U.S. patent application number 16/922394 was filed with the patent office on 2021-01-14 for composition useful in sulfate scale removal.
The applicant listed for this patent is Fluid Energy Group Ltd.. Invention is credited to Stig Magnor Nordaa, Clay Purdy, Markus Weissenberger.
Application Number | 20210009890 16/922394 |
Document ID | / |
Family ID | 1000004974336 |
Filed Date | 2021-01-14 |
United States Patent
Application |
20210009890 |
Kind Code |
A1 |
Purdy; Clay ; et
al. |
January 14, 2021 |
Composition Useful in Sulfate Scale Removal
Abstract
The present invention discloses a novel aqueous composition for
use in removing mixed sulfate scale from a surface contaminated
with such, said composition comprising: a chelating agent and a
counterion component selected from the group consisting of:
Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA; Cs.sub.5DTPA;
Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and TBAH.sub.5DTPA; a
dissolution enhancer; optionally a compound such as sodium
gluconate or the like and a carboxyl-containing fructan such as
carboxymethyl inulin. There is also disclosed methods to use such
compositions.
Inventors: |
Purdy; Clay; (Medicine Hat,
CA) ; Weissenberger; Markus; (Calgary, CA) ;
Nordaa; Stig Magnor; (Sandnes, NO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fluid Energy Group Ltd. |
Calgary |
|
CA |
|
|
Family ID: |
1000004974336 |
Appl. No.: |
16/922394 |
Filed: |
July 7, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C09K 8/528 20130101 |
International
Class: |
C09K 8/528 20060101
C09K008/528 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 11, 2019 |
CA |
3049343 |
Claims
1. A method of removing mixed sulfate scale, said method
comprising: providing a liquid composition comprising: a chelating
agent selected from the group consisting of: Li.sub.5DTPA;
Na.sub.5DTPA; K.sub.5DTPA; Cs.sub.5DTPA; Na.sub.4EDTA; K.sub.4EDTA;
TEAH.sub.4DTPA; and TBAH.sub.5DTPA; optionally, a scale removal
enhancer; a carboxyl-containing fructan; and a compound selected
from the group consisting of: sodium gluconate; gluconic acid;
glucono-delta-lactone; sodium gluconate; calcium gluconate;
potassium gluconate; exposing a surface contaminated with said
mixed sulfate scale to the liquid composition; and allowing
sufficient time of exposure to remove said mixed sulfate scale from
the contaminated surface.
2. The method according to claim 1, wherein the scale removal
enhancer is selected from the group consisting of: potassium
carbonate; potassium formate; cesium formate; cesium carbonate; and
combinations thereof.
3. The method according to claim 3, wherein the carboxyl-containing
fructan is carboxymethylinulin (CMI).
4. An aqueous composition for use in removing mixed sulfate scale
from a surface contaminated with such, said composition comprising:
a chelating agent and a counterion component selected from the
group consisting of: Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA;
Cs.sub.5DTPA; Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and
TBAH.sub.5DTPA; optionally, a scale removal enhancer;
carboxyl-containing fructan; and a compound selected from the group
consisting of: sodium gluconate; gluconic acid;
glucono-delta-lactone; sodium gluconate; calcium gluconate;
potassium gluconate.
5. The aqueous composition according to claim 4, wherein the scale
removal enhancer is selected from the group consisting of:
potassium carbonate; potassium formate; cesium formate and cesium
carbonate and combinations thereof.
6. The aqueous composition according to claim 4, wherein the scale
removal enhancer is present in the composition in an amount ranging
from 5 to 20% wt of the weight of the composition.
7. The aqueous composition according to claim 4, wherein the scale
removal enhancer is present in the composition in an amount ranging
from 10 to 15% wt of the weight of the composition.
8. The aqueous composition according to claim 4, wherein the scale
removal enhancer is present in the composition in an amount of
approximately 10 wt % of the weight of the composition.
9. The aqueous composition according to claim 4, wherein the
chelating agent and counterion are present in the composition in an
amount ranging from 5 to 40 wt % of the weight of the
composition.
10. The aqueous composition according to claim 4, wherein the
chelating agent and counterion are present in the composition in an
amount ranging from 10 to 30 wt % of the weight of the
composition.
11. The aqueous composition according to claim 4, wherein the
chelating agent and counterion are present in the composition in an
amount ranging from 10 to 20 wt % of the weight of the
composition.
12. The aqueous composition according to claim 4, wherein the pH of
the composition ranges from 10 to 11.
13. The aqueous composition according to claim 4, wherein the scale
removal enhancer is selected from the group consisting of:
K.sub.5DTPA; Cs.sub.5DTPA; Na.sub.4EDTA; and K.sub.4EDTA.
14. The aqueous composition according to claim 4, wherein the
carboxymethyl inulin is present in the composition in an amount
ranging from 0.5 to 15% wt of the weight of the composition.
15. The aqueous composition according to claim 4, wherein the
sodium gluconate is present in the composition in an amount ranging
from 1% to 20% wt of the weight of the composition.
16. A method of solubilizing barium sulfate into particles of less
than 1 micron in size, said method comprising: providing a surface
contaminated with a scale containing barium sulfate; providing a
liquid composition comprising: a chelating agent selected from the
group consisting of: Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA;
Cs.sub.5DTPA; Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and
TBAH.sub.5DTPA; optionally, a scale removal enhancer; a
carboxyl-containing fructan such as carboxymethyl inulin; and
exposing said surface contaminated with said barium sulfate scale
to the liquid composition; allowing sufficient time of exposure to
remove particles of barium sulfate scale from the contaminated
surface; wherein said particles of barium sulfate complexted with
the carboxymethyl inulin and have a particle size of less than 1
micron; and allowing the pH of the solution to drop from a pH
ranging from 10 to 11 to a pH ranging from 7 to 8 thereby causing a
reprecipitation of the solubilzed barium sulfate to a particle size
of less than 1 micron.
17. The method according to claim 16, wherein the
carboxyl-containing fructan is carboxymethylinulin (CMI).
18. The method according to claim 16 wherein the composition
comprises: a chelating agent selected from the group consisting of:
Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA; Cs.sub.5DTPA;
Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and TBAH.sub.5DTPA;
optionally, a scale removal enhancer; carboxymethyl inulin; and
sodium gluconate
19. The method according to claim 16 wherein the scale removal
enhancer is selected from the group consisting of: potassium
carbonate; potassium formate; cesium formate and cesium carbonate
and combinations thereof.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of and priority to
Canadian Application No. 3,049,343, filed Jul. 11, 2019. The entire
specification and figures of the above-referenced application are
hereby incorporated, in their entirety by reference.
FIELD OF THE INVENTION
[0002] The present invention is directed to a composition and
method for use in oilfield and industrial operations, more
specifically to compositions used in the removal of mixed sulfate
scale.
BACKGROUND OF THE INVENTION
[0003] Scaling, or the formation of sulfate mineral deposits can
occur on surfaces of metal, rock or other materials. Scale is
caused by a precipitation process as a result of thermodynamic and
hydrodynamic factors or changes in pressure, velocity rates and
temperature and the subsequent change in the composition of a
solution (commonly water).
[0004] Typical homogenous scales consist of e.g. calcium carbonate,
calcium sulfate, barium sulfate, strontium sulfate, iron sulfide,
iron oxides or iron carbonate. Sulfate scales and in particular the
common Barium sulfate scale is a major challenge for industry and
in particular the oilfield industry.
[0005] In some cases, scale deposits restrict or even shut-off the
production conduit if the produced water composition flow dynamics
are interrupted by changes in pressure and/or temperature. In many
cases this is due to wellbore components, such as downhole chokes,
safety valves or flow-controls. In addition to produced formation
brine water scaling issues due to the mineral content, also other
sourced water utilized in well workover or completions operations
can be potential sources of scaling minerals, including water flood
operations or geothermal operations.
