MT survey kicks off in Victoria

Mining Chronicle
August, 2008 Page: 103

Hot Rock has commenced a 250-station magnetotelluric (MT) survey at its geothermal permits in Victoria. These comprise four tenements covering 18,294sq km over the most prospective areas of the Otway Basin for commercial development of hot sedimentary rock geothermal resources. They are close to electricity transmission lines, population centres, infrastructure and industry.

Previous oil and gas drilling in the Otway Basin to depths of up to 3500m has recorded temperatures as high as 143C. "Temperatures of more than 120C can be commercially viable for geothermal electricity generation, depending on well productivity, drilling depths and production pumping requirements," says managing director Dr Mark Elliott.

"We are seeking HSR geothermal resources that can be developed at lower cost than hot dry rock or enhanced geothermal systems, as HSR resources contain extensive in-situ reservoir water, are naturally permeable and are exploitable at shallower drilling depths." Over the past decade MT surveys have become the standard method for the exploration of high-temperature geothermal systems.

The method involves the measurement of naturally occurring variations in the magnetic field and concurrent electric field induced in the Earth's shallow crust through atmospheric effects. This allows for ground resistivity to be determined and, by applying calculations at different frequencies, the technique can measure the resistivity of rock units down to depths of 5km or more.

The resistivity of rocks in geothermal systems varies depending on porosity, the level of mineralisation of hot geothermal fluids contained in pores and fractures in the host rocks, and the extent and type of clay alteration produced by the interaction of the geothermal fluids with the surrounding reservoir rocks. Resistivity values determined by the MT method can therefore be used to locate and delineate geothermal systems.

The results of a MT trial carried out by Hot Rock last December in the Koroit area in the vicinity of the Killara 1 and Taralea 1 oil and gas wells showed a good correlation with measured data from these wells and interpretation of geological and seismic data with regards to the distribution and hydrology of geothermal waters at depth.

Based on the encouraging results obtained from the MT trial. HRL has now commenced a 200-station MT survey to be carried out over 250sq km of the Koroit project in GEP-8. The operations plan has been approved by the Victorian Department of Primary Industries allowing the survey to proceed. Quantec Geoscience of Canada is carrying out the survey, using its Spartan MT deep resistivity sounding equipment, which collects data over a large frequency range and long recording periods, providing accurate imaging at depths up to 4-5km.

The objective of the detailed MT survey is to provide specific information on the subsurface hydrology of the geothermal fluids. This will be combined with the results of the seismic and structural geological re-evaluation work to develop an optimum location for the drilling of the test production appraisal well. A preliminary estimation of the geothermal resources potential for Hot Rock's HSR targets in the Otway Basin has been given by the company's geothermal consultant, Sinclair Knight Merz, as 1750 megawatts of electricity over a power plant life of 25 years.

This estimation is based on the assumptions that geothermal fluid is produced to a binary cycle power plant with an average inlet temperature of 140C and an exit temperature of 70C, for a production zone thickness of 100m (within a clean sandstone rock unit known as the Crayfish Subgroup) using a recovery factor of 35 per cent and a plant availability of 90 per cent.

The significance of this preliminary estimate is better appreciated when compared to the installed geothermal power plant capacity in the US of 2544MWe (2005), the largest geothermal power user in the world - about 35 per cent of Victoria's current baseload power demand.