| Title | Geological Analysis of the Volcanic Subsurface Using Borehole Resistivity Images in the Ngatamariki Geothermal Field, New Zealand |
|---|---|
| Authors | Leon Halwa, Irene C. Wallis and German Torres Lozada |
| Year | 2013 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Borehole Imaging, Formation Micro-imaging, Resistivity Imaging, Fracture Analysis, Drilling Induced Fracturing, Drilling Enhanced Fracturing, Volcanic Fabric, Secondary Porosity |
| Abstract | Deploying wireline technology to evaluate the subsurface in New Zealand geothermal fields poses significant challenges owing to the dominantly volcanic setting and hostile well conditions. Despite this, a number of wireline logs were successfully collected during the 2011-2013 Ngatamariki Geothermal Field development drilling, including two highquality resistivity-type borehole imaging logs. These kinds of borehole images are primarily used to characterize geological features such as in-situ stress direction, fractures (natural, drilling induced and enhanced) and the rock fabric. The fullbore formation micro-imager (FMI*) used at Ngatamariki is a pad-based tool that measures the electrical properties of rock and converts button resistivities to oriented electrical imagery of the borehole wall. This tool worked well at Ngatamariki because of the contrast between the low-resistivity drilling fluids and high-resistivity rock types present. Image analysis included determining fracture type, orientation, density, aperture, fracture volumes, network relationship and, to a certain extent, fracture fill type. Resistivity characterisation allowed differentiation between potentially open and cemented fractures, as well as identification of both conductive and resistive cements. Because of the high resistivity contrast between the rock and drilling fluid, the physical properties of conductive and resistive clasts within the volcanic deposits could be described, and this facilitated dividing the images into textural zones. The presence and morphology of banding in pyroclastic and lava flow deposits was characterized by comparing sine wave morphology with textural heterogeneity. The FMI* logs collected at Ngatamariki provide an on-depth, high-resolution picture of the geological subsurface which is comparable to continuous core. As such they are a significant contribution to the understanding of the relationship between geological features and permeability, providing details that constrain both conceptual models and numerical simulations. The present study also represents advancement in resistivity |