Fractures Interpreted from Acoustic Formation Imaging Technology: Correlation to Permeability

Abstract

Permeable feed zones in geothermal wells are commonly identified using well profiles of temperature, pressure and fluid velocity measured at different injection rates during well completion testing and heat-up. While this data gives some indication of the depth and relative strength of the feed zones it does not give any information on the nature of the permeability in those zones, be it primary or secondary. Fracturing is thought to contribute to permeability in areas targeted for deep reinjection in the Wairakei geothermal system, within the Tahorakuri and Waikora Formations. By characterizing those deep fractures in terms of orientation, density and aperture, as well as determining the orientation of the horizontal stress field it is possible to interpret the fracture component of the well permeability. This has implications both for well targeting and reservoir modelling.

The recent use of high temperature acoustic formation imaging technology (AFIT) can provide the necessary fracture and stress data to assess the contribution of fractures to feed zone permeability. As part of an ongoing AFIT logging program at Wairakei, data has been collected from the open hole of a number of deep wells in the southern part of the field. The location of feed zones in these wells has been interpreted from the completion test data and then correlated with AFIT fracture density and aperture data to provide more accurate feed zone depths and to characterise the nature of the permeability. Only fractures with optimal orientation within the local stress field are considered as potentially open to fluid flow. While the correlation between feed zones and fracture density is poor, good correlation is observed with the location of individual wide-aperture fracture zones. These zones may represent significant flow paths in the reservoir.