A combined laboratory and field test program was performed for long-term cyclic load effects on rock salt. Test conditions were selected to simulate compressed air energy storage (CAES) effects on salt surrounding pressurized caverns. Data are presented and discussed for permeability, mechanical response, and hydraulic fracturing of salt related to CAES effects.
Im Labor und an Ort wurde eine zweifache Teststudie unternommen, um langfristige Wirkungen del'' zyklischen Last auf Steinsalz festzustellen. Testverhaltnisse wurden ausgewahlt, um die Wirkungen von Druckluft-Energielagerung (CAES, d.h. "compressed air energy storage") auf Druckhölhlen umgebendes Salz zu simulieren. Daten werden vorgelegt und diskuttiert in Bezug auf Permeabilitat, mechanische Reaktion und hydraulische Zersplitterung in Beziehung mit CAES-Wirkungen.
Un programme experimental d''etudes en laboratoire et sur le terrain a ete realise afin d''analyser les effets à. long terme de la pression cyclique sur le sel gemme. Les conditions des experiences ont ete determinees de facon à simuler les effets de l''utilization de l''air comprime pour le stockage d''energie (UASCE) sur le sel gemme se trouvant autour de cavites pressurisees. Les donnees sont presentees et analysees en fonction de 1a permeabilite de l''effet mecanique, et de la fracturation hydraulique du sel, lies aux effets de l''UACSE.
Background of compressed air energy storage (CAES) in salt. Rock salt frequently occurs naturally in a relatively homogeneous and impermeable type of rock formation. Thus formations of both dome and bedded salt have been used for a number of decades as a geological media for storage (geostorage) of gas and liquid hydrocarbons. The concept of using underground openings in salt for energy storage in the form of compressed air apparently dates back to the late 19th century, according to Aufricht. and Howard (1961), as compressed air storage was attempted in a mine near Ileilbronn in Germany in 1899. Almost eighty years later the first CAES facility became operational near Huntorf in the Federal Republic of Germany (FRG), and reportedly has been high- Jy successful (Herbst, 1981). This well known facility utilizes two caverns in the underlying Huntorf salt dome as CAES reservoirs (Quast, 1981). 1.2 Test program at Louisiana State University (LSU) for CAES in salt. The general objectives of the LSU test program were to provide a data base for mechanical response of rock salt under conditions similar to CAES applications, and to formulate stability criteria for CAES reservoirs (caverns) in U.S. salt formations. Results from this program, when completed, should permit efficient design and utilization of CAES caverns in salt over long time periods. This paper deals mainly with data from the test program, and does not include stability criteria which will be presented in a later report. From the beginning, the LSU test program was designed to include two phases;
laboratory tests specialized for CAES, and
in situ tests in salt mines. Limited numerical modeling also was performed to couple the two test phases, and for test planning and data analysis.
Both test phases will be outlined in this paper, and a summary section will include some general conclusions based on both phases of the CAES test program for salt.
Design of laboratory test plan The novel consideration for CAES reservoirs is the long-term cyclic character of pressure and thermal loadings on the rock salt surrounding the caverns (Thoms and Gehle, 1981a). An extensive amount of literature exists for salt subjected to effects of monotonic loading followed by long-term creep at medium and elevated temperatures. In some cases, relatively few cycles of loading and unloading were performed as part of the initial phases of testing. The number of parameters that can be varied are significant when simulating time dependent effects for salt caverns subjected only to static loads. Assessing cyclic pressure and thermal load effects on salt for a wide range of conditions over long periods with only one or two general purpose test machines would present a lifetime task. Thus a laboratory was designed and developed with test capabilities particularly applicable to CAES caverns in salt. To accumulate adequate data from longterm tests, parallel testing in time was essential. Accordingly, six specialized and relatively simple test units were assembled for long-term testing, and one small general purpose test machine was purchased for quick index tests and calibration of field instrumentation. The long-term test units consisted of four basic elements as indicated in the schematic of
triaxial (Von Karman) cell and platens,
a pair of airto- oil pressure intensifiers, and,
a control unit for two channels of inlet air to the pressure intensifiers.
A previous design by Obert (1963) was used as the basis for design of the triaxial cells and selection of a collunercially available and relatively inexpensive concrete test machine for the load frames.