Light weight high temperature well cement compositions and methods

The present invention provides light weight high temperature well cement compositions and methods. The compositions are basically comprised of calcium aluminate, ASTM class F fly ash and water.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to light weight high temperature 
well cement compositions and methods, and more particularly, to such 
compositions and methods which are suitable for cementing high temperature 
wells containing carbon dioxide. 
2. Description of the Prior Art 
In the completion of high temperature subterranean wells containing carbon 
dioxide, eg., geothermal wells, the use of conventional hydraulic cement 
compositions often results in early well failure. Because of the high 
static well bore temperatures involved coupled with the presence of brines 
containing carbon dioxide, conventional hydraulic well cements rapidly 
deteriorate due to alkali carbonation, especially sodium carbonate induced 
carbonation. In geothermal wells which typically involve very high 
temperatures, pressures and carbon dioxide concentrations, conventional 
well cement failures have occurred in less than five years causing the 
collapse of the well casing. 
It has heretofore been discovered that a cement material known as calcium 
phosphate cement formed by an acid-base reaction between calcium aluminate 
and a phosphate-containing solution has high strength, low permeability 
and excellent carbon dioxide resistance when cured in hydrothermal 
environments. However, calcium phosphate cement has a relatively high 
density, eg., a density in the range of from about 15 to about 17 pounds 
per gallon, which is too high for geothermal applications. That is, in 
geothermal wells the hydrostatic pressure exerted by the high density 
calcium phosphate cement often exceeds the fracture gradients of 
subterranean zones penetrated by the well bore which causes the formation 
of fractures into which the cement is lost. While calcium phosphate 
cements have been developed which include hollow microspheres and as a 
result have densities of about 10 pounds per gallon, such light weight 
compositions are relatively expensive and the presence of the microspheres 
in the cured cement reduces its compressive strength. 
Thus, there is a need for improved less expensive well cement compositions 
useful in cementing high temperature wells containing carbon dioxide. 
SUMMARY OF THE INVENTION 
The present invention provides improved cement compositions and methods 
which meet the needs described above and overcome the deficiencies of the 
prior art. The compositions are particularly useful in high temperature 
wells containing carbon dioxide such as geothermal wells. A composition of 
the present invention is basically comprised of calcium aluminate, fly ash 
and sufficient water to form a pumpable slurry. 
Another composition of this invention is comprised of calcium aluminate, 
fly ash, sufficient water to form a pumpable slurry, a foaming agent, a 
foam stabilizer and a gas sufficient to form a foam having a density in 
the range of from about 9.5 to about 14 pounds per gallon. 
Yet another composition of this invention is comprised of calcium 
aluminate, sodium polyphosphate, fly ash, sufficient water to form a 
pumpable slurry, a foaming agent, a foam stabilizer and a gas present in 
an amount sufficient to form a foam having a density in the range of from 
about 9.5 to about 14 pounds per gallon. 
The methods of the present invention for cementing a high temperature 
subterranean zone containing carbon dioxide penetrated by a well bore 
basically comprise the steps of forming a well cement composition of this 
invention, pumping the cement composition into the subterranean zone by 
way of the well bore and allowing the cement composition to set into a 
hard impermeable mass therein. 
It is, therefore, a general object of the present invention to provide 
light weight high temperature well cement compositions and methods. 
A further object of the present invention is the provision of improved 
carbonation resistant well cement compositions and methods. 
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 
As mentioned above, high temperature wells containing carbon dioxide such 
as geothermal wells generally require the use of well cement compositions 
which do not deteriorate in the presence of carbon dioxide containing 
brines. The term "high temperature" is used herein to mean wells wherein 
the static bottom hole temperature is above about 300.degree. F. up to as 
high as about 700.degree. F. When conventional hydraulic cements are 
utilized in such wells, carbonation causes dissolution of the cement which 
is converted into water-soluble salts. Further, severe corrosion of steel 
pipe takes place thereby resulting in the total disruption of the 
conventional cement supported well structure. 
When conventional normal density cement slurries are utilized in geothermal 
and other similar wells, loss of circulation problems are often 
encountered. This is due to the weak unconsolidated formations in the 
wells having very low fracture gradients. When a relatively high density 
cement slurry is pumped into such a well, the hydrostatic pressure exerted 
on the weak unconsolidated subterranean zones therein causes the zones to 
fracture. This in turn causes the cement slurry being pumped to enter the 
fractures and lost circulation problems to occur. To avoid such problems, 
the cement compositions utilized in geothermal and other similar wells 
must be of light weight, i.e., have densities in the range of from about 
9.5 to about 14 pounds per gallon. 
