Cement compositions containing a polysaccharide and a salt and method of cementing casing in a well

A novel cement composition comprising (1) cement, (2) at least one salt, and (3) at least one polysaccharide or mixture of polysaccharides having a solution time greater than about 10 minutes; aqueous slurries prepared from these novel cement compositions and a method of cementing casing in the borehole of a well using such novel aqueous cement slurries.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
This invention is directed to a novel cement composition for the 
preparation of a novel aqueous cement slurry particularly useful in 
cementing casing in the borehole of a well. The novel cement composition 
comprises (1) an API Class "A" through "H" cement, (2) at least one 
polysaccharide having specially defined solubility characteristics, and 
(3) at least one salt. 
2. Description of the Art: 
After a borehole of an oil or gas well has been drilled, casing is run into 
the well and is cemented in place by filling the annulus between the 
borehole wall and the outside of the casing with a cement slurry, which is 
then permitted to set. The resulting cement provides a sheath surrounding 
the casing that prevents, or inhibits, communication between the various 
formations penetrated by the well. In addition to isolating oil, gas and 
water-producing zones, cement also aids in (1) bonding and supporting the 
casing, (2) protecting the casing from corrosion, (3) preventing blowouts 
by quickly forming a seal, (4) protecting the casing from shock loads in 
drilling deeper, and (5) sealing off zones of lost circulation. The usual 
method of cementing a well is to pump a cement slurry downwardly through 
the casing, outwardly through the lower end of the casing with a shoe 
and/or float valve and then upwardly into the annulus surrounding the 
casing. The upward displacement of the cement slurry through the annulus 
can continue until some of the cement slurry returns to the well surface, 
but in any event will continue past the formations to be isolated. 
If the primary cementing of the casing, as described above, does not 
effectively isolate the formations, it may become necessary to perforate 
the casing at intervals along its length and then squeeze a cement slurry 
under high pressure through the perforations and into the defined annulus 
to plug any channels that may have formed in the cement sheath. Squeezing 
is an expensive operation that requires bringing perforating and cementing 
service companies back to the well and is therefore to be avoided, if 
possible. 
It is critical in preparing cement compositions useful in cementing casing 
in the borehole of a well that they be characterized by a viscosity 
designed for optimum mixing at varying ambient temperatures, even at, or 
near, freezing temperatures, flow properties sufficient to facilitate and 
maintain lower laminar and/or plug flow and adequate gel strength to 
provide thixotropic properties to the slurry when pumping ceases. 
Cement slurries, using conventional polysaccharide additives, thin or 
become less viscous with increasing temperatures, result in turbulent flow 
at high displacement rates, lose their suspension properties or 
capabilities as they become thinner or less viscous; and have a tendency 
to commingle with drilling fluids. If mixed with high-density additives, 
separation may occur at high temperatures, and such slurries are difficult 
to mix at low temperatures. 
Slurries of decreased viscosities may channel through drilling fluids. 
Turbulent flow may also erode the wall of the borehole. If suspension 
properties are lost, water channels can be created in the slurries that 
allow gas, oil or water to migrate up or down (depending on the 
differential pressure and direction of pressure). Commingling of cement 
and drilling fluid results in contamination of the cement and will result 
in a poor cement bond and lower compressive strengths. When a slurry is 
mixed under freezing conditions, the slurry density can be lower than 
desired and will result in lower compressive strengths and insufficient 
hydrostatic pressures to contain formation pressures. 
SUMMARY OF THE INVENTION 
We have found that the above difficulties can be obviated using the novel 
cement compositions defined and claimed herein, since the novel aqueous 
cement slurries prepared from our novel cement compositions (1) will have 
flow properties that will prevent or substantially inhibit turbulent flow 
in the annulus during displacement of the slurry, (2) will provide 
superior suspensions, (3) does not affect particle separation at high 
temperatures when slurry density is increased, and (4) can easily be mixed 
at low temperatures. All of these desired features are achieved because 
the initial viscosity at ambient conditions of the aqueous cement slurries 
of this invention increase with time and temperature, which means the 
slurries are easy to mix at ambient temperatures on the surface but 
increase in viscosity which is desirable under downhole conditions where 
the temperature gradient increases. 
The novel cement composition that will provide the above desired 
characteristics comprises (1) an API Class "A" through "H" cement, (2) at 
least one polysaccharide having a solubility characteristic such that the 
time for an aqueous solution of the polysaccharide to reach 90 percent of 
its final viscosity value at a pH from 9.2 to 9.6; a temperature of about 
23.degree. C. and a shear rate of 300 rpm is greater than about 10 
minutes, and (3) at least one salt. Preferably the compositions will 
contain, in addition, a dispersant. 
