Borehole breakout and fracture analyses: Jwaneng kimberlite mine, Southern Botswana, Africa

Borehole breakouts, so named by Babcock (1978), are enlargements and elongations of a borehole in a preferential direction. These are formed by spalling of borehole fragments in a direction parallel to the minimum (least) horizontal stress. The breakouts are typically elongated in the direction of the borehole axis and can be expressed by three parameters; orientation in the borehole, the opening angle and the radial depth. Knowledge of the orientation of horizontal earth stresses derived from analysis of borehole breakouts is important to geotechnical and rock mechanical studies. The country rock model for the Jwaneng Mine is extremely complex with a number of major structural faults and splays. The main central fault that intersects the kimberlite bodies has the same orientation as the Zoetfontein fault system 55km to the north (azimuth/dip of 339°/85°). The related fault splays or fractures, are tensile with the main structures and therefore may be fluid conduits. Utilizing downhole imagery in conjunction with the more typical suite of in-situ physical rock property measurements provides constraints on these structures and associated stress fields. Inferences on water mobility through the fracture systems can be made at depth and form an integral part of mine planning, development and optimization.