FIELD OF THE INVENTION
The present invention relates generally to oil well cements. In particular, the present invention relates to a
process for preparing an improved oil well cement to be used in a subterranean formation.
BACKGROUND OF THE INVENTION
[0002] Oil well cement is intended for use in the oil and gas industry. The main function of the oil well cement is to make
the oil and gas wells function properly. The oil well cement
plays a significant role in improving productivity and efficiency
of the drilling operations in the oil and gas wells. The oil
well cement may be used for various reasons including, but not
limited to, to create an isolation between the zones, to provide
a structural support to the well, to protect the well from
corrosion, and to act as a hydraulic seal in the well.
[0003] There are several oil well cement companies worldwide
producing oil well cements that fulfil the American Petroleum
Institute (API) specifications. However, it has been observed
that cement slurries made using these existing oil well cements
suffer from various shortcomings. Some of these problems are
very severe in nature like high gelation of cement slurries,
inconsistent behavior of cement slurries, consumption of larger
amount of cement additives, less thickening time of cement
slurry, inferior compressive strength, and erratic behavior of
cement slurry when used in high temperature and high pressure
conditions.
[0004] These shortcomings of the existing oil well cements
can be due to several reasons like change in chemical, physical
and microscopic properties of the cement from plant to plant.
The main cause of physical variability is the change in particle
size distribution of the oil well cement. The chemical
variability is due to a change in content of alkali, free lime,
and tricalcium aluminate (C3A) in the oil well cement.
[0005] In light of the above, there exists a need for
developing an improved oil well cement that overcomes the
limitations of the existing oil well cements.
S-Y OF THE INVENTION
[0006] A process for preparing an improved oil well cement is
provided. The process comprises obtaining a plurality of raw
materials for preparing the improved oil well cement. The raw
materials comprise up to 63% BWOC of lime, up to 21% BWOC of
silica, up to 4.5% BWOC of alumina, up to 5.8% BWOC of iron
oxide, up to 3-4% BWOC of limestone and sand, and up to 3% BWOC
of iron ore. The raw material further comprises up to 0.75% BWOC
alkali.
[0007] Thereafter, a predefined composition of the raw
material is prepared for calcination by grinding and blending
the obtained raw material. The composition is then heated to
form cement clinker. The cement clinker comprises tricalcium
silicate (C3S) in a range of 48-65% BWOC, up to 3% BWOC of
tricalcium aluminate (C3A), and up to 24% BWOC of tetracalcium
aluminoferrite (C4AF). In an embodiment of the present invention,
up to 1.4% BWOC of free lime content is maintained in the improved
oil well cement during formation of the cement clinker.
Thereafter the cement clinker is cooled.
[0008] 'Finally, the cement clinker is grinded with gypsum to
prepare the improved oil well cement. The improved oil well
cement has particle size distribution in terms of D-value in the
range of Dl0 = 5pm to 8pm, D50 = 25.0pm to 35.0pm and D90 = 80pm
to 135pm. The improved oil well cement has surface area in the
range of 0.1379 to 0.1811 m2/g. The improved oil well cement
facilitates an initial consistency of up to 25 Bc. Further, the
improved oil well cement has a thickening time between 250-330
minutes.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0009] The present invention is described by way of
embodiments illustrated in the accompanying drawings wherein:
[OOlO] FIG. 1 is a flowchart illustrating a process for
preparing an improved oil well cement in accordance with an
embodiment of the present invention;
[OOll] Fig. 2 shows a graph indicating test result for a
cement slurry prepared using an existing oil well cement in
accordance with an embodiment of the present invention;
[0012] Fig. 3 shows a graph indicating test result for a
cement slurry prepared using the improved oil well cement in
accordance with an embodiment of the present invention; and
[0013] Fig. 4 shows a graph indicating test result for a
cement slurry prepared using improved oil well cement in
accordance with an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0014] The following disclosure is provided in order to enable
a person having ordinary skill in the art to practice the
invention. Exemplary embodiments are provided only for
illustrative purposes and various modifications will be readily
apparent to persons skilled in the art. The general principles
defined herein may be applied to other embodiments and
applications without departing from the spirit and scope of the
invention. Also, the terminology and phraseology used is for the
purpose of describing exemplary embodiments and should not be
considered limiting. Thus, the present invention is to be
accorded the widest scope encompassing numerous alternatives,
modifications and equivalents consistent with the principles and
features disclosed. For purpose of clarity, details relating to
technical material that is known in the technical fields related
to the invention have not been described in detail so as not to
unnecessarily obscure the present invention.
