Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
lOS8854
This invention is directed to the removal of calcium
sulfate and/or calcium carbonate scale deposits from oil and
gas wells and associated subsurface and surface equipment.
More particularly, the invention is directed to a novel composition
useful in the treatment of oil and gas wells to remove accumulated
water-insoluble scale deposits from the surface of subterranean
rock strata, well bore casing, tubing and associated equip-
ment.
In oil and gas wells, objectionable calcium sulfate
deposits in the form of gypsum accumulate in the pores and
channels of the fluid-producing strata and in the well bore
equipment such as the tubing, screens, chokes and pumps. These
gypsum deposits may also occur in oil field surface equipment
including flow lines, separators, and emulsion treaters. The
deposits are crystalline masses which adhere tightly to the
formations and equipment and, as the masses build up, fluid
flow is hindered and finally stopped.
This scale also causes many problems in oil and
gas well treating operations, particularly when it builds
up in the well tubing. Such scale deposits inhibit the flow
of fluids such as oil, water and/or treating fluids through
the piping. If left unchecked, a complete blockage of the pipe
will result.
Removal of calcium sulfate from wells is difficult,
particularly when the scale is in a massive, dense, impermeable
form. Removal of such scale is presently accomplished by
several methods including scraping and chemical treatment.
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One chemical method involves treatment of the scale
with a scale converter which converts the scale to an acid-
soluble material, followed by treatment with a mineral acid
such as HCl. For example, insoluble sulfate scales are
generally first reacted with a converter such as a carbonate
to yield a water-insoluble--acid soluble carbonate scale which
is then treated with a mineral acid.
A second conventional method employs strongly caustic
solutions of chelating or sequestering agents, such as ethylene~
diamine tetraacetic acid or nitrilotriacetic acid. The use of
the sequestering agent is conventionally a one-step operation.
The sequestering agents act very slowly and require very high
pH conditions to dissolve the scale and relatively long well
shut-downs are necessary for the treating operation.
In a variation on these methods, a sulfate scale con-
verting solution is used which contains a water soluble con-
verting agent such as a bicarbonate for converting the scale
to a more readily-ionizable form, and a chelating agent for
dissolving and complexing the converted scale. Scale removal
in this instance appears to be facilitated by the concurrent
converting and complexing reactions and the action is somewhat
improved over the action of the chelating agent alone. Ad-
ditionally, at least under low pressure conditions, carbon
dioxide evolution occurs, causing agitation and apparently
giving a mechanical assist to the scale removal. As with the
use of sequestering agents alone, this method requires opera-
tion under controlled pH conditions. Additionally, where the
scale converter employed is a carbonate or bicarbonate, in-
creased corrosion of the iron surfaces of the casing and as-
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sociated well-bore equipment may occur by way of the well-known
carbonate corrosion phenomenon.
A scale removing composition for effecting a rapid removal
of scale deposits formed in an oil well bore and associated equip-
ment which does not require potentially hazardo~s and corrosive,
strongly caustic solutions, which can be employed with less pre-
:~ cise control of pH and which avoids the use of potentially cor-
rosive carbonates or bicarbonates is clearly needed.
This invention may be briefly described as a composition
and method useful in the removal of water-insoluble scale deposits
from the surface of an oil well bore and associated well bore
equipment. More particularly, the.instant composition comprises
an aqueous solution of a polyaminocarboxylic acid and a borate
salt which is highly effective in removing calcium sulfate de-
posits such as gypsum and anhydrite from the surface of oil well
tubing, casing and rock strata, as well as a method for the re-
moval of scale deposits from oil wells.
The scale-removing composition of this invention generally
comprises a water solution containing an effective amount of a
polyaminocarboxylic acid chelating agent, a borate salt, and
optionally, surfactants and freezing point depressants, which is
adjusted in alkalinity to a pH above a value of 8 by addition of
a suitable caustic.
In one particular aspect the present invention provides a
composition useful in the removal of water-insoluble scale de-
posits from an oil well b.ore and the surfaces of associated well
bore equipment comprising an a~ueous solution, having a pH greater
than 8, containing from about 5 to about 12 weight percent of a
chelating compound selected from ammonium, amine and alkali
metal salts of aminopolycarboxylic acids and from about 1 to
about 5 weight percent of a borate salt selected from the group
consisting of.ammonium, amine and alkali metal borates, tetra-
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``` 105~854
borates and metaborates.
