Tuesday, 20 April

Technology for larger EGS wells

An article in the New Scientist last week was reporting on a proposal that is being discussed in the USA for future disposal of nuclear waste. What has this got to do with geothermal energy you ask. Just read on.

After abandonment of the Yucca mountain nuclear waste storage project, for a while, the hottest proposal to dispose of nuclear waste is to bury it deep in the basement rock. Holes to hold the nuclear waste canisters need to be half a meter wide. The hole will need to penetrate at least 3-km into the basement rock. Canisters of spent fuel lowered into the borehole would end up stacked one on top of the other, filling the bottom 2 kilometres. This stack would then be sealed in place with a cap of clay, asphalt and concrete.

A brainstorming meeting between Sandia and MIT groups on 15 March apparently discussed this proposal but it is not clear where it goes after that. This idea is not new but it looks like it gained new wind after Yucca Mountain. A government-owned British nuclear waste disposal consultancy company, Nirex, issued a report in 2004, on "Deep Borehole Disposal Concept for Radioactive Waste". This Report, accessible off the web, reviews past proposals in this area. One of the proposals considered in the report is shown in the following figure:

The capability to drill to 6000 m with a bottom hole diameter of 20 in (0.6 m approx.) was claimed to be within current drilling capabilities. According to one of the studies quoted this report, the maximum depth was constrained by the stability of the borehole, with a borehole containing heavy weight drilling mud being unstable in crystalline basement below a depth of approximately 6000 m.

A 1989 study by Juhlin and Sandstedt mentioned in the Nirex report examined the drilling technology and the costs at that time. Three different options were considered in this study:

• Option A: a borehole with an ID of 800 mm in the disposal zone where waste would be emplaced in a zone from 2 – 4 km.
• Option B: a borehole with an ID of 375 mm in the disposal zone where waste would be emplaced in a zone from 2 – 5.5 km
• Option C: a borehole with an ID of 375 mm in the disposal zone where waste would be emplaced in a zone from 2 – 4 km.

Option A was thought to be the most suitable for the purpose of nuclear waste disposal but also the one with most challenges, especially in the area of casing design. Nevertheless, Juhlin and Sandstedt thought that the technology existed that the borehole could be drilled, the waste emplaced and the borehole sealed in less than three years. The costs for one such borehole were estimated to be 388MSEK (approximately £30 million) at 1988 prices.

In our studies into the feasibility of the supercritical CO2 geothermal siphon, the friction up the production well turned out to be a major loss factor. This is not as much an issue with water but with the higher flow rates required for the CO2 the pressure drop gets very substantial. One solution is to have a larger-diameter well. That is why I got interested in the New Scientist article and the development in the deep borehole waste disposal proposals. If the technology is developed in that sector, it may have spin-offs for the hot rock geothermal industry.

 

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