[0006] The precipitation of sulfate scales can occur at any point
in a production, injection or abandonment-disposal well and
associate equipment cycle, and can also be caused by
incompatibilities of injected water and formation water, in
addition to the changes in temperature and pressure dynamics
mentioned above, as well as wellbore additives or upsets in the
flow equilibrium. Scale on surface equipment (e.g. heat exchangers,
piping, valves, flow-control devices) are also a catalyst for
sulfate scales. In oil & gas operations, seawater or brine is
often injected into reservoirs for pressure maintenance, and as
these have a high content of sulfate ions and formation water or
drilling fluids often have a high content of barium, calcium,
and/or strontium ions donated from the formation, these waters
often cause sulfate mineral precipitation. Sulfate scaling on
surface equipment, such as heat exchangers and the associated
piping, is a major issue for industry as well, as it typically
needs to be managed by mechanical means such as disassembling the
equipment in question, manually cleaning the scale and reassembling
which is very time consuming and expensive. Having a chemical
solution that can treat these sulfate scales with minimal agitation
and at lower temperatures would be very advantageous for industry.
As the multiple sulfate composition scaling challenges occur
offshore-onshore are typically very difficult to manage efficiently
as a whole. Having a sulfate dissolver that solubilizes all typical
sulfate scales encountered either individually or as a composition
is advantageous for industry versus having to deploy specific
chemistry for each type or scale or manage the scaling issues with
mechanical means.
[0007] The most obvious way of preventing a scale from forming
during production is to prevent the creation of super saturation of
the brine being handled and manage the flow path of fluids to
minimize pressure and rate differentials and to also add scale
inhibitors, which themselves are minimally effective and expensive.
It may also sometimes be possible by altering the operating
conditions of the reservoir, for example by ensuring that the
wellbore pressure is sufficient to prevent the liberation of gas
and by injecting water which is compatible with formation water.
However, economics usually dictate that the use of inhibitors or
treating any precipitated scale is preferred to manage costs
[0008] Controlling scale by the use of inhibitors as well as
understanding and mitigating scaling tendencies is important for
both production and injection wells, but so also is having a
solution or economical means of treating any scaling that does
occur, even after best practices have been implemented during the
production cycle.
[0009] The design of scale treatment programs requires extensive
knowledge of scaling/chemistry theory and a broad base of practical
operational experience to be successful. Applications occasionally
present themselves in which the ideal selection of chemicals and
fluids may be beyond the scope of a wellsite engineer's experience
or theoretical knowledge. Rules of thumb and general formulas may
not be adequate, and selection procedures based on broader
experience and more in-depth knowledge may be required. Analysis of
deposits and dissolver screening ideally when considering a
potential scale dissolving application, the scale that is causing
the problems will have to be analyzed.
[0010] The most common sulfate scales are barium, calcium, and
strontium. These alkaline earth metal salts have many similar
properties and often precipitate together forming problematic
sulfate scales. The deposition of this scale is a serious problem
for oil and gas producers globally, causing fouling in the wellbore
and surface related processing equipment These scales not only
restrict the hydrocarbon flow from the formation resulting in lost
production, and since the formation or injection water is saturated
with sulfates, the continued deposition causes fouling and
potentially failures of critical equipment such as perforations,
casing, tubes, valves, and surface equipment, all with the
potential to reduce the rate of oil production and result in
substantial lost revenue. There is a need in industry for an
effective solution to an ongoing challenge. Sulfate scales such as
radium sulfate, barium sulfate, calcium sulfate etc.--are sometimes
referred to as NORM scale due to their solubility
characteristics--typically 0.0023 g/l in water--are more difficult
to deal with than carbonate scales. Sulfate scales are not soluble
in traditional acid scale dissolvers. Radium sulfate, while not
being the most common sulfate scale represents a challenge in its
removal as it is often imbedded in barium sulfate scale and is also
radioactive and thus can carry an exposure risk and cause very
expensive clean-up or disposal costs of tubing and down-hole
equipment etc. when brought out of the well for replacement,
general service or abandonment. Having a chemical that can be used
to wash these components while still in the well and effectively
clean/remove the NORM materials leaving them down-hole, allowing
the operator to greatly reduce handling/disposal costs related to
NORM containing wells is very advantageous.
[0011] Once this water/acid insoluble scale has formed, it is
extremely difficult to remove with existing chemical options on the
market.
[0012] The solubility of barium sulfate is reported to be
approximately 0.0002448 g/100 ml (20.degree. C.) and 0.000285 g/100
ml (30.degree. C.). Existing methods to remove sulfate scale
include mechanical removal and/or low performance scale dissolvers
currently on the market, but both have limitations and
disadvantages. Mechanical removal involves the use of milling
tools, scraping, or high-pressure jetting and/or disassembly of key
production equipment causing substantial down time of production
and processing equipment. These methods have limited efficiency as
the scale is extremely hard to remove, often forming in areas
beyond the reach of the mechanical equipment as many facilities
have welded joints and limited access. High pressure jetting will
typically only remove the surface of the scale.
[0013] Sulfate scale dissolvers were developed to overcome the low
solubility of these types of scale. Sulfate scale dissolvers work
by chelating/mopping up the dissolved sulfate that is present in
the water allowing more to be dissolved. To assist the rate of
reaction/increase the speed and efficiency of dissolution these
products are typically deployed at elevated temperatures of
50.degree. C. to 90.degree. C. Sulfate scale dissolution will as a
result take far longer than for example carbonate scale dissolution
utilizing and acid. Typical scale dissolvers such as
ethylenediaminetetraacetic acid (EDTA), and variations of this
molecule (such as DTPA) are used by the industry to dissolve
sulfate scale with some success, and sequestering the barium,
calcium, and strontium ions. However, this process requires higher
temperature (usually above 75.degree. C.), is time-consuming, and
has limited dissolution capacity.
[0014] The following include some patent disclosures of sulfates
scale removers. U.S. Pat. No. 4,980,077 A teaches that alkaline
earth metal scales, especially barium sulfate scale deposits can be
removed from oilfield pipe and other tubular goods with a
scale-removing composition comprising an aqueous alkaline solution
having a pH of about 8 to about 14, a polyaminopolycarboxylic acid,
preferably EDTA or DTPA and a catalyst or synergist comprising
oxalate anion. It is stated that when the scale-removing solution
is contacted with a surface containing a scale deposit,
substantially more scale is dissolved at a faster rate than
previously possible.
[0015] WO 1993024199 A1 teaches the use of low frequency sonic
energy in the sonic frequency range for enhancing the dissolution
of alkaline earth metal scales using a scale-removing solvent
comprising an aqueous alkaline solution having a pH of about 8 to
about 14 and containing EDTA or DTPA and a catalyst or synergist,
preferably an oxalate anion. It is stated that when the
scale-removing solvent is contacted with a surface containing a
scale deposit while simultaneously transmitting low frequency sonic
energy through the solvent, substantially more scale is dissolved
at a faster rate than previously possible.
[0016] U.S. Pat. No. 4,030,548A teaches a barium sulfate scale or
solid can be dissolved economically by flowing a stream of
relatively dilute aqueous solution of aminopolyacetic acid salt
chelating agent into contact with and along the surfaces of the
scale while correlating the composition and flow rate of the
solution so that each portion of solution contains an amount of
chelant effective for dissolving barium sulfate and the upstream
portions of the scale are contacted by portions of the solution
which are unsaturated regarding the barium-chelant complex.
[0017] U.S. Pat. No. 6,613,899 B1 teaches carboxyl-containing
fructans such as carboxymethylinulin used to prevent deposition of
scale composed of, for example, calcium, barium and strontium salts
of sulphuric acid and carbonic acid in oil extraction. In the oil
extraction method, 0.5-200 ppm of a carboxyl-containing fructan
that contains 0.3-3 carboxyl groups per mono-saccharide unit is
incorporated in the process water, in the process equipment or in
the oil-containing formation.
