New developments in hydrofracturing stress measurement techniques and a comparison to the HTPF method

Sammanfattning: This thesis describes two hydraulic methods for rock stress measurements; hydraulic fracturing and hydraulic tests on pre-existing fractures (the HTPF method). It includes theories to evaluate stress field information for somewhat extraordinary stress and field situations, experimental investigations and field measurements. A theory for estimation of the horizontal rock stresses from hydraulic fracturing measurements has been developed. It takes into account the influence of the radial and vertical stresses acting at the borehole wall in the interpretation by use of the Hoek and Brown failure criterion. The method is based on a three-dimensional stress analysis around a borehole. The method allows for an estimate of the stress state when horizontal fractures have been formed. Application of this method of evaluation on data from stress measurements at Lavia, Finland, indicates high horizontal stresses down to 500 m depth. The results also show a large difference in magnitude between the horizontal stress components. When a borehole is inclined with respect to the principal stresses, the classical hydraulic fracturing theory does not strictly apply to determine the stress state in the plane perpendicular to the axis of the borehole. A method is presented which permits the orientation of the maximum horizontal stress to be determined for such situations. The stress measuring technique of hydraulic tests on preexisting fractures was introduced in Sweden in 1986. To this date it has been used at three different sites. This thesis presents the outcome from two of these field cases, and for both measurements a comparison has been made with the technique and stress results-from hydraulic fracturing.