Unusual Drilling-Induced Stress Features - What To Do With You?

Sarah Dawn Milicich, Cécile Massiot, Angela Griffin

Drilling-induced features identified from borehole image logs provide direct constraints on in-situ stress orientations and magnitudes. Classic features such as drilling-induced tensile fractures and borehole breakouts have been extensively documented since the 1980s, predominantly in sedimentary rocks within hydrocarbon reservoirs. More recently, petal-centreline fractures have also been recognised. Their formation has been modelled as functions of stress state, borehole trajectory, rock properties, and drilling fluid parameters. In high-temperature geothermal borehole images of the rifted Taupō Volcanic Zone, Aotearoa New Zealand, we commonly identify a range of non-typical drilling-induced features alongside typical features. Those wells intersect diverse lithologies including silicic to andesitic volcanic and volcaniclastic rocks, and metasedimentary basement. We hypothesise that these unusual features arise from the interaction of drilling with complex rock textures, mechanical anisotropy, natural fractures and veins, fault density, thermal stress, and variations in hydrothermal alteration. Globally, recent studies have modelled some non-typical features on a well-by-well basis to derive stress constraints. With the increasing availability of borehole image data in geothermal environments, we aim to engage the broader geoscience community in recognising and interpreting these unusual features. They hold promise for refining stress and permeability models critical for drilling stable and productive geothermal wells. This understanding is vital for optimising conventional geothermal development and advancing next-generation systems such as superhot and enhanced geothermal systems.