In a hydrocarbon recovery system including CO.sub.2 pressurization of an underground formation, a sealing cement comprised of Portland cement and Class C fly ash.

BACKGROUND 
This invention pertains to carbon dioxide (CO.sub.2)-enhanced recovery 
systems for crude oil and natural gas with improved CO.sub.2 -corrosion 
resistant cement and to methods of providing CO.sub.2 -resistant 
encasement of metal tubular members in a CO.sub.2 -rich environment. 
Enhanced production of crude oil or natural gas (generally referred to as 
hydrocarbons) from partially depleted underground deposits thereof by the 
injection or pressurization of the underground formation including the 
deposit with carbon dioxide (CO.sub.2), injected alternately or together 
with water, is known. Such processes are described, for example, in 
"Carbon Dioxide: Miscible Flooding Methods", "Effective Water Injection," 
SPE (June, 1975), 217, Oil and Gas Journal, Dec. 27, 1982. The CO.sub.2 
corrosive attack on well cement and metal tubular goods in the injection 
or production wells of such systems has been identified as a significant 
potential problem. See for example, "Effects of Super-Critical Carbon 
Dioxide on Well Cements"--Onanan, Halliburton Services, SPE 12595 and 
"Carbon Dioxide Corrosion Resistance in Cements"--Bruckdorfer, Dowell 
Schlumberger, CIM 85-36-61. These publications detail the chemical 
mechanism of the CO.sub.2 -corrosion attack on Portland cement and 
secondarily on metallic members exposed to a CO.sub.2 -water environment. 
These corrosive factors may be adversely affected by the high pressure and 
high temperature often present at the depth of subterranean formations in 
which hydrocarbon deposits are found. 
It is, therefore, a general object of the present invention to provide 
enhanced hydrocarbon recovery systems including means for injecting 
CO.sub.2 into subterranean hydrocarbon formations and including means for 
minimizing CO.sub.2 -corrosion attack on system components particularly 
including well sealing cements. 
BRIEF DESCRIPTION OF THE INVENTION 
Briefly, the present invention comprises a method of providing a CO.sub.2 
-corrosion resistant encasement for a tubular member in a bore hole, 
particularly in an injection or product recovery well of a CO.sub.2 
-enhanced hydrocarbon recovery system, by surrounding the tubular member 
in the bore hole with a Portland cement slurry including, on a dry weight 
basis, about 30-65% Class C fly ash and about 70-35% Portland cement. Upon 
hardening, the CO.sub.2 -corrosion resistant cement composition provides 
an effective seal of the tubular member and enhanced resistance of that 
sealing cement to CO.sub.2 -corrosion, notwithstanding the corrosive 
environmental influences of heat, temperature, CO.sub.2 and water. This is 
important since the system also includes means for injecting CO.sub.2, and 
often water, for pressurization of a hydrocarbon-containing subterranean 
formation. 
The Portland cement slurry should have a density of about 15-17 pounds per 
gallon with about 16-16.4 pounds per gallon preferred. In general, the dry 
weight ratio of Class C fly ash to Portland cement in the slurry may be 
from about 1:2 to about 2:1. While essentially any Portland cement may be 
used, Class H Portland cement is preferred. 
For a better understanding of the present invention, reference may be made 
to the following detailed description thereof, taken in conjunction with 
FIG. 1 and the subjoined claims.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention pertains to CO.sub.2 -enhanced hydrocarbon recovery 
processes wherein the corrosive influence of high concentrations of 
CO.sub.2 in combination with water adjacent well-sealing cements is 
counteracted by the provision of a sealing cement composition specifically 
adapted to resist the CO.sub.2 corrosive influence. A diagrammatic 
illustration of such a system in which CO.sub.2 -enhanced recovery is 
utilized is shown in the figure. Diagramatically illustrated there is an 
underground rock formation 8 in which is entrapped a 
hydrocarbon-containing formation 10, typically sand and shale, in which is 
trapped crude oil and/or natural gas. Formation 10, sometimes referred to 
as a pay zone, may be tapped for primary recovery of the hydrocarbon fluid 
by an oil well, typically comprising a bore hole 12, including a central 
metallic tubular member 14 and a sealing cement 16 occupying the space and 
surrounding and sealing that space between tubular member 14 and 
hydrocarbon-bearing deposit or formation 10. Naturally occurring pressure 
in the underground formation forces the hydrocarbon out of the formation 
and through perforations 15 in the tubular member 14 and surrounding 
sealing cement 16 and up through tubular member 12 to the surface. 