The cement, or first, component of the novel cement composition suitable 
for preparing the novel aqueous slurry can be any of the cements defined 
as Classes "A" through "H" in API, Spec. 10, First Edition, page 6, or in 
ASTM Bulletin C150. 
The second necessary component of the novel cement compositions claimed 
herein is a polysaccharide or a mixture of polysaccharides which have a 
solubility characteristic such that the time for an aqueous solution of 
the polysaccharide or mixture of polysaccharides to reach 90 percent of 
their final viscosity value at a pH of 9.2 to 9.6 at a temperature of 
about 23.degree. C. and a shear rate of 300 rpm is greater than about 10 
minutes, preferably from 12 to about 120 minutes, and more preferably from 
about 14 to about 40 minutes. 
A polysaccharide is a polymer made up of repeating units of 
monosaccharides. The latter are the simplest members of the carbohydrate 
family and can be defined by the formula C.sub.n H.sub.2n O.sub.n, wherein 
n is equal to or greater than 4, with n preferably being below 10. These 
polysaccharides can have molecular weights in the range of about 10,000 to 
about 10,000,000, but generally will be in the range of about 100,000 to 
about 3,000,000. By "polysaccharide," we mean to include unsubstituted as 
well as substituted derivatives thereof, examples of which include 
hydroxyalkyl substituents, such as hydroxyethyl, hydroxypropyl and 
hydroxybutyl, carboxymethyl, cyanoethyl, etc. Examples of polysaccharides 
that can be employed herein include cellulose, guar gum, starch, 
alginates, cargeenan, gum agar, gum arabic, gum ghatti, gum karaya, gum 
tragacanth, locust bean gum, pectins, tamarind gum and xanthan gum. In 
addition, derivatives, such as hydroxyethylcellulose ether, 
hydroxypropylcellulose ether, carboxymethylhydroxyethylcellulose ether, 
carboxymethylcellulose ether, hydroxypropylguar, hydroxypropylstarch, 
hydroxyethylstarch, cyanoethylguar, cyanoethylcellulose, etc., can be 
used. Of these, we prefer to use hydroxyethylcellulose ether, 
hydroxypropylcellulose ether, xanthan gum and 
carboxymethylhydroxyethylcellulose ether. 
The polysaccharide can be man-made, such as those prepared and sold by such 
companies as Hercules Inc. under the trade name Natrosol or Union Carbide 
Corporation under the trade name Cellosize. The preferred 
hydroxyethylcellulose ether (HEC) is available in varying viscosities and 
preferably the HEC has a viscosity above about 200 centipoises when 
measured in a 5 weight percent aqueous solution but less than about 6000 
centipoises when measured in a 1 weight percent aqueous solution. In a 
preferred range the viscosity will be from about 1000 to about 10,000 
centipoises when measured in a 2 weight percent aqueous solution. The 
critical viscosity of the hydroxypropylcellulose ether must be above about 
100 centipoises when measured in a 2 weight percent aqueous solution, but 
less than about 10,000 centipoises when measured in a 1 weight percent 
aqueous solution. In a preferred range, the viscosity will be from about 
1000 to about 3000 centipoises when measured in a 1 weight percent aqueous 
solution. 
The degree of substitution and the molar substitution of the 
hydroxyalkylcellulose ethers used herein are also important. By "degree of 
substitution" we mean the average number of total substituents present per 
glucose unit, while by "molar substitution" we mean the number of mols of 
ethylene oxide or propylene oxide that are attached to each glucose unit. 
The degree of substitution can be in the range of about 0.5 to about 3.0, 
preferably from about 0.9 to about 2.8. The molar substitution can be in 
the range of about 0.5 to about 10.0, preferably from about 1.0 to about 
6.0. It is understood that the hydroxyethylcellulose ether can also carry 
some propylene oxide substituents and, similarly, hydroxypropylcellulose 
ether can also carry some ethylene oxide units. 
The polysaccharide can also be prepared by microbial action. That is, the 
polysaccharide or mixture of polysaccharides, preferably extracellular 
polysaccharides, produced as a result of microbial action, which 
polysaccharides are generally hydrophilic colloidal materials. 
The important feature of this invention is that the polysaccharide or 
mixture of polysaccharides employed must possess a critical solubility 
characteristic in order to prepare the cement compositions or aqueous 
slurries of this invention which are especially useful in cementing casing 
in the borehole of a well. The desired solubility characteristic is such 
that the solution time of the polysaccharide or mixture of polysaccharides 
is greater than about 10 minutes. 
