Petratherm said yesterday it had now successfully completed drilling Paralana-1 in northern South Australia to the target depth of 1807m.

The drilling program was executed to plan and achieved all key time, cost and hole-depth targets.

Temperature gradients already recorded between surface and 491m during initial drilling of Paralana-1 in September 2005 were around 81.5C per kilometre, among the highest reported in Australia, the company said.

“Measurement of the temperature gradient at the new bottom-hole depth of 1807m will be taken in about three weeks once the hole reverts backs to its normal temperature equilibrium with the surrounding rock strata,” Petratherm managing director Terry Kallis said.

“This new temperature gradient data will enable more detailed and accurate assessment of the quality of the thermal resource at Paralana.

“The rate of temperature increase at depth is the critical factor for a hot rocks well to be part of economical electricity generation and Paralana-1 has so far been in line with our expectations for resource temperatures of 200C or more at approximately 3.5 kilometres depth.”

Geothermal energy is used to superheat water circulated through hot rock at depth, returning to surface as steam where it drives electricity generating turbines.

Petratherm has developed, in partnership with university researchers, a specific exploration model to select the optimum drill area for identifying hot granites covered with high insulating rocks at relatively shallow depth. This compares with the more conventional approach of using data from old oil and gas wells to determine a hot rocks drill site.

Petratherm has identified a depth of 3.5km as its benchmark for economically viable electricity generation under current market conditions.

Paralana-1, located 130km east of South Australia’s main electricity grid infrastructure at Leigh Creek and 11km from the existing Beverley Uranium Mine, has been selected using this exploration strategy.

“The well has been drilled through sequences of highly variable Cambrian aged rock strata and then into a representative section of more homogenous older Adelaidean rock strata below,” Kallis said.

“While we await the capture of temperature data now for this lowest section of the well, we are highly encouraged by the fact that the rock strata in this lower interval is predominantly fine-grained shale and is expected to have good insulating properties.

“Such insulation should maintain a high temperature gradient at the deeper drilled depths.”

Paralana-1 has been secured with steel casing to ensure long-term stability and will be maintained for the future as a seismic observation well, to monitor fracture stimulation of the deep heat reservoir, should such be required.

If results from the completed drilling are positive, Petratherm will then consider drilling two new deep wells to establish the modelled thermal resource and provide input to a feasibility study for the production of large-scale baseload electricity generation.