In this video, we take a look at geothermal resource assessment. In particular, we'll be looking at global geothermal resources, types of geothermal resources, stages of exploration including surface manifestations, surface geochemistry, subsurface geophysics, and exploratory drilling. We'll talk about assessment personnel, there a lot of different skill sets that are needed for hydropower resource assessment. We'll take a look at an assessment risk profile, which is pretty unique for geothermal energy. This map shows geothermal resources around the world. As you can see, and as we talked about in the previous video, geothermal resources exist where tectonic plates collide. This is happening along the west coast of South America and North America, along the southern margins of Europe as well as along southern margins of most of Asia, India, Indonesia, and down in New Zealand. If you live in the white areas, there's very little possibility that you're going to have any geothermal resources to speak up. Geothermal resources used to generate electricity usually are found between 500 and 3,000 meters underground. There's dry steam, temperatures of 150-500 degrees centigrade. These are natural subterranean steam, it's existing as steam underground. There's flash steam, this is pressurized water that flashes to steam on the surface. These usually also have temperatures of 150-500 degrees centigrade. Then there's hot water that can be used to generate electricity between 100 degrees centigrade and 170 degrees centigrade. This is liquid subterranean water. It comes up as water. It's the most common resource used for geothermal energy because it's again, most widely available. Then finally, and this is a new developing technology, is hot dry rock. These are areas where we have a lot of heat down in the ground but no water. We pump water down and bring it up hot and use it to generate electricity. Stages of geothermal exploration are several and these are important. This is a hierarchy of geothermal assessment activities. First off, we look for surface manifestations such as hot water or steam venting. We'll talk about that. Geochemistry, which is chemical analysis, geophysics, which is subsurface characteristics, and exploratory drilling, this is the exploration by drilling wells. Typically we do these in this order. We'll see why because of costs. First off, we look for surface assessments. This could be volcanoes. Now obviously we can't tap into a volcano directly for geothermal but around the area of active volcanoes or dormant volcanoes is often where we find excellent geothermal resources. We may look for hot springs, geysers, fumaroles, mud pots and pools, and just ground steaming. All of these are manifestations of subsurface geothermal energy. When we find surface manifestations of geothermal energy or undertake chemical analysis of water and steam, we'd look at the temperature, look at the chemical composition, ratios of chemical isotopes, dissolved solids, presence of mercury, and CO_2 concentrations. All of these can tell us something about the underground geothermal resource. You can see that this is really a manual activity from the pictures to the right. If our surface analysis of a potential geothermal site is favorable, we'll next go to geophysics. This is the exploration of subsurface characteristics of a perspective site. We will use seismology. This is the study of reflected subsurface sound waves created by on-surface explosives or thumpers. Gravimetry which is the measure of small changes in the force of gravity, and magnetotellurics. It's quite a mouthful where we infer subsurface electric resistivity anomalies from surface geomagnetics. Again, another mouthful. But when you put all these together, you can get a very interesting picture of what's happening underground. In this case, we can see a big salt dome and the greatest, it looks like a gray mountain. But then we also get a picture of thermal activity closer to the surface. Next, we undertake exploration drilling if things are looking positive. This is explorations of readings and results from exploration wells. The purpose of drilling is to find temperature gradients in the ground, find thermal pockets, and identify other geothermal characteristics. The types of wells that we drill are first geothermal gradient holes or TGHs. These are very small diameter and go up to 500 meters deep. Next are exploration or slim hole wells. They are 8-16 centimeters in diameter, and they can go as deep as 4,000 meters. Then finally, we can drill some initial wildcat production wells when things are looking really positive. These are 20-35 centimeters in diameter and can go down to 4,000 meters deep and deeper. Now, there's a very big difference in the costs of these different wells. TGH wells are relatively cheap and they are small diameter and they don't go down very far. Slim wells and more expensive and full-scale commercial wells are yet more expensive. You can see there's a big cost difference in the types of wells and the types of exploration that we do here. Well, what outcomes do we expect from all of this geothermal assessment? Well, we can understand the depth of the resource, the temperature and pressure of the resource, the type of resource being steam, liquid or some mix, the available volume, how much is there, rock permeability, this tells you how quickly the well can replenish itself with new water, and the chemistry of what comes up out of the well, which is important as far as operations. This all takes a lot of personnel. Takes geoscientists, geologists, geochemists, and geophysicists, and engineers, drilling engineers, reservoir engineers. All of these people have specialized roles that are needed to fully assess the geothermal resource. Well, given all of the types of exploration and the people involved, geothermal assessment is costly and risky. Here's a graph that charts failure risk against relative cost. Early on, there's a high degree of a high probability of failure. As we do more and more exploration, it becomes more and more costly. But the risk goes down until finally we prove out the site and drill for production. This looks very much like probably the risk profile for fossil fuel exploration, oil and gas. There's a lot of early exploration, it doesn't work out, but over time we find the resources we need and have a profitable venture. In summary, regarding geothermal resource assessment, we've looked at global geothermal resources, the types of geothermal resources that exist, the stages of explorations of its manifestation, surface geochemistry, subsurface geophysics, and exploratory drilling. We've looked at assessment personnel required, and we've taken a look at the risk profile for geothermal exploration. In the next video, we'll be taking a look at bioenergy in all of its manifestations. We'll see you there.