[0015] The present invention would now be discussed in context
of embodiments as illustrated in the accompanying drawings.
[0016] FIG. 1 illustrates a process for preparing an improved
oil well cement in accordance with an embodiment of the present
invention. The oil well cement may be Portland cement that may
be used for various cementing operations in oil and gas wells.
At step 102, raw materials for preparing the oil well cement are
obtained. The raw materials to prepare the oil well cement may
comprise calcareous materials and argillaceous materials. The
calcareous materials may further include naturally occurring
calcareous materials and artificial calcareous materials. The
naturally occurring calcareous materials may include sedimentary
and metamorphic lime stones, coral, shell deposits and cement
rock. The artificial calcareous materials may include
precipitated calcium carbonate and other alkali wastes from
various industrial processes. Further, the argillaceous
materials may also be artificial and naturally occurring. The
artificial argillaceous materials may include blast furnace slag
from steelworks and fly ash from coal-fired power plants. The
naturally occurring argillaceous materials may include clays,
shale, marls, mudstones, slate, schist, volcanic ashes and
alluvial silt. In an embodiment of present invention, the raw
materials for preparing the improved oil well cement may include
lime, silica, aluminum, iron oxide, limestone and sand, and iron
ore. In an embodiment of the present invention, the content of
lime may be up to 63% By Weight of Cement (BWOC), silica may be
up to 21% BWOC, alumina may be up to 4.5% BWOC, iron oxide may
be up to 5.8% BWOC, limestone and sand may be up to 3-4% BWOC,
and iron ore may be up to 3% BWOC.
[0017] Further, the choice of the raw materials may be
dependent on the presence of undesirable impurities in the raw
materials. These impurities may comprise magnesia or magnesium
oxide (MgO), fluorine compounds, phosphates, lead oxide, zinc
oxide and alkalis in the form of sodium oxide (NanO). These
impurities may cause a disruptive delayed expansion of the set
cement, a significant decrease in cement strength may cause other
deleterious effect upon the cement properties. The alkalis,
however, have been found to have positive effect as well on
compressive strength of the cement when present in desired
predefined quantities in the cement. In an embodiment of the
present invention, the desirable content of alkali in the raw
material may be up to 0.75% BWOC.
[0018] At step 104, the raw materials are prepared for
calcination. The raw materials may be prepared by grinding and
blending them uniformly to obtain a desired chemical composition.
In an embodiment of the present invention, the raw materials may
be prepared by a dry process. In other embodiment of the present
invention, the raw materials may be prepared by a wet process.
In the dry process, the raw materials are crushed, dried in
rotary driers and proportioned to obtain the correct bulk
composition. In the wet process, the raw materials are initially
proportioned in the dry state. Water is added, and further size
reduction occurs in a grinding mill.
[0019] After appropriate degree of size reduction,
classification and blending of the raw materials has been
achieved, at step 106, the composition is heated in a kiln.
Inside the kiln a series of reactions occur between lime, silica,
alumina and iron oxide at temperatures around 1500"~. The
reactions result in the formation of cement clinker that
comprises tricalcium silicate (C3S), dicalcium silicate (C2S),
tricalcium aluminate (C3A), and tetracalcium aluminoferrite
(C4AF). In an embodiment of the present invention, the desired
quantity of C3S is in a range of 48-658 BWOC, desired quantity
of C3A is up to 3% BWOC, and desired quantity of C4AF is up to
24% BWOC. Further, the content of the free lime (CaO) is closely
monitored in clinker to ensure it does not cross the predefined
limit. Excess free lime content results in undesirable effects
such as volume expansion, increased setting time or reduced
strength of the cement. In an embodiment of the present
invention, the predefined limit for the free lime content in the
improved oil well cement is up to 1.4% BWOC.
[0020] At step 108, the clinker is cooled. The quality of the
finished cement is a function of the rate of cooling of the
clinker. In an embodiment of the present invention, the clinker
is cooled slowly to about 1250"~f ollowed by rapid cooling which
is in a range of 18-20°c/minute.
[0021] At step 110, the clinker is grinded with gypsum to a
predefined particle size distribution and a predefined surface
area to form the improved oil well cement. The particle size
distribution, also known as fineness, is an important parameter
with respect to cement reactivity and slurry rheology. The
various properties of the well cement that are affected by the
particle size distribution include, without any limitation,
strength, setting time, permeability, hydration temperature,
water demand, and microstructure. Thus, the particle size
distribution is critical for controlling the rate at which a
cement sets and gains strength. In an embodiment of the present
invention, the predefined particle size distribution in terms of
D-value is in the range of Dl0 = 5pm to 8pm, D50 = 25.0pm to
35.Opm and D90 = 80pm to 135pm. Further, the fineness of the
cement is also described in terms of the surface area of cement
particles. The surface area of the cement particles may be
obtained from the particle size distribution of the cement
particles. In an embodiment of the present invention, the surface
area of the improved well cement particles may be in the range
of 0.1379 to 0.1811 m2/g.