When the naturally occuring pressure in the pay zone has been relieved, 
secondary recovery may be effected by water or polymer flooding of the pay 
zone. The present invention pertains to another recovery method where a 
hydrocarbon deposit is recovered by means of pressurized injection of 
fluids into the pay zone, particularly the pressurized injection of carbon 
dioxide. 
As shown in FIG. 1, this is typically accomplished by injecting the fluid 
through one or more injection wells and recovering the hydrocarbon through 
one or more production wells which are in fluid communication with said 
injection wells. The injection wells are also comprised of a bore hole 18, 
a central tubular member 20 and a sealing cement 22 occupying the angular 
space between tubular member 12 and bore hole 18 with perforations 19 to 
permit injection of CO.sub.2 and water through tubular member 20 and into 
the hydrocarbon-bearing formation 10. 
Typically, the CO.sub.2 -enhanced recovery project encompasses an oil field 
covering many acres and includes a plurality of injection wells and 
production wells spaced from one another so that the hydrocarbon deposit 
within the pay zone or formation 10 is driven by pressure created at the 
injection wells toward the producing wells. For this purpose, CO.sub.2 may 
be injected in alternating cycles with water or together with water under 
pressure sufficient to dissolve at least a portion of the CO.sub.2 into 
the oil in much the same way as CO.sub.2 is dissolved in a bottle of soda 
water, thus facilitating the hydrocarbon to flow more easily toward the 
producing wells. 
Necessarily, this process exposes a sealing cement surrounding metallic 
tubular members in the various wells to the corrosive influence of carbon 
dioxide in combination with water, forming carbonic acid, often under 
conditions of high temperature and pressure. 
The possible adverse consequences of corrosive attack on the sealing cement 
are loss of pressure from the hydrocarbon-bearing formation, the formation 
of seepage paths along the cement casing, and the possible breakdown of 
the sealing cement exposing the tubular member encased therein to the 
acidic CO.sub.2 -laden corrosive environment. 
To provide a CO.sub.2 -enhanced hydrocarbon recovery system in which these 
consequences are minimized and the CO.sub.2 corrosion attack inhibited, 
there is provided in accordance with the present invention a sealing 
cement with enhanced CO.sub.2 -corrosion resistance. This cement 
composition is placed in the form of a pumpable water slurry comprised of 
water, about 40-65% Portland cement, preferably Class H Portland cement, 
and about 35-60% Class C fly ash (all on a dry weight basis). Conventional 
additives, such as retarders, dispersants and fluid loss control 
additives, may also be included. Generally, the weight proportion of 
Portland cement to fly ash is in the range of about 2:1 to about 1:2. 
While the density of the slurry can be varied, the slurry generally has a 
density of about 15-17 pounds per gallon, and a slurry density of about 
16-16.4 per gallon is preferred. As defined in ASTM specification 
C618-80, Class C fly ash is that fly ash normally produced from lignite or 
sub-bituminous coal, which in addition to having pozzolanic properties 
typical of fly ash, also has some inherent cementitious properties. Some 
Class C fly ashes may contain lime contents higher than 10%. 
The utilization of various types of fly ash as additives in Portland cement 
mixes is also well known and the influence of such additives on pore size 
distribution and permeability has been specifically studied. See, for 
example, "Influence of Pozzolanic, Slag, and Chemical Admixtures on Pore 
Size Distribution and Permeability of Hardened Cement Pastes"--Manmohan 
and Mehte, Cement, Concrete and Aggregates, pages 63-67 (ASTM 1981). Class 
C fly ashes have been used specifically as additives for low-porosity 
high-strength concretes, which were studied for corrosion resistance in 
highway bridge decks, Coleman, Maage, and Diamond, Cement and Concrete 
Research, 670-678, Volume 14, No. 5, Pergamon Press, Ltd. (1984). 
Even with this information (and much more) available, there nevertheless 
remains a need for an improved well sealing cement with 
corrosion-resistance properties suitable for use in an underground 
CO.sub.2 -water-laden environment. 
This need is met, in accordance with the present invention by encasing a 
bore hole tubing with a Portland cement slurry adapted to harden into a 
CO.sub.2 -corrosion resistant cement, the slurry comprising, in addition 
to water in Portland cement, at least about 35% Class C fly ash, the 
slurry having a density of about 15-17 pounds per gallon. Preferably, this 
Portland cement is a Class H cement. Preferably, the density of the slurry 
is about 16-16.4 pounds per gallon. 
