Patent Application
20240327739
Not applicable.
The disclosure generally relates to methods for H2S treatment, more specifically a novel method of using a loop line to inject H2S scavenger chemicals into hydrocarbon lines to achieve efficient removal of H2S from hydrocarbons, such as natural gas. The loop line method allows the hydrocarbon to pass through a pipeline with multiple loops thereby increasing the contact time of the H2S scavenger with the hydrocarbon in the pipeline and in turn increasing the efficiency of the removal of H2S.
Hydrogen Sulfide (H2S) is a naturally occurring gas that has the odor of rotten eggs. It is colorless, flammable, and highly toxic—acting by inhibiting cellular respiration in a manner similar to hydrogen cyanide. It is commonly found in petroleum and natural gas, which are called “sour” if they have a high percentage of H2S (≥4 ppm).
In addition to being very poisonous, H2S causes corrosion in carbon steel pipelines, and this can significantly reduce the service life of the pipeline and processing facilities. The interstate pipeline specifications for H2S in the US varies between states but falls between 0.25 grains (4.12 ppm) per 100 cubic feet to 0.1 grains (1.65 ppm). However, natural gas can contain up to 28% H2S gas, thus it is important to remove H2S from the natural gas for longevity of pipelines in oil and gas processing facilities, as well as to meet regulatory, safety and environmental concerns.
H2S removal or mitigation—commonly known as H2S scavenging or sweetening—can be classified in 2 categories—regenerative and non-regenerative methods. Regenerative methods include amine wash using a scavenger tower. In this popular method, sour gas is passed through a column filled with an amine solution which reacts with H2S to form salts. A multiple step reaction process ultimately produces a gas stream for disposal with reduced amounts of H2S. Due to the large size of the columns and reactor space, this method is used for high gas volume removal process, as well as in higher gas flow rate systems (>5 MMSCF).
Another regenerative method for removal of H2S involves the use of redox reactions. In this method, the gas with H2S is passed through a column containing redox systems of iron and chelating agents. The redox columns contain metal oxide beds and H2S reacts with the metal oxides to produce metal sulfides, as shown in equation Eq. 1 below. The sulfide salts are then sent to regeneration tower to get regenerated metal oxides for H2S treatment.
H2S+MO*→H2O+MS Eq. 1.
*The metal oxide MO here does not strictly refer to stoichiometric ratios of compounds, for example, MO may also stand for Fe2O3.
The application of regenerative methods generally requires the use of large scavenger towers (also known as scavenger columns or bubble towers) on site. The cost of building the scavenger towers is high, and thus scavenger towers are typically used in facilities where more sour gas is produced, and where there are no space constrictions.
Non-regenerative methods for the removal of H2S include the use of chemicals like aldehydes, triazines, buffered nitrate/nitrite solutions, solutions of NaOH and KOH, biosulfides, and amine-aldehyde combination chemicals, which are generally applied via direct injection (DI) to a pipeline and thus used at sites with existing piping. However, a minimum length of straight pipeline is needed (at least 100 feet in length and preferably about 200 feet or more) so the chemicals have sufficient contact time with the hydrocarbons in order to provide efficient scavenging. In addition, the chemicals are spent during H2S scavenging and need to be constantly replenished. Thus, non-regenerative methods are mostly used when the concentration of H2S in the gas is lower because the chemical costs may be prohibitive, and where there are existing pipelines of sufficient length for gas injection.
What is needed in the art are efficient and more cost-effeetive methods for treatment of H2S in pipelines. The ideal method would be cost effeetive, require limited space, eliminate, or reduce significantly the H2S concentration, would be easy to install, reliable and have high efficiency. This invention addresses one or more of these needs.
Described herein is a novel method for the remediation of H2S in e.g., natural gas from oil and gas production facilities by the use of a loop line wherein the H2S scavenger is injected into the hydrocarbon stream through an injection quill and is mixed with the gas phase while flowing through the loop line. Existing DI injection facilities can be fitted with connections and loop line of varying number of loops, lengths, and material to increase the efficiency of H2S scavenger used in the facility. The loop line method described herein can also be applied in facilities with low gas flow rates or applied in conjunction with a direct injection line.
