It is impossible to avoid drill deviation when drilling using a spindle-type drill. There are many causes of drill hole deviation. However, they can be divided into two main categories: geological conditions and mechanical conditions.
It is important to understand drill hole deviation particularly in the detailed drilling stage, when exploring for high-grade uranium deposits, because such deposits are relatively small in size and vary internally in grade. This paper presents the results of measurements of hole deviation in holes drilled in homogeneous and isotropic granitic rocks near Mizunami in Gifu Prefecture, Japan, and in sandstone of the Athabasca Basin, Saskatchewan, Canada. Holes were drilled to depths in excess of 400 m.
The hole deviation of two vertical holes drilled in the granitic rock at Mizunami were systematically measured at intervals of 2.5 m, upon completion of drilling. Six vertical holes and ten angle holes were drilled through the sandstone in the Athabasca Basin, and hole deviation was measured at 50 m or 30 m intervals as drilling progressed. The results of the hole deviation were projected in plan view and are shown as changes in X, Y, Z directions respectively, with respect to the apparent curvature and apparent strain energy density. The relationship between the average inclination angle and average strain energy density for each hole is also discussed.
The deviations of the two vertical holes drilled in the granitic rock at Mizunami and of the six vertical holes drilled in the Athabasca Basin show a cyclic meandering pattern, while the deviations of the ten angle holes in the Athabasca Basin show a constant clockwise pattern. The boundary between the two patterns appears to be at 4° from vertical.
The inclined holes, which were more than 4° from vertical, show a linear relationship between the average angle of inclination and average strain energy density.
Given these characteristics of drill hole deviation, angle holes would appear to be more useful than vertical holesin testing optimum targets.