Mechanically Assisted Thermal Type LaserJet Process for Deep Hard Rock Drilling

Future resources like unconventional hydrocarbons, geothermal energy etc. tend to be found in deeper and harder geologic formations than what has been tapped into over the past 100 years. Although oil & gas drilling technologies and processes have been improved constantly to yield a more efficient and economic drilling process, yet rate of penetration (ROP) of those conventional 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 lower wear to reduce drilling costs and trip time. Today's tools still heavily rely on technologies based on mechanical breakage of rock such as tricone or Polycrystalline Compact Diamond (PCD) bits to mainly overcome its high compressive strength and, furthermore, requiring large amounts of energy and time. An all new method for drilling is needed for the future, as no fundamental changes or alternatives to the mechanical breaking of hard rock have been introduced to address the exponentially increasing challenges in deep drilling of hard, crystalline rock formations, e.g. as in geothermal reservoirs. Problems mainly include very low rate of penetration (1 m/h or less), very high bit/tool wear and thus, low service life of e. g. under 50 h. This all makes for numerous, long and expensive round trips and hence, very high overall drilling costs. Attempts to develop alternative means for delivering more and/or different energy to the bit and break the rock differently have been under way worldwide in the past 20 to 30 years. Thermal drilling of rock, e. g. Laser (supported) Jet drilling (LJD), could potentially become such a next fundamental change and thus, greatly improve drilling of hard rock. The International Geothermal Center in Bochum (GZB) is investigating such innovative thermal drilling technologies, especially mechanically assisted LaserJet drilling. Hereby the Laser beam is sent via water jet, protected by a gas shield, onto the rock surface, causing the local temperature to increase instantaneously, weakening the rock structure, causing thermal stresses and spallation. This weakening process in the rock due to induced thermal stresses also results in fractures, mineral dehydration and thus, reduction in the rock's Youngs and shear modulus. Subsequently, the now weakened rock may be drilled or ground down without any effort using specially optimized mechanical bit technologies. This process continues on a new surface by removing the cuttings and fragments with the help of drilling fluid as a flushing system. This article discusses principles behind thermal drilling technologies including LJD. Preliminary lab and field tests of LaserJet thermal drilling are being analyzed, showing multiple advantages compared to conventional methods including: additional energy being sent to the bit, high ROP, longer bit life due to less or no mechanical wear, etc. Hence, a thermal drilling process conducted by using Laser Jet technology delivering additional thermal energy for rock removal purposes followed by limited and decreased required mechanical work may be a breakthrough for the deep, hard rock drilling process of the future.