Conventional Hydrothermal Resources Compared with Alternative Geothermal Resources
Large scale commercial developments of geothermal fields for heat and/or electricity generation have been based on hydrothermal resources in New Zealand. However, internationally there is now interest in the development of a wider range of resources, many of which are at the demonstration stage or have undergone a measure of development, though at subsidised levels.
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, liquefied, converted to methanol, or to 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 is as yet sub-economic at currently 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.
There is a general view in New Zealand that our hydrothermal systems will be more attractive commercially than any of these alternatives, because of the accessible energy at relatively shallow depths and the success rate at which productive wells are drilled, all of which work together to reduce the total project cost of wells. However, this view should be tempered by the recognition that the prime hydrothermal systems are limited in location, that demand for energy exists outside these areas and that large scale heat demand can be an adequate commercial driver for investment in such projects. Within the next 20 years most of our prime available hydrothermal resources will be under development, after which developers will be looking to the next tier of possible projects. However, there are some immediate prospects for development for large scale heating or cooling purposes.
The following graph compares temperatures of a range of alternative geothermal developments around the world with the Boiling Point for Depth (BPD) curve often encountered at shallow depths in New Zealand fields. Also shown on the graph are some of the gradients observed in several New Zealand fields which were considered disappointing at the time, but which in fact compare favourably with many of these alternative resources world-wide.
The development of these alternative resources has often been strongly influenced by people with an oil and gas background rather than a geothermal background. The following table makes some comparisons between the respective operating systems.
Table: Comparison of Operating Systems for Resource Development
|Wells target maximum temperatures||Wells target temperatures less than or equal to 200°C because of present pump limitations|
|Drill with mud through shallow levels then water or air through the reservoir to avoid mud damage in the formation||Drill with mud all the way with a view to post-treatment to overcome mud damage|
|Well casings are selected assuming BPD conditions||Well casings based on Oil & Gas design|
|A large intervals of the bottom hole is left accessible via a slotted liner||The well may be open hole at base or more recently solid-cased to the bottom and only limited sections perforated to better control initiation of hydraulic fracture stimulation intervals to maximise production/injection rates and focus on the hottest section|
|There will generally be little attempt at enhancement of production||Production is a strong function of enhancement techniques and/or well pumping|
|Completion tests consist of Temperature/ Pressure/Spinner (TPS) runs at various injection rates then heating over a period of several weeks followed by output testing||Testing regime may use TPS but also consists of typical oil and gas wire line suite to identify zones more amendable to hydraulic stimulation|
|Long established principles for material selection||Material selection has not always benefited from geothermal experience|
|Often use directional wells to target specific structures||Often use vertical wells, broader targets|
As ideas cross-pollinate between the two operating systems it is expected that both systems and the associated resource developments will benefit. The fledgling Australian geothermal industry is proving to be an interesting melting pot of ideas by virtue of its location, whereby the mature geothermal knowledge and experience is merging with EGS concepts brought over from Europe