| Title | Innovative Thermal Drilling Technologies based on mechanically assisted LaserJet Drilling (LJD) for hard rock (geothermal) applications |
|---|---|
| Authors | Jamali, S; Wittig, V; Bracke, R |
| Year | 2016 |
| Conference | European Geothermal Congress |
| Keywords | Hard rock drilling, Geothermal drilling, Thermal Drilling, Laser, LaserJet, Spallation |
| Abstract | Geothermal resources tend to be found in deeper and harder geologic formations than typical hydrocarbon reservoirs. Therefore, drilling technologies and processes from the oil & gas field have been improved constantly to make for more efficient and economic drilling. However, drilling speeds or rate of penetration (ROP) of classic drilling technologies, e.g. tricone bits, suffer greatly in deep and hard formations. Thus, there is a great need for tools with higher ROP and low wear to reduce drilling, trip time and cost. Today’s tools still heavily rely on technologies based on tricone / PCD bits to mechanically break the rock and thus, having to mainly overcome its high compressive strength and, furthermore, requiring large amounts of energy and time. A new paradigm of drilling is needed for the future, as no fundamental, “paradigm” changing improvement or alternative to the mechanical breaking of hard rock have since been introduced to address the exponentially increasing challenges in deep drilling of hard, crystalline rock formations (like geothermal reservoirs). Problems mainly include very low rate of penetration (1 m / hr. or less), very high bit / tool wear and thus, low service live of e.g. under 50 hrs. This all makes for numerous, long and expensive round trips and thus, very high overall drilling cost. Attempts to develop alternative means for delivering more / different energy to the bit and break the rock differently have been under way worldwide in the past 20 to 30 years. Thermal drilling or breaking of rock, e.g. Laser (supported) Jet Drilling (LJD), could potentially be such next fundamental change and thus, greatly improve drilling of hard rock. GZB in Bochum is investigating such innovative thermal drilling technologies, especially mechanically assisted LaserJet drilling. Hereby, Laser beams are being sent via water jet, protected by a gas shield, onto rock´s surface, where it concentrates locally causing the local temperature to increase instantaneously in order to weaken the rock structure through spallation and thermal stresses. The high power intensity laser beam in contrast to low rock thermal conductivity causes the local temperature to increase at once which results in a local induced thermal stress that spalls the rock. This weakening process in rock due to induced thermal stresses also results in fractures, mineral dehydration and thus, reduction in rock´s Young’s and shear modulus. Subsequently, the now weakened rock will be drilled or “ground down” if needed using specially optimized mechanical bit technologies. This process continues on a new surface by removing the cuttings and fragments with help of drilling fluid as a flushing system. This paper discusses the principle behind thermal drilling technologies, The LJD working principle and preliminary lab and field tests of Laser initiated thermal drilling, showing multiple advantages compared to conventional methods including: additional energy being sent to the bit, high ROP, longer bit life due to less wear, etc. Furthermore, a new LaserJet drill bit, required changes and modifications to be implemented in the present drill string equipment and economic and technical analyses of the possible advantages and disadvantages are being described. Hence, a thermal drilling process conducted by using LaserJet technology delivering additional thermal energy for rock removal purposes followed by limited and decreased required mechanical work seems to be a breakthrough in the deep, hard rock drilling process of the future. |