The test procedure to determine the solution time of a given polysaccharide 
or mixture of polysaccharides under the above conditions is quite simple 
and well known to those having ordinary skill in the art. It is known, for 
example, that the solution time of a polysaccharide is greatly influenced 
by temperature, pH, and the rate of shear (see, for example, Hercules 
Technical Information Bulletin VC-507, also labeled HER 27477). The 
solution time is defined for this application as the time in minutes 
required for an aqueous solution of a given polysaccharide or mixture of 
polysaccharides to develop 90 percent of its final viscosity at a 
temperature of 23.degree. C..+-.2.degree. C., a pH of 9.4.+-.0.2 and a 
shear rate of 300 rpm. The solution times which are required for the 
polysaccharide or mixture of polysaccharides to prepare the novel 
compositions of this invention are greater than about 10 minutes, 
preferably from 12 to about 120 minutes, and most preferably from about 14 
to 40 minutes. It has also been found that the solution times for the 
various polysaccharides are roughly additive so that mixtures of 
polysaccharides can be employed so long as the mixture has a solution time 
of greater than about 10 minutes. 
As noted above, the polysaccharides useful in the preparation of the novel 
compositions of this invention can be obtained from companies such as 
Hercules and Union Carbide Corporation. It has been found that the 
preferred hydroxyethylcellulose ethers, which initially may fail to have 
the proper solution times, can be heat treated under conditions set forth 
below in a shallow-dish heating vessel to ensure even heating throughout 
to produce an HEC having the desired solution times. Yet another technique 
which has been found valuable in producing an HEC having the desired 
solution times is to crosslink the HEC during its preparation or after 
with a 1,2-dicarbonyl such as glyoxal. In many instances the HEC which is 
available from Hercules or others contains glyoxal which is intended to 
aid in preventing the HEC from clumping when the HEC is admixed with 
water. In accordance with this invention an HEC or other polysaccharide or 
mixture of polysaccharides can be treated with a sufficient amount of a 
1,2-dicarbonyl to result in a final polysaccharide or mixture of 
polysaccharides which have a solution time of greater than about 10 
minutes. Alcoholic hydroxyl groups such as are present on HEC are known to 
react with aldehydes to produce hemi-acetals under acid or base 
conditions. The hemi-acetals can be further reacted with alcohol to form 
an acetal plus water. The formation of acetals and hemi-acetals are 
reversable reactions. However, while the hemi-acetal linkage is hydrolized 
under both acid and base conditions, the hydrolysis of an acetal linkage 
is slow under basic conditions. The cement slurry environment is, of 
course, highly basic. It is theorized that the polysaccharides which have 
solution times greater than about 10 minutes are polysaccharides which are 
crosslinked with acetal rather than hemi-acetal linkages formed perhaps by 
the reaction of the hydroxal groups on the polysaccharide with the 
aldehyde group on the 1,2-dicarbonyl, i.e., glyoxal. Other means of 
crosslinking the polysaccharides where the linkages are slowly hydrolized 
in basic media so that the solution times of the polysaccharide are within 
the limit set forth above, are also acceptable. For example, a sample of 
HEC was obtained from Hercules Inc. and was found by analysis to contain 
0.65 percent by weight glyoxal. This material was found to have a solution 
time of 6 minutes. As will be shown in Example 27 below, the solution time 
of this same HEC was found to increase significantly when the material was 
treated with glyoxal. 
The 1,2-dicarbonyl suitable for use can be defined by the following 
formula: 
##STR1## 
wherein R.sub.1 and R.sub.2, the same or different, are members selected 
from the group consisting of hydrogen, alkyl radicals having from one to 
eight, preferably from one to four, carbon atoms; aryl radicals having 
from six to 20, preferably from six to 10, carbon atoms; alkenyl radicals 
having from two to 12, preferably from two to four, carbon atoms; 
cycloalkyl radicals having from three to 10, preferably from three to six, 
carbon atoms; and aralkyl and alkaryl radicals having from six to 20, 
preferably from six to 12, carbon atoms. Specific examples include glyoxal 
(R.sub.1 .dbd.R.sub.2 .dbd.H), biacetyl (R.sub.1 .dbd.R.sub.2 
.dbd.CH.sub.3) and benzil (R.sub.1 .dbd.R.sub.2 .dbd.phenyl). 
