Note: Descriptions are shown in the official language in which they were submitted.
HIG~-DENSITY BRINE FLUID
AND USE IN
SERVICING WELLBORES
This invention relates to a solids-free solu-
tion for introduction into wellbores and a method of use
therefor.
Subterranean formations may contain valuable
mineral deposits such as hydrocarbon oils and gases, sul-
fur and other valuable natural resources. Contact may
be made with these formations by drilling a wellbore from
the surface to the subterranean formation. During the
course of this drilling operation a fluid is in~roduced
into the wellbore to lubricate and cool the drilling bit,
to carry away material removed from subterranean forma-
tions by the drilling process, and to seal the walls of
the borehole against loss of the fluid or invasion of
the borehole by fluids in subterranean formations. Such
a fluid or drilling "mud" is described, for example, in
US 2,073,413. These drilling muds achieve the high den-
sity which is necessary to avoid intrusion of high pres-
sure subterranean fluids into the borehole by weighting
27,224-F -1- ~
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9~9
the mud with solid materials suc:h as clays or barytes.
High densities in the order of magnitude of 18-20 lb/gal
(2.1-2.4 g/cm3) are achievable with such drilling muds.
However, at certain times during the well
drilling and completion process, it is desirable to
have solids-free solutions contacting the subterranean
formations in the borehole. For example, see the arti-
cle by J. L. Kennedy, The Oil and Gas Journal, (August 2,
1971) pp. 62-64. Many procedures re~uire a solids-free
fluid. For example, packing and completion procedures
can be injured by solids suspended in the fluid within
the wellbore. See Drillinq and Production Practice,
C. M. Hudgins e-t al. (American Petroleum Institute)
1961; Hudgins et al., The Oil and &as Journal, (July 24,
15 1961) pp. 91-96; and J. H. Plonka, World Oil (April,
1972) pp. 88-89; Neal Adams, The Oil ~ Gas Journal
(November 9, 1981), pp. 254-275. These articles
describe the use of high density, solids-free brines
as fluids. Sodium chloride can be used to make brines
20 from 8.33 to 9.8 lb/gal (1.0-1.17 g/cm3). Calcium
chloride can be used to make brines from 8.3 to 11.5
lb/gal ~1.0-1.38 g/cm3). Calcium chloride/zinc chlo-
ride brines can be used to make brines from 11.5 to
14.0 lb/gal (1.38-1.68 g/cm3). The ~udgins articles
describe zinc-containing fluids with densities greater
than 14.0 lb/gal (1.68 g/cm3) as being too corrosive for
- practical use. The Plonka article describes a solids-
-free calcium bromide/calcium chloride solution which
can achieve a density of 15.0 lb/gal (1.80 g/cm3).
US 4,292,183 describes a zinc bromide/cal-
cium bromide solids-free solution having a density lying
27,224-F 2-
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in the range from about 14.5 up to about 18.0 lb/gal
(1.74-2.16 g/cm3). The Adams article describes a
CaC12/CaBr2/ZnBr2 brine system with densi~les up to
19.2 lb/gal (2.30 g/cm3), as well as characteristics
of the various fluids and methods of use.
It is sometimes desirable to have a solids-
free solution which has a density of gxeater than 15.0
lb/gal (1.80 g/cm3). Until this invention, this required
the use of a zinc-containing fluid.
A desirable well-servicing fluid needs to
have several characteristics concurrently. The density
of the fluid should be such that the hydrostatic head
formed by the column of fluid in the wellbore properly
balances the fluid pressure of the subterranean forma-
tion. Appropriate balance may at times be slightly below
the pressure of the subterranean formation, i~ often
greater than the pressure of the subterranean formation,
but most often is ~et to exactly match the hydraulic pres-
~ure of the fluid in the subterranean formation.
Fluids used during workovex and completion
range from low-density gases, such as nitrogen, to high-
-density muds and pacXer fluids. The applications and
reguirements for each fluid are different.
Fluids used during the reworking of a well
after its initial completion are termed workover fluids.
These fluids may be gases (such as nitrogen or natural
gas 3, brine waters, or muds. The functions performed
by the workover fluid include well killing, cleaning
27,224-F -3-
;eg;;~9
out a well by removing sand, rock, or metal cuttings,
and other foreign objects, drilling into a new produc-
tive interval, or plugging back to complete a shal-
lower interval.
Completion fluids are used during the opera-
tions that establish final communications between the
productive formation and the well bore. The fluids may
be a commercial workover fluid, nitrogen, or a clean,
low-solids brine water and may be used for a short per-
iod of time such as well perforating or for extended
periods such as in gravel packing. The primary require-
ment placed on the fluid is that it does not damage nor
block the producing formation.
