Accessing Earth’s heat using Geothermal Radial Drilling for borehole heat exchangers.
Tomasz Śliwa, Marek Kucper
Vol. 34, no. 2 (2017), s. 495-511, 
Sometimes there is not enough room for drilling vertical borehole heat exchangers. Hence, the idea of slant boreholes emerged. They can provide access to a rock mass, being an underground heat reservoir, located beneath infrastructure. To optimize the average value of thermal conductivity and use those layers in a given geological profile that possess best properties in the view of thermal efficiency, Geothermal Radial Drilling is used, being a very efficient technology. A GeoDrill 4R drill rig, specially designed for the Geothermal Radial Drilling technology, enables drilling at an angle between 30 and 65 degrees, which in turn, together with data on the geological profile, allows for designing the layout of exchangers so as to obtain longest possible section of a given exchanger within most energetic layers. The paper includes a description of a few investments which use Geothermal Radial Drilling for heating purposes. It also presents a comparative study between Geothermal Radial Drilling and conventional vertical borehole exchangers.