Abstract

Bedded salt formations are located throughout the United States, providing valuable storage capacity for natural gas and other hydrocarbons. In order to increase gas storage capabilities and provide operators with improved geotechnical design and operating guidelines for these caverns, stability analyses of single bedded salt cavern have been completed and are described in this paper. This work is a part of integrated efforts initiated and sponsored by the US Department of Energy, Gas Technology Institute, 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 non-salt 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 one 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 one year of pressure cycling with a minimum, mean, and maximum cavern pressure of 6.1 MPa (884.5 psi), 8.8 MPa (1276 psi) and 14.9 MPa (2160.5 psi), respectively. Different operation conditions, e.g. hydrostatic, cyclic, and a direct pressure drawdown, are compared and evaluated in terms of cavern stability.

These analyses can serve as a basis to select the best salt cavern candidate for gas storage and operations. They can also help to assess critical cavern design parameters for thin bedded salt formations.

Introduction

The US Depart. Of Energy (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.[1] This poses 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 they hold much less gas volume. In 2001, the US total gas storage capacity was about 2.38×10[11]m[3] (8.4tcf), 82% of which was stored in depleted gas reservoirs, 15% in aquifers, and 3% in salt caverns.[2] Despite the fact that deplete 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 US.[2] Salt caverns storage hold the advantages related to higher deliverability, lower cushion (or base) gas requirement, less development cost, faster to initiate the gas flow, and quicker to refill.

There are two types of underground salt deposits: salt domes and bedded salts. Bedded salts are the primary focus in this study because they are shallower and thinner formations than natural salt domes, and therefore more prone to stability risks during gas extraction and injection. Bedded salt formations are found in several areas throughout the United States and Canada.[3,4] The largest basins include the Permian Basin across Texas, Oklahoma, Kansas, Colorado, and New Mexico, the Gulf Coast Basin across Southern Texas, Louisiana, Mississippi, and Alabama, and the Michigan and Appalachian Basins across the states of Michigan, Ohio, Pennsylvania and New York. These areas have experienced different deposition and tectonic history, resulting in some differences in depth, lithology and typical geologic structure for the dominant bedded salt intervals. Bedded salt formations in all these areas, however, are layered and interspersed with non-salt sedimentary materials such as anhydrite, shale, dolomite, and limestone. The "salt" layers themselves also often contain significant impurities.

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