In one example, a loop line of suitable corrosion resistant material is installed in a facility with an existing direct H2S scavenger injection point. The natural gas first passes through a straight pipeline (usually 100-200 ft in length, but may be much shorter since combined with a downstream loop line) with H2S scavenger injection, followed by H2S detection at a sampling point. After the first injection, the H2S scavenger is introduced to another injection point, typically with an amount less than that at the direct injection point since some H2S has already been removed. The gas passes through a loop line after the second injection.
The loop line is preferably wound around a spool with a diameter of at least 4-5 or 6 feet or more. This avoids tight turns that may be experienced in existing lines, when not specifically developed for this application, and thus avoids the loss of chemical that occurs when turbulent flow occurs at tight bends and junctions or at abrupt slope changes.
To minimize slope changes, we tested horizontal loops and the data presented herein was generated with horizontal loops. However, we have also tested vertically oriented loops, and these were possibly even more efficacious than horizontal loops. Thus, vertical loops may be preferred.
The incoming gas should have high enough flow velocity to carry all the liquid scavengers into the gas phase and mix well during the flow, yet not fall out of the gas phase at bends or at areas of abrupt slope change. Gas flow velocity is controlled by choosing different pipe diameters based on the gas production rate. The goal is to make sure the gas flow rate is within the optimized range (1-10 MMSCF, preferably about 5 MMSCF), and this can be tested in a lab or in the field, and flow rate, piping length and diameter, spool core size, etc. adjusted as needed to optimize scavenging efficiency.
The time required for the gas to pass through the loop is several minutes to hours, based on the number of loops, length of the pipe and flow rate. H2S concentration is again determined at a second sampling point downstream of the loop line. The concentration of H2S at the second sampling point after the gas passes through the loop line is significantly lower than at the first sampling point after the straight pipe section. Ideally, no H2S is detected after the gas passes through the loop line.
In another example, a standalone loop line is installed, with a scavenger direct injection point just upstream of the loop or at or near the beginning of the loop. After the gas passes through the separator, and an initial amount of H2S is measured, H2S scavenger is injected and the gas directly sent to loop line with varying number of loops, as already described. The H2S concentration is then again measured after the gas exits the loop line. This type of standalone loop line injection is beneficial in facilities with limited space, or with low to medium H2S levels, or low gas flow rate (<5 MMSCF).
Any known or to be developed H2S scavenger injection can be used in the loop line method. Water-soluble H2S scavengers are commonly used. Examples include triazine-based chemicals. Triazines from monoethanolamine (MEA) react efficiently with the H2S in liquid phase to produce dithiazine compounds and an amine that can be transported in the pipeline.
Other H2S scavengers include polymeric, nitrogen-based products, aldehyde-based as well as non-nitrogen compounds. Most commonly used aldehydes include formaldehyde, acrolein, glyoxal, and glutaraldehyde. Functionalized aldehydes are also widely used. Combination products include amine-containing compound and a hemiacetal compound that can produce an aldehyde in situ. These are commonly used H2S scavenging mixtures. Combination of triazine and glyoxal are also increasingly used in oil and gas facilities.
H2S scavenger dose rate (injection rate) varies with the concentration of H2S in the gas, the diameter and length of the loop line, the chemical used and the flow rate. Tests can be performed to determine the minimum residence time needed for a given sour gas/chemical combination at a given flow rate and these results used to determine the minimum length of loop line needed for full remediation.
Dose rate at a loop line is at least 4-5 times lower than a direct injection line for the same length, diameter, and flow rates, and can even be as low as 10% the levels used in straight line gas injection. For example, MEA-triazine chemistry can be dosed at a rate of 1-10 gallons per day for hydrogen sulfide levels of less than 250 ppm or less than 200 ppm, flow rates of 1-10 or 5 MMSCF, pipe lengths of at least 25 ft, at least 50 ft, at least 100 feet, and diameters of 1-10 or 2-6 inches. Dose rate of up to 15 gallons per day may also be used, prepending on length, diameter, flow rate and ppm of H2S. Amine-glyoxal chemistry can also be dosed with similar dose rates. These dose rates are generally determined by experimental lab tests in simulated systems and confirmed by field trials.
