ABSTRACT

INTRODUCTION

Since 1987 nearly 300 hydrofrae tests have been carded out in 5 mines in 39 boreholes at the Ruhr coal district. The mine locations range from Kamp Linffort in the west to Harem in the East, a distance of around 100 kin. The maximum depths range to 1300 m. Knowing the stress field makes it possible to optimize the design of cross cuts and future mine lay-out. Investigating the stress field and strength properties also raises the security of mining, lowers reparation costs and inicates the mechanics of rock bursts [1]. Such deep hydrofrac-tests in highly consolidated coal bearing rock formations (sandstone / shales) required high hydraulic pressures to induce and reopen fractures. This paper summarizes the results of the numerous in situ hydrofrac tests and tries to derive a general picture on the stress situation in the Ruhr mining area.

UNDERGROUND TESTING PROCEDURE

All underground hydrofrac tests were carded out using the MeSy PERFRAC I hydrofrac tool. The system is equipped with nylon reinforced packer elements and friction sleeves and is designed for borehole diameters of between 48 and 66 mm. The system can be used at extreme hydraulic pressures to induce and reopen hydraulic fractures up to 60 MPa. The length of each packer element is about 1 m. Between the packers the interval has a length of about 0.6 m. In all cases the tool is operated via high pressure tubings for interval pressurization and a high pressure hose for packer pressurization. The interval pressure is recorded with a high resolution mechanical time pressure strip chart recorder. The use of an electronical digital data acquisition unit is not possible because of coal mining conditions.

ESTIMATION OF THE STRESS FIELD

The vertical stress field component S., the major and minor horizontal stress field components SI_ I and S h were calculated by using the characteristic hydrofrac pressure values and the orientation of the induced or reopened fractures [2].

This content is only available via PDF.
You can access this article if you purchase or spend a download.