed with relative ease. 
The gel employed in the present invention possesses a broad range of highly 
controllable and predictable set-up times and strengths. The process is 
applicable to a broad range of temperatures, salinities, rock formations, 
and environments. The practitioner can customize or tailor a gel for 
specific operational constraints, downhole characteristics and subsequent 
performance demands. One can predetermine the gelation rate and resultant 
gel strength and stability which are required of a gel to plug the 
wellbore. Thereafter, a gel having the required predetermined properties 
is produced under controlled conditions at the surface by utilizing 
observed correlations between specific controllable gelation parameters 
and resultant gel properties. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is described in the context of specific terms which 
are defined as follows. "Gel" as used herein is directed to a continuous 
three-dimensional crosslinked polymeric network having an ultra high 
molecular weight. The gel contains a liquid medium such as water which is 
confined within the solid polymeric network. The fusion of a liquid and a 
solid component into a single-phase system provides the gel with a unique 
phase behavior. Gels employed by the present invention have sufficient 
structure so as not to substantially propagate from the confines of a 
wellbore into a less permeable region of the formation adjoining the 
wellbore when injected into the wellbore. "Plugging" is the substantial 
prevention of fluid flow from a subterranean formation and through the 
wellbore. 
"Partially gelled" solutions are also referred to herein. A partially 
gelled solution is at least somewhat more viscous than an uncrosslinked 
polymer solution such that it is incapable of entering a less permeable 
region where no treatment is desired, but sufficiently fluid such that it 
is capable of displacement into a desired treatment zone. The crosslinking 
agent of the partially gelled solution has reacted incompletely with the 
polymer with the result that neither all of the polymer nor all of the 
crosslinking agent in the gelation solution is totally consumed by the 
crosslinking reaction. The partially gelled solution is capable of further 
crosslinking to completion resulting in the desired gel without the 
addition of more crosslinking agent. 
"Crosslinked to completion" means that the gel composition is incapable of 
further crosslinking because one or both of the required reactants in the 
initial solution are consumed. Further crosslinking is only possible if 
either polymer, crosslinking agent, or both are added to the gel 
composition. 
The gel composition utilized in the present invention is comprised of a 
carboxylate-containing polymer and a crosslinking agent. The 
carboxylate-containing polymer may be any crosslinkable, high molecular 
weight, water-soluble, synthetic polymer or biopolymer containing one or 
more carboxylate species. The average molecular weight of the 
carboxylate-containing polymer is in the range of about 10,000 to about 
50,000,000 and preferably about 100,000 to about 20,000,000, and most 
preferably about 200,000 to about 15,000,000. 
Biopolymers useful in the present invention include polysaccharides and 
modified polysaccharides. Exemplary biopolymers are xanthan gum, guar gum, 
carboxymethylcellulose, o-carboxychitosans, hydroxyethylcellulose, 
hydroxypropylcellulose, and modified starches. Useful synthetic polymers 
include inter alia acrylamide polymers, such as polyacrylamide, partially 
hydrolyzed polyacrylamide and terpolymers containing acrylamide, acrylate, 
and a third species. As defined herein, polyacrylamide (PA) is an 
acrylamide polymer having substantially less than 1% of the acrylamide 
groups in the form of carboxylate groups. Partially hydrolyzed 
polyacrylamide (PHPA) is an acrylamide polymer having at least 1%, but not 
100%, of the acrylamide groups in the form of carboxylate groups. The 
acrylamide polymer may be prepared according to any conventional method 
known in the art, but preferably has the specific properties of acrylamide 
polymer prepared according to the method disclosed by U.S. Pat. No. Re. 
32,114 to Argabright et al incorporated herein by reference. 
The crosslinking agent is a chromic carboxylate complex. The term "complex" 
is defined herein as an ion or molecule containing two or more 
interassociated ionic, radical or molecular species. A complex ion as a 
whole has a distinct electrical charge while a complex molecule is 
electrically neutral. The term "chromic carboxylate complex" encompasses a 
single complex, mixtures of complexes containing the same carboxylate 
species, and mixtures of complexes containing differing carboxylate 
species. 
The complex of the present invention includes at least one or more 
electropositive chromium III species and one or more electronegative 
carboxylate species. The complex may advantageously also contain one or 
more electronegative hydroxide and/or oxygen species. It is believed that, 
when two or more chromium III species are present in the complex, the 
oxygen or hydroxide species may help to bridge the chromium III species. 
Each complex optionally contains additional species which are not 
essential to the polymer crosslinking function of the complex. For 
example, inorganic mono- and/or divalent ions, which function merely to 
balance the electrical charge of the complex, or one or more water 
molecules may be associated with each complex. Representative formulae of 
such complexes include: 
[Cr.sub.3 (CH.sub.3 CO.sub.2).sub.6 (OH).sub.2 ].sup.+1 ; 
[Cr.sub.3 (OH).sub.2 (CH.sub.3 CO.sub.2).sub.6 ]NO.sub.3.6H.sub.2 O; 
[Cr.sub.3 (H.sub.2 O).sub.2 (CH.sub.3 CO.sub.2).sub.6 ].sup.+3 ; 
[Cr.sub.3 (H.sub.2 O).sub.2 (CH.sub.3 CO.sub.2).sub.6 ](CH.sub.3 
CO.sub.2).sub.3.H.sub.2 O; etc. 