[0018] U.S. Pat. No. 3,625,761A teaches a method of removing a
deposit of alkaline earth metal sulfate scale in an aqueous system
which comprises contacting said scale deposit with a treating
composition heated to a temperature in the range of from about 86
to about 194.degree. F. consisting essentially of an aqueous
alkaline solution containing from about 4 to about 8 percent by
weight of disodium hydrogen ethylenediaminetetraacetate dihydrate
and having a pH in the range of about 10 to 13 for a period
sufficient to dissolve at least some of the said scale, acidifying
said solution to decrease the pH thereof to a pH in the range of
from 7 to 8 with an acid selected from the group consisting of
sulfuric acid, hydrochloric acid, oxalic acid, a mixture of
sulfuric acid and oxalic acid, and a mixture of hydrochloric acid
and oxalic acid, to precipitate any alkaline earth metal ion
present.
[0019] U.S. Pat. No. 5,084,105A teaches that alkaline earth metal
scales, especially barium sulfate scale deposits can be removed
from oilfield pipe and other tubular goods with a scale-removing
composition comprising an aqueous alkaline solution having a pH of
about 8 to about 14, preferably about 11 to 13, of a
polyaminopolycarboxylic acid, preferably EDTA or DTPA and a
catalyst or synergist comprising a monocarboxylic acid, preferably
a substituted acetic acid such as mercaptoacetic, hydroxyacetic
acid or aminoacetic acid or an aromatic acid such as salicylic
acid. The description states that when the scale-removing solution
is contacted with a surface containing a scale deposit,
substantially more scale is dissolved at a faster rate than is
possible without the synergist.
[0020] U.S. Pat. No. 7,470,330 B2 teaches a method of removing
metal scale from surfaces that includes contacting the surfaces
with a first aqueous solution of a chelating agent, allowing the
chelating agent to dissolve the metal scale, acidifying the
solution to form a precipitant of the chelating agent and a
precipitant of the metal from the metal scale, isolating the
precipitant of the chelating agent and the precipitant of the metal
from the first solution, selectively dissolving the precipitated
chelating agent in a second aqueous solution, and removing the
precipitated metal from the second solution is disclosed. This is
understood to be a multi-step process which would cause longer
shutdown in production and is not determined to actually be
applicable in the field.
[0021] Despite the existing prior art, there are very few
commercial compositions available to remove barium sulfate scale,
the situation is made even more complex since most barium sulfate
scale occurs in wellbores, pipes and other equipment associated
with either oil production and/or oil exploration in offshore or
highly regulated jurisdictions such as the North Sea. Thus, the
removal of petroleum-contaminated barium sulfate scales presents an
even more challenging task for oilfield operators. it is highly
advantageous to industry to have a chemical option that meets these
stringent environmental and HSE parameters.
[0022] There thus still exists a profound need for compositions and
methods capable of removing very difficult to remove mixed sulfate
scales present in oilfield equipment.
SUMMARY OF THE INVENTION
[0023] According to an aspect of the present invention, there is
provided a method of removing mixed sulfate scale, said method
comprising:
[0024] providing a liquid composition comprising:
[0025] a chelating agent selected from the group consisting of:
Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA; Cs.sub.5DTPA;
Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and TBAH5DTPA;
[0026] optionally, a scale removal enhancer;
[0027] a carboxyl-containing fructan such as carboxymethyl inulin;
and
[0028] a compound selected from the group consisting of: sodium
gluconate; gluconic acid; glucono-delta-lactone; sodium gluconate;
calcium gluconate; potassium gluconate;
[0029] exposing a surface contaminated with said mixed sulfate
scale to the liquid composition;
[0030] allowing sufficient time of exposure to remove said mixed
sulfate scale from the contaminated surface.
[0031] Preferably, the scale removal enhancer is selected from the
group consisting of: potassium carbonate; potassium formate; cesium
formate; cesium carbonate; and combinations thereof.
[0032] Preferably, the carboxyl-containing fructan is a derivative
of inulin or another fructan that contains 0.3-3 carboxyl groups
per anhydrofructose unit. Preferably, the derivative of inulin or
another fructan that contains 0.3-3 carboxyl groups per
anhydrofructose unit contains at least 0.8 carboxyl groups per
anhydrofructose unit. More preferably, the carboxyl-containing
fructan is carboxymethylinulin (CMI).
[0033] According to an aspect of the present invention, there is
provided an aqueous composition for use in removing mixed sulfate
scale from a surface contaminated with such, said composition
comprising:
[0034] a chelating agent and a counterion component selected from
the group consisting of: Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA;
Cs.sub.5DTPA; Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and
TBAH.sub.5DTPA;
[0035] optionally, a scale removal enhancer;
[0036] carboxyl-containing fructan such as carboxymethyl inulin;
and
[0037] a compound selected from the group consisting of: sodium
gluconate; gluconic acid; glucono-delta-lactone; sodium gluconate;
calcium gluconate; potassium gluconate; and combinations
thereof.
[0038] Preferably, the scale removal enhancer is selected from the
group consisting of: potassium carbonate; potassium formate; cesium
formate and cesium carbonate and combinations thereof. Preferably,
the scale removal enhancer is present in the composition in an
amount ranging from 5 to 20% wt of the weight of the composition.
More preferably, the scale removal enhancer is present in the
composition in an amount ranging from 10 to 15% wt of the weight of
the composition. Even more preferably, the scale removal enhancer
is present in the composition in an amount of approximately 10 wt %
of the weight of the composition. More preferably, the scale
removal enhancer is selected from the group consisting of:
K.sub.5DTPA; Cs.sub.5DTPA; Na.sub.4EDTA; and K.sub.4EDTA.
[0039] According to a preferred embodiment of the present
invention, the chelating agent and counterion are present in the
composition in an amount ranging from 5 to 40 wt % of the weight of
the composition. More preferably, the chelating agent and
counterion are present in the composition in an amount ranging from
10 to 30 wt % of the weight of the composition. Even more
preferably, the chelating agent and counterion are present in the
composition in an amount ranging from 10 to 20 wt % of the weight
of the composition.
[0040] According to a preferred embodiment of the present
invention, the pH of the composition ranges from 10 to 11.
[0041] According to a preferred embodiment of the present
invention, the carboxymethyl inulin is present in the composition
in an amount ranging from 0.5 to 15% wt of the weight of the
composition.
[0042] According to a preferred embodiment of the present
invention, the sodium gluconate is present in the composition in an
amount ranging from 1% to 20% wt of the weight of the
composition.
[0043] According to an aspect of the present invention, there is
provided a method of removing calcium sulfate anhydrate scale
present on a contaminated surface, said method comprising:
[0044] providing a liquid composition comprising:
[0045] a chelating agent selected from the group consisting of:
Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA; Cs.sub.5DTPA;
Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and TBAH.sub.5DTPA;
[0046] optionally, a scale removal enhancer;
[0047] carboxymethyl inulin; and
[0048] a compound selected from the group consisting of: sodium
gluconate; gluconic acid; glucono-delta-lactone; sodium gluconate;
calcium gluconate; potassium gluconate;
[0049] exposing said surface contaminated with said mixed sulfate
scale to the liquid composition;
[0050] allowing sufficient time of exposure to remove said calcium
sulfate anhydrate scale from the contaminated surface.
[0051] Preferably, the composition comprises: a chelating agent
selected from the group consisting of: Li.sub.5DTPA; Na.sub.5DTPA;
K.sub.5DTPA; Cs.sub.5DTPA; Na.sub.4EDTA; K.sub.4EDTA;
TEAH.sub.4DTPA; and TBAH.sub.5DTPA; optionally, a scale removal
enhancer; carboxymethyl inulin; and sodium gluconate. Preferably,
the scale removal enhancer is selected from the group consisting
of: potassium carbonate; potassium formate; cesium formate and
cesium carbonate and combinations thereof.
