Note: Descriptions are shown in the official language in which they were submitted.
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FORMULATIONS AND METHODS FOR MINERAL SCALE REMOVAL
Field of the Invention
The present invention relates to compositions and methods for removal of
mineral
scales.
Background of the Invention
Mineral scale formation in surface and subsurface oil and gas production
equipment is
recognized as major operational problem in oil and gas production. Mineral
scale build
up is also an issue in other industries, such as pulp and paper making. Group
11 metal
ions, with the exception of perhaps magnesium, can all form sparingly soluble
scales.
Scale deposits have been identified as complexes of calcium such as carbonate,
oxalate, sulfate, silicate; aluminum such as silicate, hydroxide, phosphate;
barium such
as chloride, carbonate, sulfate and various other alkaline earth and
transitional metal
salts. The most common scales are: calcium carbonate; sulfate salts of
calcium,
strontium and barium; and sodium chloride.
In the oil and gas industry, sulfate scales are formed when subsurface
formation water
is mixed with injected sea water. Many subterranean waters contain alkaline
earth metal
cations, such as barium, strontium, calcium and magnesium. Sea water has high
concentration of S042- and formation waters, with high concentrations of Ca2+,
Ba2+ and
Sr2+.The injection of seawater into oilfield reservoirs is necessary to
maintain reservoir
pressure and improve secondary recovery. When two incompatible waters are
mixed
such as seawater and formation water and interact chemically, a precipitate or
scale is
formed. Two waters are called incompatible if they interact chemically and
precipitate
minerals when mixed. Mixing of these waters, therefore, could cause
precipitation of
CaSO4, BaSO4 and SrSO4.
Three common mechanisms for scale formation on onshore and offshore oil field
system are:
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1) Decrease in pressure and/or increase in temperature of a water, leading to
a
reduction in the solubility of the salt (most commonly these lead to
precipitation of
carbonate scales, such as CaCO3).
2) Mixing of two incompatible water, most commonly formation water rich in
cations
such as barium, calcium and/or strontium, mixing with sulfate rich seawater,
leading to
the precipitation of sulfate scales, such as BaSO4.
3) Water evaporation, resulting in the salt concentration increasing above the
solubility
limit and leading to salt precipitation. This may occur in high pressure, high
temperature
gas wells where a dry gas stream may mix with a low grade brine stream
resulting in
dehydration and most commonly the precipitation of NaCI.
Scale may prevent effective heat transfer, interfere with fluid flow,
facilitate corrosive
processes, or harbor bacteria. Scale is an expensive problem in many
industrial water
systems, in production systems for oil and gas, in pulp and paper mill
systems, and in
other systems, causing delays and shutdowns for cleaning and removal.
Barium and strontium sulfate scale deposits present a unique and particularly
intractable problem. Of all the scales in the oil industry, barium sulfate
scales are the
most easily precipitated due to the very low solubility (2.3 mg/L). In oil and
gas
production operations, deposition of calcium, barium and strontium sulfate
scales are
especially problematic as these species are unreactive to typical chemical
processes
used for scale dissolution.
Scales and deposits can be formed to such an extent that the permeability of
the
formation is impaired resulting in lower flow rates, higher pump pressures,
and
ultimately abandonment of the well.
Another problem associated with the formation of barium and strontium sulfate
scales is
that radium, another member of the alkaline earth group of metals, tends to be
deposited at the same time so that the equipment becomes radioactive, and may
eventually have to become unusable for safety reasons alone.
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Scale treatments include scale inhibition, sulfate ion removal and scale
removal. Scale
inhibitors try to prevent or delay scale formation at substoichiometric levels
in solution.
Nucleation inhibitors disrupt and redissolve the scale protocrystals formed
due to
supersaturation and prevent deposition. Crystal growth inhibitors adsorb on or
interact
with the crystal growth sites (growing edges or spirals) and retard the
crystal growth by
"poisoning" the growth of scale nuclei. Some chemicals chelate or tie up the
reactants in
a soluble form. If inhibitors cannot be used, scale formation can be prevented
by
removing the sulfate ions from seawater by approaches like membrane
distillation,
nanofiltration, and reverse osmosis.
