July 2017, Vol. 244, No. 7

Features

Setting New Standards in Natural Gas

Have you ever heard the joke that asks, “Where do the writers of dictionaries go to look up a word’s spelling?” Or have you ever wondered how people truly know how long a meter is? Well, the answer in both cases is, there are standards that are used as references.

Now, sitting in an unassuming laboratory in Denver, CO, there is a new global standard for the chemical composition of the primary constituents in natural gas. The product of a multi-year, international effort, these natural gas standards will serve as references in research and technology development by industry, academia and governmental agencies.

First Standards

In the mid-1980s, Chevron Oil Field Research Co. worked with the International Atomic Energy Agency to develop a suite of three natural gas standards for molecular and stable isotopic analyses. These reference materials included a coal-associated gas (NGS-1), an oil-associated gas (NGS-2) and a biogenic gas (NGS-3).

NGS-1 was collected by G. Hut of the University of Groningen, The Netherlands, from the Groningen gas field, and consists of over 80% CH₄ with a δ¹³C value of approximately -29‰ (VPDB). NGS-2 was collected by T. Ricchiuto of Agip S.p.A., Milano, Italy from southern Italy and contains over 50% CH₄ with a δ¹³C value of approximately -45‰ (VPDB). NGS-3 was also collected by Ricchiuto from the Porto Corsini field, from northern Italy, and contains over 95% CH₄ with a δ¹³C value of approximately -73‰ (VPDB).

In the early 1990s, these reference gases were transferred to the U.S. National Institute of Standards and Technology (NIST), which provided recertified isotopic values, assumed responsibility for distribution, and renamed them RM 8559, RM 8560, and RM8561, respectively. For over 20 years, these materials were distributed by NIST and used in research and technology development by industry, academia and governmental agencies.

Transition Period

By the mid-2000s, the supply of these gases had diminished and NIST discontinued the distribution of these materials. The residual gas, cylinders and manifold assembly were originally transferred to Power, Environmental, and Energy Research Institute (a private research institution in Covina, CA) but subsequently were moved to the U.S. Geological Survey (USGS) Energy Resources Geochemistry Laboratories in Denver. Around the same time, a “round-robin” type inter-laboratory study of some natural gases from China was conducted, with the hope that these might replace the NIST standards. This study was not a true calibration exercise and the gases were not made available for general use as reference standards.

In response to this situation, Geoffrey Ellis and Robert Dias of the Energy Resources Program (ERP) of the USGS initiated a project to develop a new suite of gas standards to replace the old NIST gas standards. They quickly saw that an undertaking of this magnitude was beyond the capabilities of the USGS alone and reached out to researchers from academic, governmental and industrial organizations worldwide for assistance.

USGS personnel led in administering and coordinating the work related to development of these standards. Much credit is due to many from outside the USGS who contributed valuable time and resources to complete the project.

Standard Development

Technical Advisory Committee – The first step in developing these gas standards was organizing an advisory committee of natural gas experts from global academic, industrial and governmental agencies. Twenty-eight individuals agreed to serve as formal members of this committee, which helped review and advise on all important aspects of the project including:

• The determination of the number and the molecular and isotopic composition of the gases used as reference materials.

• The choice of subsampling and sample distribution methods.

• The selection of the analytical methods used for the calibration of the gases.

• The evaluation of the analytical data.

All aspects of the standard reference gas development were reviewed by the committee. Due consideration was given to all recommendations, and every effort was made to comply with the consensus view of the committee. Although the committee provided valuable oversight throughout the development of the standards, ultimately, the USGS took sole responsibility for the scientific integrity of the standard gases and the process leading to their development.

Gas Sample Selection

After extensive consultation with the technical advisory committee, the ideal molecular and stable isotopic compositions of the new standard reference materials were identified (Table 1). The targeted compositions and values represent a series of compromises among selection of specific compounds, identification of the most useful isotopic range and the sheer volume of work required to provide a proper calibration.

The inclusion of non-hydrocarbon gases (particularly N₂ and CO₂) was considered and determined not to be a priority, given the availability of other standards for these compounds. Heavier hydrocarbon homologues (specifically “iso-” and normal butanes and pentanes) were discussed as additional potential compounds of interest for inclusion in these standards.

Although there are no standard reference materials for the stable isotopic composition of butane and pentane gases, the consensus view was that the potential negatives associated with inclusion of these compounds outweighed the benefits. In particular, the heavier hydrocarbon components present a risk of phase separation under certain pressure and temperature conditions, which complicates the storage, handling and distribution of the standards.

Moreover, it was difficult to justify the significant increase in the amount of analytical work required to calibrate these additional components when most analytical methods used to determine their stable isotopic composition are directly tied to methane, ethane or propane.

It was determined that “synthetic” gas mixtures were more likely than produced natural gas samples to be able to meet the desired criteria (Table 1), and that, to the extent possible, the new gas standard mixtures should be fabricated from pure components with the desired isotopic compositions. The molecular compositions of the three mixtures were selected such that the gas components were equimolar on a carbon basis to facilitate the stable isotopic analysis of the standards by modern continuous-flow gas chromatography isotope ratio mass spectrometry (GCirMS) methods.