[0022] In an embodiment of the present invention, TABLE 1
lists down various parameters which are enhanced in the improved
oil well cement.
TABLE 1
S . No.
1.
2
[0023] In an embodiment of the present invention, after the
improved oil well cement is prepared, the improved oil well
cement is tested on a plurality of parameters to ascertain if
the improved oil well cement overcomes the limitations of the
existing oil well cements or not. In an embodiment of the present
D e s c r i p t i o n
ppp
Magnesium Oxide
ppp
Sulphur Trioxide
R e s u l t
R e q u i r e d (%)
3. 3.0 (Max)
0.75 (Max)
pp
48-65
3.0 (Max)
24.0 (Max)
0.75 (Max)
1.4 (Max)
Loss on Ignition
RESULTS OBTAINED
E x i s t i n g
c e m e n t
6.0 (Max) 1.58 1.34
1.48
ppp
4 0.48
Improved
c e m e n t
-
3.0 (Max)
1.50
Insoluble Residue 0.42
2.03
5 56.45
1.79
21.54
0.38
2.4
-
2.05
Tricalcium Silicate 57.22
1.66
20.92
0.18
-
1.4
6
I
Tri calcium
Aluminate
7
Tetra Calcium
Aluminoferrite
8
9
Total Alkali
Content As Sodium
Oxide (NaaO)
Equivalent
ppp
Free lime
invention, the plurality of parameters comprise gelation,
consistency, and thickening time.
[0024] Gelation of a cement slurry is one of the most serious
problems occurring in oil well cementing. It appears as a
premature viscosification or as gel strength build-up of the
cement slurry in the well. The gelation is one of the primary
reasons for cementing job failures. Gelation may occur
immediately after mixing or during the displacement, when the
slurry has reached circulating temperature. One of the reasons
for gelation is particle size distribution. Thus, the test for
gelation comprises preparing cement slurries with the improved
oil well cement and an existing oil well cement. The two cement
slurries may be prepared using equal amounts of water, and
similar types of quantities of additives. Further, the test
conditions are kept identical for the two cement slurries. The
readings for gelation at 10 seconds and 10 minutes may be
recorded using a Fann Viscometer. Table 2 provides the readings
of the gelation test.
TABLE 2
Cement used
Existing cement
Improved oil well
cement
[0025] As it can be seen from the readings of Table 2 that no
gelation was observed in the slurry prepared with the improved
10 seconds Dial
reading
13-15
15-17
10 minutes Dial
reading
Out of scale
(highly viscous)
20-65
oil well cement. Absence of gelation indicates that an
improvement in the particle size distribution of the oil well
cement results in elimination of the gelation problem.
[0026] A second test is conducted to analyse the effect of
improved oil well cement on consistency of a cement slurry
prepared using the improved oil well cement. In an embodiment of
the present invention two cement slurries are prepared, one with
the improved oil well cement and the other one with an existing
oil well cement. The two cement slurries may be prepared using
equal amounts of water, and similar types of quantities of
additives. Further, the test conditions are kept identical for
both the cement slurries. In an exemplary embodiment of the
present invention, the Bottom Hole Circulating Temperature
(BHCT) for testing both the cement slurries is kept at 170 degree
centigrade. On comparing the test results it is noted that
despite using the same quantities of retarders in both the
slurries, the cement slurry made using the improved oil well
cement shows significant improvement in the consistency of the
cement slurry. In an embodiment of the present invention, the
improved oil well cement provided an initial consistency of up
to 25 Bc. Fig. 2 shows a graph indicating test result for a
cement slurry prepared using existing oil well cement in
accordance with an embodiment of the present invention. Fig. 3
shows a graph indicating test result for a cement slurry prepared
using improved oil well cement in accordance with an embodiment
of the present invention. The two cement slurries are tested at
170 degree BHCT. As it can be seen from the graphs in Fig. 2 and
Fig. 3, the cement slurry prepared using the existing oil well
cement has an initial consistency of 60 Bc which is highly
viscous and doesnf t allow the slurry to be pumped in the oil
wells effectively. On the other hand, the cement slurry prepared
using the improved oil well cement shows drastic improvement in
the initial consistency. Fig. 3 shows that an initial consistency
of 25 Bc is achieved using the improved oil well cement that
facilitates easy pumping of cement slurry with same dosages of
cement additives.