By the present invention, improved well cement compositions are provided 
which resist high temperature carbonation deterioration. A cement 
composition of this invention which can be non-foamed or foamed is 
basically comprised of calcium aluminate, fly ash and sufficient water to 
form a pumpable slurry. When foamed, the cement composition includes a 
foaming agent, a foam stabilizer and a gas present in an amount sufficient 
to form a foam having a density in the range of from about 9.5 to about 14 
pounds per gallon. 
Another composition of this invention is comprised of calcium aluminate, 
sodium polyphosphate, fly ash, a foaming agent, a foam stabilizer and a 
gas present in an amount sufficient to form a foam having a density in the 
range of from about 9.5 to about 14 pounds per gallon. 
The calcium aluminate can be any commercial grade calcium aluminate 
suitable for use as a cement. A suitable such calcium aluminate is 
commercially available from the Lehigh Portland Cement Company of 
Allentown, Pennsylvania, under the trade designation "REFCON.TM.." The 
calcium aluminate is generally included in the cement composition in an 
amount in the range of from about 15% to about 45% by weight of the 
composition. 
When used, the sodium polyphosphate includes sodium hexametaphosphate and 
sodium triphosphate as well as vitreous sodium phosphates. A suitable 
sodium polyphosphate for use in accordance with the present invention is 
commercially available from Calgon Corporation of Pittsburgh, Pa. The 
sodium polyphosphate can be included in the cement composition in an 
amount in the range of from about 5% to about 20% by weight of the 
composition. When included, the sodium polyphosphate combines with the 
calcium aluminate to form calcium phosphate in the form of hydroxyapatite. 
Fly ash is the finally divided residue that results from the combustion of 
ground or powdered coal and is carried by the flue gases generated. A 
particular fly ash that is suitable in accordance with the present 
invention is a fine particle size ASTM class F fly ash having a Blaine 
fineness of about 10,585 square centimeters per gram which is commercially 
available from LaFarge Corporation of Michigan under the trade designation 
"POZMIX.TM.." Another fly ash that is suitable is an ASTM class F fly ash 
which is commercially available from Halliburton Energy Services of 
Dallas, Texas under the trade designation "POZMIX.TM. A." The fly ash is 
generally included in the composition in an amount in the range from about 
25% to about 45% by weight of the composition. 
The major crystalline phase of ASTM class F fly ash is mullite (3Al.sub.2 
O.sub.3.2SiO.sub.2). It reacts with calcium aluminate to form calcium 
alumino silicate (CaO.Al.sub.2 O.sub.3.2SiO.sub.2). Also, iron and quartz 
in the fly ash react with the calcium aluminate to form andradite 
(Ca.sub.3 Fe.sub.2 SiO.sub.4).sub.3. These reactions increase the 
compressive strength of the set cement as compared to set calcium 
aluminate cement alone. 
The water utilized can be from any source provided it does not contain an 
excess of compounds that adversely affect other compounds in the cement 
composition. For example, the water can be fresh water or saltwater. 
Generally, the water is present in the cement composition in an amount 
sufficient to form a pumpable slurry, i.e., an amount in the range of from 
about 10% to about 60% by weight of the composition. 
In order to facilitate the foaming of the cement composition, a foaming 
agent is included in the composition. A particularly suitable and 
preferred such foaming agent is an alpha-olefinic sulfonate having the 
formula 
EQU H(CH.sub.2).sub.n --CH.dbd.CH--(CH.sub.2).sub.m SO.sub.3 Na 
wherein n and m are individually integers in the range of from about 6 to 
about 16. The foaming agent is generally included in the cement 
composition in an amount in the range of from about 1% to about 2% by 
weight of the water in the composition. The most preferred foaming agent 
of this type is an alpha-olefinic sulfonate having the above formula 
wherein n and m are each 16, i.e., a sulfonic acid C.sub.16-16 alkane 
sodium salt. 
A foam stabilizer is also included in the cement composition to enhance the 
stability of the composition after it is foamed. A particularly suitable 
and preferred stabilizing agent is an amidopropylbetaine having the formul 
a 
EQU R--CONHCH.sub.2 CH.sub.2 N.sup.+ (CH.sub.3).sub.2 CH.sub.2 CO.sub.2.sup.- 
wherein R is a radical selected from the group of decyl, cetyl, oleyl, 
lauryl and cocoyl. The foam stabilizer is generally included in the cement 
composition in an amount in the range of from about 0.5% to about 1% by 
weight of the water in the composition. The most preferred foam stabilizer 
of this type is cocoylamidopropylbetaine. 
The gas utilized to foam the composition can be air or nitrogen, with 
nitrogen being the most preferred. The amount of gas present in the cement 
composition is that amount which is sufficient to form a foam having a 
density in the range of from about 9.5 to 14 pounds per gallon, most 
preferably 12 pounds per gallon. 
In order to provide resiliency to the set cement composition of this 
invention, the composition may optionally include inert ground rubber 
particles. Such particles are produced from worn out tires and are 
commercially available from Four D Corporation of Duncan, Okla. 
