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Tuesday, 29 July 2014

Measurement standards in the geothermal industry

Historically, many of the early geothermal projects were developed in a non-systematic manner. It came with a time, that the stages and phases of a development process of geothermal resources have become more clearly defined. Even today there are differences in methodologies and techniques between different countries and agencies.
  
There are seven phases in the process of developing geothermal projects, including: preliminary survey, exploration, test drilling, project review and planning, field development, power plant construction, commissioning and operation, however this article will focus only on the two first phases, in which measurements data are the most important.
 
The Preliminary Survey Phase involves a work program to assess the already available evidence for geothermal potential within a specific area. The initial surveying involves a literature review of geological, hydrological or hot spring/thermal data, drilling data, anecdotal information from local populations, and remote sensing data from satellites, if available.
Most countries have existing databases of geological and hydrological data.  These are useful for guiding early geothermal surveying and exploration.  Information should be also gathered on legal issues, like the national, regional and local regulations that might allow or restrict access for exploration activities.
Basic information for the preliminary survey covers: the power market and possible power purchase agreements or feed in tariff, infrastructure issues (roads, water, communication, transmission), resource ownership issues, environmental and social issues, institutional and regulatory frameworks, issues related to political and financial stability and information from available literature on the resource itself.

The second phase Exploration, may start by looking at a regional level and, as more data is gleaned, focus to a more localized analysis. Exploration typically begins with gathering data from existing, nearby wells and other surface manifestations, and goes on to surface and sub-surface surveying using geological, geochemical, and geophysical methods. Surveying techniques used in this phase are following: 

    Surface studies - gathering local knowledge, locating active geothermal surface features, assessing surface geology.
    Geochemical surveying - geothermometry, electrical conductivity, pH, flow rate of fluids from active features, soil sampling.
    Geophysical surveying - gravity, electrical resistivity, magnetotelluric, temperature gradient drilling, 2D & 3D seismic sampling.

By the end of this phase, sufficient exploration data should have been collected and analyzed to select sites and targets for the first few deep exploration wells.  A preliminary estimate of the magnitude of the resource, and developing initial conceptual and numerical models should also be possible at this stage.

Geological data
In the field exploration it is very important to locate and characterize all existing geothermal features within the project area and within a relevant distance from the project area.
Geological data for the project area should be presented in the form of geological maps, structural maps, stratigraphic columns, and cross sections for the project areas.

Geochemical data
Geochemical studies focus on understanding the geothermal fluid sources and flow paths and assessing potential operational issues that will come with development, such as wellbore scaling, corrosion, and concentrations of non-condensable gases. Regional CO2 gas surveys are becoming increasingly popular, because elevated CO2 at the surface may indicate the presence of permeable faults or the extent of an active geothermal system. Fluid and gas geochemical data are presented on maps, tables, drawings, and plots for the project area. 
A good outcome of the geochemistry studies would be an indication of temperature distribution within the geothermal system, a maximum temperature range for the resource, and a fluid-mixing model.

Geophysical data
There are many types of geophysical surveys that can be carried out. They include gravity surveys, temperature gradient drilling (also referred to as heat flow surveys), electrical and electromagnetic resistivity surveys (particularly magneto-telluric (MT), but there are also several others), 2D and 3D seismic techniques. Geophysical data collection points should be presented on maps with license boundaries and cross section lines clearly labeled. Maps should be provided as geo-referenced digital files or have a grid overlain on them that allows for easy geo-referencing. Gravity data should be presented as contour maps with the appropriate reduction densities indicated. Resistivity data should be presented in a similar way, with contour maps of resistivity or conductivity at a particular elevation/depth, or isoresistivity contour maps showing the elevation of a particular resistivity or conductivity value. Seismic data requires more detailed processing and interpretation than either resistivity or gravity data. For temperature gradient drilling data should be presented graphically, with a legend listing the dates that each profile (temperature log) was made.
Each geophysical survey should be carried out soundly and the acquired data should be interpreted by an experienced operator with deep insight in the region’s tectonics and geology.

Satellite imagery
More and more data from satellite and airborne sensors are becoming readily available. Many of them can be applied to geothermal exploration, like satellite or aircraft-based infrared scans, and thermal data acquired by Thematic Mapper (TM) and Enhanced Thematic Mapper Plus (ETM+) sensors on-board Landsat-5 and Landsat-7 satellites. It may be also appropriate to develop a Geographic Information System (GIS) database of potential geothermal sites based on aerial photos and satellite data. The technique may be especially useful in difficult terrain where ground access is difficult. Relevant remote sensing data can be downloaded into GIS software for integration with data compiled from surface surveys to produce detailed maps for each project area.

References:
GeothermEx Inc., USA, and Dr. Colin Harvey: "Geothermal Practices Best Practices" (PDF download)

Authors:
Aneta Żądło
Tomasz Sasin