Geologic controls on permeability revealed by borehole imaging: Case studies from Sumatra, Indonesia and the Taupo Volcanic Zone, New Zealand

Abstract

We use borehole images, which provide a continuous, highresolution record of the borehole wall, to evaluate wellborescale geologic controls on permeability. Typically, borehole imaging studies include one or more wells from a single reservoir. Ours is unique for its cross-comparative nature, with seven case study wells from four volcanic-hosted geothermal reservoirs, of which two are in Sumatra, Indonesia, and two are in the Taup? Volcanic Zone, New Zealand.
A quantitative well capacity (injectivity index) is included for each case study, which is also novel for a borehole image study. This enabled a key insight: wells with the largest capacity have fracture frequencies that are two to three times greater than lower capacity wells. As well as the influence of fracture frequency, our results reveal the impact of host rock properties and fracture arrangement, hydrothermal alteration, and stress rotation. We introduce a novel category of drilling induced damage that correlates with the distribution of feedzones.