Geopressured resources are examples of one such alternative resource, often associated with natural gas or other hydrocarbons. They are a normal phase of basin evolution. They have three energy forms: thermal, hydraulic and methane gas. The thermal energy can be converted to electricity using binary cycle plant or used directly for heat. The hydraulic energy can be converted to electricity with a hydraulic turbine. Dissolved methane gas can be separated and sold, burned, compressed and liquefied, or converted to methanol or electricity by fuelling a turbine. Such resources can be encountered at depths greater than 4kms. The US Department of Energy has committed US$5M to a Louisiana demonstration project. There are no known opportunities in New Zealand.
Hot Sedimentary Aquifers (HSAs) have been developed in Europe and are part of the suite of proposed developments in Australia. There are likely to be opportunities for applications in New Zealand. Drilling for oil and gas in our sedimentary basins has established the presence of a normal natural thermal gradient of the order of 30°C/km in basins located in Taranaki and southwards. As some wells have been drilled to 5km, temperatures exceeding 150°C have been measured in New Zealand outside the traditional hydrothermal resources and this is adequate for a range of heat applications and for electricity generation. Generation of electricity is, as yet, sub-economic at current electricity prices, even using existing oil wells.
Enhanced or Engineered Geothermal Systems (EGS) include those systems previously labelled “Hot Dry Rock”. Developments are generally based on environments in which the thermal gradients are better than the standard 30°C/km, assisted by the proximity of radioactive rock (e.g. radiogenic granite) or possibly magma, which increases the local heat flow in the upper crust. EGS is of particular interest in the US and Australia from a research and development perspective. Developments require the creation or support of a limited reservoir through a range of enhancing techniques. EGS targeting elevated crystal temperatures and without the restriction of locating discrete zones with high permeability, typically occupy large areas in excess of 500 km2 (compared with New Zealand hydrothermal systems with an area of 5-40 km2). Whilst they can be considered low grade, their large size necessitates the total thermal energy in place is an order of magnitude greater than a typical hydrothermal system.