Packer fluids are placed in the annulus
lS between the production tubing and casing. The most
common requirements for packer fluids are to maintain
pressure co~trol, be nontoxic and noncorrosive, remain
pumpable, and minimize formation damage.
The well-servicing fluid should be solids-
-free. That means substa~tially free from suspended
solids of greater than about 5 ~m in diameter. It also
means that the solution should have a crystallization
point lower than the use temperature. Generally, the
crystallization point is the minimum temperature at
which the soluble solids are dissolved to form a solids-
-free solution.
Corrosivity is also an important factor. The
corrosivity of the brine solution should be such that no
27,224-F -4-
12~ iZ9
significant corrosion of metal piping or drilling imple-
ments occurs. This generally re~uires an uninhihited cor-
rosion rate of less than 0.005 inch/year (0.127 mm/year).
This is partisularly true when the fluid is used to shut
in a well. The fluid may be in contact with the downhole
piping for long periods of time. It is also important to
minimize harm to the environment.
When one requires a solution with a density
of greater than 15.0 lb/gal (1.80 g/cm3) to balance the
formation pressure, each of the previous recited fluids
has shortcomings. All of the brines except the zinc-
-containing brines will not form a solids-free solution
at greater than 15.0 lb/gal (1.80 g/cm3) with a crystal-
lization point of less than 20C. The densities and
solubilities of the solid salts will not permit it.
While the zinc-containing brines may form solids-free
solutions at greater than 15.0 lb/gal (1.80 g/cm3),
these solutions are generally more corrosive and may
harm the environment. It is therefore an object of
this invention to provide a solids~free well-servicing
fluid with a density of greater than 15.0 lb/gal (1.80
g/cm3) and a crystallization point of less than 20C
that does not require the use of zinc salts.
This and other objects of the invention are
achieved in a method for servicing a wellbore comprising
introducing into the wellbore a fluid comprising an admix-
ture of water, calcium bromide and methanol. The density
of this solution is at least about 15.0 lb/gal (1.80 g/cm3)
and has a crystallization point of no more than about 20C.
27,224-F -5-
~121(~9;~
The composition consisting essentially of an
admixture of water, calcium bromide and methanol wherein
the admixture has a density of at least about 15.0 lb/gal
(1.80 g~cm3) and a crystallization point of no more than
about 20C is also considered novel.
A feature of this invention is that it incor-
porates commercially available and relatively environ-
mentally safe materials to achieve densities which have
heretofore been unachievable with such low environmental
risk and corrosivity.
The drawing represents a phase diagram for
the methanol/CaBr2/water system.
~ he high density, well-servicing fluid of the
present invention may consist essentially of a substanti-
ally solids-free admixture of water, calcium bromide and
methanol wherein the admixture has a density of at least
about 15.0 lb/gal (1.80 g/cm3) and a crystallization point
of no more than about 20C.-
In addition to calcium bromide and methanol,
certain hydrates and methanates may be present in theadmixture. Also, other water-soluble materials may be
used in the well-servicing fluid in an amount les~ than
that which would adversely affect the utility of the
solution. Such materials include, for ~xample: organic
corrosion inhibitors such as triethanolamine, propargyl
alcohol, pyridine and its derivatives, and other organic
corrosion inhibitors known to those in the art; viscosity
- 27,224-F -6-
~21(~9~
adjusting reagents such as, for e~ample, hydroxymethylcel-
lulose and others known to those in the art; pH control-
ling materials such as sodium hydroxide, calcium hydrox-
ide and the like. In some applications it may be desir-
able to blend the fluid of the invention with other inor-
ganic salt solutions to adjust the final density of the
well-servicing fluid. Such solutions include, for exam-
ple, solutions of the inorganic salts, calcium chloride,
sodium chloride, mixtures thereof and the like.
Well-servicing fluids of this invention would
preferably not include metallic ions not commonly found
in sea water at a concentration greater than 1 ppm. Metal-
lic ions commonly found in sea water in greater than 1 ppm
concentration include sodium, magnesium, calcium, potas-
sium, and strontium. Metallic ions which would be absent
from the composition of the invention include zinc, lith-
ium, and chromium which would only be present in small
quantities as impurities or as a treating agent such as
a zinc oxide corrosion inhibitor. In any case, these
materials may be present at less than 1 percent, pref-
erably at less than 0.5 percent and most preferably at
less than 0.1 percent concentrations by weight based on
the total solution.
The presence of any substantial amount of
solids in the fluid of the invention may lead to damage
to the subterranean formation. Suspended solids may
effectively block production from the formation. Pref-
erably the fluid is free of solids of a particle size
larger than 5 ~m. Most preferably the solution is
completely solids-free. Solids present may include gen-
eral impurities found in the materials used to make the
27,224-F -7-
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fluid, dust and dirt adhering to the equipment used or
solids precipitated by cooling of the solution or other
chemical process such as pH adjustment.
