Oceanic Natural Gas Hydrate (NGH) has Very Low Environmental Risk of any conventional or unconventional natural gas resource. Exploration and production risk is an aspect of NGH that has received little attention. This is because most of the literature dealing with NGH risk concerns its potential impact on the global greenhouse in the supposed event in which a large release of natural gas could reach the atmosphere and increase warming potential.
NGH can be regarded as an environmentally secure resource. It often has been characterized wrongly as an environmental hazard. NGH risk factors relevant to its exploitation as a major resource of natural gas can be best understood on an attribute-by-attribute basis of its inherent composition and disposition, its amenability to exploration, and the relative safety during production, especially compared with deepwater conventional hydrocarbon deposits that occur in the same operating environment.
Contrasting NGH and Conventional Gas: When NGH forms in the gas hydrate stability zone (GHSZ), it does so through a highly reversible chemical reaction that is largely controlled by supply of the natural gas reactants, which are dominated by methane (Max and Johnson, 2017a, b). Because of the large scale venting of natural gas from the seafloor, it is likely that only a small part of the methane produced throughout the geological history of the global biogeochemical system is sequestered as NGH.
Purity-NGH: The natural gas hydrate cycle (Fig. 1) consists of the formation and conversion of NGH back to its constituent gas and water. In situations where the base of the GHSZ moves up and down in response to environmental changes, gas can be recycled many times. The same amount of gas that is accreted during formation can be released rapidly enough for technical production of the NGH resource. When NGH forms without significant reaction with sediment host, the endothermic reaction produces heat and rejects impurities including dissolved or droplets of oil, small particulate material, and dissolved ionic matter (e.g., salts) that that are dispersed by pore waters within a permeable reservoir.
As most of the NGH is dominated by biogenic methane, the dispersed fluid flow into focused flow and migration of the mineralizing solutions tends to have no association with oils that are produced at higher temperatures. There is rarely associated oil, particularly on thickly sedimented passive continental margins. This results in a relatively pure material, with few other chemicals. Oceanic NGH tends to be relatively pure and to have about the same purity worldwide. If NGH should convert and escape its reservoir into the sea, it will be essentially without pollutants and the flow water will be pore water that is diluted with the converted fresh water.
Figure 1. The NGH cycle.
Purity-Conventional Oil and Gas: In contrast, oil produced under thermogenic conditions can have a complex chemical composition including heavy metals, sulfur, and other materials that can make both gas and oil impure, chemically corrosive, and notorious for creating natural pollution should it breach the reservoir and industrial piping / processing equipment.
Purity – Other Unconventional Oil and Gas: Natural gas effuses from coal deposits when they are depressurized. Other gases such as SOx will also effuse and fine carbonaceous particles may also effuse into the gas stream. In addition, considerable water is produced as part of the continuous depressurization process and the water can contain chemicals and metals from the pore water, which has had considerable time to equilibrate chemically. Temperature of the produced gas is not an issue and pressure is lower than formation pressure. Treatment and handling of produced water from coalbed methane is a problem.
The shattering of shale source beds that is rich in oil and gas, which did not migrate away during when the combination of pressure and temperature in the basin allowed the sediments to enter the ‘oil window’. The oil, gas, and water produced, in addition to flowback water that may contain chemicals that were injected as part of the fracking process, have created well known pollution issues that are commonly dealt with by a combination of water treatment, injection wells, and reuse of frack water. In any case, the hydrocarbon product and water can be highly contaminated, although usually not at a very high temperature.
Physical Situation-NGH: Oceanic NGH occurs only in the GHSZ, which is a zone of thermodynamic stability most stable near the seafloor and less stable downward, with the base determined by the relatively shallow depth (on the order of 1 km or less) at which the geothermal gradient crosses the phase boundary of hydrate stability (Max and Johnson, 2017a). The materials within prospective sediments within the GHSZ thus tend to be relatively close to the cold seafloor. The sediments that provide the likely hosts for NGH concentrations and their bounding sediments are roughly the same semi-consolidated marine sediments having relatively low-powered drilling requirements worldwide.
Temperatures in NGH concentrations will rarely exceed 40 °C and will usually be about 20 °C or lower as depressurization conversion consumes heat. Reservoir formation pressures will be close to a hydrostatic pressure below seafloor. If NGH deep in the GHSZ were to be exposed to seafloor water during drilling, for instance, the NGH would become more stable as its temperature point would move further into the field of stability and away from the phase boundary (Max and Johnson, 2017a).
