Retrospect Resource Assessment Study Using Experimental Design and Response Surface Methodology: the Ohaaki Geothermal Field, New Zealand

Abstract

The Ohaaki geothermal field celebrated its 30 years of production in 2018. The field has suffered from several production issues, including the inability of the resource to meet the installed generation capacity at a sustainable production level for a certain prolonged period. Numerical reservoir simulations have been instrumental in understanding the shortfall in the supply of steam and in predicting the future deliverable power of the field at various operating scenarios. Given the substantial amount of production information, it would be interesting to look back and examine how the field would have behaved if it were assessed differently during early exploration. In this study, the performance of the Ohaaki geothermal resource is investigated using three different approaches: the volumetric stored heat method, deterministic prediction from the calibrated natural state model and experimental design (ED) and response surface methodology (RSM). A probabilistic distribution of power potential will be generated both from the volumetric method and ED/RSM approach by applying Monte Carlo simulation to the equation and polynomial model, respectively. The results suggest that; among the three competing models, the ED/RSM approach yields the most conservative MWe estimate suggesting that the Ohaaki geothermal field has a power potential of 66 to 69 MWe for 50 years of operation. This reveals the potential of ED/RSM as a good risk-averse approach and as a useful tool in estimating the sustainable development of a geothermal resource in an efficient and reliable manner.