In theory, constant, base-load electricity could be available 24 hours a day, 7 days a week, with near-zero carbon emissions, almost anywhere in the world. This is because there is hot rock beneath our feet, no matter where we are. Unfortunately, because of the thermal gradient in most parts of the world, the layers of rock that are hot enough to be exploited to produce electricity are only found at depths of over 5 kilometres (see article in this issue on Enhanced Geothermal Systems). And supercritical water (SCW), an equally abundant geothermal source, with temperatures of over 374°C, is usually found at depths of over 10 km and at pressures of up to 1000 bar. Drilling to these depths and in such extreme conditions tests the limits of currently available technology and, above all, becomes prohibitively expensive. But revolutionary ultra-deep drilling technologies, such as high-pressure water jets combined with electric-discharge plasma, could be set to change this.
The most widespread geothermal deep drilling technology is the rotary drill, which has been in use since the introduction of the tri-cone rotary bit in 1909. Diesel-electric drilling rigs are used to create boreholes protected by steel casings. These are arranged one inside the other as drilling proceeds, leading to a progressive tapering, which limits the final size of the borehole. Typically, the cost of deploying this technology rises exponentially with depth. Indeed, drilling can represent more than half the cost of developing Enhanced Geothermal Systems (EGS).
Despite a number of improvements, such as the polycrystalline diamond compact (PDC) bits developed in the 1970s, the search is on for radically different, breakthrough technologies to lower these costs substantially, and to allow drilling to depths of 10 km, to exploit the geothermal energy potential of supercritical water . This is steam that has become so dense that its liquid water and steam phases merge into a single fluid. At present, drilling for supercritical geothermal resources is only feasible where there are unusually high geothermal gradients, such as in Iceland, where supercriticality occurs at a depth of about 4 – 5 km.
As many as 20 new ultra-deep drilling technologies are currently being explored, such as plasma-jet rock cutting, the use of high-energy laser beams to disintegrate the rock by melting it (thermal spallation) and electrical discharge. But none of these has so far proven to be effective in the severe conditions found at depths of 10 km.
One of the reasons for the exponentially rising costs of using rotary bit technology for deep drilling is the need to replace worn-out drill bits. This involves withdrawing the ‘string’ - the interconnected drilling rods - bringing it to the surface, dismantling and then reassembling it. The deeper the well, the longer this process takes. The more interesting new developments are technologies where there is no drill bit and no direct contact with the rock.
One promising technology, which could be the breakthrough that the industry is looking for, is a variant of hydrothermal spallation drilling, where a jet of supercritical water is directed at rock at the bottom of a water-filled borehole (which is usually the case beyond depths of 6 km). Because the rock is a poor conductor of heat, only the upper layer is heated rapidly, eventually leading it to fracture into small fragments. In this case the corrosive nature of SCW, which is usually a handicap to conventional drilling techniques, is turned to good use, by aiding the disintegration process.
Along similar lines, with the help of a EUR 2.4 million grant from the EU Structural Fund, a Slovakian engineering company, GA Drilling (formerly Geothermal Anywhere) has developed a pulsed-plasma and ultra-sound cutting tool, known as Plasmabit, which is attached to the head of a long, coiled tube. As the cutting tool is fed down the borehole, the plasma and the ultra-sound pulses break up and disperse the rock without direct contact. Water is then pumped down the tubing and rises back up the borehole, enabling buoyancy to be used to bring capsules of drilled disintegrated rock to the surface.
Because there is no direct contact with the rock, there is no bit to wear out. GA Drilling claims that, by using its technology, the cost of drilling will only increase linearly with depth, instead of exponentially, thus drastically reducing the costs of ultra-deep drilling. Also, because of other new technology the company has developed, the inevitable tapering of the borehole with depth, found with conventional rotary bit systems, no longer applies, leading to the possibility of boreholes with a constant diameter from surface to rock.
The technology has passed its proof of concept stage and is now ready to be tested at moderate depths. GA Drilling has recently teamed up in a strategic partnership with UK geothermal engineering company EGS Energy, to use its contactless plasma drilling technology for a proposed enhanced geothermal systems initiative at the Eden Project in Cornwall (UK), which involves drilling two boreholes to a depth of about 4.5 km. Cornwall was also the location for Europe’s first deep geothermal research and development facility, the Hot Dry Rocks project, at Rosemanowes between 1976 and 1991.
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