Phase 2: Exploration

The purpose of the Exploration Phase is to cost-effectively collect new geoscientifi c data to minimize uncertainty related to estimates of key reservoir parameters (temperature, depth, extent, permeability, etc.) prior to the Test Drilling Phase. Exploration may start at a regional level and progressively focus on smaller target areas as data reveal the most attractive locations. Exploration typically begins with gathering new samples and data from existing surface manifestations (and perhaps wells if they are available). Exploration then proceeds to surface and sub-surface surveying using geological, geochemical, and geophysical methods. Environmental studies during the Exploration Phase establish key background (or baseline) information. Some countries require detailed environmental impact statements as an early outcome of any exploration program.

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For most projects, the decision to mobilize and contract equipment for the Test Drilling Phase is a signifi cant fi nancial commitment. For this reason, uncertainties about the characteristics of the drilling target and conditions should be reduced as much as cost-effectively practical during the Exploration Phase. In order to make an exploration program cost effective while reducing uncertainty, this typically begins with relatively low cost regional reconnaissance methods and then proceeds to more complex and expensive surveys over smaller identifi ed areas of interest. Table 3.1gives some examples of surveying techniques often used in the Exploration Phase. Chapter 4 provides a more comprehensive list.

GEOLOGICAL AND

• Collect samples from all thermal features for analysis

• Geothermometry (water and gases) (Ellis, 1979;

Giggenbach and Goguel, 1989)

• pH + electrical conductivity

• Flow rate and temperature of fl uids discharging from active thermal features

• Gravity and magnetics (Pálmason, 1975)

• Electrical resistivity

• Magnetotellurics (Ander-son et al., 2000)

• Passive seismic monitor-ing

• 2D and 3D seismic refl ec-tion

• Temperature gradient and conductive heat fl ow

Of all these techniques, drilling for temperature gradient (or conductive heat fl ow) measurements is usually one of the most expensive activities. Such drilling may be well justifi ed, however, if surface geological, geochemical and geophysical surveys have been completed and substantial uncertainty remains about the nature of the target reservoir. Temperature gradient drilling might then provide a cost-effective approach to risk mitigation by obtaining additional subsurface information about the temperature and extent of the potential reservoir (Coolbaugh et al., 2007).

Table 3.1.

44 BEST PRACTICES GUIDE FOR GEOTHERMAL EXPLORATION

By the end of the Exploration Phase, suffi cient data should have been collected and analyzed to prepare a pre-feasibility study and select sites and targets for the Test Drilling Phase, in which the fi rst deep wells are drilled directly into the predicted reservoir.

Figure 3.1shows an example of a time line for the preliminary survey and exploration phases of a project. However, timelines can vary signifi cantly between projects: if any obstacles to a smooth execution of the program are encountered; if the geological setting is complex; or if the interpretation of geoscience results remains ambiguous after the initial planned exploration program (requiring additional exploration). The timeline can stretch beyond two years.

2 4 6 8 10 12 14 16 18 20

Figure 3.1.

Example of a timeline for a well-planned and effective preliminary survey and exploration work program in a relatively straight forward geothermal project

Time (Month)

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3.3.1. Conceptual Model

A conceptual model (discussed further in Section 4.4) is a schematic representation of the current best understanding of a geothermal system, consistent with all known data and information. The fi rst iteration of a conceptual model for any new project might be little more than a generic representation of the type of geothermal play under investigation.

While the initial conceptual model is expected to be crude or incomplete, it is important to have an initial model that can be refi ned and improved as the exploration, test drilling and fi eld development phases proceed and more data become available. During the Exploration Phase, the conceptual model of the geothermal system is continually updated as new data are gathered. The model needs to contain suffi cient geological, hydrological, and tectonic information to allow a fi rst pass estimate of reservoir depth, temperature, and extent. This is used during the Test Drilling Phase to target production scale wells toward lithological units and/or geological structures with the highest probability of delivering commercial fl ow rates of geothermal fl uids.

3.3.2. Non-Technical Data Compilation

At the completion of the exploration program, the developer will be at a decision point, whether or not to proceed with the project. This is the time to update or confi rm current information (Cassel et al., 1981) relating to these factors:

• Power market and possible PPAs

• Purchase agreements for district or greenhouse heating

• Infrastructure issues (roads, water, communication, transmission)

• Resource ownership issues

• Environmental and social issues

• Institutional and regulatory frameworks

• Issues relating to political and fi nancial stability

3.3.3. Pre-feasibility Study

The fi nal product of the Exploration Phase is a “pre-feasibility study” – an assessment of all the technical and non-technical data within the framework of a risk-weighted fi nancial model of the project prior to committing to the Test Drilling Phase. This is a very signifi cant milestone since proceeding to test drilling involves major fi nancial commitments to the project. This is at a time when uncertainty about the reservoir characteristics is still high and the expenditure curve is steep. The pre-feasibility study should recommend either for or against continuing the project after considering all relevant factors.

46 BEST PRACTICES GUIDE FOR GEOTHERMAL EXPLORATION