Note: Descriptions are shown in the official language in which they were submitted.
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DOWNHOLE REMOVAL OF H2S AND CO2
BACKGROUND OF THE INVENTION
[0001] Environmental concerns have driven the need to
reduce acid gas containing compounds from subterranean fluids
because they are known to produce acid rain and airborne
particulate material. State and local governments have enacted
regulations including timetables for the development of one or
more treatment methods which are capable of abating the
problem. Further, the presence of acid gases such as CO2 and
H2S is detrimental in the use of various commercial fluids.
During processing, fluids containing acid gas are also
responsible for accelerated corrosion times in tooling and
instrumentation. The need for a really effective method of
removing acid gases from geothermal fluids is clear.
[0002] Na2CO3 and other sodium salts are known to be used in
aqueous solutions to scrub H25 and CO2. Methods exist today
where additives are injected in multiple stages in facilities
that are above ground in order to purge the fluids of the
acidic gases. Although these methods have been improving over
time, to ensure the removal of the gases, the fluids still
have to be transported to other facilities. This subjects the
machinery and transportation facility and equipment to
corrosive gases while endangering the people who operate them.
SUMMARY OF THE INVENTION
[0003] In a hydrocarbon production environment, one
possible solution would be to find a way to perform the
scrubbing operation downhole, inside the well. However the
issue that prevents such use is the possibility of damage to
the formation. The contact of the Na2CO3 with the formation
can lead to changes of the properties of the porous media,
resulting in damage to the formation and reduction in well
production. The primary object of the invention is to further
address the need for effective acid gas scrubbing.
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[0004] According to the invention, the foregoing objects
and advantages have been attained.
[0005] According to the invention, a process for the
removal of acid gas from fluid produced from a subterranean
formation into a subterranean well is provided which comprises
the steps of: producing fluid from the subterranean formation
into an area selected from the group consisting of the
subterranean well, a production tube in the well, an annular
space defined therebetween, and combinations thereof so as to
produce a production fluid containing acid gas; and
introducing an aqueous solution of an active ingredient
selected from the group consisting of Na2003, NaOH, and
combinations thereof into said area, so as to contact the
aqueous solution with the fluid as the fluid travels from a
down-hole production tube to a surface well facility so as to
contact the material with the acid gas and thereby remove the
acid gas from the production fluid.
[0006] By controlling the amount of solution and material
injected, and maintaining a proper pressure balance in the
well, contact between the solution and the formation can be
prevented.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] A detailed description of preferred embodiments of
the invention follows with reference to the attached drawing,
wherein:
[0008] Figure 1 illustrates an application of a method
according to the invention inside the well.
DETAILED DESCRIPTION
[0009] The invention relates to improvements in methods of
scrubbing hydrocarbon fluids for H25, CO2 and other acid gases.
[0010] As set forth above, surface scrubbing of fluids to
remove acid gases can be effective at removing the acid gases,
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but exposes facilities for transportation of the fluids to the
location where acid gas is removed to damage and corrosion
from the acid gases. In accordance with the present
invention, in the environment of downhole production of
hydrocarbon fluids, many such fluids contain acid gases as
discussed. In accordance with the present invention, the
fluid with acid gas can be treated downhole with a scrubbing
additive, and careful monitoring of the amount of additive to
be used and of the pressure in the well as compared to that of
the formation can result in effective downhole treatment
without exposing the formation to damage from the scrubbing
solution.
[0011] According to the invention, aqueous solutions of
Na2CO3 and NaOH are injected continuously into contact with the
continuous subterranean fluid stream that is rising up after
being produced from the formation.
[0012] Referring generally to Figure 1, a subterranean well
1 is shown having a production tube 2 with an annular space 3
defined between the walls of the subterranean well 1 and the
production tube 2. The production fluid 4 while being
continuously pumped up through the production tube 2 is
scrubbed with an aqueous solution 6 that is continuously
sprayed at a specified depth 7 from coil tubing 5 which is
inside the production tube 2. The aqueous solution 6 mixes
with the production fluid 4 creating a reaction product 8.
[0013] One advantage of the invention is the use of
stoichiometry and pressure differences to ensure that after
injection, the aqueous solution 6 does not reach the formation
9 of the well. If the solution 6 comes in contact with the
formation 9, it could lead to damage, a change in the porous
media properties, and reduction of hydrocarbon production
rates from the well.
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[0014] Permeation of either the NaOH or Na2003 into the
formation is a key source of concern, and is carefully
prevented as disclosed herein.
