Geothermal resources are underground reservoirs of hot water at varying depths and temperatures. Wells can be drilled into a geothermal reservoir to tap the fluids, bringing them to the surface for a variety of uses (see: What can geothermal energy be used for?)
But how do we “see” beneath the surface? How do we measure properties of the deep underground to understand the conditions there and what might be happening?How can we forecast what will happen in the future if the fluids are withdrawn from deep underground and returned?
To understand a geothermal system and assess its energy potential, geoscientific measurement and interpretive techniques, exploratory drilling and testing, and mathematical modelling are undertaken.
Geothermal exploration often commences with surface-based geoscientific surveys and interpretive assessment. Three main disciplines are applied on the surface to explore for geothermal resources, including geology, geochemistry and geophysics.
Geological field methods map the surface rocks types and underlying structures (e.g. faults and folds which indicate crustal movements that have shaped the present terrain). To obtain rock samples from deep in the crust without having to drill deep underground, it is sometimes possible to find these rock types on the surface at other locations. Two examples being used in the GNG programme are:
Geological information is combined with information on the location, distribution and type of geothermal surface expressions and features. Geothermal surface features (e.g. hot springs, steam vents) can be surveyed, and sampled for analysis of their fluid geochemistry. Underground temperatures are estimated using chemical geothermometers, which are calculated using the relative concentrations of particular chemical species. The species present also provide insight into the underground fluid conditions, such as pH.
Geophysical methods investigate the physical properties of the deep to shallow crust. In the GNG programme, we are using data and interpretations from seismic tomography, magnetotellurics and magnetics to search for and identify possible locations of supercritical geothermal resources. Geophysical techniques can also assess variations in properties, such as rock density and gravitational strength.
Drilling exploration wells is essential for providing validation of the indirect geoscientific assessments done at the surface. During drilling and well testing, rock samples are collected, and temperature and pressure measurements are made down the hole. Wells are discharged to assess the productive capability of the well, and fluid samples are retrieved for chemical analysis. These rock and fluid samples can be assessed to characterise the subsurface geology, the geochemistry, and the physical conditions of the reservoir to gain a thorough understanding of the geothermal system.
Geothermal drilling technology has developed since the late 1940s and has greatly enhanced our understanding of New Zealand’s geothermal resources. The early geothermal drilling used rigs designed to drill wells for shallow geotechnical investigations or prospecting for mineral resources. Over time, the drilling depth has increased with improvement of technology and as equipment has become available in New Zealand. Geothermal wells are now routinely drilled to 3500 metres. Looking forward, we anticipate that exploratory supercritical wells will be drilled to 5 – 6 kilometres depth.
Maps, models and interpretations are developed from the spatial datasets, aiming to visualise the underground, and understand the subsurface processes. Reservoir numerical models and simulations provide forecasts on the energy and fluid that might usefully be extracted from the reservoir, and its energy capacity, as well as the effects of fluid withdrawal that might be experienced in the reservoir, and by the surrounding environment (at depth, in the shallow subsurface and at the surface). Both production and injection operations can be modelled in this way.
There will always be more to learn about geothermal resources – deeper wells will continue to reveal new knowledge, and advances in computational capacity will open up new modelling capabilities that continually refine our ideas and augment our understanding.