In another aspect the present invention provides a method
for removing water-insoluble scale from the surfaces contained
within an oil well bore comprising contacting the sur~aces
with an aqueous solution, having a pEI greater than 8, compris-
ing from about 5 to about 12 weight percent of a chelating
agent selected ~rom the alkali metal, amine and ammonium salts
of amino polycarboxylic acids, and from about 1 to about 5
weight percent of a borate salt selected from the amine, ammon-
ium and alkali metal borates, tetraborates and metaborates.
Examples of suitable polyaminocarboxylic acid chelating
agents are widely known, and include alkylene diamine tetra-
acetic acids such as ethylenediamine tetraacetic acid (EDTA),
and aminotriacetic acids such as nitrilotriacetic acid (NTA),
as well as the corresponding ammonium and alkali metal salts.
Aqueous solutions of these compounds
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alone have long been employed for scale removal.
sorate salts useful in the practice of this invention
include the alkali metal tetraborate and metaborate salts.
Preferably, sodium tetraborate or the hydrated analog Na2g407. lO
H20 (borax) is employed by virtue of ready availability, however,
sodium metaborate and the analogous ammonium, lithium and
potassium salts are also useful in the practice of this invention.
In most instances, the solution should contain at
least about 5 weight percent of the chelating compound to be
effective, together with greater than about 1 weight percent
of the borate salt. While increased concentration tends to
promote the rate of dissolving and scale removing, the solubility
of these compounds is limited, and aqueous solutions will normally
contain no more than about 12 weight percent of the chelating
compound and about 4 to 5 weight percent of the borate salt
in dissolved form at room temperature. The preferred concentration
range of each component is from about 5 weight percent to about
12 weight percent of the chelating compound and from about
l weight percent to about 5 weight percent of the borate
salt. Although even lower concentrations may be somewhat
effective, the rate of scale dissolving and removal becomes
considerably slower and therefore impractical for most purposes.
The composition of this invention is most effective
in removing scale deposits when alkaline. More particularly,
when the pH of the composition is above a value of about 8,
and more preferably in a range of values of from about 9 to
11, the rate of scale removal is rapid and the resulting com-
position is not so caustic as to present a high degree of
potential danger in handling. Adjustment of the pH may be
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readily accomplished by the addition of caustic. While any
of the common caustics such as alkali metal oxides or ammonium
or alkali metal hydroxides may be employed for this purpose,
soda lye or sodium hydroxide is conveniently available and may
be added either in the solid form or as a concentrated aqueous
solution. The precise amount employed to produce the desired
pH will of course depend upon the type and amounts of the
chelating agent and borate salts.
The effectiveness of the compositions of this in-
vention in dissolving and removing scale deposits is demonstrated
by the test data in the Table I below. These data report the
results of a series of comparative tests run with a variety of
compositions and employing gypsum board samples as the test
specimen. The weight of the gypsum dissolved is taken as a
measure of relative effectiveness of the particular composition.
The test data in Table I were obtained by the
following procedure: Squares of 1/2" gypsum board
measuring approximately 1/4" x 1/4" were weighed into a
100 ml beaker, to a total sample weight of six grams. A
50 ml. portion of the "scale remover" formulation was then
added to the beaker, and the beaker containing the test mixture
was placed in an air oven at 100 F for 24 hours. A sheet of
aluminum foil was loosely placed over the beaker to retard
evaporation. The beaker was then removed from the oven and
the remaining gypsum board sample was collected by filtration,
washed, and then dried at 150 F. for a period of 2 to 3 hours.
The cooled sample was then reweighed to determine the weight
loss. The weight loss for a series of six such tests is reported
in Table I, expressed in the form lbs. of gypsum board dissolved
per gallon of "scale remover" employed.
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The formulation for each "scale remover" is also
given in Table I, where the amounts indicated are expressed
in weight percent of the total formulation.
Table I. Testing of Aqueous sOrate - EDTA
Mixtures with Gypsum Board Samples
TEST RUN NO.
Composition 1 2 3 4 5 6 7
EDTA (1) 10.7 1~.712.8 10.710.7 10.7 10.7
NaOH(2) 6.4 6.4 6.4 3.9 6.4 6.4 6.4
Na2B407 10H20 (3) 2.12.12.1 2.1 2.1 ---- ----
H2O (4) 80.8 80.378.7 83.380.3(6)82.9 82.9
Surfactant (5) ---- 0.5 ---- ---- 0.5 ---- ----
pH 12.8 11.7 9.4 7.511.6 12.8 10.7
Test Results
Gypsum Board
Dissolved,
(lbs/gal) 0.57 0.550.55 0.100.65 0.48 0.24
... ..