Trivalent chromium and chromic ion are equivalent terms encompassed by the 
term chromium III species as used herein. The carboxylate species are 
advantageously derived from water-soluble salts of carboxylic acids, 
especially low molecular weight mono-basic acids. Carboxylate species 
derived from salts of formic, acetic, propionic, and lactic acid, lower 
substituted derivatives thereof and mixtures thereof are especially 
preferred. The carboxylate species include the following water-soluble 
species: formate, acetate, propionate, lactate, lower substituted 
derivatives thereof, and mixtures thereof. The optional inorganic ions 
include sodium, sulfate, nitrate and chloride ions. 
A host of complexes of the type described above and their method of 
preparation are well known in the leather tanning art. These complexes are 
described in Shuttleworth and Russel, Journal of The Society of Leather 
Trades' Chemists, "The Kinetics of Chrome Tannage Part I.," United 
Kingdom, 1965, v. 49, p. 133-154; "Part III.," United Kingdom, 1965, v. 
49, p. 251-260; "Part IV.," United Kingdom, 1965, v. 49, p. 261-268; and 
Von Erdman, Das Leder, "Condensation of Mononuclear Chromium (III) Salts 
to Polynuclear Compounds," Eduard Roether Verlag, Darmstadt, Germany, 
1963, v. 14, p. 249; and are incorporated herein by reference. Udy, Marvin 
J., Chromium, Volume 1: Chemistry of Chromium and its Compounds, Reinhold 
Publishing Corp., N.Y., 1956, pp. 229-233; and Cotton and Wilkinson, 
Advanced Inorganic Chemistry 3rd Ed., John Wiley & Sons, Inc., N.Y., 1972, 
pp. 836-839, further describe typical complexes which may be within the 
scope of the present invention and are incorporated herein by reference. 
The present invention is not limited to the specific complexes and 
mixtures thereof described in the references, but may include others 
satisfying the above-stated definition. 
The gel is formed by admixing a carboxylate-containing polymer and 
crosslinking agent at the surface to form an injectable gelation solution. 
Surface admixing broadly encompasses inter alia mixing the solution in 
bulk at the surface prior to injection or simultaneously mixing the 
solution at or near the wellhead by in-line mixing means while injecting 
it. Admixing is accomplished for example by dissolving the starting 
materials for the crosslinking agent in an appropriate aqueous solvent. 
Exemplary starting materials include solid CrAc.sub.3.H.sub.2 O, solid 
Cr.sub.3 Ac.sub.7 (OH).sub.2 or a solution labeled "Chromic Acetate 50% 
Solution" commercially available, for example, from McGean Chemical Co., 
Inc., 1250 Terminal Tower, Cleveland, Ohio 44113, U.S.A. The crosslinking 
agent solution is then mixed with an aqueous polymer solution to produce 
the gelation solution. Among other alternatives, the starting materials 
for the crosslinking agent can be dissolved directly in the aqueous 
polymer solution to form the gelation solution in a single step. 
The aqueous solvent of the gelation solution may be fresh water or a brine 
having a total dissolved solids concentration up to the solubility limit 
of the solids in water. Inert fillers such as crushed or naturally fine 
rock material or glass beads can be added to the gelation solution to 
reinforce the gel network structure, although a solid-free solution is 
preferred. Special chemicals, such as biocides, can also be added to the 
gelation solution as required to solve specific problems. 
The present process enables the practitioner to customize or tailor-make a 
gel having a predetermined gelation rate and predetermined gel properties 
of strength and stability from the above-described composition. The 
gelation rate is defined as the degree of gel formation as a function of 
time or, synonymously, the rate of crosslinking in the gelation solution. 
The degree of crosslinking may be quantified in terms of gel viscosity 
and/or strength. Gel strength is defined as the coherence of the gel 
network or resistance to deformation under external forces. Stability is 
defined as either thermal or phase stability. Thermal stability is the 
ability of a gel to withstand temperature extremes without degradation. 
Phase stability is the ability of a gel to resist syneresis which can 
detract from the gel structure and performance. 
Tailor-making or customizing a gel in the manner of the present invention 
to meet the demands of a particular plugging application is provided in 
part by correlating the independent gelation parameters with the dependent 
variables of gelation rate and resultant gel strength and stability. The 
independent gelation parameters are the surface and in situ gelation 
conditions including: temperature, pH, ionic strength and specific 
electrolytic makeup of the solvent, polymer concentration, ratio of the 
weight of polymer to the combined weight of chromium III and carboxylate 
species in the mixture, degree of polymer hydrolysis, and average 
molecular weight of the polymer. 
The operable ranges of the gelation parameters are correlated with the 
dependent variables of gelation rate and resultant gel properties by means 
including qualitative bottle testing and quantitative viscosimetric 
analysis. The operable ranges of a number of gelation parameters and their 
correlation with the dependent variables are described below. 
The lower temperature limit of the gelation solution at the surface is the 
freezing point of the solution and the upper limit is essentially the 
thermal stability limit of the polymer. The solution is generally 
maintained at ambient temperature or higher at the surface. The 
temperature may be adjusted by heating or cooling the aqueous solvent. 
Increasing the temperature within the prescribed range increases the 
gelation rate. 
The initial pH of the gelation solution is within a range of about 3 to 13 
and preferably about 6 to 13. Although gelation can occur at an acidic pH, 
lowering the initial pH of the solution below 7 does not favor gelation. 