The third necessary component is at least one salt. By "salt" we intend to 
include organic as well as inorganic salts. Particularly effective are 
inorganic monovalent and polyvalent metal salts, such as magnesium 
chloride, ammonium chloride, potassium chloride, sodium chloride, calcium 
chloride, aluminum chloride, stannous chloride, and sodium borate. Of 
these we prefer potassium chloride. The amount of salt to employ is 
suitably from 0.001 to 4.5 weight percent based on the weight of cement 
and is preferably from 0.1 to 3 weight percent based on the weight of 
cement. 
Optionally, a dispersant is also present to improve flowability and assist 
the water in wetting the cement particles. By "dispersant" we mean to 
include any anionic surfactant, that is, any compound which contains a 
hydrophobic (for example, any hydrocarbon substituent, such as alkyl, aryl 
or alkaryl group) portion and a hydrophilic (for example, any negatively 
charged moiety, such as O.sup.-, CO.sub.2.sup.- or SO.sub.3.sup.-) 
portion. We prefer to use sulfonic acid derivatives of aromatic or 
aliphatic hydrocarbons, such as naphthalene sulfonic acid formaldehyde 
condensation product derivatives, particulary their sodium or potassium 
salts. Examples of dispersants that can be used include lignosulfonates; 
CFR-2, a sulfonate dispersant sold by the Haliburton Company; sodium 
naphthalene sulfonate formaldehyde condensation products, such as DAXAD-19 
of W. R. Grace Company, Lomar D. of Diamond Shamrock Company, D 31 of B. 
J. Hughes Company, and D 65 of Dowell Company; and potassium naphthalene 
sulfonate formaldehyde condensation products, such as DAXAD 11 KLS of W. 
R. Grace Company. 
Other additives conventionally added to cement compositions useful in 
cementing casings in the borehole of a well can also be added to the novel 
cement composition herein. These additives can include, for example, (1) 
heavy-weight additives, such as hematite, ilmenite, silica flour and sand; 
(2) cement retarders, such as lignins and lignosulfonates; and (3) 
additives for controlling lost circulations, such as walnut hulls and 
cellophane flakes. 
The novel aqueous cement slurry can be prepared in any suitable or 
conventional manner, for example, by mixing the dry ingredients before 
addition to the aqueous solution or by adding the individual components to 
an aqueous slurry of cement. 
Table I below defines the amounts of each of the components that can be 
used to prepare the novel aqueous cement slurry claimed herein, based on 
the weight of the dry cement. 
TABLE I 
______________________________________ 
Weight Percent 
Broad Range 
Preferred Range 
______________________________________ 
Polysaccharide 
0.001-3.0 0.01-2.0 
Salts* 0.1-7 0.5-5 
Water 30-65 33-60 
Dispersant 0-3.0 0.1-2.0 
______________________________________ 
*Based on the weight of water 
A method of cementing casing in the borehole of a well herein can comprise 
suspending the casing in the borehole of a well, whether vertical or 
slanted, pumping into the well the novel aqueous cement slurry herein 
until said slurry fills that portion of the space desired to be sealed and 
then maintaining said slurry in place until the cement sets. In a 
preferred embodiment the novel cement slurry herein can be pumped 
downwardly into the casing that has been suspended in the borehole of a 
well, and then circulated upwardly into the annulus surrounding the 
casing. Circulation can continue until the slurry fills that portion of 
the annular space desired to be sealed and can continue until the cement 
slurry returns to the surface. The cement slurry is then maintained in 
place until the cement sets. The cement so produced will result in a 
strong, continuous, unbroken bond with the outside surface of the casing 
and with the wall of the formation.

DESCRIPTION OF PREFERRED EMBODIMENTS 
A number of cement slurries were prepared and tested using Class H cement. 
In all runs, the purpose was to prepare a cement slurry containing varying 
amounts of a polysaccharide(s) plus other additives and thus determine the 
rheological properties of the slurry at 300 R. The cement slurries were 
prepared in accordance with API Spec 10, page 16, First Edition, January 
1982. The runs are summarized in Table II below. In each of Runs Nos. 1, 
and 3 through 12, in Table II below, the amount of cement used in 
preparing the slurries was 800 grams. The viscosity of each slurry in the 
above runs at ambient and elevated temperatures were determined in 
accordance with API Spec. 10, First Edition, January 1982, Appendix H, 
page 77, except that the slurry was transferred to the sample cup for 
ambient temperature readings immediately after mixing. The high pressure 
consistometer was used for the downhole readings at 140.degree. F. In all 
of the remaining Runs in Table II below, 500 grams of Class H cement were 
used except Example 35, where no cement was employed. In all of these 
remaining Runs, the API viscosity was determined using the atmospheric 
pressure method described in Appendix H (page 77) to API Spec. 10, First 
Edition, January 1982, except that the slurry was transferred to the 
sample cup for ambient temperature (70.degree. F.) readings immediately 
after mixing. In addition, downhole readings were obtained after heating 
the sample with stirring on a water bath at atmospheric pressure to 
140.degree. F. (usually 8 to 10 minutes required). In Runs Nos. 7 through 
32 and 34 and 35, the carboxymethylhydroxyethylcellulose ether (CMHEC) was 
sold by Hercules Co. as CMHEC 37L. In Runs Nos. 1 and 3 through 32 and 35, 
the hydroxyethylcellulose ether (HEC) was sold by Hercules as Natrosol 250 
MBR. In Runs No. 2 through 32 and 34 and 35, the xanthan gum was sold as 
Kelzan XCD by Kelco Co. Run 33 used hydroxypropyl starch and Run 34 used 
guar gum instead of hydroxyethylcellulose. 
