Fractures in an Andesite Lava Flow at Mt Ruapehu and its Implications for Fracture Modelling in the Rotokawa Geothermal Reservoir, New Zealand.

Cecile Massiot and David D. McNamara and Andrew Nicol and Garth Archibald and John Townend. ( 2014 )
in: Proc. 34th Gocad Meeting, Nancy

Abstract

Fluid flow in the high-temperature (320◦ C), andesite-hosted Rotokawa geothermal reservoir (New Zealand) is largely controlled by fractures and faults. In preparation for developing fracture models and aid with the targeting of permeable fractures, this paper presents the analysis of fractures in a surface analogue on the Mt Ruapehu volcano, New Zealand, using two complementary datasets: (1) a 100 m long scanline where fracture location, orientation, and length were measured by hand, and (2) a terrestrial laser scanner (TLS) survey acquired over the entire outcrop. The majority of fractures form weakly clustered sub-vertical cooling joints, with six dominant dip directions identified on the TLS survey, suggesting the hexagonal shape typical of columnar joints in basaltic lava flows. The scanline survey presents only three of these orientations, which is inferred to be due to the sampling of different fractures. Preliminary analysis of fracture length on the scanline survey highlights the high degree of fracture connectivity and suggests a log-normal distribution. A subset of sub-horizontal, highly clustered fractured is observed only on the scanline and may be linked to intra-flow layering. This paper also summarises findings from the statistical analysis of fractures observed in acoustic borehole televiewer (BHTV) logs and drill-cores from the Rotokawa andesites. There, fractures are predominantly steeply dipping and striking NE–SW, parallel to the maximum horizontal compressive stress direction and the regional structural trend. Fracture width is best fitted by an exponential distribution. The fracture spacing of the main fracture sets follow a log-normal, powerexponential or gamma distribution, apart from the N-S striking fracture set which is best modelled by a power-law. A change of fracture spacing distribution is noted at c. 1–5 m, with exponential and log-normal best fitting lower and higher spacing, respectively, and may correspond to the threshold at which fracture interaction occurs. The spacing and width distributions differ from the power-law distribution found in crystalline-hosted geothermal fields. These distributions indicates the presence of mechanical layers within the Rotokawa andesites, either associated with unidentified faults, or with the intercalation of permeable, mechanically heterogeneous breccia layers and massive lava flow interior. Though fractures display a preferred strike consistent with them being of tectonic origin, their geometries are likely to have been affected by fractures formed during the emplacement and cooling of the lava flow. Integrating these observations will be fundamental for developing reliable fracture models at the Rotokawa Geothermal Field, and in other volcanic-hosted geothermal reservoirs.

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BibTeX Reference

@INPROCEEDINGS{Massiot1GM2014,
    author = { Massiot, Cecile and McNamara, David D. and Nicol, Andrew and Archibald, Garth and Townend, John },
     title = { Fractures in an Andesite Lava Flow at Mt Ruapehu and its Implications for Fracture Modelling in the Rotokawa Geothermal Reservoir, New Zealand. },
 booktitle = { Proc. 34th Gocad Meeting, Nancy },
      year = { 2014 },
  abstract = { Fluid flow in the high-temperature (320◦ C), andesite-hosted Rotokawa geothermal reservoir (New Zealand) is largely controlled by fractures and faults. In preparation for developing fracture models and aid with the targeting of permeable fractures, this paper presents the analysis of fractures in a surface analogue on the Mt Ruapehu volcano, New Zealand, using two complementary datasets: (1) a 100 m long scanline where fracture location, orientation, and length were measured by hand, and (2) a terrestrial laser scanner (TLS) survey acquired over the entire outcrop. The majority of fractures form weakly clustered sub-vertical cooling joints, with six dominant dip directions identified on the TLS survey, suggesting the hexagonal shape typical of columnar joints in basaltic lava flows. The scanline survey presents only three of these orientations, which is inferred to be due to the sampling of different fractures. Preliminary analysis of fracture length on the scanline survey highlights the high degree of fracture connectivity and suggests a log-normal distribution. A subset of sub-horizontal, highly clustered fractured is observed only on the scanline and may be linked to intra-flow layering.
This paper also summarises findings from the statistical analysis of fractures observed in acoustic borehole televiewer (BHTV) logs and drill-cores from the Rotokawa andesites. There, fractures are predominantly steeply dipping and striking NE–SW, parallel to the maximum horizontal compressive stress direction and the regional structural trend. Fracture width is best fitted by an exponential distribution. The fracture spacing of the main fracture sets follow a log-normal, powerexponential or gamma distribution, apart from the N-S striking fracture set which is best modelled by a power-law. A change of fracture spacing distribution is noted at c. 1–5 m, with exponential and log-normal best fitting lower and higher spacing, respectively, and may correspond to the threshold at which fracture interaction occurs. The spacing and width distributions differ from the power-law distribution found in crystalline-hosted geothermal fields. These distributions indicates the presence of mechanical layers within the Rotokawa andesites, either associated with unidentified faults, or with the intercalation of permeable, mechanically heterogeneous breccia layers and massive lava flow interior. Though fractures display a preferred strike consistent with them being of tectonic origin, their geometries are likely to have been affected by fractures formed during the emplacement and cooling of the lava flow. Integrating these observations will be fundamental for developing reliable fracture models at the Rotokawa Geothermal Field, and in other volcanic-hosted geothermal reservoirs. }
}