PROGRESS IN GEOTHERMAL ENERGY DEVELOPMENT, COOPER BASIN, SOUTH AUSTRALIA

Abstract

Geodynamics Limited is nearing the completion of its ‘proof of concept’ hot fractured rock (HFR) program to extract superheated hot water for electricity generation from granite buried beneath the Cooper Basin. Difficult drilling conditions were discovered in the target granite when the Habanero–1 well penetrated permeable sub-horizontal fractures at more than 4,000 m depth. The well was completed at 4,421 m with overpressures in the fractures around this depth exceeding pressures projected from a hydrostatic gradient by more than 5,000 psi. The static rock temperature at the bottom of the well is about 250°C.

The overpressures assisted in the development of the world’s largest underground heat exchanger, a volume of rock more than 0.7 km3 defined by more than 11,700 microseismic events located on-site during the injection of 23 ML of fresh water into the granite fracture network. The horizontal heat exchanger is more than 2 km north–south, more than 1 km east–west and more than 300 m thick. During its development there was no evidence of direct upwards growth towards the sedimentary cover, which is at about 3,700 m, though a small number of events were observed above the main cloud of events. From production logging surveys, a major fracture at a depth of 4,254 m is interpreted to have taken most of the flow during the injection.

The second well (Habanero–2) was located 500 m southwest of the first. Before intersecting a major fracture, interpreted to be an extension of the dominant fracture in Habanero–1, it was drilled to a depth of 4,325 m. At this depth, total drilling circulation losses were encountered which were only partially overcome with the pumping of calcium carbonate lost circulation material. During the operation the lower 245 m of the drill stem was irretrievably lost, and the well was subsequently sidetracked to a total depth of 4,358 m, just below the main fracture.

Flow and circulation testing between the two wells in early 2005 is designed to demonstrate the economic potential of the far-field geothermal system and the heat exchange volume between the two wells.