At static well bore temperatures above about 125.degree. F., a set retarder 
is required. The set retarder functions to lengthen the time in which the 
cement composition starts to thicken and set so that the composition can 
be pumped into the well bore and into the zone to be cemented before such 
thickening takes place. Preferred such set retarders for use in accordance 
with this invention are gluconic acid and citric acid. When used, the set 
retarder is included in the cement composition in an amount in the range 
of from about 0.5% to about 2% by weight of the composition. 
A preferred composition of the present invention is comprised of calcium 
aluminate present in an amount of about 30% by weight of the composition, 
ASTM class F fly ash present in an amount of about 50% by weight of the 
composition and water present in an amount sufficient to form a slurry. 
Another preferred composition of the present invention is comprised of 
calcium aluminate present in an amount of about 30% by weight of the 
composition, ASTM class F Fly Ash present in an amount of about 50% by 
weight of the composition, sufficient water to form a pumpable slurry, a 
foaming agent comprised of a sulfonic acid C.sub.16-16 alkane sodium salt 
present in an amount of about 1.5% by weight of the water in the 
composition, a foam stabilizer comprising cocoylamidopropyl-betaine 
present in an amount of about 0.75% by weight of the water in the 
composition and a gas present in an amount sufficient to form a foam 
having a density in the range of from about 9.5 to about 14 pounds per 
gallon. 
Yet another preferred composition of this invention is comprised of calcium 
aluminate present in an amount of about 28% by weight of the composition, 
sodium polyphosphate present in an amount of about 19% by weight of the 
composition, ASTM class F fly ash present in an amount of about 49% by 
weight of the composition, sufficient water to form a pumpable slurry, a 
foaming agent comprised of a sulfonic acid C.sub.16-16 alkane sodium salt 
present in an amount of about 8% by weight of the water in the 
composition, a foam stabilizer comprising cocylamideopropylbetaine present 
in an amount of about 4% by weight of the water in the composition and a 
gas present in an amount sufficient to form a foam having a density in the 
range of from about 9.5 to about 14 pounds per gallon. 
As previously mentioned, the above described cement compositions can 
include ground rubber particles present in an amount in the range of from 
about 10% to about 40% by weight of the compositions to improve the 
resiliency of the compositions. Further, when the static well bore 
temperature is above about 125.degree. F., a set retarder selected from 
the group of gluconic acid and citric acid is included in the cement 
compositions in an amount of about 1.0% by weight of the compositions. 
The cement compositions of this invention may be prepared in accordance 
with any of the mixing techniques utilized in the art. In one preferred 
method, a quantity of water is introduced into a cement blender followed 
by the sodium polyphosphate (if used), calcium aluminate and fly ash. The 
mixture is agitated for a sufficient period of time to form a pumpable 
non-foamed slurry. 
When the cement slurry formed as above is foamed, the slurry is pumped to 
the well bore and the foaming agent and foam stabilizer followed by the 
gas utilized are injected into the slurry on the fly. As the slurry and 
gas flow through the well bore to the location where the resulting foamed 
cement composition is to be placed, the cement composition is foamed and 
stabilized. Other liquid additives utilized, if any, are added to the 
water prior to when the other components of the cement composition are 
mixed therewith and other dry solids, if any, are added to the water and 
cement prior to mixing. 
The methods of this invention of cementing a high temperature subterranean 
zone containing carbon dioxide penetrated by a well bore are basically 
comprised of the steps of forming a foamed cement composition of this 
invention, pumping the foamed cement composition into the subterranean 
zone to be cemented by way of the well bore and then allowing the foamed 
cement composition to set into a hard impermeable mass therein.

In order to further illustrate the improved cement compositions and methods 
of this invention, the following examples are given. 
EXAMPLE 1 
In a controlled test, API Class G Portland Cement was mixed with 40% silica 
flour and water to form a cement slurry. The slurry was allowed to set for 
24 hours at a temperature of 190.degree. F. Thereafter, the set cement was 
placed in an aqueous 4% by weight sodium carbonate solution for 28 days at 
600.degree. F. 
A calcium phosphate cement composition was prepared comprised of 23.3% 
water; 17.5% calcium aluminate; 15.6% sodium polyphosphate; 40.8% ASTM 
class F fly ash, 1.9% sulfonic acid C.sub.16-16 alkane sodium salt foaming 
agent and 0.9% cocoylamidopropylbetaine foam stabilizer, all by weight of 
the composition. After mixing, the resulting slurry was allowed to set for 
24 hours at a temperature of 190.degree. F. Thereafter, the set cement was 
placed in a 4% by weight aqueous sodium carbonate solution for 28 days at 
600.degree. F. 