In its most general form, the invention comprises a CO.sub.2 -enhanced 
hydrocarbon recovery system including means for injecting carbon dioxide 
into an underground hydrocarbon-bearing deposit formation. This system 
includes at least one well comprising a bore hole and a tubular member 
extending through the bore hole from the surface of the earth to the 
zone(s) in an underground formation containing hydrocarbon or in fluid 
communication with such zones. The tubular member is surrounded by a 
cement which fills the annular space between the tubular member and the 
formation. The cement is implaced as a pumpable slurry and forms a seal 
upon hardening. The Portland cement slurry contains a composition as 
defined above. 
The efficacy of this invention has been demonstrated by laboratory tests in 
which cement formulations of the type used in the present invention have 
been compared with similar formulations using Class F rather than Class C 
fly ash and/or no fly ash at all. Carbonation penetration depth and 
strength retention in a CO.sub.2 -saturated water environment have been 
the primary criteria for evaluating the CO.sub.2 -corrosion resistance of 
the cement formulation and upon which it has been concluded that 
formulations specifically including at least about 35% Class C fly ash are 
particularly adapted for use in the underground pressurized wet CO.sub.2 
exposure of well sealing cements. 
EXAMPLE 1 
This series of experiments demonstrates the effect of Class C fly ash 
theron. 
For this example, cylindrical macrosamples of 1-in. OD.times.2-in. [2.54-cm 
OD.times.5.05-cm] were cast using a 50-cc disposable plastic syringe as a 
mold. Before preparing the cement slurry and casting the sample, the 
plunger was removed and the syringe tip was sealed. All cement slurries 
tested were prepared according to API Specification 10, Section 5, using 
distilled water. Before mixing the slurry, a defoaming agent was added to 
the mix water to reduce air entrainment. The cement samples were cast by 
slowly pouring the slurry down one side of the syringe barrel, then 
puddling the cement with a stirring rod (API Specification 10, Section 5) 
to remove any trapped air. After sealing the syringe with a rubber 
stopper, the samples were cured for 72 hours at 3,000 psi [20.68 MPa] and 
175.degree. F. [79.4.degree. C.]. The samples were demolded by cutting off 
the syringe's tip end, placing the mold in warm water to expand the 
plastic and pushing the cured cement cylinder out of the barrel with a 
syringe plunger. 
Cement samples were carbonated by loading them in the autoclave in a sample 
holder. After filling the chamber with water and sealing, liquid carbon 
dioxide was injected into the water using a sparge tube connected to a 
carbon dioxide tank until 900 psi [6.21 MPa] was recorded. The test 
chamber was recharged every two to three days over the four- to six-week 
carbonation period. Temperature and pressure were maintained at 
175.degree. F. [79.4.degree. C.] or 125.degree. F. [51.7.degree. C.] and 
3,000 psi [20.68 MPa]. To eliminate any effects temperature and pressure 
had on the results, a second autoclave was used for a control sample set. 
This was run under the same conditions as the test set except that the 
chamber was not charged with carbon dioxide. 
Carbonic acid corrosion effects on the macrosample sets were determined 
using compressive strength and carbonation penetration depth measurements. 
Compressive strengths of the 1-in. OD.times.2-in. [2.54-cm 
OD.times.5.08-cm] samples were measured by first cutting off 0.5-in. 
[1.27-cm] of each cylinder end, then milling the end surfaces 
perpendicular to the cylinder's axis. This was done to remove any 
deteriorated cement at the sample ends and to ensure that the cylinder was 
perpendicular to the instrument's platens, both of which cause scattering 
in the test results. Compressive strengths were determined using a Baldwin 
instrument. Because the length-to-diameter ratio was less than two, the 
resultant strengths were multiplied by 0.91 to obtain corrected 
compressive strengths (ASTM Standard C42-68 ). 
The depth the carbonic acid penetrated into the cement was determined by 
staining the freshly cut end of a cylinder with a 1% phenolphthalein 
solution. By measuring the unstrained area, which indicates no calcium 
hydroxide is present in the cement matrix, the depth of carbonic acid 
invasion into the matrix was determined. 
With respect to dimensional stability, in all cases tested, carbonation 
resulted in a net positive expansion as compared to control sample. Two 
formulations were prepared with Class C cement mixed at a slurry density 
of 15.6 pounds per gallon. One of these mixtures included 40% Class C fly 
ash. This mixture also included 0.4% (by weight of cement) of other minor 
additives. To achieve constant density the fly ash formulation included 
37% water, and the 100% Portland cement (Class C) formulation included 47% 
water. 
