Tectonic structure and permeability in the Taupô Rift: new insights from analysis of LiDAR derived dems
McNamara, David D.
Mil, Sarah D.
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Villamor, P. , Nicol, A. ,Seebeck, H. ,Rowland, J., Townsend, D., Massiot, C., McNamara, D.D., Milicich, S.D., Ries, W., Alcaraz, S. (2017). Tectonic structure and permeability in the Taupô Rift: new insights from analysis of LiDAR derived dems. Paper presented at the 39th New Zealand Geothermal Workshop, Rotorua, New Zealand, 22 – 24 November.
The location of permeable zones, and preliminary quantification of a geothermal well’s production or injection capacity are routinely interpreted from well pressure, temperature, and flow measurements made at different injection rates and during heating after shut-in (also referred to as PTS or completion test data). The spatial resolution of feed zones interpreted from completion test data typically ranges from 10 – 100 m thick. By itself, the completion test dataset cannot inform on the nature of the permeability, i.e. fracture versus formation permeability, and is subject to uncertainties. On the other hand, borehole image logs provide a direct description of the fractures (location, morphology and orientation) and of rock textures intersected by the borehole, to a spatial resolution of ~0.01 – 1 m. Traditionally, analyses of completion test and borehole image log data are done independently by the reservoir engineer and geologist, respectively. We present a joint interpretation of completion test data and borehole image logs (acoustic and resistivity) from two boreholes in the Wairakei Geothermal Field, New Zealand, highlighting the advantages of a cross-discipline interpretation of a borehole’s permeable zones. In the two studied boreholes, we show that feed zones generally correlate with (1) fractures of low acoustic amplitude and visible on the traveltime image of the acoustic image log, and (2) low resistivity fractures with a high resistivity halo on the resistivity image log. In the latter case, fractures with haloes coincide with a reservoir-scale fault modelled independently from 3D modeling of stratigraphic offsets. However, not all fractures of such appearance observed on the image logs correlate with permeable zones. The integrated interpretation of completion test data and image log increases the robustness in the feed zone interpretation (location, extent, fracture or matrix permeability components), and thus can contribute to improving fluid flow numerical models used to sustainably manage geothermal resources.
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