The initial pH of the solution is most preferably alkaline, i.e., greater 
than 7 to about 13. Increasing the pH within the prescribed range 
increases the rate of gelation. 
The polymer concentration in the solution is about 500 ppm up to the 
solubility limit of the polymer in the solvent or the rheological 
constraints of the polymer solution, preferably about 1000 to about 
200,000 ppm, and most preferably about 3000 to about 100,000. Increasing 
the polymer concentration increases the gelation rate and ultimate gel 
strength at a constant ratio of polymer to crosslinking agent. 
The ionic strength of the solvent can be from that of deionized distilled 
water to that of a brine having an ion concentration approaching the 
solubility limit of the brine. Increasing the ionic strength of the 
solution can increase the gelation rate. 
The weight ratio of polymer to chromium III and carboxylate species 
comprising the mixture is about 1:1 to about 500:1, preferably about 2.5:1 
to about 100:1, and most preferably about 5:1 to about 40:1. Decreasing 
the ratio generally increases the gelation rate and up to a certain point 
generally increases the gel strength, especially at a constant high 
polymer concentration. 
The degree of hydrolysis where an acrylamide polymer is employed is about 0 
to 60% and preferably about 0 to 30%. Within the preferred range, 
increasing the degree of hydrolysis increases the gelation rate. 
Increasing the molecular weight of the polymer increases the gel strength. 
It is apparent from these correlations that one can produce gels across a 
very broad range of gelation rates and gel properties as a function of the 
gelation conditions. Thus, to effect an optimum plugging job according to 
the present process, the practitioner predetermines the gelation rate and 
properties of the resultant gel which meet the demands of the given 
wellbore and thereafter produces the gel having these predetermined 
characteristics. The demands of the wellbore include the in situ gelation 
conditions such as temperature, connate water properties, size of the 
treatment volume, the pressure drop and permeability of the adjoining 
matrix as well as the post treatment conditions such as shut-in pressures. 
Analytical methods known to one skilled in the art are used to determine 
these demands which provide criteria to predetermine the gelation rate and 
resultant gel properties in the manner described above and continuing 
hereafter. 
The gelation rate is advantageously sufficiently slow to enable preparation 
of the gelation solution at the surface and injection of the solution as a 
uniform slug into the wellbore. Too rapid a gelation rate produces 
excessive gelation of the solution at the surface which results in a 
solution that may be difficult, if not impossible, to inject into the 
wellbore to be plugged due to its rheological properties. At the same 
time, the gelation rate must be sufficiently rapid to enable completion of 
the reaction within a reasonable period of time to satisfy regulatory 
requirements. 
The solution may be substantially ungelled before reaching the wellbore. 
However, at least partial gelation of the solution may be advantageous 
before the solution reaches the wellbore face to prevent the solution from 
penetrating the permeable rock bounding the wellbore. Substantial 
penetration of such material by the solution results in unnecessary and 
wasteful consumption of the solution. The solution advantageously gels to 
completion in the wellbore. The values of the independent variables in the 
process are carefully selected to achieve a gelation rate meeting these 
criteria. 
The amount of solution injected into the wellbore is a function of the 
volume of the wellbore to be plugged and its performance demands. One 
skilled in the art can determine the required amount of a gel to plug a 
given wellbore. 
The injection rate is a function of the gelation rate and operational 
constraints of injection pressure and pumping limits. The required 
injection rate is fixed such that all of the solution can be practically 
injected into the volume before it becomes unpumpable. The gelation time 
of the gel ranges from near instantaneous up to 48 hours or longer. Longer 
gelation times are limited by practical considerations. 
Gels having a predetermined gelation rate and resultant gel properties to 
meet the demands of a given well are produced by adjusting and setting the 
surface gelation conditions as they correlate to the gelation rate and gel 
properties. Accordingly, the gels are produced in a manner which renders 
them insensitive to most extreme formation conditions. The gels can be 
stable at formation temperatures as high as 130.degree. C. or more and at 
any formation pH contemplated. The gels are relatively insensitive to the 
stratigraphy of the rock, metal tubulars and other materials and chemicals 
employed in cementing operations. The gels can be employed in carbonate 
and sandstone strata and unconsolidated or consolidated strata having 
varying mineralogy. The gels are substantially insoluble in the formation 
fluids. Once the gels are in place, they are substantially permanent and 
resistant to in situ degradation. 
If it is desired to reactivate a plugged well, the gels may be reversible 
on contact with sodium hypochlorite, hydrogen peroxide or any other 
suitable peroxo compound. Alternatively, the gels can be drilled or bailed 
out of the wellbore. 
The process is applicable to most plugging applications where Portland 
cement is presently used, simply by substituting the gel for the Portland 
cement and selecting the gelation conditions in the manner described 
herein. Although the invention has been described in the context of oil 
field injection and production wells, the plugging process is applicable 
to virtually any well one desires to plug, including abandoned waste 
injection wells, water wells, etc. 
The strength of the gel can vary from an elastic jelly-like material to a 
rigid rubber-like material. The stronger materials are generally preferred 
where extreme shut-in pressures are encountered which could cause a weak 
plug to fail. PA is often preferred for such formulations because it has a 
slower gelation rate than PHPA which enables one to inject it into the 
wellbore before it sets up. 