The procedure for measuring the "Solution Time" in all runs was as follows: 
To 340 ml water vigorously stirring at room temperature is added 7.00 g 
DAXAD-19 to form a first solution. 2.00 g of the sample to be tested are 
then added to the first solution in such a way that the sample is poured 
into the vortex of the first solution to facilitate dispersion of the 
sample. After about 30 seconds, the stirring is stopped and the vessel is 
placed on a viscosity measuring device such as a Fann VG meter or the 
equivalent. The viscosity is recorded as a function of time. Using the 
Fann VG meter, and setting the shear rate at 300 rpm, readings were taken 
at 1, 2, 3, 4, 5, 10, 15, 20 minutes, etc. until a constant viscosity was 
obtained. 
It is important to control the pH and the temperature during this 
determination. The pH obtained by dissolving 7.00 g DAXAD-19 in 340 ml 
water is 9.4.+-.0.2. A valid test is obtained only if the pH remains at 
9.4.+-.0.2 after the sample to be tested is added. This essentially means 
that the sample to be tested must be per se near neutral pH. 
The temperature must also be held constant during the test. A valid test is 
obtained if the temperature is maintained at 23.+-.2.degree. during the 
test. 
The value of the solution time is obtained by determining the time 
necessary to reach 90 percent of the final viscosity value. 
TABLE II 
__________________________________________________________________________ 
Solution 
HEC Lot.sup.2 
Time of 
Xanthan 
CMHEC 
Dispersant 
KCl API 
Example 
WT. % Designation 
HEC Gum WT. % 
WT. % 
BWOC WT. % 
WT. % Temp. Viscosity 
Number.sup.1 
BWOC for HEC 
in Minutes 
BWOC BWOC DAXAD-19 
BWOW .degree.F. 
Cp at 300 
__________________________________________________________________________ 
R) 
1 0.208.sup.3 
A 25 0 0 0 3 72 (22.2) 
168 
140 
(60) 
184 
2 0 -- -- 0.208.sup.4 
0 0 3 72 (22.2) 
127 
152 
(67) 
93 
3 0.208 A 25 0.0113 0 0.729 3 72 (22.2) 
105 
140 
(60) 
135 
4 0.208 A 25 0.0113 0 0.729 0 72 (22.2) 
131 
140 
(60) 
113 
5 0.208 A 25 0.0113 0 0.729 1 72 (22.2) 
113 
140 
(60) 
129 
6 0.208 A 25 0.0113 0 0.729 5 72 (22.2) 
70 
140 
(60) 
121 
7 0.208 A 25 0.0113 0.05 0.729 3% MgCl.sub.2 
72 (22.2) 
114 
140 
(60) 
180 
8 0.208 A 25 0.0113 0.05 0.729 3% NH.sub.4 Cl 
72 (22.2) 
105 
140 
(60) 
147 
9 0.208 A 25 0.0113 0.05 0.729 3% NaCl 
72 (22.2) 
80 
140 
(60) 
170 
10 0.208 A 25 0.0113 0.05 0.729 3% CaCl.sub.2 
72 (22.2) 
87 
140 
(60) 
190 
11 0.26 D 9 0.0163 0.0638 
.911 3 70 (22.1) 
163 
140 
(60) 
152 
12 0.26 E 7 0.0163 0.0638 
.911 3 70 (22.1) 
136 
140 
(60) 
134 
13 0.26 F 18 0.0163 0.0638 
.911 3 70 (22.1) 
140 
140 
(60) 
166 
14 0.26 G 25 0.0163 0.0638 
.911 3 70 (22.1) 
124 
140 
(60) 
205 
15 0.26 H 20 0.0163 0.0638 
.911 3 70 (22.1) 
146 
140 
(60) 
182 
16 0.26 I 19 0.0163 0.0638 
.911 3 70 (22.1) 
176 
140 
(60) 
198 
17 0.26 J 20 0.0163 0.0638 
.911 3 70 (22.1) 
143 
140 
(60) 
194 
18 0.26 K 21 0.0163 0.0638 
.911 3 70 (22.1) 
154 
140 
(60) 
203 
19 0.26 L 22 0.0163 0.0638 
.911 3 70 (22.1) 
142 
140 
(60) 
196 
20 0.26 M 24 0.0163 0.0638 
.911 3 70 (22.1) 
153 
140 
(60) 
187 
21 0.26 N 25 0.0163 0.0638 
.911 3 70 (22.1) 
70 
140 
(60) 
123 
22 0.208 P 14 0.0113 .05 .729 3 70 (22.1) 
31 
152 
(67) 
208 
23 0.26 -- -- 0 0 0 3 70 (22.1) 
65 
HPS.sup.5 140 
(60) 
111 
24 0.26 -- -- 0.0113 .05 .729 3 70 (22.1) 
