N O T E S. Environmental Forensics. Identification of Natural Gas Sources using Geochemical Forensic Tools. Dispute Scenarios

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1 Environmental Forensics N O T E S V o l u m e 2 9 Identification of Natural Gas Sources using Geochemical Forensic Tools By Paul Boehm, Ph.D. and Tarek Saba, Ph.D. F o r m o r e i n f o r m a t i o n o n E x p o n e n t s e n v i r o n m e n t a l s e r v i c e s, c o n t a c t : Paul D. Boehm, Ph.D. Principal Scientist and Group Vice President, Environmental Group (978) pboehm@exponent.com Natural gas is found in geological formations at varying depths and in varying chemical compositions. After the discovery of a gas field and installation of gas extraction wells, the supply of natural gas native to the formation continues for a number of years until the gas field is depleted. The existence of a gas field attests to the ability of a structure and rock sequences to trap and retain commercial quantities of gas over significant periods of geological time (many millions of years). Therefore, depleted gas fields offer the potential for re-injecting and storing natural gas underground, which can then be withdrawn at a later date via operating wells. In, 394 permitted or certificated underground gas storage fields were operating in the U.S. as a vital part of the natural gas infrastructure. Dispute Scenarios On occasion, actual leakage and migration of storage gas from fields beyond their certificated boundaries, or an alleged migration of the gas from a storage field, have resulted in several kinds of disputes. These include: Disputes over the origin of natural gas found in formations and production wells adjacent to storage fields These disputes typically arise between owners/operators of storage fields and neighboring companies that produce natural gas. The key question in these disputes is whether the gas in the nearby production wells is stored gas that has migrated from the storage field or whether the gas is naturally occurring native gas. Under the law of capture, some states provide that an injector of natural gas retains title to and possession of gas that has migrated to adjoining properties, if the injector can prove that such gas was originally injected in an underground formation for the purpose of storage. Those states also give the injector the right to conduct tests on gas wells surrounding the storage formation to determine the ownership of the disputed gas. 1

2 Disputes over the origin of natural gas impacting residential water wells in the vicinity of underground storage fields The occurrence of natural gas in groundwater and water wells, and accumulation in homes is, of course, a safety concern. Figure 1 shows an extreme, but actual example of flaming tap water from a well impacted by natural gas. The main issue in these situations is whether the source of the gas is from a leaking nearby storage field, usually in a deep geological formation, or whether the gas is native to the area and from shallow formations (e.g., shale formations). Some areas in the U.S., like the southern region of New York State and large parts of Pennsylvania, Ohio, and West Virginia, sit above one of the largest natural gas deposits, called the Marcellus shale deposit, and the presence of native gas in residential water wells in some of these areas is not uncommon. For example, helium is formed from the decay of naturally occurring uranium in the earth, which then migrates and mixes with natural hydrocarbons in the formation. Table 1 presents the typical chemical composition of natural gas. Table 1. Typical chemical composition of natural gas Compound Symbol Percent in Natural Gas Methane CH Ethane C 2 H 6 Propane C 3 H 8 Butane C 4 H Carbon dioxide CO 2 8 Oxygen O 2.2 Nitrogen N 2 Hydrogen sulphide H 2 S Rare gases A, He 2 Figure 1. Impacted residential water wells represent a safety hazard. In both of these types of disputes, the use of geochemical fingerprinting techniques has been proven to be a valuable tool in determining the origin of the natural gas. Geochemical Source Fingerprinting Composition of Natural Gas Natural gas was formed in the earth when organic matter (e.g., plant and animal debris) was buried and subjected to pressure and increased temperature over millions of years. The chemical composition of natural gases includes hydrocarbons (methane, ethane, propane, butane, etc.) and non-hydrocarbon gases (e.g., nitrogen, carbon dioxide, helium). Oil (the liquid part of the hydrocarbon formation) is usually formed at the same time as natural gas. However, the gas often migrates deep within the earth, sometimes apart from the oil. Oil and gas become trapped by geological formations and remain in place until discovered and produced by oil and gas companies. The non-hydrocarbon compounds could form at the time of natural gas formation or at a different time period. 2 Geochemical Fingerprinting The term geochemical fingerprinting refers to the use of established chemical and interpretational methods to fully characterize the hydrocarbon composition of a natural gas or hydrocarbon liquid sample. The purpose of this characterization is to establish the unique chemical composition or fingerprint of a gas or an oil sample. This fingerprint can be used to differentiate the sample in question from other gas or liquid hydrocarbon samples. Natural gas compositions vary from place to place, and from formation to formation in the earth. However, the composition of natural gas samples in a particular geological formation in a particular area will have very similar compositions that are different from natural gas samples in other areas. Gas compositions also may vary at the same location as a function of the depth that defines the actual formation zones. Geochemical fingerprinting techniques determine the exact amounts of the various hydrocarbon gases, non-hydrocarbon gases, and light hydrocarbon liquids in a particular sample. This detailed chemical fingerprint can be compared to other samples to determine if they are from the same source or from different sources.

