ABSTRACT
Research associated with the deformation and failure of underground natural gas storage facilities has been underway at Penn State now for some 10 years. Analysis of the stress fields around pressurized underground cavities have indicated that at some locations a tension-tension-compression principal stress state exists. Relatively little information is available on the failure criteria for such a stress state and recent experiments have been conducted in an attempt to establish suitable criteria. In these studies apparatus was developed for simultaneously loading of hollow cylindrical rock specimens under axial tensile load and internal pressure. Acoustic emission techniques were employed to detect yield and initial failure, which occur first on the inner surface of the test specimen. Results of a detailed series of tests, carried out on Indiana Limestone, are presented and the suitability of various failure criteria are discussed. One important conclusion of the study is that material behavior in such a stress state is extremely sensitive to the magnitude of the intermediate principal stress.
INTRODUCTION
Studies associated with optimization of pressures associated with the underground storage of natural gas have been underway in the Mineral Engineering Department at The Pennsylvania State University now for a number of years (Hardy et al., 1972, 1976; Hardy, 1976). These studies have involved laboratory, analytical, and field investigations. The aim of these studies is to develop criteria which will permit the underground storage of gas at the maximum possible level consistent with a high degree of safety and operating efficiency. At present in the USA, natural gas is mainly stored in "containers" consisting of depleted gas and petroleum reservoirs and aquifers. Use is also made of a limited number of salt caverns, and in one case a specially modified coal mine. Regardless of the type of storage facility involved its mechanical stability depends on the storage pressure, the in situ stress field, the geometry of the storage "container," and the mechanical properties of the associated rocks. Analytical studies have indicated that the stress states associated with a pressurized underground storage facility involve both tensile (T) and compressive (C) components. Studies to date indicate that under various conditions T-T-C, T-C-C and C-C-C stress states may exist. The present paper describes the results of a recent study (Khair, 1972) carried out to investigate yield and failure criteria associated with the T-T-C stress state. These studies were of a limited nature since only one rock type (Indiana Limestone) was investigated and the effect of test specimen geometry was not evaluated. The results, however, are of considerable interest since they indicate that the failure is strongly influenced by the intermediate principle stress, which is tensile in sign.
TEST CONDITIONS
It is a well established fact that by suitably loading specimens in the form of hollow, thickwalled, cylinders it is possible to investigate failure in geologic materials resulting from a wide range of stress states. Figure 1 illustrates the loading conditions involved in the current study, namely: axial stress and internal pressurization.
