UPFLOW ALONG A BASEMENT FAULT REVEALED BY GEOTHERMAL NUMERICAL PRESSURE TRANSIENT ANALYSIS

Abstract

The well BR49 was drilled in 1995 into the basement of the East Bank of Ohaaki Geothermal field, New Zealand. Recent pressure transient datasets measured in 2015 reveal a distinctive linear flow response. This is strong evidence that fluid flow in BR49 is controlled by flow along a fault plane in the otherwise-impermeable greywacke basement, which is highly significant for the conceptual model of this field.
It has not always been clear whether there are one or two upflows at Ohaaki. The main upflow under the West Bank is certain, however the presence of a secondary upflow under the East Bank has been a subject of ongoing debate. In the most recent conceptual model the greywacke basement has low permeability. A major liquid upflow is located in the northwest part of the field, which flows up through the West Bank and laterally to the East Bank; and a secondary upflow is located in the southeast underneath the East Bank. In spite of the implication for fault-related permeability in the greywacke, direct evidence for faults have been elusive. This study presents cumulative reservoir engineering, numerical modelling and geochemistry evidence, all of which supports the conclusion that BR49 must, in fact, be connected to a deep fault-controlled fluid source.
This study focuses on pressure transient analysis (PTA) results from well BR49 which provide direct evidence for a permeable fault in the basement. This powerfully demonstrates the value of performing PTA early, during completion testing, so the results can be used as inputs to the conceptual model. It is also a demonstration of the numerical PTA framework.