Comparison of different methods for the estimation of primary stresses in rock salt mass with respect to cavern design

From the rock mechanical point of view a most accurate knowledge of the magnitude of the primary stresses is of major importance for the design of caverns in rock salt mass. This fact has become more and more essential especially with reference to rock mechanical optimisation concepts for the layout of gas storage caverns. Here, the primary state of stress has a substantial influence on the maximum permissible internal cavern pressure within normal operation periods. Further, when investigating the post operation period abandoned cavern, the primary state of stress still is an essential, parameter that has to be determined accurately. As has been shown by experience the assumption of standard average densities or fixed gradients for the salt rock mass as well as for the overlying rock mass may lead to remarkable deviations of the adopted primary stress state in the calculation model compared with the in situ situation. For estimation the primary state of stress in: a salt dome four different procedures can be applied: determination of the densities as a result from geological interpretation evaluation of in situ pneumatic fracturing tests evaluation of in situ hydraulic fracturing tests and interpretation of borehole gravity measurements. The simultaneous application of these different procedures in the framework of two gas storage projects provides the data base for a comparison, worked out in this publication. As a main result it will be shown that not only the accuracy of the measurements itself but also a proven competent interpretation is necessary.