33 
Guar Gum.sup.6 140 
(60) 
19 
.sup. 25.sup.7 
0.59 A 25 0.0125 0.142 
2.08 2.85 
72 (22.2) 
45 
150 
(65.5) 
15 
__________________________________________________________________________ 
.sup.1 All slurries had 46% water BWOC (Based on the weight of cement). 
.sup.2 Lot Designation indicates a different Lot or batch of 
hydroxyethylcellulose made by Hercules, Inc. All Lots bore the designatio 
Natrosol 250 MBR. 
.sup.3 Glyoxal present as a coating on the polysaccharide (from about 0.2 
to about 2.0 weight percent of polysaccharide. 
.sup.4 Solution Time for Xanthan Gum was 5 minutes. 
.sup.5 Solution Time for HPS (hydroxypropylstarch) was 25 minutes. No HEC 
was used in Example 23. 
.sup.6 Solution Time for Guar Gum was 6 minutes. No HEC was used in 
Example 24. 
.sup.7 No cement present; other components based on weight of water which 
was 350 cc. 
Referring to Table II above, Example 1 shows that the use of an HEC having 
a solution time of 25 minutes results in a cement slurry exhibiting 
inverse rheology. By inverse rheology is meant that the slurry has an API 
viscosity at 140.degree. F. which is greater than the viscosity of the 
same slurry at 70.degree. F. Preferably the viscosity increases at least 
ten percent and the viscosity increase can be significantly greater such 
as increases of 50 to 200 percent or 50 to 500 percent or more. Example 2 
shows that the use of a polysaccharide (xanthan gum) having a solution 
time of 5 minutes results in a cement slurry which does not exhibit 
inverse rheology. 
Examples 3 through 10 show the effect on inverse rheology of using varying 
amounts and types of a salt (e.g., KCl). When no salt is present (Example 
4), inverse rheology was not observed. The remaining Examples 3 and 5 
through 10 show varying amounts of KCl or other salts can be used to 
obtain a cement slurry having inverse rheology. 
Examples 11 through 24 illustrate the preparation of cement slurries using 
various Lots of HEC and other polysaccharides. These Examples further 
illustrate that polysaccharides having the defined solution times in 
accordance with this invention are required in order to prepare cement 
slurries having the desired inverse rheology. 
Example 25 shows that a cement is required to obtain inverse rheology. 
EXAMPLE 26 
In the Run for this Example, a cement slurry having the same composition as 
Example 7 above (except KCl was used in place of MgCl.sub.2) was prepared 
as described above for Example 7 and the rheology at 70.degree. F. and 
152.degree. F. was determined. The results were as follows: Viscosity at 
300 R and 70.degree. F. was 169 and the viscosity at 300 R and 152.degree. 
F. was 118, showing no inverse rheology. The HEC used for this Example 26 
was a Natrosol 250 MBR Lot C and had a solution time of 6 minutes. 
EXAMPLE 27 
26.8 grams of a mixture of polysaccharides, i.e., 20.8 grams of a Natrosol 
250 MBR Lot C hydroxyethylcellulose; 5 grams of CMHEC and 1 gram of 
xanthan gum was first added to a solution of 8 grams of glyoxal (40 
percent aqueous solution) in 80 ml of isopropyl-alcohol. After stirring 
for 10 minutes, the solvent was removed in vacuo at 50.degree. C. for 80 
minutes. The sample was then dried at 82.degree. C. for 16 hours. This 
mixture of polysaccharides was found to have a solution time of 50 
minutes. 