The components used to make the fluid of the
invention are generally commercially available materials.
The calcium bromide used may be of general commercial
quality and may be a material commonly referred to as
calcium bromide spike which is an 80 percent calcium
bromide/20 percent water solid. The calcium bromide
may also be generated in agueous medium, for example,
by reacting hydrogen bromide with calcium hydroxide,
for example, in the method described in US 4,234,556.
Any available water may be used to make the
fluid as long a~ it does not contain materials deleteri-
ous to the properties. Sea water may be used, thoughfresh water is preferred. The components of the admix-
ture may be mixed in any order but`generally it is more
convenient to add the solids to a mixture of the two
liquid components. The percentage ranges of the com-
ponents in the fluid of the invention generally rangefrom 55 to 70 percent calcium bromide, 15 to 35 percent
water, and 5 to 30 percent methanol. This can ~e seen
in the drawing as the area between the two curves, to wit,
between the isodensity line of 15.0 lb/gal and the curve
showing the compositions with a 20UC crystallization point.
Referring now to the dxawing, we see a stand-
ard triangular composition diagram for a 3-component sys-
tem~ At the uppermost corner which is l~beled CaBr2 for
calcium bromide, the point at the apex of the triangle
represents a composition of 100 percent calcium bromide
27,224-F -8~
~Z~92g
and each line which is parallel to the opposite side of
the triangle represents a 10 percent increment in the per-
centage by weight of calcium bromide on the phase diagram.
Similarly a~ the two lower corners of the triangle, meth-
anol designated MeO~ and water designated H20 label theapexes which stand for the 100 percent methanol composi-
tion and the 100 percent water composition, respectively.
The lines within the triangle running parallel to the
opposite sides from each of these two apexes also repre-
sent 10 percent increments o composition.
Four curves are presented on this composi-
tional chart for discussion. Three of the curves approx-
imate straight lines and represent isodensity curves.
That is each of the points on the curve represent the
composition which has a given density. The three curves
are labeled 15.0, 15.5 and 16.0 lb/gal (1.80, 1,86, and
1.92 g/cm3) (kg/l)). ~he points on these curves may
represent compositions which are ~olids or liquids or
mixtures thereof. The drawing was made as accurately
as the data permits. However, differences of several
percent may not be significant and are considered to
be within experimental error.
The fourth curve represents an isocrystalliza-
tion point line. That is, each point on the line repre-
sents a composition which has a crystallization point ofabout 20C or 68F. For purposes of discussion in this
application, the crystallization point of the solution is
determined by placing 2 to 5 ml of the solution in a test
tube and then cooling it in a dry ic~/methylene chloride
bath until solids appear. The mixture with the suspended
solids is then heated slowly with stirring until the last
27,224-F -9-
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~Z~ ;2~
crystal dissolves. The temperat.ure at which the last
crystal dissolves is the crystallization point.
Other workers in this field have used an exper-
imental method similar to this one wherein the crystalli-
zation point is taken to be the temperature at which thecrystal first appears upon cooling. This was not done
wi~h this system because of the tendency of this system
and similar systems to supercool. The result of this is
that the temperature at which the first crystal appears
may be several degrees lower than the temperature at
which the last crystal disappears. Therefore, for pur-
poses of the disclosure and the claims of this applica-
tion, the temperature at which the last crystal disap-
pears is taken to be the crystallization point.
Referring to the drawing, ~he isocrystalli- -
zation point line extends from a composition of approxi-
mately 35 percent calcium bromide/65 percent methanol
to a point just on the opposite side of the 16 lb iso-
density line with a composition of approximately 67 per
cent calcium bromide, 13 percent methanol and 20 percent
water. This approximates the maximum density which can
be achieved with this 3-component mixture while still
retaining a crystallization point of 20C. ~he isocrys-
tallization point curve continues from this maximum to a
point which represents approximately 58.8 percent calcium
bromide and 41.2 percent water.
The portion of this compositional chart which
is bounded by the isocrystallization point curve at 20C
lying in the direction of the apex labeled calcium bro-
mide, represènts that portion of the compositions of
27,224-F -10-
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these three components which are not solids-free at 20C
and lower temperatures. T~at portion of the compositional
chart which lies between the isocrystallization point
curve at 20C and the side opposite the calcium bromide
apex represents the compositions of these three components
which will be solids-free at 20C or higher. The fluid
composition of the invention is represented by points in
~hat area of the phase diagram lying on or within the area
enclosed by the 15.0 lb/gal (1.80 g/cm3) isodensity line
and the 20C isocrystallization point line.
