ABSTRACT

In the research project the stress distribution in the vicinity of the future version of an underground fuel storage in Gora, Poland was determined. The stress distribution was established by FEM simulations of a 3D model of a viscous rock mass. As the structure of the salt dome in G6ra is not fully recognized yet, several Simplifications in model geometry and internal structure were made and discussed in the paper. Computer Simulations for 20 years period reveal that the stress intensity tends to increase in nearly linear function of depth in caverns area and decrease with time. Results of modelling are graphed as horizontal and vertical cross-sections showing the stress distribution round the caverns filled with brine or stored liquid carbohydrates. There are also the profiles of stress intensity and effort ratio on the cavern walls and in the pillars between the caverns.

INTRODUCTION

Depending on the actual mining and geological conditions, the caverns remaining after the solution mining might be sometimes utilized as underground storage reservoirs of hydrocarbons. The safety of longterm operation of an underground storage reservoir is determined on the basis of the stress distribution in its direct vicinity, which in turn is associated With mechanical properties of rocks and the deposit structure.

In case of nearly vertical bedding of salt rocks In some salt dome formations the internal structure of the deposit is almost unidentifiable in the vertical drill holes and hardly recognizable in the course of geophysical investigations. The main rock types are distinguishable, though their actual position with respect to one another is unknown. An example of such mining condition is the salt dome in Gora, where the caverns remaining after the solution mining are now used as underground storage reservoirs of oil and fuels. The schematic of the solution mining area is shown in Figure 1. The average dip of the salt strata is found to be 87%, Mechanical properties of rocks were determined In laboratory conditions.

The paper outlines a method for evaluating stress and effort ratio around the salt caverns basing on laboratory tests and modelling of rock strata stresses and Ort/stress ratio value over the last 20 years.

(Figure in full paper)

PROPERTIES OF SALT DIAPIR ROCKS IN GORA

Mechanical properties of salt rocks: instantaneous and long-term strength are established on the basis of laboratory tests. The stress-strain σijij) and compressibility σiiii) relationships are determined in the conditions of fast loading increase. Rheological tests were performed to investigate the creeping behavior εij(t) for the constant stress σ11= const and relaxation σij(t) for the fixed strain ε11 = const. Uni-axial and tri-axial tests were applied to find the Young modulus, the Poisson ratio and the ultimate differential stress of σ1 – σ2 differential strength Rl - σ2). The laboratory research has established that the creep of salt rocks is governed by the power formula of time. for σ2 = σ3 expressed by the ultimate

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