[0052] According to an aspect of the present invention, there is
provided a method of solubilizing barium sulfate into particles of
less than 1 micron in size, said method comprising:
[0053] providing a surface contaminated with a scale containing
barium sulfate;
[0054] providing a liquid composition comprising:
[0055] a chelating agent selected from the group consisting of:
Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA; Cs.sub.5DTPA;
Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and TBAH.sub.5DTPA;
[0056] optionally, a scale removal enhancer;
[0057] a carboxyl-containing fructan such as carboxymethyl inulin;
and
[0058] exposing said surface contaminated with said barium sulfate
scale to the liquid composition;
[0059] allowing sufficient time of exposure to remove particles of
barium sulfate scale from the contaminated surface;
[0060] wherein said particles of barium sulfate complexted with the
carboxymethyl inulin and have a particle size of less than 1
micron;
[0061] allowing the pH of the solution to drop from a pH ranging
from 10 to 11 to a pH ranging from 7 to 8 thereby causing a
reprecipitation of the solubilzed barium sulfate to a particle size
of less than 1 micron.
[0062] Preferably, the scale removal enhancer is selected from the
group consisting of: potassium carbonate; potassium formate;
Cs.sub.2COOH; Cs.sub.2CO.sub.3; and combinations thereof.
Preferably, the carboxyl-containing fructan is a derivative of
inulin or another fructan that contains 0.3-3 carboxyl groups per
anhydrofructose unit. According to a preferred embodiment of the
present invention, the derivative of inulin or another fructan that
contains 0.3-3 carboxyl groups per anhydrofructose unit contains at
least 0.8 carboxyl groups per anhydrofructose unit. Most
preferably, the carboxyl-containing fructan is carboxymethylinulin
(CMI).
[0063] According to a first aspect of the present invention, there
is provided an aqueous composition for use in removing mixed
sulfate scale from a surface contaminated with such, said
composition comprising:
[0064] a chelating agent and a counterion component selected from
the group consisting of: Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA;
Cs.sub.5DTPA; Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and
TBAH.sub.5DTPA;
[0065] a scale removal enhancer;
[0066] sodium gluconate or the like; and
[0067] carboxymethyl inulin.
[0068] According to another aspect of the present invention, there
is provided a method of removing mixed sulfate scale, said method
comprising the steps of:
[0069] providing a liquid composition comprising:
[0070] a chelating agent selected from the group consisting of:
Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA; Cs.sub.5DTPA;
Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and TBAH.sub.5DTPA;
[0071] a scale removal enhancer;
[0072] sodium gluconate or the like; and
[0073] carboxymethyl inulin;
[0074] exposing a surface contaminated with mixed sulfate scale to
the liquid composition;
[0075] allowing sufficient time of exposure to remove the mixed
sulfate scale from the contaminated surface.
[0076] According to another aspect of the present invention, there
is provided an aqueous composition for use in removing mixed
sulfate scale from a surface contaminated with such, said
composition comprising:
[0077] a chelating agent and a counterion component selected from
the group consisting of: Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA;
Cs.sub.5DTPA; Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and
TBAH.sub.5DTPA;
[0078] a scale removal enhancer;
[0079] sodium gluconate or the like; and
[0080] carboxymethyl inulin.
[0081] Preferably, the scale removal enhancer is selected from the
group consisting of: potassium carbonate; potassium formate; cesium
formate and cesium carbonate and combinations thereof. Preferably,
the scale removal enhancer is present in the composition in an
amount ranging from 5 to 20% wt of the weight of the composition.
More preferably, from 10 to 15% wt of the weight of the
composition. Also preferably, the scale removal enhancer is present
in the composition in an amount of approximately 10% wt of the
weight of the composition.
[0082] Preferably, the chelating agent and counterion are present
in the composition in an amount ranging from 5 to 40% wt of the
weight of the composition. More preferably, from 10 to 30% wt of
the weight of the composition. Also preferably, the chelating agent
and counterion are present in the composition in an amount ranging
from 10 to 20% wt of the weight of the composition.
[0083] Preferably, the pH of the composition ranges from 10 to
11.5. More preferably, the composition has a pH ranging from 10 to
11.
BRIEF DESCRIPTION OF THE FIGURE
[0084] The invention may be more completely understood in
consideration of the following description of various embodiments
of the invention in connection with the accompanying figures, in
which:
[0085] FIG. 1 is the graphical representation of the dissolution
performance of a composition according to a preferred embodiment of
the present invention in comparison to various other commercially
available sulfate scale dissolver over a time period of 24 hours at
60.degree. C.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0086] By the addition of potassium carbonate to K.sub.5DTPA, the
same solubility numbers can be attained at a lower pH. Instead of
13.5 a pH of 11 was sufficient to get comparable solubility
numbers. This represents a considerable difference to typical
commercially available products. This allows an operator to conduct
scale removal operations at a lower pH and therefore increases the
safety for the personnel handling the remover or anyone in the
surrounding area as well as environmental risks and clean up costs
in the case of an uncontrolled release.
[0087] According to a preferred embodiment of the present
invention, the mixed sulfate scale removing composition provides
greatly improved rates of scale dissolution. This, in turn, reduces
the down time for wells where the scale is being removed and
associated the costs. It also reduces the cost of such treatment,
by limiting the treatment time by allowing production to
recommence.
[0088] As shown below, the compositions tested for removing
non-contaminated barium sulfate scale permits the removal thereof
at a much lower pH than what has been practiced to date. Indeed,
such a composition can effectively remove barium scale under
conditions where the pH is 11, rather than other scale removal
compositions which require conditions where the pH is 13 or higher.
Accordingly, a preferred composition according to the present
invention may remove, at pH=10 up to 30 kg/m.sup.3 of
non-contaminated BaSO.sub.4 scale. When using the term
"non-contaminated BaSO.sub.4 scale", it should be understood to the
person skilled in the art, that what is meant is that the barium
sulfate scale is not contaminated by petroleum product or a
petroleum-based product.
[0089] According to a preferred embodiment of the present
invention, a composition for removing mixed sulfate scale permits
the removal thereof with a higher dissolution capacity. This, in
turn, allows reducing the volume of scale remover necessary. This
also decreases transport costs and many other related items
resulting from the usage of lower volumes of scale remover.
[0090] According to a preferred embodiment of the present
invention, the additional sulfate scale dissolver comprising sodium
gluconate or the like. Compounds having a gluconate component or
portion are understood to fall in the later category but do not
comprise the entire category as other sugars are also considered to
be within this description. Preferably, the compounds having a
gluconate component or portion include but are not limited to:
gluconic acid (CAS #526-95-4); glucono-delta-lactone (CAS
#90-80-2); sodium gluconate (CAS #527-07-1); calcium gluconate (CAS
#299-28-5/18016-24-5); potassium gluconate (CAS #299-27-4).
Potassium gluconate being preferred.
[0091] According to a preferred embodiment of the present
invention, the carboxyl-containing fructans are understood to be a
derivative of inulin or another fructan that contains 0.3-3
carboxyl groups per anhydrofructose unit. In particular, the
derivative contains at least 0.8 carboxyl groups per
anhydrofructose unit. Preferably, the carboxyl groups can be
present in the form of carboxyalkyl groups, such as carboxymethyl,
carboxyethyl, dicarboxymethyl or carboxyethoxycarbonyl groups.
According to a preferred embodiment of the present invention, mixed
carboxyfructans can also be used. Preferably, the number of
carboxymethyl groups is greater than the number of other carboxyl
groups. The most preferred of the carboxyl-containing fructans is
carboxymethylinulin (CMI). Carboxymethylinulin (CMI) having a
degree of substitution of of 0.15-2.5 is disclosed in WO 95/15984.
Mixed carboxyl derivatives the inulin can have been first
carboxymethylated and then oxidised or vice versa. According to a
preferred embodiment of the present invention, the
carboxyl-containing fructan has an average chain length (=degree of
polymerisation, DP) ranging between 3 and 1000, but preferably, the
average chain length ranges from 6-60 monosaccharide units.