Example 26 was repeated using the above treated mixture of polysaccharides 
to prepare the cement slurry. The cement slurry was found to exhibit 
inverse rheology (i.e., 300 R readings of 105 and 206 cp. at 70.degree. F. 
and 140.degree. F., respectively). 
A number of additional runs were made to illustrate the effect of heating 
on the solution times and rheological properties of certain 
hydroxyethylcellulose materials used in preparing the cement compositions 
of this invention. In Tables III and IV below, the HEC (Natrosol 250 MBR 
Lot B) either as received (Example 28) or after heat treatment (Examples 
29-41) was used to formulate a cement slurry wherein the amount of cement 
was 500 g, the weight percent water based on the weight of cement was 46; 
the amount of KCl was 3 percent based on the weight of water; the amount 
of HEC was 0.26 percent based on the weight of cement; and the amount of 
xanthan gum was 0.0163 percent based on the weight of cement; and the 
amount of CMHEC was 0.0638 percent based on the weight of cement; and the 
amount of dispersant (DAXAD-19) was 0.911 percent based on the weight of 
cement. Such slurries were then tested for their API viscosities at 300 R 
and temperature of 70.degree. F. and 140.degree. F., respectively, and the 
results are set forth in Tables III and IV below. 
TABLE III 
______________________________________ 
TEMPERATURE EFFECT OF HEC HEATED 1.5 HOURS 
Example Rheology Solution 
No. Time/Temperature 
Temp. 300 R Time 
______________________________________ 
28 As Received 70.degree. F. 
143 5 minutes 
140.degree. F. 
139 
29 1.5 h/90.degree. C. 
70.degree. F. 
175 6 minutes 
140.degree. F. 
152 
30 1.5 h/100.degree. C. 
70.degree. F. 
184 6 minutes 
140.degree. F. 
161 
31 1.5 h/110.degree. C. 
70.degree. F. 
184 6 minutes 
140.degree. F. 
159 
32 1.5 h/120.degree. C. 
70.degree. F. 
159 8 minutes 
140.degree. F. 
153 
33 1.5 h/130.degree. C. 
70.degree. F. 
87 25 minutes 
140.degree. F. 
154 
34 1.5 h/140.degree. C. 
70.degree. F. 
102 -- 
140.degree. F. 
151 
35 1.5 h/150.degree. C. 
70.degree. F. 
61 -- 
140.degree. F. 
83 
______________________________________ 
Referring to Table III, heating the HEC at a temperature from 90.degree. C. 
to 110.degree. C. for 1.5 hours still results in solution times for the 
HEC of only 6 minutes and the resulting slurries exhibited normal 
rheology, i.e., the viscosity decreased as the temperature increased 
(Examples 29-31). Example 32 shows that at a temperature of about 
120.degree. C. inverse rheology is almost achieved. It is to be noted that 
the solution time for the HEC from Example 32 was 8 minutes. 
Example 33 illustrates that inverse rheology is achieved by heating to a 
temperature of 130.degree. C. and the solution time of the HEC was 25 
minutes. 
In Examples 34 and 35, the solution times of the heated HEC's were not 
taken, but the rheological properties of the cement slurries indicate that 
heating to 140.degree. and 150.degree. C. results in inverse rheology. 
The procedure used for the heat treatment of the samples of HEC used in the 
runs for the examples in Table III above and Table IV below was as 
follows: 
The sample to be treated (50 g) was placed on a watch glass or in a flat 
pan and spread out so that a large surface area was exposed. Typically, 
the sample was less than one-half inch thick. The sample was then placed 
in an oven at the desired temperature in air or under an atmosphere of 
nitrogen. After heating for the desired time, the sample was cooled in air 
and then broken up with a spatula and passed through a 20-mesh screen. 
In yet another series of runs, an HEC (Natrosol 250 MBR-Lot C) was heated 
at a constant temperature of 130.degree. C. for varying times. These runs 
are summarized in Table IV below. It should be noted that the use of the 
designation of various Lot numbers of HEC, i.e., Lot B, Lot C, indicate 
that separately prepared batches of HEC were obtained from Hercules. 
TABLE IV 
______________________________________ 
TIME STUDY OF HEC RHEOLOGY EFFECT 
Example Rheology Solution 
No. Time/Temperature 
Temp. 300 R Time 
______________________________________ 
36 0.5 h/130.degree. C. 
70.degree. F. 
266 -- 
140.degree. F. 
222 
37 1.0 h/130.degree. C. 
70.degree. F. 
228 -- 
140.degree. F. 
242 
38 1.5 h/130.degree. C. 
