Summary
Bedded salt formations are located throughout the U.S., providing valuable
storage capacity for natural gas and other hydrocarbons. To increase
gas-storage capabilities and provide operators with improved geotechnical
design and operating guidelines for these caverns, stability analyses of
single-bedded salt caverns have been completed and are described in this paper.
This work is a part of integrated efforts initiated and sponsored by the U.S.
Department of Energy (DOE), the Gas Technology Institute (GTI), and Pipeline
Research Council International, Inc.
Numerical geomechanical models have been developed to investigate
single-cavern deformation and bedding-plane slip for a variety of cavern
configurations. A viscoplastic salt model has been developed based on an
empirical creep law developed for the Waste Isolation Pilot Plant (WIPP)
Program and combined with a Drucker-Prager model for damage and failure. The
nonsalt materials are described with either a traditional Mohr-Coulomb model,
or an elastic model, depending on layer properties.
A baseline model with specified geometric dimensions is first selected and
subjected to 1-year cyclic pressure operations. The amount of damage around the
cavern wall and roof is evaluated and used as a comparison in the study. Then,
the operations are extended to 15 years to study cavern stability for long-term
gas storage and operations. In addition to the baseline model, parametric
studies have been performed to investigate cavern damage as a function of salt
roof thickness, overburden stiffness, interface properties, and cavern
geometries. Each cavern simulation includes 1 year of pressure cycling with a
minimum, mean, and maximum cavern pressure of 6.1 MPa (884.5 psi), 8.8 MPa
(1,276 psi) and 14.9 MPa (2,160.5 psi), respectively. Different operation
conditions (e.g., hydrostatic, cyclic, and direct-pressure drawdown) are
compared and evaluated in terms of cavern stability.
These analyses can be a basis to selecting the best salt cavern candidate
for gas storage and operations as well as helping to assess critical cavern
design parameters for thin-bedded salt formations.
Introduction
The DOE forecasts global natural gas consumption increasing nearly 70%
between 2002 and 2025, with the strongest growth coming from Asia, Eastern
Europe, and the former Soviet Union (Donnelly 2005). Gas demand in mature
economies, such as North America and Europe, also remains strong. The increase
in consumption and strong demand pose significant challenges to gas reservoir
development, gas transportation, and storage. Currently, there are three main
types of natural underground storage facilities for natural gas: depleted
reservoirs, aquifers, and salt caverns. Salt caverns are typically much smaller
than depleted reservoirs and aquifers and, therefore, hold much less gas
volume. In 2001, the U.S. total gas storage capacity was approximately
2.38´1011 m3 (8.4 Tcf), 82% of which was stored in
depleted gas reservoirs, 15 percent in aquifers, and 3% in salt caverns (EIA
2001). Despite the fact that depleted reservoirs are the dominant storage
method for natural gas, it is estimated that salt caverns represent 15% of
daily deliverability (a measure of the amount of gas that can be withdrawn from
a storage facility) in the U.S. (EIA 2001). Salt-cavern storage holds the
advantages related to higher deliverability—lower-cushion (or base) gas
requirement, less development cost, efficienty to initiate the gas flow and
refill.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
1 February 2006
- Meeting paper published:
15 May 2006
- Revised manuscript received:
3 October 2006
- Manuscript approved:
6 October 2006
- Version of record:
20 August 2007
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