[0092] Absolute Solubility of Barium Sulfate Scale
[0093] The inventors have previously noted that chelating agents
such as EDTA (Ethylenediaminetetraacetic acid) or DTPA
(diethylenetriaminepentaacetic acid) and the ability to dissolve
non-contaminated barium sulfate depends substantially on the size
and ion strength of the counterion.
[0094] In Tables 1 and 2 (absolute solubility testing) the absolute
(or maximum) solubility of non-contaminated increases with the size
of the counterion from lithium to cesium. TEAH (Tetraethylammonium
hydroxide) and TBAH (Tetrabuthylammonium hydroxide) as organic
bases (counterions) are showing the same trend. Information
indicates that the size of the TBAH cation (including the hydrate
layer) is comparable to potassium.
[0095] The solubility numbers for both were found to be very
similar. In order to quantitatively compare the kg/solubility
properly, the BaSO.sub.4: chelating agent ratio was calculated in
g/mol and the Ba.sup.2+:chelating agent ratio was calculated in
mol/mol. The mol:mol ratio indicates the number of molecules of the
chelating agent needed to dissolve one ion of Ba.sup.2+ (complex).
The highest ratio which was found was almost 0.5, which means that
there needs to be, on average, 2 molecules of DTPA to dissolve 1
Ba.sup.2+ ion but mostly it can be much less.
[0096] Tests performed have indicated that, besides the nature of
the counterion, an excess of the counterion also improves the
solubility. K.sub.5DTPA was tested in conjunction with KCl,
K.sub.2CO.sub.3 and KOOCH (potassium formate). It seems that the
counterion play also a large role as K.sub.2CO.sub.3 (with the
larger anion) was much more effective than KCl (with a small
anion).
TABLE-US-00001 TABLE 1 Absolute solubility of non-contaminated
barium sulfate scale (when using a 40% solution of the scale
removing composition) 40wt % sol BaSO4 BaSO4 Ba2.sup.+ pH (kg/m3)
(g/mol) (mol/mol) Li.sub.5DTPA 2 Na.sub.5DTPA 13.01 17 20.24 0.088
K.sub.5DTPA 13.25 46 62.16 0.266 K.sub.5DTPA + 13.21 38 51.35 0.22
10wt % K.sub.2CO.sub.3 Cs.sub.5DTPA 13.4 52 72.2 0.309 Na.sub.4EDTA
13.11 9 7.89 0.034 K.sub.4EDTA 13.32 31 32.98 0.141 TEAH.sub.4DTPA
13.1 14 43.75 0.187 TBAH.sub.5DTPA 13.33 18 64.28 0.275
TABLE-US-00002 TABLE 2 Absolute solubility of non-contaminated
barium sulfate scale (when using a 20% solution of the scale
removing composition) at 60.degree. C. 20wt % sol BaSO4 Ba2.sup.+
pH BaSO4 (kg/m3) (g/mol) (mol/mol) K.sub.5DTPA 13.19 27 72.97 0.313
K.sub.5DTPA + 13.32 41 110.81 0.475 5 wt % K.sub.2CO.sub.3
K.sub.5DTPA + 11.25 40 108.11 0.463 5 wt % K.sub.2CO.sub.3
K.sub.5DTPA + 10 33 89.19 0.3821 5 wt % K.sub.2CO.sub.3 Cs5DTPA +
35 5 wt % CsCO3 Cs5DTPA + 35 10 wt % CsCO.sub.3 Cs5DTPA + 30 10 wt
% HCOOCs TEAH4DTPA + 21 10 wt % K.sub.2CO.sub.3 TBAH5DTPA + 25 10
wt % K.sub.2CO.sub.3
[0097] Moreover, the K.sub.5DTPA composition (at 40%) was
determined to dissolve 30 kg/m.sup.3 of FeS for a g/mol total of
40.54.
[0098] Preferably, the dissolution of non-contaminated barium
sulfate in an amount above 20 kg/m.sup.3. More preferably,
dissolution of barium sulfate above 30 kg/m.sup.3 is desired.
[0099] Speed of Barium Scale Dissolution
[0100] A second set of tests were performed to study the speed of
dissolution of non-contaminated barium sulfate scale. In order to
determine the speed, a relatively small amount of BaSO.sub.4 (0.25
g--this equates to 10 kg/m.sup.3) was used and the time was
measured until the solution became clear. Large differences were
noted. The best results involved the combination of K.sub.5DTPA
with K.sub.2CO.sub.3. This combination provided a dissolution time
which was almost 4 times faster than K.sub.5DTPA alone.
[0101] The speed of dissolution of compositions according to
preferred embodiment of the present invention were tested and
studied. Table 3 summarizes the findings of the testing. The
experiment involved the dissolution of 0.25 g of BaSO.sub.4 in a
volume of 50 ml fluid at 60.degree. C. under gentle stirring by
magnetic stir bar.
TABLE-US-00003 TABLE 3 Speed of dissolution of non-contaminated
barium sulfate scale Fluid Time pH K.sub.5DTPA (40%) 1 h 44 min
13.26 K.sub.5DTPA (40%) + 10% TBAH 1 h 38 min 13.4 K.sub.5DTPA
(40%) + 20% TBAH 1 h 21 min 13.43 K.sub.5DTPA (40%) + 30% TBAH 1 h
20 min 13.49 K.sub.5DTPA (40%) + 10 wt % KCl 1 h 24 min 13.27
K.sub.5DTPA (40%) + 10% K.sub.2CO.sub.3 30 min 13.22 K.sub.5DTPA
(20%) + 5% K.sub.2CO.sub.3 22-23 min 10.5-11
[0102] This testing indicates that both the extent of barium scale
dissolution and the speed at which it is dissolved represent marked
improvements over known compositions.
[0103] Preferably, the scale removal enhancer is selected from the
group consisting of: K.sub.2CO.sub.3; KOOCH; CsCO.sub.3; CsCOOH and
combinations thereof. Preferably, the scale removal enhancer is
K.sub.2CO.sub.3. Preferably also, the scale removal enhancer is
present in an amount ranging from 5 to 30% by weight of the scale
removal composition. More preferably from 10 to 20% by weight and
even more preferably, the scale removal enhancer would be present
in an amount of approximately 10% by weight.
[0104] Impact of Temperature
[0105] The speed of dissolution of a barium scale dissolver
composition was tested and studied under different temperature
conditions on non-contaminated barium sulfate scale. Table 4
summarizes the findings of the testing. The experiment involved the
dissolution of 0.25 g of BaSO.sub.4 in a volume of 50 ml fluid at
various temperatures under gentle stirring by magnetic stir bar.
The composition tested comprised a 20 wt % solution of K.sub.5DTPA
and 5 wt % K.sub.2CO.sub.3.
TABLE-US-00004 TABLE 4 Impact of Temperature on the Dissolution of
Barium Sulfate Temperature Time in .degree. C. (.degree. F.)
(minutes) 25 (77) 225 40 (104) 50 60 (140) 22 80 (176) 3.5 90 (194)
1.5
[0106] Laboratory Testing of Scale Dissolution
[0107] The sample selected for the solubility testing origins from
an oilfield tubular containing sulfate scale crystals originally
used for demonstration purposes. Crystals of non-contaminated
barium sulfate scale were removed from the tubular to be used for
the solubility testing. 200 cc of composition (K.sub.5DTPA 20 wt %
and 5 wt % K.sub.2CO.sub.3) was used. A weighted portion of
oilfield sulfate scale sample was submerged in 200 cc of each
de-scaling composition. A small magnetic stirrer is added to create
a very minimal vortex, creating a small movement of fluid without
rigorously stirring the fluid. The fluid was heated to 70.degree.
Celsius.
