Big Bore Well Drilling in New Zealand - A Case Study

Abstract

A typical standard geothermal well in New Zealand is a 9-5/8” cemented production or injection casing completed with a 7” perforated liner in an 8.5” hole. In 2006, one geothermal operator began looking at big bore completions as a way to improve the cost per megawatt of its drilling operations by reducing the number of wells required per field. After an initial review, it was concluded that a big bore well completed with 13-3/8” cemented casing and 9-5/8” perforated casing could increase the productivity or injectivity of wells by 66% for a cost increase of 17% and minimal additional risk. Starting in 2007, big bores were included in the drilling program in the Mokai, Kawerau and Rotokawa geothermal fields.

Nine standard wells and twelve big bore wells, both production and injection, were drilled from 2007 to the beginning of 2009. The results of the drilling and testing program show that big bore wells can successfully reduce drilling costs as well as the number of wells required, but are dependent on the characteristics of the field.