[0027] A third test is conducted to analyse the effect of
improved oil well cement on thickening time of a cement slurry
prepared using the improved oil well cement. The thickening time
of a cement slurry is analysed to determine the time required to
prepare and place the cement slurry in the oil well. In an
embodiment of the present invention, thickening time is the time
elapsed from the initial application of temperature and pressure
to the time required for the cement slurry to reach a consistency
of 100 Bc. The thickening time is a function of temperature, the
type of ingredients used, and is determined at a BHCT which is
in a range of 6 0 " ~to 230"~.T he thickening time of the cement
slurry may be calculated or tested using High-Pressure, High-
Temperature (HPHT) consistometer. Generally, the thickening time
of the slurry with existing oil well cement is between 88-105
minutes. The thickening time of the existing cement indicates
that cement sets very early and doesn't give desired time for
placement of cement slurry. On the other hand, the thickening
time for the cement slurry prepared using the improved oil well
cement is more than 300 minutes. Fig. 4 shows a graph indicating
test result for a cement slurry prepared using improved oil well
cement in accordance with an embodiment of the present invention.
As it can be seen from the test result, the thickening time of
the cement slurry prepared using the improved oil well cement is
between 300-330 minutes. In an embodiment of the present
invention, the thickening time of 250-330 minutes has been
successfully achieved for the improved oil well cement. Thus,
the test results indicate an improvement in the thickening time
that facilitates a convenient and effective pumping of cement
slurry with same dosages of cement additives.
[0028] After the improved oil well cement is tested on various
critical parameters, the tested oil well cement is stored in
large airtight silos to protect the oil well cement from humidity
and carbon dioxide.
[0029] Thus, based on the tests conducted on the improved oil
well cement it may be apparent to a person of ordinary skill in
the art that the improved oil well cement overcomes the critical
problem of gelation of API standards based existing oil well
cements. In an exemplary embodiment of the present invention,
the improved oil well cement of the present invention was tested
in predetermined oil and gas wells and it gave desired and better
results. The improved oil well cement also did not show any
gelation and gave desired consistency and thickening times.
[0030] While the present invention has been shown and
described with reference to preferred embodiments, it will be
understood by those skilled in the art that various changes in
form and detail may be made therein without departing from or
offending the spirit and scope of the invention as defined by
the appended claims.

We claim:
l.A process for preparing an improved oil well cement, the
process comprising:
obtaining a plurality of raw materials for preparing the
improved oil well cement, the raw materials comprise up to 63%
BWOC of lime, up to 21% BWOC of silica, up to 4.5% BWOC of
alumina, up to 5.8% BWOC of iron oxide, up to 3-4% BWOC of
limestone and sand, and up to 3% BWOC of iron ore;
preparing a predefined composition on the raw material
for calcination by grinding and blending the obtained raw
material;
heating the composition to obtain predefined quantities
of tricalcium silicate (C3S), dicalcium silicate (C2S),
tricalcium aluminate (C3A), and tetracalcium aluminoferrite
(C4AF) in formed cement clinker;
cooling the cement clinker; and
grinding the cement clinker with gypsum for preparing the
improved oil well cement, wherein the improved oil well cement
has a predefined particle size distribution and a predefined
surface area.
2. The process as claimed in claim 1, wherein the raw material
further comprises up to 0.75% BWOC alkali.
3. The process as claimed in claim 1, wherein the predefined
quantities of C3S is in a range of 48-65% BWOC, C3A is up to
3% BWOC, and C4AF is up to 24% BWOC.
4.The process as claimed in claim 1, wherein up to 1.4% BWOC of
free lime content is maintained in the improved oil well cement
during formation of the cement clinker.
5.The process as claimed in claim 1, wherein the predefined
particle size distribution in terms of D-value is in the range
of Dl0 = 5pm to 8pm, D50 = 25.0pm to 35.0pm and D90 = 80pm to
135pm.
6. The process as claimed in claim 1, wherein the predefined
surface area of the improved well cement particles is in the
range of 0.1379 to 0.1811 m2/g.
7. The process as claimed in claim 1, wherein initial consistency
of a slurry prepared using the improved oil well cement is up
25 Bc.
8. The process as claimed in claim 1, wherein thickening time of
a slurry prepared using the improved oil well cement is between
250-330 minutes.