The nature of fracture permeability in the basement greywacke at Kawerau Geothermal Field, New Zealand

Abstract

The Mesozoic basement at Kawerau Geothermal Field comprises well indurated, inter-bedded sandstones and argillites with a complex structural history. These rocks have very low matrix porosity but nonetheless host both geothermal production and injection. Fluid flow therefore is localized in fault and fracture networks. The geometry of, and potential controls on, these fluid pathways is revealed by an integrated study of borehole acoustic image logs, geologic, drilling and reservoir data from two deep geothermal injection wells. Well permeability, as interpreted from pressure, temperature, and fluid velocity logs acquired during well completion testing, correlates with large aperture fractures and zones where cross-cutting fractures are densely distributed. The occurrence of large aperture fractures also correlates with the occurrence of high sandstone proportions in the drill cuttings. A similar spatial relationship between fracture aperture and rock type occurs in exposed basement greywacke hosted Kuaotunu epithermal deposit, Coromandel Peninsula, New Zealand. These observations demonstrate the importance of understanding material properties when exploiting and stimulating fracture permeability. Despite the complex structural history of the basement greywacke, nearly all large aperture fractures identified from image logs were found to be optimally orientated for reactivation within the modern stress field. Comparison between the orientations of fractures observed down-hole and the orientation of field-scale faults interpreted from vertical displacement between wells reveals a structural relationship across scales. An understanding of the relationship between the modern stress field, field-scale structures and fractures that contribute to wellbore flow can be applied to the mapping of reservoir fracture permeability.