[0108] Results
[0109] 25.165 grams of non-contaminated oilfield sulfate scale was
weighted and added to the fluid. The stirrer and heater were
started. After 1 hour a slight colouring of the fluid was observed.
After 4 hours at temperature when no continued visual reduction of
scale was observed, the fluid has been filtered and the filter
rinsed with water, dried and weighted back. The maximum scale
solubility was reached and subsequently calculated.
[0110] The base barium scale dissolver composition (used in later
testing and referred to as "base BSD") comprises a 20 wt % solution
of K.sub.5DTPA and 5 wt % K.sub.2CO.sub.3. The base BSD was able to
dissolve 52.97 grams per litre of scale at 70.degree. C. The
testing was also carried out with a commercially available product
(Barsol NS.TM.), which is alkali/EDTA based and with EDTA. The
Barsol NS.TM. product was capable of dissolving 24.19 grams per
litre. While EDTA alone only dissolved around 6 grams per litre.
Under identical conditions, the base BSD was shown to have more
than double the performance of Barsol NS.TM..
[0111] According to a preferred embodiment of the present
invention, there is provided a one--step process for removing mixed
sulfate scale inside a wellbore, said process comprising:
[0112] providing a liquid composition comprising:
[0113] a chelating agent selected from the group consisting of:
Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA; K.sub.5DTPA; Cs.sub.5DTPA;
Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and TBAH.sub.5DTPA;
[0114] a carboxyl-containing fructan or a salt thereof such as
carboxymethyl inulin; and
[0115] a compound selected from the group consisting of: sodium
gluconate; gluconic acid; glucono-delta-lactone; sodium gluconate;
calcium gluconate; potassium gluconate;
[0116] exposing a surface contaminated with mixed sulfate scale to
the liquid composition;
[0117] allowing sufficient time of exposure to remove some or all
of the mixed sulfate scale from the contaminated surface. The
person skilled in the art will understand that what is meant by
"one-step" is that there is a single treatment step in the process
(or method) to remove mixed sulfate scale.
[0118] When the surface contaminated with mixed sulfate scale is
deep undergound or a hard to access tubing or piping, the exposure
consists of circulating the liquid composition through the tubing
or piping until it has been established that the scale has been
removed beyond a desirable predetermined point. Hence, in some
cases, it is quite possible that the entirety of the scale present
is not removed but the amount of removal is sufficient to re-start
operations and provide the desired productivity and/or circulation
through the affected tubing/piping. The liquid composition can also
be heated in order to improve the removal of the scale and the
speed at which the removal is effected.
[0119] According to another preferred embodiment of the present
invention, the method of treatment of mixed sulfate scale wherein
the fluid is spotted , i.e. placed in a tube/tank/pipe/equipment in
a soaking operation. This may in some instances be somewhat less
efficient than circulating or agitating the fluid due to the
surface reaction nature of the fluid, but it is used in some cases
to remove enough scale to run tools, pull stuck tubing or free
blocked flow control equipment etc., for example.
[0120] Sulfate scales that are commonly found inside wellbores
include calcium sulfate, strontium sulfate and barium sulfate. Up
to now, it was believed that an effective barium sulfate scale
dissolver was the missing link in order to remove very difficult to
remove scales. It was surprisingly discovered that depending on the
type of calcium sulfate present in the mixed sulfate scales the
scales may be more or less easy to remove. The most common form of
calcium sulfate is the dihydrate. Upon exposure to pressure and
temperature, the dihydrate converts to hemihydrate and ultimately
to the anhydrous form. Calcium sulfate anhydrous presents a
substantially more difficult to remove scale than its dihydrate
counterpart.
[0121] Tests were carried out in order to assess the advantage of a
composition according to a preferred embodiment versus a typical
mix of sulfate scales that is encountered during oil industry
operations. As calcium sulfate is by far the most common scale
component, it is believed that these tests are quite representative
of actual mixture. The solubility of the scale mixture was
evaluated against the base BSD composition and two preferred
embodiments of the present invention. The results are listed in
Table 5 below.
TABLE-US-00005 TABLE 5 Solubility of mixed sulfate scales
comprising Calcium sulfate, Strontium sulfate and barium sulfate at
60.degree. C. Sodium 25-30 wt of wt of wt of fil Total Gluconate UP
scale filter and prod Solubility Solution (wt %) (vol %) Scale (g)
(g) (g) kg/m3 100% base BSD 5 1 80% CaSO4.cndot.2H2O 10.0003 0.2764
3.1851 70.916 1% SrSO4 19% BaSO4 100% base BSD 10 1 80%
CaSO4.cndot.2H2O 10.0006 0.2872 3.1888 70.990 1% SrSO4 19% BaSO4
100% base BSD \ \ 80% CaSO4.cndot.2H2O 10.0017 0.2904 3.6249 66.672
1% SrSO4 19% BaSO4 80% base BSD, 10 1 80% CaSO4.cndot.2H2O 10.0048
0.2829 3.9884 62.993 20% H2O 1% SrSO4 19% BaSO4 NB: 100% base BSD
refers to an undiluted solution of the composition as set out
previously. A diluted solution of base BSD is referred to in the
amount of residual stock concentration after dilution. 25-30 UP
refers to a commercial sodium carboxymethyl inulin composition
having a 30-32 wt % NaCMI content.
[0122] Tests were carried out in order to assess the advantage of a
composition according to a preferred embodiment versus a calcium
scale in both the dihydrate form and in the anhydrous (anhydrate)
form. As calcium sulfate is the most common scale component in a
mixture of sulfate scales, it was believed to be important to
assess the effectiveness of the known descaler against two
preferred embodiments of the present invention. It is known that
while calcium sulfate dihydrate and calcium sulfate anyhdrate have
greatly different properties and that the dihydrate form is both
the most common and first to be formed when depositing. What is
also known is that the dihydrate will convert to more stable forms
upon exposure to heat and pressure. The most stable form of calcium
sulfate being the anyhydrate. The results of the experiments are
listed in Table 6 below.
TABLE-US-00006 TABLE 6 Solubility of Calcium sulfate (anhydrous)
and Calcium sulfate (dihydrate) wt of Sodium 25-30 wt of wt of fil
and Total Gluconate UP scale filter prod Solubility Solution (wt %)
(vol %) Scale (g) (g) (g) kg/m3 100% Base BSD 5 1 100%
CaSO4.cndot.2H2O 10.0001 0.2773 3.1126 71.648 100% Base BSD 10 1
100% CaSO4.cndot.2H2O 10.0021 0.2653 2.7927 74.747 100% Base BSD \
\ 100% CaSO4.cndot.2H2O 10.0009 0.2647 3.0829 71.827 100% Base BSD
5 1 100% CaSO4 (Anhydrous) 10.0027 0.2873 5.8483 44.417 100% Base
BSD 10 1 100% CaSO4 (Anhydrous) 10.0007 0.2774 5.4992 47.789 100%
Base BSD \ \ 100% CaSO4 (Anhydrous) 10.0058 0.2774 7.9699 23.133
100% Base BSD \ \ 100% CaSO4 (Anhydrous) 10.0017 0.2662 7.1513
31.166
[0123] The results of Table 6 indicate that the type of calcium
scale (anydrate vs dihydrate) has a substantial and marked impact
of the dissolution efficiency of the scale dissolvers tested.
[0124] The use of sodium gluconate is an effective component in the
removal of smaller cations present in a mixed sulfate scale.
Varying the amount of sodium gluconate (or the like) can have a
direct impact on the effectiveness of the composition according to
a preferred embodiment of the present invention as it provides for
an increased dissolution power of mixed sulfates scale. Sodium
gluconate is a representative compound of sugars which have the
same properties including but not limited to the gluconate or the
like categorization and it is quite effective in the presence of
scale containing transition metal cations (such as, but not limited
to, iron, manganese, zinc, tin) and post-transition metal cations
(such as, but not limited to, aluminum, lead) as these cations have
typically a smaller ion radii. According to a preferred embodiment,
the gluconate or the like can be present in a concentration ranging
from 0.1 wt % to 20 wt % of the total weight of the composition,
more preferably from 1 to 20 wt %. According to another preferred
embodiment, the gluconate or the like can be present in a
concentration ranging from 1 wt % to l0 wt % of the total weight of
the composition, more preferably ranging from 1 to 5 wt %, even
more preferably ranging from 1 to 3 wt %.