3 Figure 2 presents an illustration showing the concept of how fingerprinting is applied to different natural gas samples. The schematic illustrates that one can determine the similarities and differences between two natural gas samples by comparing the composition of those samples. Figure 3 shows how data are typically presented. By looking at percent composition of key natural gas components, the differences between groups of natural gas samples are clearly revealed. While the compositions of gases from a particular group of samples vary somewhat within that group, the groups of samples from the same source are distinct from those from other sources. Hydrocarbons (C1/C2+) 2 1 Gas 1 Gas 1: A group of samples with lower helium and higher methane Gas 2: A group of samples with elevated helium and lower methane Gas Non-hydrocarbon Compound Helium (percent) C1 = Methane C2+ = [Ethane+Propane+Butane+Pentane+Hexane+] Figure 3. Geochemical data presentation. In this example, two key diagnostic chemical parameters are plotted one on each axis of the graph. Gas samples from two different origins form two separate groups, which differ from one another with respect to the diagnostic source parameters Gas 1 Gas 2 Helium Methane Unknown Unknown Gas 1 Gas 2 Ethane Nitrogen = MATCH = MATCH MATCH Other components: Propane, butane, pentane, hexane+, O2, CO2, H2, Argon Figure 2. Conceptual example of the basic process of comparing the compositions of gas samples to determine if the compositions of samples do or do not match known sources Stable Isotope Geochemistry Most elements (e.g., carbon) occur in nature as mixtures of stable isotope forms. The ratio of one isotope to another varies according to the source organic matter from which the gas was formed. The carbon 12 ( 12 C) isotope is the most common form of carbon (99 percent of all carbon) and has 6 neutrons in the carbon atom. The 13 C isotope (1 percent of all carbon) has one extra neutron (a total of 7 neutrons) and is therefore heavier than 12 C. Figure 4 presents an illustration of the different isotopes of carbon. The ratio of 13 C/ 12 C has been proven to be an effective discriminator in identifying natural gases from different sources. Stable isotope ratio analysis (ratios of carbon, hydrogen, and sometimes nitrogen isotopes) can be applied to an entire gas sample or to individual compounds in the gas (e.g., 13 C/ 12 C for methane or ethane). As in gas compositional analysis, one would expect that there would be a difference in the 13 C/ 12 C ratio between gases generated from different source rock formations. Proton Neutron Electron 12C 13C Figure 4. Stable isotopes are different chemical forms of the same element that exist in nature (e.g., carbon) 3 3

4 The geochemical fingerprinting techniques presented above (gas composition and isotope analysis) were used in several cases where identification of the origin of natural gas was a key. Below is a description of a representative case. In this case, geochemical fingerprinting in conjunction with geological and reservoir engineering expertise provided effective tools and defensible outcomes for our client. 2. The gas produced by the company across the southern divide was originally native gas. With time, this native gas was exhausted and replaced by the migrating storage gas (Figure 6). These findings were instrumental in enabling the two companies to reach a settlement that was satisfactory to both parties. Case Study: Identification of the Origin of Natural Gas Produced Near an Underground Storage Reservoir 2 Storage gas In this case, processed natural gas (storage gas) from a number of states had been transported to a permitted storage facility and injected into an underground storage formation 1,9 ft below ground surface. This formation is surrounded by geological structures initially thought to be sealing structures, which prevent gas from escaping the storage field boundaries. However, several years after the operation of the storage field, pressure monitoring in observation wells around the field at its perimeter indicated that the southern divide was leaking storage gas. At the same time, property owners south of the field began producing natural gas and claimed that they were extracting native gas and not storage gas escaping the field. Exponent experts were retained to determine the origin of the disputed gas. To determine if storage gas was migrating to and beyond the southern divide, the compositions of storage and native gases were compared. First, all chemical data for native gas that was withdrawn from the storage field area or from the surrounding counties were assembled and reviewed. Second, a field gas sampling program was carried out, which obtained and analyzed gas samples from the storage field and from wells south of the field. Third, established techniques of geochemical fingerprinting were applied to determine whether the gas withdrawn from wells outside and south of the field was storage gas, native gas, or other. The extensive work that was done in these three steps enabled us to determine that: 1. Storage gas samples from the field were chemically very different from native gas. Compared to storage gas, the native gas had a much lower methane content and a higher ( times as high) helium content. Figure presents the geochemical fingerprints of storage versus native gas. Native and storage gases were clearly two distinct groups. 4 C1/C2 + Range of C1/C2 + in native gas C1/C Range of helium in native gas Helium (percent) 2 1 Storage gas n n n Native gas Figure. Comparison between storage gas from the field and native gas Range of C1/C2 + in native gas Native gas Range of helium in native gas Helium (percent) Figure 6. Geochemical fingerprinting data can be used to track the transition of native to storage gas, as gas was added to the storage field

5 Central to Exponent s environmental expertise is a deep capability in environmental forensics. We have applied our expertise and experience to a wide variety of situations: refineries, former manufactured gas plants, mines, smelters, foundries, pulp and paper mills, wood treatment facilities, oil spills, fuel terminals, and many manufacturing facilities with contaminants in air, groundwater, surface water, sediment, and soil. We have more than 3 scientists and engineers with a variety of experience in environmental forensics. Points of contact for specific chemical classes are Walt Shields (metals and dioxins), Brian Murphy (TCE and other chlorinated solvents and MGP wastes), Paul Boehm (petroleum), Paul Boehm and Tarek Saba (PAHs, PCBs, natural gas), Gary Bigham (mercury), Peter Mesard (perchlorate), and Stephen Mudge (isotope analysis). Please contact Tarek Saba or Paul Boehm at Exponent if you would like additional information on this issue of our Environmental Forensics Notes. Click here for more information on our Environmental Forensics services. Click here to view our previous Environmental Perspectives Newsletters.