Patent Application
20170101859
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.
Na2CO3 and other sodium salts are known to be used in aqueous solutions to scrub H2S 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.
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.
According to the invention, the foregoing objects and advantages have been attained.
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 Na2CO3, 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.
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.
A detailed description of preferred embodiments of the invention follows with reference to the attached drawing, wherein:
The invention relates to improvements in methods of scrubbing hydrocarbon fluids for H2S, CO2 and other acid gases.
As set forth above, surface scrubbing of fluids to remove acid gases can be effective at removing the acid gases, 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.
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.
Referring generally to
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.
Permeation of either the NaOH or Na2CO3 into the formation is a key source of concern, and is carefully prevented as disclosed herein.
In a preferred embodiment of the invention, as a result of contact of Na2CO3 and H2S, sodium sulfide is produced along with water according to the following reactions:
H2S+Na2CO3→Na2S+H20+CO2
Na2CO3+2CO+H20→2HCOONa+CO2
After the H2S is consumed by Na2CO3 and produces sodium salts, the equilibrium is moved to the right in the reaction formula, resulting in the following reactions:
CO2+H2O→CO3−+2H+
Na2CO3+2H2O→Na++H2CO3+OH−
OH−+2H+→H2O
Thus, contact of H2S and CO2 with Na2CO3 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.
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.
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.
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.
The aqueous solution (Na2CO3) 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%) CO2.
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%) CO2.
The above two paragraphs provide ranges for particular cases of how much aqueous solution (Na2CO3) is to be used in order to stoichiometrically avoid contact of the active ingredient (Na2Co3) of the solution with the formation.
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.
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 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.
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.
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.
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 of operation. The invention rather is intended to encompass all such modifications which are within its spirit and scope as defined by the claims.