At the end of the test periods, samples from the interiors of the set 
Portland Cement composition and calcium aluminate cement composition were 
tested. The tests showed that the Portland Cement composition contained 
1.5% by weight calcium carbonate and the calcium phosphate cement 
contained none. Samples were also tested taken from the exteriors of the 
set cements which showed that the Portland cement composition contained 
10.6% calcium carbonate while the calcium phosphate cement contained none. 
EXAMPLE 2 
Test calcium phosphate cement slurry samples were prepared by mixing 240 
grams of water with 180 grams of calcium aluminate, 160 grams of sodium 
polyphosphate and 420 grams of fly ash for each sample. Various Portland 
cement set retarding additives were combined with the test samples. After 
mixing, each test sample was tested for thickening time at 125.degree. F. 
in accordance with the test procedure set forth in API Specification For 
Materials And Testing For Well Cements, API Specification 10, 5th ed., 
dated Jul. 1, 1990 of the American Petroleum Institute. The set retarders 
tested are identified and the thickening time test results are set forth 
in Table 1 below. 
TABLE I 
______________________________________ 
Thickening Time Tests.sup.1 
Amount Added to 
Thickening Time 
Set Retarder Tested Test Sample, grams hrs.:mins. 
______________________________________ 
None -- 1:35 
Acrylic Acid Polymer 6 2:02 
Tartaric Acid 6 1:12 
Gluconic Acid 6 4:05 
Citric Acid 6 6:00+ 
______________________________________ 
.sup.1 API Tests at 125.degree. F. 
From Table I, it can be seen that gluconic acid and citric acid are the 
most effective set retarders for the calcium aluminate cement composition 
at a temperature of 125.degree. F. 
EXAMPLE 3 
Two additional calcium aluminate cement slurry samples were prepared as 
shown in Table II below. After mixing, the resulting slurries were allowed 
to set for 24 hours at 190.degree. F. Thereafter, the set samples were 
placed in 4% by weight aqueous sodium carbonate solutions for 28 days at 
600.degree. F. At the end of the 28 day periods, the samples were tested 
for compressive strengths in accordance with the above mentioned API 
Specification 10. The results of the tests are also set forth in Table II 
below. 
TABLE II 
__________________________________________________________________________ 
Compressive Strength Tests 
Sample Components, grams Compressive 
Sample Calcium 
Sodium 
Fly 
Foaming 
Foam Density 
Strength, 
No. Water Aluminate.sup.1 Phosphate.sup.2 Ash.sup.3 Agent.sup.4 
Stabilizer.sup.5 lb/gal. psi 
__________________________________________________________________________ 
1 465.5 
350 311.5 815.5 
37.3 18.6 12.1 
570 
2 266 200 178 466 21.3 10.6 15.1 1060 
__________________________________________________________________________ 
.sup.1 "REFCON .TM." from Lehigh Portland Cement Co. 
.sup.2 Calgon Sodium Polyphosphate 
.sup.3 ASTM class F fly ash from LaFarge Corp. 
.sup.4 Sulfonic acid C.sub.16--16 alkane sodium salt 
.sup.5 Cocoylamidopropylbetaine 
From Table II, it can be seen that the calcium aluminate cement 
compositions of the present invention maintained their compressive 
strengths after 28 days in the presence of sodium carbonate solutions at 
600.degree. F. 
EXAMPLE 4 
An API Class G Portland cement was mixed with 40% silica flour and water to 
form a cement slurry. The slurry was allowed to set for 48 hours at a 
temperature of 500.degree. F. Thereafter, the set cement was placed in an 
aqueous solution containing 2.4% dry ice and 0.8% sulfuric acid. A calcium 
aluminate cement composition was prepared comprised of 30% water, 37% 
calcium aluminate and 33% ASTM class F fly ash. The resulting slurry was 
allowed to set for 48 hours at a temperature of 500.degree. F. Thereafter, 
the set cement was placed in an aqueous solution containing 2.4% dry ice 
and 0.8% sulfuric acid. The above described test cement samples were kept 
in the carbonate-acid solutions for 53 days at 500.degree. F., after which 
the Portland cement lost 33% of its weight while the calcium aluminate 
cement gained 9.1% in weight. 
Calcium aluminate (Lehigh "REFCON.TM.") was mixed with 59% by weight water 
and cured for 24 hours at 500.degree. F. The same calcium aluminate was 
mixed with ASTM class F fly ash in an amount of 75% by weight of the 
calcium aluminate and with water in an amount of 34% by weight of calcium 
aluminate and cured for 24 hours at 500.degree. F. The set samples were 
tested for compressive strengths in accordance with the above mentioned 
API Specification 10. The set sample formed with calcium aluminate alone 
had a compressive strength of only 410 psi while the sample formed with 
calcium aluminate and fly ash had a compressive strength of 2120 psi. 
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.