Gas storage in a pinnacle reef

Monday, May 24, 1993

In 1988, Union Gas entered into negotiations for the development of a pinnacle reef to be used for storage of natural gas. The pinnacle reef, known as Dow Sarnia Block "A" Pool, is five kilometres south of Sarnia in the Silurian Guelph Formation (Fig. 1). The reef was discovered using seismic surveys in 1980 and was completed as a gas producer. From 1981 to 1986, the reef produced 87.45 million cubic metres (3.087 billion cubic feet) of gas. Production from the reef ceased in July 1986.

As the operator and developer, Union Gas used an integrated approach to produce a geological model of the reef which embraced geophysics, reservoir engineering and petrophysics. Initial study of the geology and geophysics and reservoir enalysis at the time of discovery left several significant issues to be resolved. The reservoir volume and geometry needed refinement before the reef could be used for storage. Three dimensional seismic analysis, core analysis and advanced petrophysics provided the required information needed to proceed with the development of the reef.

The development of the natural gas storage pool involved the input and coordination of a multidisciplinary team. This approach to engineering design and construction of the pool and its facilities was the key to meeting the "in service" date of July 15, 1992. Figure 2 is the project schedule chart; from this the "parallel processing" of tasks is evident. Mathematical modelling of the pool started early in the project and continued until all the data had been incorporated. Seismic study was scheduled early in the project and other tasks were scheduled as much as three years in advance.

Until the edge of the reservoir could be properly delineated, Union Gas negotiated a large buffer of space around the reef to provide protection once the reef was converted to storage. This was to ensure the integrity of the main reef and any surrounding subsurface structure that might be in hydrologic communication with the reef. When the data from the seismic and well-logged programs were acquired, the size of the buffer zone was reduced. This data was used to support an application to the Ontario Energy Board for approval to use the reef for storage.

To estimate the potential storage, the amount of original gas in the reef was estimated using the historical pressure decline and production method. The original pressure was 5723 Kpaa and the final pressure was 1292 kPaa. This relationship was used to calculate the original amount of gas in place, which was 109.59 million cubic metres (3.869 billion cubic feet). The maximum planned storage wellhead pressure is 10,310 Kpaa (149.3 psia) and the total capacity of the pool is 210.5 million cubic metres (7.431 billion cubic feet).

Condensed from "The Integrated Approach to Modelling of the Dow Sarnia Block "A" Pool," by Wayne Passmore, Garry Ayres and J.P. Desrochers printed in the Technical Proceedings of the 31st Annual Conference of the Ontario Petroleum Institute, Niagara Falls, Ontario, October 1992.

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