[0125] While the presence of sodium gluconate or the like is
preferable in some cases, the presence of such a compound has its
limitations. For operations carried out at temperatures of
150.degree. C. or more, gluconates have a tendency of being less
stable and degrading, and thus would be not desirable for such
applications. For this reason, its use in high temperature
applications has limitations. Hence, in situations where the
operating temperature encountered by the compositions used are of
150.degree. C. or higher, a preferred composition of the present
invention will not necessarily require the presence of a gluconate
compound (or the like). Thus, a preferred embodiment of the present
invention to be used for the removal of mixed sulfate scale at high
temperatures (i.e. above 150.degree. C.) will comprise: a chelating
agent selected from the group consisting of: Li.sub.5DTPA;
Na.sub.5DTPA; K.sub.5DTPA; Cs.sub.5DTPA; Na.sub.4EDTA; K.sub.4EDTA;
TEAH.sub.4DTPA; and TBAH.sub.5DTPA; optionally, a scale removal
enhancer; a carboxyl-containing fructan such as carboxymethyl
inulin. Thus, a preferred embodiment of the present invention to be
used for the removal of predominantly barium sulfate scale will
comprise: a chelating agent selected from the group consisting of:
Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA; Cs.sub.5DTPA;
Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and TBAH.sub.5DTPA;
optionally, a scale removal enhancer; a carboxyl-containing fructan
such as carboxymethyl inulin.
[0126] Moreover, the compositions according to preferred
embodiments of the present invention used are environmentally safer
than many other dissolvers. This represents a major advantage over
any known chemically-based methods of mixed sulfate scale. Another
advantage to the compositions according to preferred embodiments of
the present invention includes the speed of dissolution which is
considerably faster than any known commercial compositions. Another
advantage of preferred compositions according to the present
invention is that they can be employed on wells according to a
one-step process and thus are very desirable to operators which
deal with mixed sulfate scale issues on a regular basis, such as in
the North Sea.
[0127] According to another aspect of the present invention, there
is provided a method of solubilizing barium sulfate into particles
of less than 1 micron in size, said method comprising:
[0128] providing a surface contaminated with a scale containing
barium sulfate;
[0129] providing a liquid composition comprising:
[0130] a chelating agent selected from the group consisting of:
Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA; Cs.sub.5DTPA;
Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and TBAH.sub.5DTPA;
[0131] optionally, a scale removal enhancer;
[0132] a carboxyl-containing fructan such as carboxymethyl inulin;
and
[0133] a compound selected from the group consisting of: sodium
gluconate; gluconic acid; glucono-delta-lactone; sodium gluconate;
calcium gluconate; potassium gluconate;
[0134] exposing said surface contaminated with said barium sulfate
scale to the liquid composition;
[0135] allowing sufficient time of exposure to remove particles of
barium sulfate scale from the contaminated surface; wherein said
particles of barium sulfate are complexed with the carboxymethyl
inulin and have a particle size of less than 1 micron;
[0136] allowing the pH of the solution to drop from a pH ranging
from 10 to 11 to a pH ranging from 7 to 8 thereby causing a
reprecipitation of the solubilzed barium sulfate to a particle size
of less than 1 micron (due to the low solubility product of barium
sulfate at low pH (7 to 8).
[0137] The interaction of the CMI with the dissolved barium sulfate
is not completely clear but it seems that the CMI
interacts/intereferes with the crystal surface of the barium
sulfate in order to prevent/minimize/inhibit crystal growth and
thus maintain barium sulfate at a particle size of less than 1
micron. It is hypothesized that the CMI-barium sulfate complex
creates a sort of nanoparticle which, because of its small size is
capable of undergoing brownian motion and thus never quite settling
(i.e. does not reprecipitate, at least during the period of
duration (up to 7 days) which the testing herein seems to support).
According to a preferred embodiment, the carboxyl-containing
fructan can be present in a concentration ranging from 0.01 wt % to
15 wt % of the weight of the composition, more preferably from 0.5
wt % to 15 wt %. According to another preferred embodiment, the
carboxyl-containing fructan can be present in a concentration
ranging from 0.01 wt % to 1 wt %. According to a more preferred
embodiment, the carboxyl-containing fructan can be present in a
concentration ranging from 0. 1 wt % to 0.5 wt %. According to yet
another preferred embodiment, the carboxyl-containing fructan can
be present in a concentration ranging from 0.01 wt % to 0.4 wt %,
more preferably from 0.1 wt % to 0.35 wt %, even more preferably
from 0.25 wt % to 0.32 wt %
[0138] Once a composition according to a preferred embodiment of
the present is exposed to a surface contaminated with sulfate
scale, the scale is removed over a period of time but the dissolved
scale is at risk of reprecipitating upon exposure to formation
water. Since the scale dissolver has a pH preferably ranging from
11 to 11.5, the barium sulfate and other scales will dissolve but
as the dissolved scale is increasingly exposed to the formation
water which typically has a pH of about 6 to 7, the pH of the water
around the dissolved scale will decrease. The reprecipitation of
barium sulfate at around pH=8 is unavoidable as the ksp for barium
sulfate is very low at such pH. This is one of the main reasons why
the above mentioned preferred composition is desirable as it will
prevent reprecipitation of barium sulfate and thus allow fluids to
flow after scale removal.
[0139] Comparative Testing of a Preferred Scale Dissolver of the
Present Invention
[0140] In order to assess the efficiency of a preferred scale
dissolver of the present invention, it was compared to three other
commercially barium sulfate scale dissolver at 50.degree. C. and at
90.degree. C. The results of the testings are found in tables 7 and
8 below. Compositions A, B, and C are commercially available barium
sulfate scale dissolvers. Composition D is a preferred scale
dissolver of the present invention which comprises the base BSD+10
wt % sodium gluconate and 1% vol of CMI (at approximately 30-32 wt
%).
TABLE-US-00007 TABLE #7 Amounf of barium sulfate scale
dissolved after 24 hours at a temperature of 50.degree. C. Barium
Sulfate Scale dissolved Composition (in wt %) A 57 B 16 C 72 D
84
TABLE-US-00008 TABLE #8 Amounf of barium sulfate scale
dissolved after 24 hours at a temperature of 90.degree. C. Barium
Sulfate Scale dissolved Composition (in wt %) A 77 B 22 C 79 D
87
[0141] In both experiments, the composition according to a
preferred embodiment of the present invention (Composition D)
performed better than all three commercially available barium
sulfate scale dissolvers. Another non-negligible observation is
that all three commercially available barium sulfate scale
dissolvers (A, B and C) have a pH above 12 (some close to 13),
while Composition D has a pH ranging between 11 and 11.5. This
difference in pH is significant for operators and any personnel
handling this type of caustic product, and thus it is highly
desirable to have a product with a pH as close to neutral as
possible.
[0142] Comparative Testing with Other Sulfate Scale Inhibitors
[0143] In another round of testing, various commercially available
barium sulfate scale dissolvers (compositions A, B, D, E, F, I, J,
K, L, N, and P) were tested for mixed sulfate scale dissolving
efficiency (at 60.degree. C.) compared to a composition according
to a preferred embodiment of the present invention (Composition Q
which comprises base BSD+CMI (1% vol. (at approximately 30-32 wt
%))+sodium gluconate (10 wt %)).