[0015] In a preferred embodiment of the invention, as a
result of contact of Na2003 and H25, sodium sulfide is produced
along with water according to the following reactions:
H2S Na2 CO3 Na2 S H20 CO2
Na2003 + 200 + H20 - 2HCOONa + 002
[0016] After the H2S is consumed by Na.2003 and produces
sodium salts, the equilibrium is moved to the right in the
reaction formula, resulting in the following reactions:
CO2 + H20 ---> CO3- + 2H+
Na2003 + 2H20 Na + + H2003 + OH
OH- + 214+ 4 H20
[0017] Thus, contact of H2S and 002 with Na2003 ultimately
results in water soluble sodium salts containing the sulfur
and carbon from the acid gas, and these water soluble salts
can be removed from the well in the water stream produced
therefrom.
[0018] As mentioned above, it is a key concern to avoid
contact between the solution and the formation, as the
sodium-containing additives are known to cause formation
damage. The problem of contacting the solution with the
formation is therefore addressed, as mentioned above, through
manipulation of process stoichiometry and through manipulation
of differential pressure between the well and the formation.
[0019] With an understanding of the amount of acid gas
present in the produced fluid, an amount of additive can be
utilized such that the additive is entirely or at least
substantially consumed chemically during the reaction, and
therefore there is no additive remaining to contact the
formation. This type of manipulation is referred to as
controlling the additive through stoichiometry.
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[0020] Alternatively, or in addition, the pressure within
the well can be kept slightly lower than the pressure in the
formation such that fluids are not forced into the formation,
and thereby the solution does not actually contact the
formation.
[0021] The aqueous solution (Na2003) is preferably injected
at a concentration of active ingredient of less than about 11%
weight for a typical formation fluid. The solution, which
could be for example a 10% wt. solution, can be injected at
rates, based on the incoming fluid flow, as follows.
Approximately 12-30 cm3 (cc) of solution can be used per 1 ft3
of subterranean fluid containing between 1,000 and 4,000 ppm
(0.1-0.4%) H2S and/or between 10,000 and 30,000 ppm (1 and 3%)
002.
[0022] Further, approximately 25-30 cm3 (cc) of solution can
be used per 1 ft3 of subterranean fluid containing between
10,000 and 20,000 ppm H2S and/or between 40,000 and 50,000 ppm
(4 and 5%) 002.
[0023] The above two paragraphs provide ranges for
particular cases of how much aqueous solution (Na2003) is to be
used in order to stoichiometrically avoid contact of the
active ingredient (Na2003) of the solution with the formation.
[0024] Similar reactions occur when the active ingredient
is NaOH. The important parameter, which can be determined by
a person skilled in the art using known chemical
relationships, is to introduce only as much sodium as will be
consumed by reaction with the H2S, such that the active
ingredient is stoichiometrically presented from contacting the
formation. This determination can preferably also involve a
prior determination of the amount of H2S in the produced
reservoir fluids.
[0025] Thus, according to a preferred embodiment of the
invention, a sample of formation fluid is obtained and
analyzed to determine its H2S content, preferably both H2S and
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Co2. This content can then be used, for example on a per
volume basis, to determine a proper amount of active
ingredient, or sodium, to introduce in the aqueous solution
such that the active ingredient or sulfur will be consumed in
reaction with the H2S and Co2 and thereby be stoichiometrically
prevented from reaching the formation. In this way, the
harmful components of H2S and Co2 are converted to water
soluble products that are easily separated from water soluble
products that are easily separated from water produced from
the well, without exposing the well head, pipelines, etc., to
corrosive H2S and also while protecting the formation from
being damaged by the active ingredient.
[0026] In accordance with a further embodiment of the
invention, contact of the active ingredient in solution with
the formation can be prevented by maintaining hydrostatic or
dynamic pressure in the well at a lower level than formation
pressure. This pressure imbalance, which should be small,
keep fluid flow in the direction away from the formation and
thereby keeps the solution and active ingredient from damaging
the formation.
[0027] Excellent results have been obtained in accordance
with the present invention, without exposing the formation to
the expected significant damage due to contact with
sodium-based scrubbing materials. Thus, in accordance with
the present invention, a solution is provided whereby
hydrocarbon fluids containing acid gases can be treated
downhole to remove the acid gas before leaving the well, such
that the well, surface facilities and pipelines are not
exposed to acid gas.
[0028] It is to be understood that the invention is not
limited to the illustrations described and shown herein, which
are deemed to be merely illustrative of the best modes of
carrying out the invention, and which are susceptible of
modification of form, size, arrangement of parts and details
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of operation. The invention rather is intended to encompass
all such modifications which are within its spirit and scope
as defined by the claims.
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