However, if scale formation cannot be prevented, or if the strategy to prevent
its
formation fails, scale deposits are removed either with mechanical means like
milling,
jetting, ultrasound or chemical means like sequestration with a chelating
agent.
Mechanical means can be effective in the well bore, but are not of much use,
if deposits
are in the formation.
Chemical removal treatments are considerably less expensive than mechanical
methods and effective for scale removal from the formation. Barium sulfate
scale is
insoluble in most mineral acids like hydrochloric acid, nitric acid, etc.
Hence the only way to dissolve the barite scale is by using complex organic
acids called
chelating agents or chelants. These chelating agents are ethylamine molecules
having
multiple carboxylic acid arms which can pick up barium molecules from solid
state and
bring them into the solution. Common chelating agents include diethylene
triamine
pentaacetic acid (DTPA) and ethylene diamine tetraacetic acid (EDTA). These
formulations will remove scale deposits, however, the amount of scale removed
is small
and the rate of dissolution is slow. Additionally, as the cations in question
are almost
non-complexible, the use of chelants results in excess chelant that is
disposed
essentially wasted.
Prior art attempts at dissolving barium sulfate include utilizing boiling
sulfuric acid. This
is impractical and dangerous for dissolving downhole barium sulfate. Another
method
for removing barium sulfate is to fracture it with extremely high pressure.
However, this
process is quite costly in that it requires special equipment and extra
manpower, and
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results in lengthy well downtimes. Another prior art process for removing
barium sulfate
is to perforate the scale, which is not very effective. In some instances,
when scales
present too much of a problem, the well is simply abandoned and a new well is
drilled
when scales present too much of a problem. Obviously, this is a very costly
solution.
US 5,068,042 (Hen, 1991): utilizes sulfate scale (barium, strontium, radium)
dissolution
by compositions comprising an aqueous solution of an aminopolycarboxylic acid
(APCA) or its salt, and a second component which is
diethylenetriaminepenta(methylenephosphonic) acid (DTPMP) or its salt, or
aminotri(methylenephosphonic) acid (ATMP), or its salt. US 5,762,821 (Tate,
1998) and
US 5,685,918 (Tate, 1997): teaches scale deposits removal by contact with an
aqueous
solvent of a polyaminocarboxylic acid chelant (EDTA) in an alkaline
environment
produced by the addition of a potassium base (KOH), a hydroxycarboxylic acid
synergist, a wetting agent, and optionally a sodium base. US 6,494,218 (Zaid
et al.,
2002) teaches scale dissolver compositions comprise an aqueous dispersion
which
includes a salt of a chelating agent, a carbonate, a base, and an
organophosphorus
compounds, carboxylic acids and mixtures thereof. The chelating agent is a
salt of
EDTA. The dissolver composition is heated to a temperature of from 100-170 F.,
and
then contacted with the scale. US 7,470,330 B2 (Keatch, 2008) discloses a
method for
removing scale from surfaces such as barium sulfate, strontium sulfate, and
radium
sulfate using aqueous solution of EDTA and potassium carbonate. US Patent no.
7,343,978 (John et al., 2008) proposes a scale dissolver fluid for dissolving
scale
(barite) in a subterranean hydrocarbon-bearing formation which comprises an
aqueous
solution of EDTA and a salt of EDTA. Surfactant is used to reduce the
viscosity of the
fluid. U.S. Patent No. 4,973,201 (Paul et al., 1990) discloses a method for
decontamination by applying an aqueous chemical composition comprising a
chelating
agent (EDTA or DTPA) and a synergist (oxalate or monocarboxylic acid anion
such as
salicylate), which increases solubility of sulfates in an aqueous solution.
W01990011972 A1 (Morris and Paul, 1990) teaches that alkaline earth metal
scales,
especially barium sulfate scale deposits are removed with a composition
comprising an
aqueous alkaline solution having a pH of about 11 to 13, of a
polyaminopolycarboxylic
acid, EDTA or DTPA and a catalyst or synergist comprising, oxalate,
thiosulfate,
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nitriloacetate or monocarboxylic acid anions. US 4,190, 462 (De Jong et al.,
1980)
teaches barium sulfate scale removal by contacting the scale with an aqueous
solution
consisting essentially of water, a monovalent cation salt of a monocyclic
macrocyclic
polyamine containing at least two nitrogen-linked carboxymethyl groups and
enough
monovalent basic compound to provide a solution pH of about 8. US #4,708,805
(D'Muhala, 1987) discloses the removal of barium and strontium sulfate scales
by
treatment with a sequestering composition comprising citric acid, a
polycarbazic acid,
and an alkylenepolyaminopolycarboxylic acid, DTPA or EDTA.