The following examples demonstrate the practice and utility of the present 
invention but are not to be construed as limiting the scope thereof. 
Most of the examples are formatted as tables of data which describe the 
formulation and maturation of one or more gels. Each gel is represented in 
a table by a single experimental run. Data include the conditions for 
producing the gel and the quantitative or qualitative strength of the 
produced gel. The tables display data in a three-tier format. The first 
tier is the values of the fixed gelation conditions which are constant and 
common to every run in the table. The second tier is the values of the 
gelation conditions which vary among the different runs in the table but 
are constant for any given run. The third tier is the gel strength which 
varies as a function of time within each run. Qualitative gel strength is 
expressed in alphabetic code. Quantitative gel strength is simply the 
numerical value of apparent viscosity. 
The following gel strength code and nomenclature are useful for 
interpreting the tables. 
Gel Strength Code 
A No detectable continuous gel formed: the bulk of the solution appears to 
have the same viscosity as the original polymer solution although isolated 
local gel balls may be present. 
B Highly flowing gel: the gel appears to be only sightly more viscous than 
the initial polymer solution. 
C Flowing gel: most of the gel flows to the bottle cap by gravity upon 
inversion. 
D Moderately flowing gel: only a small portion (5-10%) of the gel does not 
readily flow to the bottle cap by gravity upon inversion (usually 
characterized as a tonguing gel). 
E Barely flowing gel: the gel can barely flow to the bottle cap and/or a 
significant portion (&gt;15%) of the gel does not flow by gravity upon 
inversion. 
F Highly deformable nonflowing gel: the gel does not flow to the bottle cap 
by gravity upon inversion. 
G Moderately deformable nonflowing gel: the gel deforms about half way down 
the bottle by gravity upon inversion. 
H Slightly deformable nonflowing gel: only the gel surface slightly deforms 
by gravity upon inversion. 
I Rigid gel: there is no gel surface deformation by gravity upon inversion. 
J Ringing rigid gel: a tuning fork-like mechanical vibration can be felt 
upon tapping the bottle. 
Nomenclature 
% Hydrolysis: % of carboxylate groups on the acrylamide polymer based on 
the total number of acrylamide groups 
Polymer MW: average molecular weight of the acrylamide polymer 
Polymer Conc: acrylamide polymer concentration in the polymer solution 
(ppm) 
Polymer Solvent: aqueous solvent in the polymer solution 
Polymer pH: pH of the polymer solution 
Total Ion Conc: total concentration of chromium III and acetate ions in the 
gelation solution (ppm) 
Weight Ratio Polymer:Ions: weight ratio of acrylamide polymer to chromium 
III and acetate ions in the gelation solution 
Metal Ion Conc: chromium III concentration in the gelation solution 
Temp: gelation temperature (.degree.C.) 
Time: gelation time (hr) 
Gel Code: gel strength code 
Viscosity: apparent viscosity of the gelation solution at about 0.1 
sec.sup.-1 shear rate (cp) 
Pressure: viscometer pressure (kPa)

The polymer solutions of the following examples are prepared by diluting 
aqueous acrylamide polymer solutions with an aqueous solvent. Where 
qualitative data are obtained, the dilute polymer solution is combined 
with a crosslinking agent solution in a 0.12 liter widemouth bottle to 
form a 0.05 liter sample. The sample is gelled in the capped bottle and 
the qualitative gel strength is determined by periodically inverting the 
bottle. 
Where quantitative data are obtained, the polymer solution and crosslinking 
agent solution are combined in a variable pressure and temperature 
rheometer (viscometer), having an oscillatory mode of 0.1 rad/sec and 100% 
strain. The apparent viscosity at a shear rate of about 0.1 sec.sup.-1 is 
recorded as a function of time. 
In all of the examples, the crosslinking agent solution is that used in the 
present invention (i.e., a complex or mixture of complexes comprised of 
chromium III and acetate ions). The crosslinking agent solution of the 
present invention is prepared by dissolving solid CrAc.sub.3.H.sub.2 O or 
CrAc.sub.7 (OH).sub.2 in water or diluting a solution obtained 
commercially under the label of "Chromic Acetate 50% Solution". 
EXAMPLE 1 
About 2000 liters of a gelation solution is prepared by mixing a 2.2% by 
weight solution of a polyacrylamide in fresh water with a chromic 
carboxylate complex. The polyacrylamide has a molecular weight of 
11,000,000. The weight ratio of polyacrylamide to complex is 20:1. The 
solution is placed in an abandoned injection well which is 244 m deep and 
cased with 10.3 cm I.D. casing. The gelation solution cures to a gel in 
the well after 72 hours. An injection leakoff test using water is 
conducted on the gel plugged well. No leakoff (i.e., &lt;35 kPa) is detected 
in a 30 minute test at 6900 kPa applied water pressure. 
