Fractures Interpreted from Acoustic Formation Imaging Technology: Correlation to Permeability

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. 1.0 INTRODUCTION Recent deep drilling at Wairakei in the Karapiti South reinjection area has drilled beyond the relatively well-understood Waiora Formation and into the Tahorakuri formation, encountering previously unknown deep permeable zones around 2000-2500m depth. The nature of permeability in these zones and the controls on that permeability are of interest both for the reinjection strategy at Wairakei as well as understanding the nature of the connection between Wairakei and Tauhara geothermal systems. The acoustic formation imaging tool (AFIT) or ‘borehole televiewer’ has provided a fracture dataset for the deep sections of these wells. Of the hundreds of fractures imaged only a small number will be permeable as fractures in the well bore wall need to also extend a significant distance beyond the well bore and be interconnected. Correlation with feed zones identified from completion testing enables the identification of fractures associated with permeability. The spatial extent and orientation of these permeable fractures is providing new insights into the mechanics of the reservoir. 2.0 RESERVOIR SETTING Pressure interference between the neighbouring Wairakei and Tauhara geothermal fields and the fluid geochemistry indicate that they are separate fields with separate upflows which are hydrologically connected at more than one level. The deep wells WK317, WK404 and WK407 are located along the southern and eastern boundary of the Wairakei system, between the Wairakei and Tauhara systems (Figure 1). It is noteworthy that the overall permeability found in each of the wells discussed is in the “high” to “very high” range, even for Wairakei, with values in the order of 100 t/h per bar and greater. Figure 1: Well layout map for Wairakei-Tauhara. Wells in depth range 2500-3000m are in green, 1800-2500m in red, all others in yellow. As illustrated in Figure 2 the active extensional tectonic setting of the Taupo Volcanic Zone (TVZ) has produced normal faulting oriented NE-SW throughout the region (Bignall et al, 2010), perpendicular to the orientation of the connection between Wairakei and Tauhara (Figure 1). Figure 2: 3D geological model of the Wairakei system. Waiora Formation in pale blue and pink, Tahorakuri Formation in brown and rhyolite lavas in red. Wairakei and Tauhara have distinct pressure-depth profiles at shallow depths (Figure 3). While no deep drilling has been completed at Tauhara, projection of the shallow pressure gradient predicts intersection with the Wairakei pressure gradient around minus 2500mRL, implying a permeable connection around this level. Figure 3: Pressure-depth profiles for Wairakei and Tauhara fields. 3.0 GEOLOGIC SETTING The two major geologic formations relevant to this study are described below, from Bignall et al (2010) and the stratigraphic relationship between the formations is illustrated in Figure 4. 3.1 Waiora Formation This is a thick volcanic sequence of nonwelded/welded ignimbrite, tuff and breccia, with interlayered mudstones and siltstones, containing both rhyolite and andesite lavas. The current understanding of permeability in the Waiora Formation is that is it is controlled by flow unit boundaries, particularly rhyolite lava boundaries. 3.2 Tahorakuri Formation This is a pumiceous lithic tuff with intercalated partially welded ignimbrite. The tuff contains pumice, rhyolite lava and siltstone. Minor occurrences of the Waikora Formation greywackepebble conglomerate are intercalated with the Tahorakuri Formation. Figure 4: Cross-section in the vicinity of WK317 modified from Milicich et al (2010). 4.0 ACOUSTIC FORMATION IMAGING 4.1 Data acquisition The AFIT tool is an acoustic borehole televiewer that is capable of operation in conditions ≤300°C. Developed by Advanced Logic Technology (ALT) in Europe it is operated in New Zealand by Tiger Energy Services (TES). As the AFIT tool is lowered and raised in the well an acoustic transducer emits a sonic pulse. This pulse is reflected from a rotating, concave mirror in the tool head, focusing the pulse and sending it out into the borehole. The sonic pulse travels through the borehole fluid until it encounters the borehole wall. There the sonic pulse is attenuated and some of the energy of the pulse is reflected back toward the tool. This is reflected off the mirror back to the receiver and the travel time and amplitude of the returning sonic pulse is recorded. Through the use of the rotating mirror (≤5 rev/sec) 360° coverage of the inside of the borehole wall can be obtained. 4.2 Data set acquired Planar geological features such as fractures can be observed as sinusoids on the final imaged data set (Figure 5). Accelerometers and magnetometers within the tool allow accurate structural measurements (strike and dip) to be obtained using processing software RECALL. Further characterization of geological features (e.g. high or low amplitude, fracture density, fracture aperture) is also carried out using this software. The final dataset obtained is a spreadsheet including fracture depth, type, dip, dip direction and aperture. Figure 5: Example of AFIT amplitude response with sinusoidal intersection of fracture plane with wellbore. 4.3 Filters applied for this study Before attempting to correlate fractures with the permeable zones from the completion test a number of filters are applied to the dataset in order to ensure that only fractures potentially open to fluid flow are included. 4.3.1 Confidence filter Fractures in the dataset are classified as low confidence if their shape or existence is unsure. To lend a greater degree of confidence to the conclusions of this study, the low confidence fractures are filtered from the dataset. 4.3.2 Amplitude filter Fractures with a low amplitude signal (dark) on an acoustic image are often interpreted as open. However these dark fractures may also be filled with sulfide minerals and so close attention is paid to alteration geology to distinguish these from other dark fractures. A fracture with a high amplitude signal (bright) is often interpreted as a closed fracture as the high amplitude signal can be attributed to a hydrothermal mineral fill. For the purposes of this study all high amplitude fractures are filtered from the dataset. 4.3.3 Azimuth filter The maximum horizontal stress in the Taupo Volcanic Zone is generally oriented NE-SW as the orientation of the extension in this rift basin (and hence the minimum horizontal stress) is oriented NW-SE. The exact maximum horizontal stress orientation Shmax at the well can be determined directly from measurement of drilling induced tensile fractures (DITF, Figure 6) observed on the AFIT image. The orientation of Shmax ranges between 035 and 045o in the Wairakei wells imaged to date (Table 1). Table 1: Summary of Shmax orientations Well Shmax orientation WK317 045o