Desurveying drillholes: Methods for calculating drillhole orientation and position, and the effects of drillhole length and rock anisotropy on deviation

Directional drilling of longer drillholes is becoming increasingly important as resources are exploited at greater depths. As drillholes lengthen, the choice of desurveying method becomes more crucial as the assumptions that are inherent to all methods are compounded. The aim of this study is to first discuss the assumptions involved in each desurveying method and their potential implications for plotting drillhole pathways, and secondly to compare the established desurveying methods to find the most precise one for plotting the drillhole pathway, using examples from Mount Isa, Australia.

The orientations (azimuth and plunge) of drillholes are required to orient drill core (also known as rock or well core), which can be used to measure the orientations of geological structures at any point. Knowledge of the 3D positions for points of interest along the drill core are required to locate drillhole intersections with geological boundaries, faults or underground mine workings. New computer code has been developed to estimate the orientations and positions of drillholes at any point along their length using the existing desurveying methods. Such orientation and location estimates from the computer codes allow the original orientations of geological structures observed in drill core to be calculated. The codes are available in both R and Python languages in an easy access repository. Results from the codes show that the Basic Tangent method is consistently the least precise, whilst the industry standard Minimum Curvature method has a high precision compared to the other desurveying methods. The impact of rock anisotropy and drillhole length on the precision of the desurveying methods was investigated. Distances between end-of-hole points for each desurveying method increase with increasing drillhole length and angle between the drillhole and anisotropy.