Rock stress measurement by sleeve fracturing technique

Sleeve fracturing is very similar to hydraulic fracturing except for the method of loading. Sleeve fracturing is a technique by which radial fractures are induced at any depth without introducing fluid into the rock during the fracturing process, and it is realized by pressurizing a jacketed borehole. Diametral deformation of a borehole is measured to examine the fracturing process of the primary radial fractures, which occur in the direction perpendicular to the minimum in-situ stress in the plane normal to the borehole axis, and the secondary radial fractures which extend perpendicular to the maximum in-situ stress.In this paper, the fracturing process with multi-stage loading is analyzed by the boundary element method and a procedure for determining in-situ rock stresses is presented. Relationships among borehole pressures, length of fractures and in-situ rock stresses on the fracturing process are formulated. It is clarified that the in-situ rock stresses are determined by the length of fractures and the borehole pressures at the stages of the completion of free separation and the re-propagation of fractures, which are evaluated from the curves of pressure-diametral displacement in multi-stage loading test. Furthermore, it is shown that the in-situ rock stresses can be successfully determined by using the presented procedure in practice.