Physical Situation-Conventional Oil and Gas: In strong contrast, deepwater oil and gas can be present at very high pressures and temperatures (over 250 °C) and can contain a wide range of polluting materials and corrosive chemicals; a veritable Hades material in its reservoir that has to be handled with extreme care and generally, cost and which has tested the limits of industrial innovation and materials.
Production: Conventional gas resides in highly compressed form within reservoirs and will flow strongly if released. NGH, in contrast is a solid material that is stable in the reservoir so long as the stability conditions (usually heating) are not disturbed. In addition, free gas within a NGH reservoir can only be produced from the stable, solid NGH by artificial stimulation or conversion (Max and Johnson, 2011). Not only can the amount of gas created within a reservoir be controlled but also the pressure within the reservoir can be controlled to conform to requirements for reservoir mechanical stability and an optimized production plan.
Conventional gas can be a very difficult material to deal with and contain because of its high overpressures, while gas produced from conversion of NGH (using the emerging best method of depressurization), will have pressures that are lower than formation pressure and may indeed have a lower pressure from within the reservoir a the wellhead at the seafloor. The temperatures and potentially corrosive nature of conventional gas require very special handling, with a significant blowout threat of very nasty material. In contrast, gas converted from NGH will be chemically purer and less chemically aggressive. This would mean that pressure-driven blowout will be impossible and that wellbore liners and materials will not need to be as robust and will thus will be less expensive than for conventional gas.
The most visible long-term environmental risk is pollution potential, and this usually means liquid hydrocarbons. Because NGH concentrations are generally not associated with oil or pollutants, leakage will involve only pure water and chemically unreactive natural gas, which will consist of mainly methane. Deepwater leakage of methane would not have significant environmental impact. Leakage in deep water is unlikely to reach the sea surface where it could reinforce the atmospheric greenhouse (Max and Johnson, 2016), but could be regarded as beneficial because methane is a primary feedstock for the base of the food chain.
Reservoir Security: Deepwater gas deposits will always have a strong potential for blow out because of their very high pressure. In contrast, leakage around well casing or due to a fracture in the gas production system up to the wellhead, and sediment movement, or faulting within the reservoir that could produce pathways for free gas to escape will not initiate a run-away situation. A breach in a NGH producing reservoir has a simple solution. Ceasing depressurization will cause free gas within the reservoir to rapidly recrystallize to NGH. The thermal deficit of forced dissociation will provide a heat sink that will absorb heat from the exothermic hydrate formation reaction. Thus, NGH production contains a built-in mechanism for preventing blowouts. There will be no need for expensive intervention through drilling relief wells for intervention or to have methods and apparatus available for sealing a wellbore, such as may be necessary for conventional, high pressure natural gas deposits.
Processing: Gas produced from NGH will be lower temperature, lower pressure, and purer than conventional deepwater gas, so materials and handling requirements can be much less robust, lengthy, and less costly. Designing optimized NGH processing equipment could be one of the ways in which dramatic reduction in the normal operating costs for NGH could be achieved.
Michael D. Max & Arthur H. Johnson
References: Max, M.D. & Johnson, A.H. 2011. Methane Hydrate / Clathrate Conversion. In: (Khan, M.R., ed), Clean Hydrocarbon Fuel Conversion Technology, Woodhead Publishing Series in Energy No. 19. Woodhead Publishing Ltd. Cambridge, U.K. ISBN 1 84569 727 8, ISBN-13: 978 1 84569 727 3, 413-434.
Max, M.D. & Johnson, A.H. 2016. Exploration and Production of Oceanic Natural Gas Hydrate: Critical Factors for Commercialization. Springer International Publishing AG, 405pp.
Max, M. & Johnson, A. 2017a. Natural Gas Hydrate: Some Basics. Oilpro 4 May 2017. http://oilpro.com/post/31097/natural-gas-hydrate-1-some-basics?utm_source=DailyNewsletter&utm_medium=email&utm_campaign=newsletter&utm_term=2017-05-04&utm_content=Article_5_txt.
Max, M.D. & Johnson, AH. 2017b. Natural Gas: How Large Concentrations of Hydrate Form. Oilpro 11 May 2017. http://oilpro.com/post/31175/natural-gas-hydrate-petroleum-systematics-large-concentrations-hy?utm_source=DailyNewsletter&utm_medium=email&utm_campaign=newsletter&utm_term=2017-05-11&utm_content=Article_9_txt.