Table 1: Target molecular and stable isotopic composition of new gas standards

Mix 1 – light: After an exhaustive attempt to identify a method for fabricating an isotopically light mixture that closely matched the target composition, it was determined that using a produced natural gas would be the most cost-effective.

The closest match to the target composition for the isotopically light gas mixture that could be found was a produced natural gas sample from a biogenic gas field in northern Colorado. It was concluded that this gas was the best material that could be obtained for the isotopically light end-member standard, given the fiscal and logistical constraints of the project. The mole percent composition of the gas is typical of a natural bacterial gas and is labeled as USGS HCG-3 (U.S. Geological Survey Hydrocarbon Gas 3).

Mix 2 – intermediate: The intermediate gas mixture was created by locating sources of the pure component gases that had stable isotopic compositions most closely matching the target compositions. These pure components were then blended together in the desired proportions to create the intermediate gas mix. Although the ²H of the ethane in this mix is significantly heavier (~50‰) than the target value, this was the lightest value that could be found. Given the difficulty associated with trying to create isotopically depleted compositions, it was determined that this was as close to the target values for the intermediate mix as was feasible. This mixture has been labeled USGS HCG-2.

Mix 3 – heavy: The heavy gas mixture was created by adding small aliquots of isotopically enriched (¹³C and ²H) methane, ethane or propane into additional cylinders of the pure components that were used to make the intermediate mix. These heavy components were then blended together to make the heavy gas mix, USGS HCG-1.

Analytical Methods

The calibrated values for the gas standards were derived from analytical work performed at the USGS Energy Geochemistry Laboratory and Isotech Laboratories, Inc. in Champaign, IL. The hydrocarbon gas analytes were directly measured against the inorganic carbonate standards NBS-19 and LSVEC for carbon and the SMOW-SLAP scale for hydrogen.

Both laboratories were provided duplicate pressurized stainless steel cylinders of the individual components (methane, ethane and propane) that comprise the mixtures USGS HCG-1 and HCG-2, for a total of 12 individual gas samples. These gases were prepared for dual-inlet isotopic analyses on a manual vacuum line. Briefly, this involved the combustion of the individual hydrocarbons to CO₂ and water and the subsequent reduction of water to H₂ by zinc reduction (400°C, 60 min).

The isotopically light gas-standard (USGS HCG-3) is a produced natural gas from a biogenic gas field in northern Colorado. As such, it contains a mixture of methane, ethane, and propane and required splitting into the component hydrocarbon gases prior to offline combustion and analysis. The component isolation from this gas mixture was done at Isotech Laboratories by means of a preparatory GC system.

This gas mixture was split into the individual hydrocarbon gases, combusted to carbon dioxide and water, and distributed to the USGS lab for analysis. A total of 18 glass ampoules (one carbon dioxide and one water sample for each of methane, ethane and propane, each in triplicate) was distributed to the USGS lab for reduction of the water to hydrogen gas for ²H analysis of the hydrogen and ¹³C analysis of the carbon dioxide. Similar sample preparation (i.e., water reduction to H₂) and analysis was performed at Isotech Laboratories.

Additionally, samples of the NGS gases (NIST RM 8559, RM 8560 and RM8561) were prepared and analyzed by the same methods as the USGS gas standards. This allows for the new reference gases to be directly tied to the NGS gases previously in circulation.

The carbon and hydrogen stable isotopic composition of the CO₂ and H₂ derived from the combustion of the hydrocarbon gases and subsequent reduction of water were measured by dual-inlet magnetic-sector isotope ratio mass spectrometry (Thermo MAT253). The CO₂ was analyzed directly against CO₂ produced from the H₃PO₄ digestion (30°C, 24h) of NBS 19. The H₂ gases were analyzed against SMOW and SLAP prepared (reduced to H₂) by the same method as the hydrocarbon-derived water. Each sample and primary standard was analyzed in replicate in order to establish the statistical significance of each result. The arithmetic mean of the replicate analyses of each individual component is taken as the calibrated value for the gas standards (Table 2).

Table 2.  Suggested reference values for the new USGS hydrocarbon gas mixtures. (These values are solely based on offline preparation methodology and dual inlet analyses.)

New Era

With completion of this project, there is a new supply of natural gas standards that is expected to last for many decades. Moreover, the remnants of the previous standards are being archived with the new standards to ensure continuity with legacy data. Scientists from industry, academia and governmental organizations again have access to an internationally recognized standard for the chemical composition of natural gas constituents.

These new natural gas standards can now be requested from the USGS. The gases must be ordered as a set of all three standard reference materials. There is a charge for ordering the standards, which covers the cost of the packaging (i.e., gas cylinders, etc.) and shipping. Orders can be placed through the USGS website. The standards are shipped twice yearly (usually in April and October).

A full report on the development and certification of the natural gas standards is in preparation.

 

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