The density of the fluid of the invention is
generally expressed in lb/gal, kg/l or g/cm3. One lb/gal
equals 0.1198 kg/l and 1 kg/l eguals 1 g/cm3 and 8.345
lb/gal. The density of water is approximately 1 kg/l or
8.34 lb/gal. Density of the solutions may be determined
in any of the ways known to those skilled in the art, for
example, by using a pycnometer or a hydrometer. Gener-
ally the fluid will have a density of at least about 15.0
lb/gal (1.80 g/cm3). This is represented by points on
the composition diagram of the figure which lie between
the 15.0 lb/gal (1.80 g/cm3) isodensity line and the pure
calcium bromide apex of the triangle. Preferably, the
density will be greater than 15.0 lb/gal (1.80 g/cm3)
or, for example, at least 15.1 lb/gal (1.81 g/cm3).
Most preferably the fluid of the invention will have
a density of at least 15.5 lb/gal (1.86 g/cm3) and may
have a density of 16.0 lb/gal (1.92 g/cm33 or greater.
All of these densities given require that the fluid of
the invention be solids-free and have a crystallization
point of 20C or less.
27,224-F -11-
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In addition to calcium bromide and methanol,
hydrates and methanates of calci.um bromide will be pres-
ent in the admixture. While the exact nature of such
materials is difficult to determine, and their presence
is difficult to detect, it is apparent that whatever
species is present, the admi~ture has the properties of
density, crystallization point, corrosivity and environ-
men~al effects as herein above described.
Examples of well-servicing techniques where
~he fluid of the invention may be employed are taught,
for example, in US 2,894,584; US 2,898,294 and US
3,126,950.
The following examples further illustrate
this invention, but ~hould not be construed as limiting
the scope.
Example 1
108 ml of CH30H (methanol) were added to
100 ml of commercially available fluid containing 53 per-
cent CaBr2 and 47 percent H2O. To this was added 150 g
20 of CaBr2 spike in increments of 10 to 50 g (CaBr2 spike
is 80 percent CaBr2 and 20 percent H2O). After mixing
on a shaker table the solution was filtered. This solu~
tion had a density of 13.19 ppg (1.58 g/cm3). The solu-
tio~ was divided in half, and to approximately 125 ml of
solution were added 175 g of spike. Again, after shaking
until all of the solids were dissolved, the solution was
filtered. The density of the solution was 16.0 ppg (1.92
g/cm3) and it had a crystallization point of 64.5F
(18.1C).
27,224-F -12-
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~L21~L1929
Example 2
To an 8-ounce (235-ml) jar were added 81 ml
of methanol and 50 ml of 53 percent CaBr2 and 47 percent
H2O. Additions of 25 or 50 g of CaBr~ spike (80 percent
CaBr2 and 20 percent ~2) were made 11 times so that the
total weight of spike added was 325 g. The solution was
then suction filtered. The density was found to be 16.0
ppg (1.92 g~cm3). The crystallization point was deter-
mined by placing 2 to 5 ml of solution in a test tube and
then cooling in a dry ice/methylene chloride bath until
solids appeared. This was then heated slowly with stir-
ring until the last crystal dissolved. The crystalliza-
tion point of the solution was 62F (16.7C).
An agueous solution of CaBr~/~20/MeOH will
have a lower crystallization point when compared to a
CaBr2/H20 fluid. The crystallization point depression
is due to the presence of MeOH in combination with the
CaBr2 and water in solution. At a density of 15.9 ppg
(1.91 g/cm3), the CaBr2/H20/MeOH solution has a crystal-
20 lization point of 64F (17.8C). This can be directly
contrasted to a solution containing only CaBr2 and water
wherein a 15.9 ppg ~1.91 g/cm3) fluid would have a crys-
tallization point of 81F (27.2C). The variation in
crystallization point temperatures (CP) of solutions of
25 CaBr2/~20~MeOH and CaBr2/H20 is illustrated by Table I.
27,224-F -13-
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.
1~1(P929
TABLE I
CaBr2/H20 vs CaBr2/H20/MeOH
Density CaBr ~2 CaBr?~H2O~MeO~
CPF CPF
q/cm3 l~/gal ~ CaBr2 (C) % CaBr~ % MeOH (C)
1.905 15.g ~1.581 64.6 12.864
(27.2) (17.8)
1.955 16~3 64.090 65.8 12.370
(32.2) (21.1)
10 1.865 15.5 60.074 63.3 13.355
(23.3) (12.8)
1.815 ~5.1 58.568 62.0 13.960
(2~.0) (15.6
CaBr2/H2O da a
Densities: International Critical Tables
CP: Seidell (Revised by Linke), Solubilities:
Inorganlc and Metal Orqanic Compounds; Van
Nostrand Co., Inc., 1958.
27,224-F -14-