EXAMPLE 2 
__________________________________________________________________________ 
% Hydrolysis: 30 
Polymer MW: 5,000,000 
Polymer Conc: 8350 
Polymer Solvent: 5,000 ppm NaCl in aqueous solution 
Polymer pH: 10.6 
Run Number 
1 2 3 4 5 6 7 8 9 
__________________________________________________________________________ 
Metal Ion 
52 105 210 
420 
630 
105 210 
420 
630 
Conc 
Total Ion 
250 500 1000 
2000 
3000 
500 1000 
2000 
3000 
Conc 
Weight Ratio 
33 16.7 
8.4 4.2 2.8 16.7 
8.4 4.2 2.8 
Polymer:Ions 
Temp 22 22 22 22 22 60 60 60 60 
Time Gel Code 
0.5 A A A A A B B C C 
1.0 A A A A A C C D E 
1.5 A A A A A D E G H 
2.0 B B B B B E F H I 
4.0 B B B C D G G H I 
8.0 B B C D E G H I J 
24 D E E F H G H I J 
48 E E E G I G H I J 
80 G G G H I G H I J 
168 G G H I J G I J J 
2040 G G H I J G I J J 
__________________________________________________________________________ 
The data show that gelation rate and gel strength increase as the 
temperature increases and as the weight ratio of polymer to ions 
decreases. 
EXAMPLE 3 
______________________________________ 
% Hydrolysis: 30 
Polymer MW: 5,000,000 
Polymer Conc: 8350 
Polymer Solvent: 
5,000 ppm NaCl in aqueous solution 
Temp: 22 
Run Number 
1 2 3 4 5 6 7 8 
______________________________________ 
Polymer pH 
10.6 10.6 10.6 10.6 8.0 8.0 8.0 8.0 
Metal Ion 
105 210 420 630 105 210 420 630 
Conc 
Total Ion 
500 1000 2000 3000 500 1000 2000 3000 
Conc 
Weight Ratio 
16.7 8.4 4.2 2.8 16.7 8.4 4.2 2.8 
Polymer:Ions 
Time Gel Code 
0.5 A A A A A A A A 
1.0 A A A A A A A A 
1.5 A A A A A A A A 
2.0 B B B B A A A A 
2.5 B B B B A A A A 
4.0 B B C D A B B B 
5.0 B C D D A B B B 
6.0 B C D E A B B B 
7.0 B C D E A B B B 
8.0 B C D E B B B B 
24 E E F G B B B C 
28 E E G I B B B C 
48 E E G I B B B C 
80 G G H I B C C G 
168 G H I J C E G H 
2040 G H I J E E G -- 
______________________________________ 
Run Number 
9 10 11 12 13 14 15 16 
______________________________________ 
Polymer pH 
7.0 7.0 7.0 7.0 6.0 6.0 6.0 6.0 
Metal Ion 
105 210 420 630 105 210 420 630 
Conc 
Total Ion 
500 1000 2000 3000 500 1000 2000 3000 
Conc 
Weight Ratio 
16.7 8.4 4.2 2.8 16.7 8.4 4.2 2.8 
Polymer:Ions 
Time Gel Code 
0.5 A A A A A A A A 
1.0 A A A A A A A A 
1.5 A A A A A A A A 
2.0 A A A A A A A B 
2.5 A A A A A A A A 
4.0 A A A A A A B B 
5.0 A A B B A A B B 
6.0 A B B B A A B B 
7.0 A B B B A B B B 
8.0 B B B B B B B B 
24 B C C C B B C C 
28 B C D E B C D F 
48 B C D E B C D F 
80 B C D E B C D F 
168 B D E H D D E G 
2040 E F G -- E F G -- 
______________________________________ 
Run Number 17 18 19 20 
______________________________________ 
Polymer pH 4.0 4.0 4.0 4.0 
Metal Ion 105 210 420 630 
Conc 
Total Ion 500 1000 2000 3000 
Conc 
Weight Ratio 
16.7 8.4 4.2 2.8 
Polymer:Ions 
Time Gel Code 
0.5 A A A A 
1.0 A A A A 
1.5 A A A A 
2.0 A A A B 
2.5 A A A B 
4.0 A A A B 
5.0 A A B B 
6.0 A A B C 
7.0 A A B C 
8.0 A B B C 
24 B C D D 
28 B C D D 
48 B C D E 
80 B C F G 
168 B D G I 
2040 D G -- -- 
______________________________________ 
The data show that gelation rate and gel strength decrease as pH of the 
polymer solution decreases. 
EXAMPLE 4 
______________________________________ 
% Hydrolysis: &lt;1 
Polymer MW: 11,000,000 
Polymer Conc: 20,000 
Polymer Solvent: Denver Tap Water 
Temp: 60 
Run Number: 
1 2 3 
______________________________________ 
Weight Ratio 
40 20 10 
Polymer:Ions 
Time Viscosity 
0.0 940,000 940,000 
940,000 
0.5 500,000 1,300,000 
1,300,000 
1.0 800,000 2,300,000 
2,300,000 
2.0 1,100,000 2,800,000 
3,500,000 
4.0 1,200,000 3,200,000 
4,300,000 
8.0 1,300,000 3,400,000 
4,700,000 
12 1,300,000 3,400,000 
4,700,000 
16 1,400,000 3,400,000 
4,700,000 
20 1,400,000 3,400,000 
4,700,000 
______________________________________ 
Viscosity data confirm the observations of Examples 2 and 3 that decreasing 
the weight ratio of polymer to ions increases the gelation rate. 
The mature gel of Run 2 exhibits properties of a Bingham plastic. Its yield 
stress is determined by attempting to flow the gel through a 1.4 mm 
diameter orifice at an applied pressure for four hours. The gel does not 
flow through the orifice at applied pressures up to 3450 kPa for tests 
conducted at temperatures ranging from 22.degree. C. to 124.degree. C. 
