SeekerBlog

TR: But haven’t people been talking about greater use of geothermal energy for years now? What’s changed?

JT:…Many [energy] technologies, whether they’re renewables or nuclear power or coal or whatever it might be, need to be continually revisited and placed in context with the current state of technology. In this case, our interest in trying to go after hydrocarbons and extract hydrocarbons has developed a lot of technology in subsurface engineering that’s useful and makes geothermal worth revisiting.

MIT Technology Review interviews MIT chemical engineering prof. Jefferson Tester on the prospects for deep geothermal energy and what it will take to make it a reality. I’m familiar with the enhanced recovery techniques used in the oil industry to increase reservoir permeability, e.g., by water pressure. I think Tester is correct that this proven technology will work to achieve connectivity between water injection and production wells. I have no information on what the likely range of costs might be for suitable heat mining locations. I do know from not always happy experience that the cost of drilling and completing wells deeper than 2 miles is formidable

Technology Review: How much geothermal energy could be harvested?

Jefferson Tester: The figure for the whole world is on the order of 100 million exojoules or quads [a quad is one quadrillion BTUs]. This is the part that would be useable. We now use worldwide just over 400 exojoules per year. So you do the math, and you know you’ve got a very big source of energy.

How much of that massive resource base could we usefully extract? Imagine that only a fraction of a percent comes out. It’s still big. A tenth of a percent is 100,000 quads. You have access to a tremendous amount of stored energy. And assessment studies have shown that this is thousands of times in excess of the amount of energy we consume per-year in the country. The trick is to get it out of the ground economically and efficiently and to do it in an environmentally sustainable manner. That’s what a lot of the field efforts have focused on.

…TR: How do you plan to harvest stored heat from more areas?

JT: What we’re trying to do is emulate what nature has provided in these high-grade systems. When we go very deep, [rocks] are crystalline. They’re very impermeable. They aren’t heat exchangers like we really need. We’d like to create porosity and permeability. [The rock] actually is filled with small fractures, so what you’re trying to do is find those weak zones and reopen them. We need to engineer good connectivity between an injection set of wells and a production set of wells, and sweep fluid, in this case, water, over that rock surface so that we extract the thermal energy and bring it up another well.

TR: What technology do you need to open up the rock and harvest the heat?

JT: All the technology that goes into drilling and completing oil and gas production systems, [such as] stimulation of wells, hydraulic fracturing, deep-well completion, and multiple horizontal laterals, could in principle be extended to deep heat mining. Hydraulic methods have been the ones that hold the most promise, where you go into the system and you pressurize the rock — just water pressure. If you go higher than the confinement stress, you will reopen the small fractures. We’re just talking about using a few thousand pounds per square inch pressure — it’s surprising how easy this is to do. This is a technique that’s used almost every single day to stimulate oil and gas reservoirs.

My bottom line is to watch for investment by the majors like BP or Shell — if they start pilot projects then we’ll know that “universal geothermal” is a real option. I doubt that a science prize would be applicable here because of the magnitude of investment and length of time to assess success. I wouldn’t favor taxpayer-funded research grants because the economics are clear, and the majors know the costs and limitations of the technology — so if they aren’t motivated I don’t think it is yet time.