Summary
A composition is provided for sulfate scale removal, said composition
comprising one or
more chelants and one or more accelerants, wherein said accelerants serve to
occupy
reactive sites of cations of the sulfate scale. The composition is preferably
provided
wherein said one or more chelants comprise a mixture of two or more
polyaminopolycarboxylic acids. The composition if further preferably provided,
wherein
said one or more accelerants promote rapid complexation of cations of the
sulphate
scale. The composition is further still preferably provided, wherein said one
or more
accelerants comprise substances that have a smaller molecule size than that of
said
one or more chelants.
It is to be understood that other aspects of the present invention will become
readily
apparent to those skilled in the art from the following detailed description,
wherein
various embodiments of the invention are shown and described by way of
illustration.
As will be realized, the invention is capable for other and different
embodiments and its
several details are capable of modification in various other respects, all
without
departing from the spirit and scope of the present invention. Accordingly the
drawings
and detailed description are to be regarded as illustrative in nature and not
as
restrictive.
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Detailed Description of the Various Embodiment
The description that follows and the embodiments described therein are
provided by
way of illustration of an example, or examples, of particular embodiments of
the
principles of various aspects of the present invention. These examples are
provided for
the purposes of explanation, and not of limitation, of those principles and of
the
invention in its various aspects.
In accordance with the present invention, alkaline earth scales, and more
particularly
sulfate scales such as barium sulfate scales can be chemically removed from
surface
equipment and oil and gas bearing formations by a chemical process employing a
composition comprising a mixture of chelants, more preferably a mixture of
polyaminopolycarboxylic acids, and accelerants to promote more rapid
dissolution.
Polyaminopolycarboxylic acids of the present invention include but are not
limited to
ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid
(DTPA),
polyaspartic acid (PAA) and methylglycinediacetic acid (MGA) among others, and
mixtures thereof
Accelerants have been found by the present inventors to be useful to promote
higher
rates of scale dissolution and markedly more rapid complexation of the cations
of the
scale deposit. Accelerants of the present invention include any number of
small
molecule groups that act as bridging compounds to facilitate occupying the
reactive
sites of the cations of the scale deposits. The accelerants of the present
invention are
relatively smaller molecules than the chelant materials. In this way they can
fill in
reactive sites on the alkaline earth scale that are otherwise not reachable by
the larger
chelant materials, due to steric hindrance. In this way, the accelerants of
the present
invention react with the alkaline earth molecules more quickly and promote
more rapid
access by the chelants.
Accelerants of the present invention include any smaller molecule groups,
including but
not limited to any member of mono or dicarboxlic acids and/or their respective
salts from
potassium, sodium or ammonium where the formula is:
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Monocarboxylic acid: R-C(0)0H, wherein R=C2 to C6
Dicarboxylic acid: HO2C-R-CO2H, wherein R is aliphatic or
aromatic
ring of C2 to C6 length. More preferably, the aromatic ring is a five-member
furan.
Single component and multiple component mixtures of accelerants would also be
understood by those skilled in the art to be useful in the present invention.
Accelerants of the present invention preferably comprise antioxidants. More
preferably,
the accelerants include, but are not limited to, tocopherol, ascorbic acid,
isothiocyanates, tannins such as gallic acid and polyphenols.
According to one embodiment of the present invention, scale removal can be
effected
with an aqueous solution of a mixture of polyaminopolycarboxylic acids such as
EDTA
and DTPA which act as chelating agents, together with the accelerant, to form
stable
complexes with the metal cations of the scale deposits.
Complexation occupies cation reactive sites of the scale deposits. Blocking
the reactive
sites of the cations prevents them from redepositing on surface equipment once
being
dissolved. The chelating agents have multiple reactive sites that serve to
react with
multiple cation sites and form more than one bond between the alkaline earth
scale
material and a molecule of the chelating agent, resulting in the formation of
a ring
structure incorporating the cation thereby dissolving the scale and preventing
it from
rescaling onto equipment surfaces.