EXAMPLE 5 
______________________________________ 
% Hydrolysis: &lt;1 
Polymer MW: 11,000,000 
Polymer Conc: 20,000 
Polymer Solvent: Denver Tap Water 
Weight Ratio Polymer:Ions: 
20 
Run Number 1 2 3 
______________________________________ 
Temp 23 43 60 
Time Viscosity 
0.0 50,000 50,000 50,000 
0.2 50,000 50,000 875,000 
0.5 50,000 100,000 
1,400,000 
1.0 60,000 200,000 
2,250,000 
2.0 75,000 600,000 
2,900,000 
4.0 100,000 1,125,000 
3,275,000 
8.0 125,000 1,800,000 
3,400,000 
12 175,000 2,100,000 
3,425,000 
16 200,000 2,300,000 
3,425,000 
20 300,000 2,500,000 
3,425,000 
______________________________________ 
Viscosity data confirm that increasing the temperature increases the 
gelation rate. 
EXAMPLE 6 
______________________________________ 
% Hyorolysis: &lt;1 
Polymer MW: 11,000,000 
Polymer Conc: 20,000 
Polymer Solvent: Denver Tap Water 
Weight Ratio Polymer:Ions: 
20 
Temp: 60 
Run Number: 
1 2 3 
______________________________________ 
Pressure 690 3450 6900 
Time Viscosity 
0.0 91,000 91,000 91,000 
0.2 250,000 800,000 
250,000 
0.5 800,000 1,400,000 
800,000 
1.0 1,700,000 2,200,000 
2,000,000 
2.0 2,300,000 2,800,000 
2,700,000 
3.0 2,500,000 3,200,000 
3,200,000 
4.0 2,700,000 3,300,000 
3,400,000 
8.0 2,750,000 3,400,000 
3,500,000 
12 2,800,000 3,400,000 
3,500,000 
16 2,800,000 3,400,000 
3,500,000 
20 2,800,000 3,400,000 
3,500,000 
______________________________________ 
Viscosity data show that gelation rate is a weak function of pressure. The 
rate increases slightly with pressure. 
EXAMPLE 7 
______________________________________ 
Polymer MW: 5,000,000 
Polymer Conc: 10,000 
Polymer Solvent: Denver Tap Water 
Polymer pH: 8.0 
Temp: 22 
Metal Ion Conc: 207 
Total Ion Conc: 990 
Weight Ratio Polymer:Ions: 
10.0 
Run Number 1 2 
______________________________________ 
% Hydrolysis 30 2 
Time Gel Code 
0.25 B A 
2.0 B A 
3.0 C A 
4.0 C A 
5.0 C B 
8.0 E B 
23 F B 
48 E B 
72 F B 
103 F B 
268 G B 
______________________________________ 
The data show that the rate of gelation of partially hydrolyzed 
polyacrylamide is considerably faster than that of substantially 
unhydrolyzed polyacrylamide. Thus, the gelation rate of an acrylamide 
polymer solution can be slowed by reducing the degree of hydrolysis of the 
acrylamide groups. 
EXAMPLE 8 
______________________________________ 
Polymer MW: 11,000,000 
Polymer Conc: 12,500 
Polymer Solvent: Denver Tap Water 
Temp: 40 
Weight Ratio Polymer:Ions: 
20 
Pressure: 3450 
Run Number 1 2 
______________________________________ 
% Hydrolysis 30 2 
Polymer pH 10 9 
Time Viscosity 
0 190,000 8,000 
0.1 255,000 10,000 
0.5 300,000 15,000 
1 350,000 25,000 
2 415,000 40,000 
3 460,000 70,000 
4 500,000 100,000 
8 575,000 210,000 
11 600,000 300,000 
14 605,000 355,000 
18 605,000 425,000 
20 605,000 460,000 
36 605,000 610,000 
______________________________________ 
Viscosity data confirm the observations of Example 7. 
EXAMPLE 9 
______________________________________ 
% Hydrolysis: &lt;1 
Polymer MW: 5,000,000 
Polymer Conc: 20,000 
Polymer Solvent: Denver Tap Water 
Polymer pH: 8.8 
Run Number 
1 2 3 4 5 6 7 8 
______________________________________ 
Metal Ion 
404 206 104 52.2 404 206 104 52.2 
Conc 
Total Ion 
1420 980 495 244 1920 980 495 249 
Conc 
Weight Ratio 
10 20 40 80 10 20 40 80 
Polymer:Ions 
Temp 22 22 22 22 60 60 60 60 
Time Gel Code 
0.5 A A A A B B A A 
1.0 A A B B G G F C 
2.0 A A B B I I I I 
3.0 B B B B J J J J 
5.0 B B B B J J J J 
6.0 C C C C J J J J 
7.0 E E E E J J J J 
8.0 G G F F J J J J 
25 H H H H J J J J 
48 H H H H J J J J 
96 I I I I J J J J 
120 I I I I J J J J 
144 J J J J J J J J 
1032 J J J J J -- -- -- 
______________________________________ 
The data show that there is a functional relation between gelation rate and 
temperature for polyacrylamide gels as well as partially hydrolyzed 
polyacrylamide gels. 
