Evaluation of the HTPF method of stress determination in two kinds of rock

Abstract

Hydraulic tests have been conducted on two sites with a wireline operated straddle packer system in vertical boreholes down to a depth of 1065m. Results obtained have been interpreted with both the classical hydraulic fracturing method and the HTPF (“hydraulic testing of pre-existing fractures”) method. At the first site, a sedimentary deposit near Aix en Provence (South of France), the tests indicate a stress discontinuity associated probably with a local rock heterogeneity. Results from the second site, a gneissic formation in the Cezallier plateau (south central France), interpreted with the HTPF method, yield a vertical gradient for the vertical stress component much lower than that expected from the rock mass density. This is attributed to topographic effects. The interpretation with the classical method gives very similar results for the σ h and σ H magnitudes and orientation. The HTPF method appears to be inadequate for the data obtained at Aix en Provence—the heterogeneity of the formation compromises the validity o the hypothesis that the stress field is a linear function of depth.