The concentration of H2S in the gas is preferably first measured after the gas exists the oil/water separator unit, but this may be omitted if the concentration is already known. After treatment with the H2S scavengers in a loop line, concentration is again preferably measured. Quantitative analysis like gas chromatography and chemiluminescence are used for the measurement of H2S gas. Commercially available H2S analyzers can also be fitted on site that can measure H2S concentration continuously and specifically quantify the H2S levels. Lead acetate tape H2S analyzers are popularly used as they are robust, compact and do not require frequent re-calibrations.
Any non-reactive, robust and flexible material can be used to make the loop line. Exemplary materials include high density polyethylene (HDPE), helically wound epoxy-free dry fiberglass, polyvinyl chloride (PVC), or polyethylene (PE). Ideal diameter for these piping in the loop line is 1-10 inches or even 12 inches, preferably about 2 inches to about 6 inches or about 4 inches. The number of loops in the loop line are dependent on factors such as the concentration of H2S in the gas, flow rate, efficiency of scavenger chemistry, diameter of the line, diameter of the loop and the length of pipe needed for a given sour gas/chemical combination.
The piping is wound to a spool to both conserve space and to avoid turbulence and the scavenger falling out of the gas phase. Ideally, sufficient loops to provide at least 25 feet of non-turbulent piping are provided, but it may be 50 or 100 or more (up to 200, but preferably less), and may be less if the loop pipe is combined with straight piping. However, increased number of loops are preferred, which increases the overall length and thus the retention time of the H2S scavenger chemicals in the pipeline. Retention time of about 4 h to about 20 h are expected with increasing number of loops. Retention times of about 6 h to about 12 h are preferred.
Plastic, wood, or preferably metal spools can be used for the loop piping. Generally, we prefer 6 to 12 ft diameter spools with a high weight rating, up to about 10-100 metric tons, would be preferred. However, 4- or 5-foot spools may also be used. If plastic piping is used, the spool may have a lower weight rating.
The invention includes any one or more of the following embodiments, in any combination(s) thereof.
Any method herein described, said pipe loop being of at least 25 ft, at least 50 ft, or at least 100 ft in length.
Any method herein described, said pipe loop being of shorter length than a straight line pipe of same diameter and flow rate and of a length that brings H2S concentration to less than 1 ppm.
Any method herein described, said pipe loop wound around a spool can be oriented horizontally or vertically, but preferably vertical.
Any method herein described, said pipe loop wound around a spool of at least 4-5 ft in diameter, preferably at least 6 ft in diameter.
Any method herein described, wherein said H2S scavenger is an amine, triazine, an aldehyde, or combinations thereof.
Any method herein described, wherein said pipe loop is composed of high density polyethylene (HDPE), fiberglass, epoxy-free dry fiberglass, polyvinyl chloride, polyethylene or combinations thereof.
Any method herein described, wherein said loop line has a diameter of 1-10 inches and a flow rate of 1-10 MMSCF, preferably about 2-6 inches in diameter and about 5 MMSCF.
Any method herein described, wherein said method is preceded by testing to optimize pipe length, diameter, flow rate, chemical identity and amount thereof before implementing the optimized values in a plant or pipeline or at a wellpad.
Any method herein described, wherein the collected H2S-free natural gas is processed to form liquified natural gas (LNG).
As used herein “gas sweetening” or “gas scavenging” are processes to remove H2S from gases. “H2S mitigation” or “H2S removal” are used interchangeably and refer to methods and strategies to remove H2S from oil and gas.
As used herein a “loop line” is a line that provided a continuous low grade curve, so as to avoid turbulence and scavenger inactivation, and yet provide the long residence times needed without a large space commitment. We suggest a minimum spool diameter of at least 4-5 and preferably at least 6 ft (72″) as large metal spools are already available in the industry for carrying coiled tubing. Indeed, SPOOLTECH® (Houston, TX) has metal spools from 2 to 23 ft diameters and rated to as much as 95 metric tons. NOV® (Houston, TX) has smaller spools ranging from a 72-inch core holding 110,000 pounds, to a spool having a 98-inch core and holding 210,000 pounds. SONOCO® (Hartsville, SC) and DWELLOP® (Stavenger, Norway) also provide large high capacity spools, and there are many other commercial suppliers.