EXAMPLE 10 
______________________________________ 
% Hydrolysis: 30 
Polymer MW: 5,000,000 
Polymer Solvent: Distilled Water 
Polymer pH: 8.0 
Temp: 22 
Weight Ratio Polymer:Ions: 
10 
Run Number 
1 2 3 4 5 6 
______________________________________ 
Metal Ion 310 207 157 
105 63 42 
Conc 
Total Ion 1,480 990 747 
499 300 
200 
Conc 
Polymer Conc 
15,000 10,000 7,500 
5,000 3,000 
2,000 
Time Gel Code 
0.25 B B B A A A 
0.5 C B B A A A 
1.0 C B B A A A 
2.0 E B B A A A 
3.0 E C B A A A 
4.0 G C B A A A 
5.0 I C B B A A 
8.0 I E C B B A 
23 I F C B B A 
48 I F C C B A 
72 I G D C B A 
103 I G F C B B 
268 I H F D C B 
______________________________________ 
The data show that decreasing the polymer concentration while maintaining 
the same weight ratio of polymer to ions substantially decreases the 
gelation rate and gel strength. 
EXAMPLE 11 
______________________________________ 
% Hydrolysis: &lt;1 
Polymer Solvent: Denver Tap Water 
Weight Ratio Polymer:Ions: 
10 
Temp: 60 
Run Number 1 2 3 
______________________________________ 
Polymer Conc 
30,000 20,000 15,000 
Time Viscosity 
0.0 3,000 3,000 3,000 
0.2 700,000 700,000 
200,000 
0.5 1,700,000 1,600,000 
400,000 
1.0 3,200,000 2,300,000 
800,000 
2.0 4,000,000 2,800,000 
1,000,000 
4.0 4,600,000 3,300,000 
1,200,000 
8.0 4,600,000 3,300,000 
1,200,000 
20 4,600,000 3,300,000 
1,200,000 
______________________________________ 
Viscosity data confirm the observations of Example 10 that gelation rate 
decreases as polymer concentration decreases. 
EXAMPLE 12 
______________________________________ 
% Hydrolysis: 30 
Polymer Conc: 3,000 
Polymer Solvent: 
3,000 ppm NaCl in aqueous solution 
Polymer pH: 10.1 
Temp: 22 
Metal Ion Conc: 155 
Total Ion Conc: 600 
Weight Ratio Polymer:Ions: 
5.0 
Run Number 1 2 
______________________________________ 
Polymer MW 5,000,000 
11,000,000 
Time Gel Code 
0.5 A A 
1.0 A B 
3.0 A B 
4.0 B C 
5.0 B E 
11 B E 
24 C F 
48 C G 
56 D G 
101 D G 
156 E G 
192 E G 
269 F G 
______________________________________ 
The data show that gelation rate and gel strength increase as the molecular 
weight of the polymer increases. 
EXAMPLE 13 
______________________________________ 
% Hydrolysis &lt;1 
Polymer Conc: 20,000 
Polymer Solvent: Denver Tap Water 
Weight Ratio Polymer:Ions: 
20 
Temp: 60 
Run Number 1 2 
______________________________________ 
Polymer MW 5,000,000 
11,000,000 
Time Viscosity 
0.0 100,000 
100,000 
0.5 300,000 
1,400,000 
1.0 800,000 
2,200,000 
2.0 1,300,000 
2,800,000 
4.0 1,800,000 
3,200,000 
6.0 2,000,000 
3,300,000 
8.0 2,100,000 
3,400,000 
12 2,200,000 
3,400,000 
16 2,200,000 
3,400,000 
______________________________________ 
Viscosity data confirm the observations of Example 12. 
Examples 2-13 show that the gelation rate of the polymer and crosslinking 
agent of the present invention can be adjusted to any desired rate and gel 
strength by selecting the values of the independent variables such as 
polymer or crosslinking agent concentration, polymer molecular weight, 
temperature, pH, etc. This is particularly useful in customizing a gel for 
a specific plugging application. 
Various salts and gases commonly found in oil field brines are added to the 
gelation solution of Examples 14 and 15 to determine the sensitivity of 
the gelation reaction to in situ fluids. 
EXAMPLE 14 
______________________________________ 
% Hydrolysis: 30 
Polymer MW: 5,000,000 
Polymer Conc: 8350 
Polymer pH: 9.6 
Temp: 22 
Metal Ion Conc: 259 
Total Ion Conc: 1,000 
Weight Ratio Polymer:Ions: 
8.4 
Run Number 1 2 3 4 
______________________________________ 
Polymer Solvent 
fresh 3,000 ppm 10,000 ppm 
29,200 ppm 
water NaCl NaCl NaCl 
Time Gel Code 
0.5 B B B B 
1.0 B C D D 
2.0 B C D D 
3.0 B D D D 
4.0 B D D D 
5.0 B E E E 
7.0 B E E E 
24 D F F F 
51 G G G G 
79 I I I I 
480 I I I I 
______________________________________ 
EXAMPLE 15 
______________________________________ 
% Hydrolysis: 30 
Polymer MW: 5,000,000 
Polymer Conc: 8350 ppm 
Polymer Solvent: 
5,000 ppm NaCl in aqueous solution 
Polymer pH: 7.0 
Temp: 22 
Metal Ion Conc: 259 
Total Ion Conc: 1000 
Weight Ratio Polymer:Ions: 
8.4 
Run Number 
1 2 3 4 5 
______________________________________ 
Additive: none CO.sub.2 
NaNO.sub.3 
MgCl.sub.2 
CaCl.sub.2 
(control) 
Additive Conc 
-- satu- 2000 2000 1000 
(ppm): rated 
solu- 
tion 
Time Gel Code 
1.0 A A A A A 
4.0 A A A A A 
5.0 B B B B B 
6.0 B B B B B 
7.0 B B B B B 
8.0 C C C C C 
24 C C C C C 
72 D D C D D 
120 E E E E E 
264 E E E E F 
288 E E E E F 
408 E E E E F 
______________________________________ 
Run Number 
6 7 8 9 10 
______________________________________ 
Additive: Na.sub.2 SO.sub.4 
NH.sub.4 Cl 
KCl NaHCO.sub.3 
Na.sub.2 CO.sub.3 
Additive Conc 
3000 100 400 2000 100 
(ppm): 
Time Gel Code 
1.0 A A A A A 
4.0 A A A A A 
5.0 B B B B B 
6.0 B B B B B 
7.0 B B C B B 
8.0 C C C B B 
24 C C C C C 
72 D D D D D 
120 E D D E E 
264 F F F F F 
288 F F F F F 
408 F F F F F 
______________________________________ 
The data of Examples 14 and 15 show that the gelation reaction is 
relatively insensitive to these additives. 