The carbonated sample of the Class C fly ash-containing formulation 
exhibited a penetration depth of 0.20 inches versus a penetration depth of 
the comparable 100% Class C Portland cement slurry (without other 
additives) of 0.54 inches. Thus the Portland cement without Class C fly 
ash seemed to be much more susceptible to CO.sub.2 attack than that with 
the fly ash formulation as used in the present invention. 
Moreover, the carbonated sample of fly ash-containing cement exhibited a 
compressive strength of 7460 psi versus a compressive strength of the 
non-fly ash comparable cement formulation of 6150 psi. 
EXAMPLE 2 
The enhanced CO.sub.2 -corrosion resistivity of Class C fly ash-Portland 
cement combinations was further demonstrated using microcylindrical 
samples. The result of using microcylindrical samples was that carbonation 
effects were observable over relative short exposure periods compared to 
macrocylindrical cement samples. 
Carbonation of these samples was effected as described in Example 1. This 
is believed to be representative of the potential CO.sub.2 attack upon 
sealing cements surrounding tubular goods in the bore hole of producing or 
injection wells in a CO.sub.2 -enhanced recovery system. 
Because in other testing it had been demonstrated that Class H Portland 
cements tend to be somewhat more CO.sub.2 -corrosion resistant than other 
types of Portland cement, and because there appears to be some discernible 
relationship between CO.sub.2 -corrosion resistivity and slurry density, 
further testing was conducted utilizing constant density slurries and 
Class H Portland cement in combination with Class F fly ash as compared to 
Class C fly ash. This is reported in the following formulation listings by 
reference to the fly ash class. 
Table I summarizes strength data on a variety of cement formulations useful 
in the present invention and comparative data for cement formulations 
without fly ash or with a different type of fly ash (i.e., Class F fly 
ash). 
TABLE I 
______________________________________ 
EFFECTS OF FLY ASH ON PORTLAND 
CEMENT STRENGTH LOSS AFTER CO.sub.2 
CARBONATION AT 175.degree. F. AUTOCLAVE 
EXPOSURE AT SLURRY DENSITY OF 16.0 
POUNDS PER GALLON 
% % 
% Strength 
Strength 
Run Portland Cement 
Fly Ash Fly Loss at 
Loss at 
No. Class and Source 
Class Ash 3 Weeks 
6 Weeks 
______________________________________ 
1. Neat Maryneal 
-- 0 33 51 
Class H 
2. Neat Maryneal 
(Class F) 
35 36 66 
Class H 
3. Neat Maryneal 
(Class F) 
50 24 57 
Class H 
4. Neat Maryneal 
(Class C) 
35 -44 32 
Class H 
5. Neat Maryneal 
(Class C) 
35 -55 26 
Class H 
6. Neat Maryneal 
(Class C) 
35 -0.7 29 
Class H 
______________________________________ 
Similar results from the same compositions were produced with carbonation 
at 125.degree. F. as shown in Table II. 
TABLE II 
______________________________________ 
EFFECTS OF FLY ASH ON PORTLAND 
CEMENT STRENGTH LOSS AFTER CO.sub.2 
CARBONATION AT 125.degree. F. AUTOCLAVE 
EXPOSURE AT SLURRY DENSITY OF 16.0 
POUNDS PER GALLON 
% % 
% Strength 
Strength 
Run Portland Cement 
Fly Ash Fly Loss at 
Loss at 
No. Class and Source 
Class Ash 3 Weeks 
6 Weeks 
______________________________________ 
7. Neat Maryneal 
-- 0 33 47 
Class H 
8. Neat Maryneal 
(Class F) 
35 36 58 
Class H 
9. Neat Maryneal 
(Class F) 
50 24 50 
Class H 
10. Neat Maryneal 
(Class C) 
35 -0.7 26 
Class H 
______________________________________ 
The foregoing data indicate that the strength retention (or strength loss 
reduction) attainable in a system including CO.sub.2 -corrosion producing 
environment is much improved by the use of pipe-sealing cement comprising 
Portland cement in combination with Class C fly ash, as seen by comparison 
to similar formulations using Class F fly ash or no fly ash at all. 
Moreover, the data show that essentially no improvement is obtained as 
compared to 100% Portland cement by the inclusion therein of Class F fly 
ash. 
While this invention has been described with reference to specific 
embodiments thereof, it is not limited thereto and the appended claims are 
intended to be construed to encompass not only those forms and embodiments 
of the invention disclosed and described above, but to such other variants 
and modifications thereof as may be made by those skilled in the art but 
which are nevertheless within the true spirit and scope of the present 
invention.