Notes: (1) EDTA = Ethylenediamine tetraacetic acid
(2) NaOH = Reagent ~rade pellets
(3) Na2B4O7. 10H20 = Reagent grade borax
(4) H20 = deionized water
(5) Proprietary alkanolamine surfactant
(6) Tap Water
The test was repeated five times with the
formulation shown for Test Run No. 1. The results were
reproduced within 5.0%. Similarly, five runs employing
the formulation of Test Run No. 6 gave results reproduced
within 3.4%.
A practical test of the instant scale solvent
compositions was carried out by measuring the dissolution
of actual scale samples collected from oil wells, using
the procedure followed for gypsum board tests. The results
of these tests are reported in Table II, and it will be
apparent from a comparison of these data with the data
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reported in Table I that an adequate correlation of tests
on gypsum board with tests on field samples is realized.
Table II. Testing of Aqueous Borate - EDTA
Mixtures with Actual Scale Samples
TEST RUN NO.
Compositions 8 9 10
EDTA (1) 10.710.7 10.6
NaOH (2) (3) 6.4 4.8 4.1
Na2B4O7 10H20 2.1 3.2 5.3
H2O 80.380.7 79 5
Surfactant ( ) 0.5 0.5 0.5
pH 10.5 8.5 8.3
Test Results
Scale Dissolved
lbs./gal. 0.520.48 0.48
Notes: (1) Technical Grade Ethylenediamine tetraacetic
acid
(2) Technical Grade
(3) Technical Grade
(4) Tap Water
(5) Proprietary alkanolamine surfactant
.
It is apparent from the data of Test Runs 1-3,
when compared with run 4, that an alkaline pH is necessary
for effective reaction and dissolution of the gypsum board,
and from Test Runs 4, 9 and 10 data that effective results
are obtained above a pH of about 8. It will also be noted,
when compared with Examples 6 and 7, that very high pH levels
near 13 are required for solutions containing EDTA alone,
but more moderate pH levels are quite effective when borax
is present in the mixture. This composition is therefore
particularly advantageous for field application in that
a highly corrosive, strongly caustic solution is no longer
necessary, thereby reducing the potential hazard to personnel,
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and in addition minimizing the possibility of caustic etching
and weakening of metal pipe, tanks, pumps and associated
equipment.
As was said previously, the instant chelating
compound--borate compositions are also desirable over the
prior art mixtures containing carbonates or bicarbonates
because of the reduced potential for carbonate corrosion
of iron surfaces. It will also be apparent that mixing
of the instant scale remover compositions to any pH value
above about 8 results in an effective scale removing mixture,
whereas common experience has shown that with mixtures of
chelating compounds and carbonates or bicarbonates, best ~ ^
results are obtained in a narrow range near a pH of 7.5.
This provides an additional advantage in that for application
in oil fields, a precise weighing of materials and measurement
of pH is quite difficult to attain.
The compositions of this invention are most useful
for removing calcium sulfate scale from the bore of an oil
well. The following illustrative embodiment is provided
to demonstrate a typical procedure to be followed and results
that could be expected in such an application. 1
Scale Removal Procedure for_Oil Well Use
450 gallons of tap water are placed in
a mix tank, together with 2 gallons of alkanolamide sur-
factant. The mixture is then agitated while 500 lbs of
technical ethylenediamine tetraacetic acid are added,
followed by 100 lbs of technical borax. When the solids are
dissolved, 300 lbs of technical grade flake caustic soda are
added slowly with continuous mixing. After all solids are
dissolved, the pH is found to be approximately 10.8. A
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pre-flush of kerosene is normally employed first to clear
the well fluids from the area. The treating mixture is pumped
into the production tubing of the oil well to substantially
fill the tubing and the casing therein below. The well is
closed in and the treating solution, optionally capped by
pumping in a small slug of kerosene, is allowed to stand for
a period of 24 hours. The treating solution is then swabbed
from the well, and the well is placed back into production.
An oil well in the San Andreas Formation near
Brownfield, Texas was treated with 500 gallons of scale
remover composition substantially according to the procedure
above. Well production was increased by approximately 50%
as a result of this scale removing treatment.
The use of well-treating compositions in oil
fields often requires the addition of surfactants such
as alkanolamides, ethoxylated phenols and the like and
fluid-friction reducing compounds such as polyacrylonitrile,
polyethylene oxide and the like to meet practical handling
requirements, for example, the reduction of interfacial tension,
the emulsification of crude oil components present in the well
fluid and the increase of the pumpability of the fluid. Ad-
ditionally, where the well treatments are carried out under
extreme weather conditions, it may be necessary to substitute
a freezing point depressant such as ethylene glycol for a
portion of the water. Such additions and substitutions may
be made without markedly affecting the properties of the novel
composition of the instant invention and so are contemplated
as being within its scope.
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