While single component chelant solutions can be employed, the present
inventors have
surprisingly discovered that mixtures of more than one polyaminopolycarboxylic
acids,
in the form of salts of polyaminopolycarboxylic acid in solution, function
more favorably
to form stable complexes of the scale deposits and can sequester high
concentrations
of alkaline earth scales at higher rates than single component chelant
solutions.
Accelerants used in the present invention can preferably be added to an
aqueous
solution of one or more polyaminopolycarboxylic acid salts to increase the
speed of
complex formation and the efficiency of scale removal. The concentration of
the
accelerant in the aqueous solution can preferably be at least 0.10% to 1.0% by
wt.
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Higher concentrations of accelerant may also be used, however, no economic nor
functional increase in performance is observed.
As is typical with most chelation reactions, scale removal is preferably
performed under
alkaline conditions ranging from a solution pH of 9-13 and most preferably at
a pH of
11-12.5. It would be well understood by a person of skill in the art that
alkaline
conditions can be created in any suitable way known in the art for example, by
the
addition of any suitable alkaline components to the solution.
The preferred compositions and solutions of the present invention comprise
about 1.0%
to 20.0% of a mixture of various polyaminopolycarboxylic acid salts. One
example of a
preferred aqueous solution for scale removal is composed of the following:
Ingredient Percentage by Weight
Ethylenediaminetetraacetic Acid, Na+or K+ Salt 1-5%
Diethylenetriaminepentaacetic Acid, Na + Salt 3-10%
Polyaspartic Acid, Na+or K+ Salt 1-5%
Polyphenol 0.1-.5%
Surfactant 0.1-0.5%
The surfactant is preferably present to reduce surface tension in the
composition, but it
would be well understood by a person of skill in the art that a surfactant
would not be
required for the compositions or methods of the present invention.
To those skilled in the art, this invention may be embodied in many different
forms and it
should not be construed as limited to the embodiment herein disclosed. It will
be
recognized to those skilled in the art that other suitable chelants and other
polyaminopolycarboxylic acids and various forms of salts and various other
accelerants
maybe employed and interchanged for those materials herein disclosed.
The present invention may be used in downhole or surface installations.
Methods of
suggested application include continuous treatment down the annulus or
treating string
of producing wells, with water flush and continuous injection into surface
lines. It can
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also be used for formation squeeze treatment of oil wells, which is a well-
known
industrial cleaning method involving using chemical compositions to protect
the well
downhole from scale deposition and formation damage.
An initial high dosage of the solution of the present invention of between 50-
100 ppm (8-
16 litres /1000 bbls of well product) based on well production rate can also
be used to
remove scale build-up in downhole or surface equipment and facilities, in
cases of
severe scaling problem. A fill and soak or circulation method of application
is
recommended, which is a well-known industrial chemical cleaning method
involving
filling up vessel or pipe system with cleaning composition and letting it soak
for several
hours in order to dissolve scaling. The duration of contact required for scale
deposit
removal will depend on the scale composition and barium sulfate content.
The previous description of the disclosed embodiments is provided to enable
any
person skilled in the art to make or use the present invention. Various
modifications to
those embodiments will be readily apparent to those skilled in the art, and
the generic
principles defined herein may be applied to other embodiments without
departing from
the spirit or scope of the invention. Thus, the present invention is not
intended to be
limited to the embodiments shown herein, but is to be accorded the full scope
consistent
with the claims, wherein reference to an element in the singular, such as by
use of the
article "a" or "an" is not intended to mean "one and only one" unless
specifically so
stated, but rather "one or more". All structural and functional equivalents to
the
elements of the various embodiments described throughout the disclosure that
are
known or later come to be known to those of ordinary skill in the art are
intended to be
encompassed by the elements of the claims. Moreover, nothing disclosed herein
is
intended to be dedicated to the public regardless of whether such disclosure
is explicitly
recited in the claims. No claim element is to be construed under the
provisions of 35
USC 112, sixth paragraph, unless the element is expressly recited using the
phrase
"means for" or "step for".
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