As geothermal energy development continues to pick up steam (pun intended) globally, we want to shed light on the inception stage of development – that is surface exploration. Thorough investigation within this stage essentially determines whether to proceed with or halt further development of a geothermal project.
In one of our previous posts, we introduced the two phases of geothermal development. Within each phase, there are various steps that must be performed as pre-requisites for those which proceed. In this post, we will delve deeper into what geothermal surface exploration involves. Surface exploration is conducted to obtain information regarding the properties of a geothermal system prior to drilling.
Exploration strategies are dependent on factors that include geological setting and the temperature of the system under investigation. The detailed exploration entails extensive geological and geochemical studies, a range of geophysical techniques including gravity, magnetic and resistivity surveys. Interpretation of these integrated geoscientific studies leads to the prioritization of targets for exploration drilling programs. The application of sound scientific method and analysis during these early phases increases the probability of success with subsequent drilling and development and will reduce the cost of later stages in development and improve the cost-effectiveness of the overall project.
Surface exploration can provide information on the temperature in the geothermal reservoir, the permeability of fractures in the reservoir, the areal extent of the thermal anomaly, depth to useful temperatures, location of the up-flow zone and chemical composition of fluids. There are scales of surface exploration which can be either regional – covering a large area in order to identify possible geothermal resources and roughly assess their size, or local – concentrating on the interior of a known geothermal field in order to identify the best production sites and targets for drilling to obtain maximum energy output at minimum cost.
Geothermal surface exploration consists of many different surveys that go hand in hand to delineate the proposed system in question. It entails a multi-geoscientific process holistically aimed at defining the geometry and characteristics of the geothermal system before drilling takes place. Broadly speaking, surface exploration is approached via geological, geophysical and geochemical methods. Geochemical methods support the development of geological and hydrological models which allow conceptualization of the physical framework within which a geothermal reservoir exits. Taken together, the shape, flow field, stage of development, location of high-temperature zones and possible drilling targets can be approximately estimated.
At the onset of a geothermal exploration project, it is normally uncertain whether results will be economically, technically and environmentally feasible. Geothermal exploration therefore invariably necessitates risk. The exploration program should be designed to suit the type of resource expected, the amount of energy expected to be produced from the project and the time frame for the development. Ideally, before commencing an exploration exercise it is imperative to collect available information from previous work, topographic, geological, structural and geothermal maps of the prospect area of interest. An inception report is normally prepared after obtaining the valuable information from the reconnaissance and a detailed exploration plan subsequently developed. Below, we briefly highlight what the various methods entail.
Geology is the study of the solid earth, the materials which it comprises, the structure of these materials and the processes which act upon them over time. Geological exploration involves geological mapping (lithology), structural geology (faults, fractures, dikes), volcanology and volcanic history, hydrogeology, geo-hazards, and environmental geology. In the surface exploration phase, it establishes the tectonic setting and history, volcanological history, likely locations and types of heat sources, likely permeability structure and provides information to conduct a geological risk assessment.
Image 1: Geochemical surveillance, source: http://www.geowarn.ethz.ch/index.asp?ID=15&cat=124
Geochemistry involves the study of soil chemical anomalies. Geochemical exploration for geothermal resources involves sampling, analysis and interpretation of discharge of thermal fluids from fumaroles, hot springs and steaming grounds and temperature in the geothermal reservoir. The main objectives of geochemical studies are to characterize the thermal fluids, establish their origin, flow direction (up-flow, outflow), evaluate mixing scenarios, estimate the equilibrium reservoir temperature and determine the suitability of the fluids for the intended use. Chemical (water and steam) geothermometers evaluate subsurface temperatures based on chemical components in water and gas and discrepancies between the nature of geothermometers can provide important information about the nature of the system.
Image 2: Soil resistivity testing, source: https://atslab.com/electrical-testing/soil-resistivity-testing/
Geophysical exploration involves measuring the different physical attributes of the earth’s crust from the surface. This is done directly via thermal, electrical or passive seismic methods or indirectly via gravity, magnetics and active seismic methods. Emphasis is placed mainly on parameters that are sensitive to temperature and fluid content of the rocks, or on parameters that may reveal structures that influence the properties of the geothermal system. Geophysical exploration assists in delineating a geothermal resource, outlining a production field, locating aquifers, or structures that may control aquifers in order to site wells, or assess the general properties of the geothermal system. The sequence in which geophysical methods are applied depends to a considerable extent on the specific characteristics of each prospect.
Collectively, the strategies and techniques outlined must be performed co-dependently to ensure the authenticity of the exploration phase. Great emphasis is placed on this phase because it governs the decision to drill exploratory wells. As such, the siting of the well must be made with good confidence. This should not be confused with drilling risks being removed, but they will indeed be lowered.
This wraps us another #GeothermalFactsandStats blog post. Want to learn more about these types of surface exploration methods? Follow us on all social media platforms for more exciting and steam-filled content!
Author: Elizabeth Bullock