Determination of stresses in salt rocks byhydraulic fracturing: Case-study

Abstract

The paper presents the experimental results of stress assessment in salt rock mass around underground excavations in Solikamsk Mines 1, 2 and 3, Uralkali, at the Upper Kama potash deposit. The experiments were a part of the geomechanical research aimed at justification of project designs and engineering solutions made in selection of safe and efficient mining methods, including safety of watertight strata. The experimental works were accomplished using the measurement and computation system Gidrorazryv designed at the Institute of Mining, SB RAS. In the salt rock testing, the system was equipped with new-design downhole survey tool and wireless communication between pressure sensors and portable computer. The sites for measurement stations were selected with regard to current mining situation, distance between the sites and stoping front, absence of undermining or overmining and installability of drilling equipment. A particular attention was paid to a drilling tool capable of geometrically accurate drilling in salt rock mass, in compliance with standards of the hydraulic fracturing stress measurement technique. In the tests, the method of directional hydrofracturing with softening of measurement hole boundary by cutting initiation slots was implemented. All in all, 75 hydraulic fracturing tests were carried in adjacent rock mass in the mines. It is found that stress state in roof rocks of underground excavations is nonuniform: the vertical stress is similar to the calculated value due to weight of overlying rocks while the horizontal stresses exceed the gravitational stress by 2–3 times.