Examples 16-18 utilize actual or synthetic field injection waters in the 
gelation solutions. 
EXAMPLE 16 
______________________________________ 
% Hydrolysis: 30 
Polymer MW: 11,000,000 
Polymer Conc: 3000 
Polymer Solvent: 
Synthetic Field Injection Water A* 
Polymer pH: 10.4 
Metal Ion Conc: 77.5 
Total Ion Conc: 299 
Weight Ratio Polymer:Ions: 
10.0 
Run Number 1 2 
______________________________________ 
Temp 22 43** 
Time Gel Code 
0.50 B B 
0.75 C C 
1.0 C D 
1.5 C D 
2.0 D E 
6.0 D E 
8.0 E E 
35 F F 
168 F F 
240 G F 
269 G G 
504 G G 
______________________________________ 
*Synthetic Field Injection Water A has the following composition: 
g/l 
CaSO.sub.4.H.sub.2 O 
0.594 
MgSO.sub.4 
0.788 
NaHCO.sub.3 
1.53 
CaCl.sub.3 
0.655 
Na.sub.2 SO.sub.4 
1.52 
K.sub.2 SO.sub.4 
0.452 
**Temperature of Field A. 
EXAMPLE 17 
______________________________________ 
% Hydrolysis: 30 
Polymer MW: 5,000,000 
Polymer Solvent: 
Actual Field Injection Water B* 
Temp: 60** 
Run Number 1 2 3 4 
______________________________________ 
Polymer Conc 3000 4000 5000 8000 
Polymer pH 8.5 8.5 8.5 9.0 
Metal Ion 54.5 72.6 64.9 90.7 
Conc 
Total Ion 240 320 286 399 
Conc 
Weight Ratio 12.5 12.5 17.5 20 
Polymer:Ions 
Time Gel Code 
0.5 A A A A 
1.0 A A A C 
1.5 A B B D 
2.0 B D D E 
3.0 C D D F 
4.0 D D D F 
5.0 D E E F 
12 D E E F 
27 D D D F 
504 D D D F 
______________________________________ 
*Actual Field Injection Water B has a TDS of 0.58%, 
H.sub.2 S &gt;100 ppm, and is comprised of the following primary ionic 
constituents: 
ppm 
Na.sup.+ 
252 
Mg.sup.2+ 
97 
Ca.sup.2+ 
501 
Cl.sup.- 
237 
SO.sub.4.sup.2- 
1500 
HCO.sub.3.sup.- 
325 
**Temperature of Field B. 
EXAMPLE 18 
______________________________________ 
% Hydrolysis: 30 
Polymer Solvent: 
Synthetic Field Injection Water C* 
Polymer pH: 7.5 
Temp: 22** 
Weight Ratio Polymer:Ions: 
15 
Run Number 
1 2 3 
______________________________________ 
Polymer MW 
11,000,000 11,000,000 
11,000,000 
Polymer Conc 
3,000 5,000 8,000 
Metal Ion Conc 
45.4 75.7 121 
Total Ion Conc 
200 333 533 
Time Gel Code 
0.25 A A A 
0.5 A A B 
4.0 A A B 
5.0 A A C 
6.0 A B C 
7.0 A C D 
24 B D D 
96 C D G 
150 D D G 
197 D D H 
936 D D H 
______________________________________ 
*Synthetic Field Injection Water C has the following compositon: 
g/l 
Na.sub.2 CO.sub.3 
0.249 
NH.sub.4 Cl 
0.085 
CaCl.sub.2 
0.821 
MgCl.sub.2.6H.sub.2 O 
1.78 
Na.sub.2 SO.sub.4 
1.09 
NaCl 4.80 
NaHCO.sub.3 
2.09 
**Temperature of Field C. 
Examples 16-18 show that gels can be formulated in saline actual field 
waters. The concentration and proportions of gel components can be 
selected to form stable gels even in complex injection waters such as B 
and C at the formation temperature. 
While the foregoing preferred embodiments of the invention have been 
described and shown, it is understood that the alternatives and 
modifications, such as those suggested and others, may be made thereto and 
fall within the scope of the invention.