As used herein a “spool” or “reel” is shaped like a spool for holding thread, with an inner core, and outer flanges on each end to protect the piping wound thereon. It may also be provided with a frame to hold the spool and/or deploy the piping wound thereon, or the “spooler”—which allows spooling to be wound/unwound evenly—may be provided separately.
The use of the word “a” or “an” in the claims or the specification means one or more than one, unless the context dictates otherwise.
The term “about” means the stated value plus or minus the margin of error of measurement or plus or minus 10% if no method of measurement is indicated.
The use of the term “or” in the claims is used to mean “and/or” unless explicitly indicated to refer to alternatives only or if the alternatives are mutually exclusive.
The terms “comprise”, “have”, “include” and “contain” (and their variants) are open-ended linking verbs and allow the addition of other elements when used in a claim. The phrase “consisting of” is closed and excludes all additional elements. The phrase “consisting essentially of” excludes additional material elements, but allows the inclusions of non-material elements that do not substantially change the nature of the invention, such as instructions for use, buffers, and the like. Any claim or claim element introduced with the open transition term “comprising,” may also be narrowed to use the phrases “consisting essentially of” or “consisting of,” and vice versa. However, the entirety of claim language is not repeated verbatim in the interest of brevity herein.
The following abbreviations are used herein:
In
The gas with the H2S scavenger travels through multiple loops of pipeline (307) for a retention time of a few minutes to a few hours. Depending on the overall length of the loop, the pipe diameter, the number of turns and the flow rate of the gas, the gas and H2S scavenger mixture can travel from 6 h to about 12 h in the loop with minimal turbulence or loss of scavenger. With the loop line method, typically 0.5 to 0 ppm of H2S would be detected at the second sampling point. Sweet gas is collected at the sweet gas outlet 309 and is sent for sale or storage as LNG.
Although only three loops are shown in
In a recent application of this loop line method, two existing US shale gas facilities were chosen for fitting with a loop line. The fields A and B already had direct injection of H2S scavenger amounting to more than 500 gallons/month of scavenger. However, the H2S level in the gas after the DI of H2S scavenger was still very high (35-40 ppm). To reach 0 ppm would have required an estimated 300-500 additional gallons of scavenger. The results of this application are shown below in Table 1.
To address the high hydrogen sulfide levels, Flexpipe™ (Farnham Quebec CA) loop made of high-density polyethylene, helically wound epoxy-free dry fiberglass, and a protective outer jacket was tested in the loop line method, although high density polyethylene or similar materials could also be used. The Flexpipe™ was installed in a natural gas wellpad, downstream of the straight pipe section.
Upon adding a loop line of same diameter, flow rate, with added injection of only about 110-120 gallons of the same chemicals, the H2S levels in the sales gas reached 0 ppm, resulting in significant cost saving on scavenger chemicals. The results of this application are shown below in Table 2. As can be seen, we were able to obtain significant cost saving due to the increased efficiency of the loop system.
In a developing oil and gas facility with medium to low H2S concentration, a loop line of appropriate material could be installed with no H2S scavenger direct injection into straight line piping, and instead employing addition of chemicals at any suitable point upstream of the loop. A multi-loop system with a low injection of H2S scavenger chemical is likely to remove all H2S from the gas resulting in a 0 ppm level, provided that the pipe diameter, length, flow rate and chemical amounts are tested to provide optimal scavenging. We anticipate that 25-200, probably 100-150 feet of loop piping, with a 2-10, preferably 4-6-inch diameter and optimized flow rates, on a spool core of at least 6 feet diameter will be sufficient to reduce 200-250 ppm of hydrogen sulfide to near zero.
The examples herein are intended to be illustrative only, and not unduly limit the scope of the appended claims. Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the scope of the disclosure as defined in the claims.
The following references are incorporated by reference in their entirety:
This application claims priority to U.S. Ser. No. 63/492,378, filed Mar. 27, 2023 and incorporated by reference in its entirety for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 63492378 | Mar 2023 | US |