70.degree. F. 
207 -- 
140.degree. F. 
238 
39 2.0 h/130.degree. C. 
70.degree. F. 
154 -- 
140.degree. F. 
239 
40 2.5 h/130.degree. C. 
70.degree. F. 
136 -- 
140.degree. F. 
218 
41 As Received 70.degree. F. 
168 6 minutes 
140.degree. F. 
153 
______________________________________ 
Referring to Table IV above, it can be seen that heating an HEC for 0.5 
hours (Example 36) at 130.degree. C. was insufficient to result in the 
production of a cement slurry where the viscosity increases with 
temperature. The composition of the cement slurries used for all of the 
Examples in Table IV was the same as the composition of the cement 
slurries used for the Examples in Table III above. Heating the HEC at 
130.degree. C. for a time of about 1.5 hours appears to be desirable to 
result in a cement slurry which increases in viscosity with increasing 
temperature (Example 38). The HEC used in all of the runs in Table IV was 
the same HEC used in Example 17 in Table II above and had a solution time 
of about 6 minutes. Referring to Example 17 in Table II, the cement slurry 
had normal rheological properties. Although the solution times for the 
heat-treated HEC materials for Examples 36-41 of Table IV were not 
obtained, it is concluded from the rheological properties of the cement 
slurries made using the heat-treated materials that a treating time of at 
least about 1 hour at 130.degree. C. is necessary to ensure that an HEC 
having an appropriate solution time is obtained. 
EXAMPLE 42 
The aqueous cement slurry having the composition of Example 3 in Table II 
above, was pumped at a rate of 420 gallons per minute (1.587 m.sup.3 
/minute) through an annular space simulating a seven-inch (17.78 cm) OD 
well casing inside a 9.875-inch (25.08 cm) well bore. As noted in Table II 
above, the HEC had a solution time of 25 minutes. The following table sets 
forth Reynolds Numbers for certain of these displacement rates. These were 
determined in accordance with API Bulletin 13 D, First Edition, August 
1980, page 6. 
______________________________________ 
Displacement Rates, 
Gallons Per Minute 
Reynolds No. 
______________________________________ 
100 161 
420 896 
840 2048 
1260 3322 
______________________________________ 
The above table shows that at a nominal displacement rate of 100 gallons 
per minute (0.378 m.sup.3 /minute) or 420 gallons per minute (1.587 
m.sup.3 /minute), the Reynolds Number is less than the threshold of 3000 
for turbulent flow. Since normal displacement rates in cementing 
operations are in the range of about 84 gallons per minute (0.317 m.sup.3 
/minute) to about 500 gallons per minute (1.89 m.sup.3 /minute), pumping 
of the novel aqueous cement slurry herein under such normal displacement 
operations will not cause the aqueous cement slurry to enter the turbulent 
region. 
EXAMPLE 43 
A blend was prepared containing 800 g of Class H cement and, based on the 
weight of the dry cement, 0.208 weight percent of hydroxyethylcellulose 
ether (Natrosol 250 MBR) having a solution time of 25 minutes (Lot A), 
0.0113 weight percent of xanthan gum (Kelzan XCD), 0.729 weight percent of 
DAXAD-19 and 1.38 weight percent of potassium chloride. This dry blend was 
then sealed in a jar and placed in a freezer for 24 hours at 15.degree. F. 
(-9.4.degree. C.). The blend was then removed from the jar and 368 cc of 
tap water cooled to 46.degree. F. (7.7.degree. C.) was added thereto and 
mixed in a commercial blender for 35 seconds. The resultant slurry 
temperature was 38.degree. F. (3.3.degree. C.). The API 300 R viscosity 
was 71 cp. An identical composition at 72.degree. C. (22.2.degree. C.) had 
an API viscosity of 98 cp. This unusual property, therefore, enables one 
to prepare the cement slurry at very low temperatures with great ease. 
This is most unusual, for conventional aqueous cement slurries containing 
polysaccharides alone or with dispersants are extremely difficult to mix 
at low temperatures, as a result of which, slurry densities may be low, 
the cements may have lower compressive strengths, and may result in 
hydrostatic pressures insufficient to contain formation pressures. 
While the novel aqueous cement slurries described and claimed herein can be 
used in cementing wells, it is understood that they can also be used in 
other situations wherein cement work is desired, for example, construction 
cements, grouts, mortars, plasters, patching cements, etc. 
Obviously, many modifications and variations of the invention, as 
hereinabove set forth, can be made without departing from the spirit and 
scope thereof, and therefore only such limitations should be imposed as 
are indicated in the appended claims.