[0144] Composition Q performed better than all of the commercially
available mixed sulfate scale dissolvers. The composition of the
scale was (major elements only): barium (44 wt %); calcium (4.4%);
strontium (8.4 wt %), the balance of the composition of the scale
being mainly made up by the anions and organic compounds. The scale
dissolution for each composition was measured at 2 hours, 4 hours 8
hours and 24 hours after start of treatment. FIG. 1 is the
graphical representation of the performance of each composition
over a time period of 24 hours at 60.degree. C. The time axis
(x-axis) was not graphed to scale as the sheer number of
compositions tested would not be easily distinguishable in the
first three measurements (2, 4 and 8 hours) and the graphical
depiction provided in FIG. 1 allows to better assess the efficiency
of each composition over time. The composition according to a
preferred embodiment of the present invention was by far by the
most effective scale dissolver in total dissolved scale (measured
in ppm). As well, contrary to Compositions A and B which started to
have reprecipitation occur after 8 hours, Composition Q maintained
the dissolved scale and managed to dissolve even more scale after
the 8 hours measurement.
[0145] Barium Sulfate Reprecipitation Laboratory Tests
[0146] Several experiments were conducted to assess the ability of
certain compositions (base BSD, base BSD+CMI (1% vol. (at
approximately 30-32 wt %)), and base BSD+sodium gluconate (10 wt
%)) to maintain the dissolved barium sulfate in solution.
[0147] Reprecipitation Experiment #1
[0148] Three blends containing the Base BSD composition (Base BSD,
Base BSD+CMI, and Base BSD+sodium gluconate) were prepared with
added barium sulfate and were observed as reprecipitation occurred
at each pH interval from 11 to 7 with the addition of 1 N
hydrochloric acid. 61.66% of the barium sulfate precipitate was
filtered out from the Base BSD+sodium gluconate solution and 12.94%
of the barium sulfate precipitate was filtered from the Base
BSD+CMI solution, with the remainder staying suspended in
solution.
[0149] Procedure: To observe the reprecipitation of barium sulfate,
2.0000 g of barium sulfate was dissolved in 100 mL of each Base BSD
compositions (Base BSD, Base BSD+CMI, and Base BSD+sodium
gluconate) at 60.degree. C. for 4 hours on a heated stir plate at
190 rpm. The solutions were then cooled to ambient temperature,
transferred into new beakers and then placed onto stir plates with
stir bars. A pH probe was placed in the solution to monitor the pH
of the solution as 1N hydrochloric acid (HCl) was added drop wise,
with a photo taken at each pH interval.
[0150] After 3 days, the reprecipitated solutions were filtered
through P8 and then P2 filter paper and then re-examined after 7
days.
[0151] Results and Observations:
[0152] Using P8 and P2 filter papers, 61.66% of the barium sulfate
precipitate was filtered out from the Base BSD+sodium gluconate
solution. For the base BSD+CMI solution, 12.94% of the barium
sulfate precipitate was filtered out while the remaining barium
sulfate stayed suspended in solution for a period of up to at least
7 days.
[0153] Barium Sulfate Reprecipitation Laboratory Tests--Experiment
#2
[0154] Base BSD+CMI test solutions were heat treated in a high
pressure/high temperature Teflon lined cell at 150.degree. C.
(302.degree. F.), at 400 psi, for 6 hours and 24 hours to simulate
downhole conditions. Barium sulfate was then dissolved in the
heat-treated Base BSD+CMI to ensure unaltered functionality.
Afterwards, the pH was lowered stepwise to 7 with the addition of 1
N hydrochloric acid. This was executed to determine the pH where
reprecipitation occurs and if the specific additive in Base BSD+CMI
is still functional after the heat treatment to suppress the
precipitation formation to a lower pH and if the formed barium
sulfate crystal size is still altered to smaller particle sizes by
Base BSD+CMI. The different solutions with the reprecipitation were
filtered through filters with different pore sizes. It was
determined that a minimum of 94 wt % of the barium sulfate
reprecipitate from both Base BSD+CMI test solutions (6 hours and 24
hours at 150.degree. C.) is smaller than 1 .mu.m. The remaining
crystal material stays suspended in solution for an extended time
(7 days) without further settling out.
[0155] Procedure: Base BSD+CMI solutions were prepared and exposed
to a heat treatment in high pressure/high temperature Teflon lined
cells at 150.degree. C., at 400 psi, for 6 hours and 24 hours.
After the heat treatment, each cell was depressurized and the
solution was allowed to cool to ambient temperature. No visual
decomposition of the additive package in the Base BSD+CMI was
observed.
[0156] To determine the functionality of the heat-treated Base
BSD+CMI , 2.0 g of barium sulfate was dissolved in 100 mL of the
two Base BSD+CMI blends (heat-treated at 6 hours at 150.degree. C.,
and 24 hours at 150.degree. C.) at 60.degree. C. for 4 hours on a
heated stir plate at 190 rpm. The solubility capability was
unaltered compared to non heat-treated Base BSD+CMI . The solutions
were then cooled to ambient temperature. The solutions were
transferred into a beaker with a stir bar and placed on a stir
plate. A pH probe was placed in the solution to monitor the pH of
the solution as IN hydrochloric acid (HCl) was added (drop wise). A
photo was taken at each pH interval. The first slight
reprecipitation of barium sulfate occurs at a pH of 8. Full
reprecipitation occurred at a pH of 7. The test solutions were
stored at room temperature for 3 days.
[0157] After 3 days, the reprecipitated barium sulfate from the
Base BSD+CMI solutions were filtered through P8 and then through P2
filter paper and allowed to rest for 7 days further days.
[0158] Results:
[0159] Using the P8 and P2 filter paper, 97.4 wt % of the barium
sulfate reprecipitate passed both filters from the Base BSD+CMI (24
hours at 150.degree. C.) solution and 94.3 wt % of the barium
sulfate reprecipitate passed both filters from the Base BSD+CMI (6
hours at 150.degree. C.) solution. The remaining material stayed
suspended in solution. Based on these results and observations the
assumption is, that the majority of the formed crystalline material
is smaller than 1 .mu.m. This is a direct effect of the utilized
additive package of the Base BSD+CMI.
[0160] Barium Sulfate Reprecipitation Laboratory Tests--Experiment
#3
[0161] To observe the reprecipitation of barium Sulfate laboratory
tests were carried out to compare three compositions (the base BSD,
base BSD+CMI, and base BSD+sodium gluconate) were prepared with
barium sulfate to be observed as reprecipitation occurs at each pH
interval from 11 to 7 with the addition of 1 N hydrochloric
acid.
[0162] Procedure: To observe the reprecipitation of barium sulfate,
2.0000 g of barium sulfate was dissolved in 100 mL of the three
tested BSD compositions (base BSD, base BSD+CMI, and base
BSD+sodium gluconate) at 60.degree. C. for 4 hours on a heated stir
plate at 190 rpm. The solutions were then cooled to ambient
temperature. The solutions were transferred into a beaker with a
stir bar and placed on a stir plate. A pH probe was placed in the
solution to monitor the pH of the solution as 1N hydrochloric acid
(HCl) was added (drop wise). A photo was taken at each pH
interval.
[0163] After 3 days, the reprecipitated solutions were filtered
through P8 filter paper and then through P2 filter paper and
allowed to rest for 7 days for further observation. Fischer
Scientific P8 filter papers have a porosity such that particles
greater than 20 microns are filtered out. The P2 filter paper (from
Fischer Scientific) has a porosity such that particles as small as
1 micron are filtered out of solution.
[0164] Observations: The barium sulfate precipitate was filtered
out from the solution containing only the Base BSD however the
precipitate in the solutions containing the Base BSD+1% vol. CMI
and the Base BSD+sodium gluconate could not be filtered and
remained suspended in solution even after 7 days.
[0165] While the foregoing invention has been described in some
detail for purposes of clarity and understanding, it will be
appreciated by those skilled in the relevant arts, once they have
been made familiar with this disclosure that various changes in
form and detail can be made without departing from the true scope
of the invention in the appended claims.
* * * * *