The effect of topography on in situ stresses in argillaceous rock at Tournemire, France.

A. Rejeb Rockmeck 2002 - VI th Regional Rock Mechanics Symposium, SELCUK University, Konya, Turquie - 9-11/10/2002.

This paper presents two series of stress measurements taken by hydraulic fracturing in boreholes in a Toarcian argillaceous sequence at the Tournemire site, France. The paper shows that interpretation of the same measurements taken in both test series gives two probable in situ stress fields. The results of finite element modelling were used to determine the stress field consistent with both the equilibrium equations and the in situ measurements, and to quantify the influence of topography on this stress field in the Tournemire massif. After two series of tests at the Tournemire site, the wide spread and lack of precision in the stress measurement results seems to be a fact of life at this site. There being no pre-existing fractures with different orientations in the boreholes tested, it would not have been possible to measure the amplitude of the stresses more accurately even if more hydraulic tests had been carried out. Moreover, the presence of different layers with different degrees of rigidity, and the marked topography of the massif, do not make it any easier to interpret the measurement data. By calculating stresses in the Tournemire massif under gravity, we were able to confirm and quantify the effect of topography on the stress field. These calculations show that the apparent specific gravity of the different facies has a major impact on the results obtained. Note that the density values obtained by laboratory tests and used in the calculations overestimate the stresses. This shows that it is often essential to take the effect of scale on these parameters into account, to obtain a better match between theoretical results and in situ measurements. However, because of the two-dimensional nature of the model developed, these numerical calculations did not enable us to decide between the different results of the orientations of the tested fractures, or between the two directions of major horizontal stress given by the two series of measurements.