Cushion gas

Oldenburg, C.M. 2003. Carbon dioxide as a cushion gas for natural gas storage. Energy and Fuels, 17, 240-246. 

The idea here is to capture carbon dioxide from power plants and then inject it into natural gas hydrate reservoirs assumed to contain primarily methane hydrate. Thus one achieves the simultaneous sequestration of carbon dioxide with the production of natural gas. Lee et al. (2003) presented laboratory data that showed the replacement of methane molecules by C02. Yoon et al. (2004) and Ota et al. (2005) confirmed these laboratory findings. Park et al. (2006a) used a CO2/N2 mixture containing 20 mol % carbon dioxide (flue gas) instead of pure C02 and noticed that the methane recoveiy increased from 64 to 85 %. A similar idea for sequestering captured C02 is to use it as cushion gas for natural gas storage in reservoirs (Oldenburg, 2003). 

Monitoring such operations implies specific measures and modelling tools which correctly handle gas mixing phenomena. When such conditions are obtained the total saving is estimated at 20 % of the cushion gas cost. 

It was either too low in certain sites, as it was the case in Eminence, Mississippi (USA) and Tersanne (France), which led to important volume losses through creep, and consequently to losses in storage capacity, or it was too high, so causing over-investment in cushion gas and losses of potential working storage capacity. 

There are numerous advantages to this improved process. First, most of the gas stored in the cavern can be produced and the cushion gas requirement is minimal. Gas deliverability can also be maintained longer at the same level, since water injection keeps gas pressure in the cavern higher. Moreover, every time the cavern is refilled with water, more capacity is created for storage. Finally, the cavern can be further developed by resuming SMUG. 

Cushion Gas Alternatives", IGU Committee A Report. Study leader V. Onderka, PhD, Geogas a.s., 20th,World Gas Conference. Copenhagen, 10-13 June 1997. 

The screening report included an initial ranking of the 8 fields by an overall assessment of the following parameters The estimated work volumes, the buffer or cushion gas volumes, the reservoir depths and pressures, the reservoir type and status and the distances to pipelines and consumers. Also the average permeability and the amount of available data have been taken into consideration. 

In all scenarios it was possible to inject the required volumes of working gas within a period of 1 - 5 months. The subsequent withdrawal of identical volumes of gas from the storage was possible in all scenarios5, except Scenario IV, without using additional cushion gas. In Scenario IV, it was possible to withdraw only 465 and not the required 480 million m3 gas, if no additional cushion gas was to be injected. It was demonstrated, however, that lowering the minimum well bottom hole pressure by 10 bar to 80 bar and optimising the individual well rates, would enable the withdrawal of the required 480 million m3 gas. 

Short-term storage in depleted gas fields is considerably cheaper than in aquifers, for the following reasons. The geology has already been established and the field surveyed. There is confidence that the gas will not leak from the store without the requirement for a detailed appraisal. Since the reservoir will already be full of gas at low pressure, there will be no need to provide an initial charge (so-called cushion gas ) so that most of the gas that is injected can be recovered. Finally, much of the necessary equipment and the transport pipeline are likely still to be in place. By contrast, aquifers provide none of these advantages and much work will be involved in their evaluation and inauguration as gas stores. 

Containment system RPV is immersed in water-filled CV the cushion gas is Ar (highly effective RPV thermal insulation is needed)... 

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