2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 156-8
Presentation Time: 3:00 PM

THE EFFECT OF HYDROTHERMAL ALTERATION ON DRILLABILITY - A STUDY LOOKING AT MECHANICAL SPECIFIC ENERGY, BINGHAM INDEX AND ALTERATION STRENGTH INDEX


WYERING, Latasha Deborah1, VILLENEUVE, Marlene C.1 and WALLIS, Irene C.2, (1)Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, 8140, New Zealand, (2)Mighty River Power, 283 Vaughan Road, P.O. Box 3040, Rotorua, 3040, New Zealand

Multiple equations have been used over the years to address drilling optimization and drillability, in the hope to improve the rate of penetration. The majority of these equations have been developed for the oil and gas industry, with little to major success, but none of them seem to adapt well to drilling in a conventional gas/liquid geothermal field. We present an equation that has been developed to use environmental factors for improving the drillability of hydrothermally altered lithologies in the geothermal industry. The Alteration Strength Index (ASI) equation was developed to use drill cuttings and known geological parameters that impact rock strength to predict a range of rock strengths for the lithologies being analyzed.

The drilling parameters in the 17inch section of NM8 in the Ngatamariki geothermal field, New Zealand, were held nearly constant. This means we have been able to conduct a drillability analysis with confidence that environmental factors are the key influence on rate of penetration. This rate of penetration was used as an analog to rock strength and has been compared to two oil and gas equations, MSE (Dupriest & Koederitz, 2005; Dupriest et al., 2005) and Bingham Index (Bingham, 1964-1965), as well as our ASI equation.

The fastest and slowest drilled of the six lithologies encountered in the 17inch section were, respectively, the tuff breccia (6.81 m/hr) and welded ignimbrite (4.56 m/hr). The Bingham index and ASI predicted the tuff breccia and welded ignimbrite to be the weakest and strongest of the drilled lithologies, while MSE predicted tuff breccia and the veined tuff breccia, to be the weakest and strongest. Our results show that the ASI equation is capable of predicting a range of rock strengths for a given lithology that can be used to improve the understanding of how hydrothermal alteration affects drilling and help improve drill bit selection.

Session No. 156

T98. Environmental and Engineering Geology Student Research Competition
Monday, 20 October 2014: 1:00 PM-5:00 PM
213 (Vancouver Convention Centre-West)
Geological Society of America Abstracts with Programs. Vol. 46, No. 6, p.395
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