Hydraulic fracturing stress measurements at Forsmark and Stidsvig, Sweden

This paper presents a new development in the use of a multi-hose for hydraulic fracturing, for estimation of in-situ stresses in the Earth's crust. The test procedure and evaluation on hydraulic fracturing in the first and second breakdown method is described. Hydraulic fracturing was performed in Precambrian gneissic rocks at depths down to 500 m in one borehole at Forsmark, Central Sweden, and down to 150 m in three boreholes at Stidsvig, Southern Sweden. Tests were conducted in both 76 and 56 mm boreholes. The hydraulic fracturing tests at Forsmark are most likely to follow a stress pattern calculated from the second breakdown method, and excluding the influence of pore pressure. The maximum horizontal principal stress at 500 m depth was found to be about 27 MPa. A major discontinuity in the state of stress is found to exist along a large fracture zone at 320 m depth. The poor rock quality and the large conductivity for the boreholes at Stidsvig speak in favour of stress determination according to the first break down method. Linear regression analysis of the data gives a maximum principal horizontal stress of 14 MPa at 150 m depth. Before hydraulic fracturing, overcoring stress measurements were conducted in one borehole at each test site. For the borehole at Forsmark, the magnitudes of the stresses found from overcoring were much larger than those found from hydraulic fracturing. The poor rock quality at Stidsvig excludes the possibility of reliable measurements with the overcoring method