Underwater laser drilling: Drilling underwater granite by CO₂ laser

The purpose of this research is to develop Underwater Laser Drilling Technology in drilling and completing oil and natural gas wells. Recent challenges in oil fields are caused by: the geometry of oil or gas wells is becoming more complicated in profile; drilling in hard formations like granite requires more tripping times for changing rotary bits made of steel; as the water depth at which floater rigs can operate increases, tripping has to take longer than ever. Underwater Laser Drilling is a key to solution of these challenges since the laser may allow for non-contact / non-rotating drilling method. Granite is one of the most difficult rocks to be drilled by lasers. Consequently, no successful report has been found on underwater granites drilled by lasers. This paper describes experimental tests of irradiating carbon dioxide (CO₂) laser to granite submerged in water/ bentonite solutions. Sample granite was submerged at 50mm from the water-surface and then a CO₂ laser with a wavelength of 10.6μm which has a high absorption coefficient in water was intermittently irradiated into the 4wt% bentonite solutions. A laser induced underwater shock waves, cavitation bubble formations, and micro-water jet formations upon bubble collapse. The high power laser beam locally melted the granite into molten glass, which successively turned to a molten glass bead. The generated glass beads were small enough to be removed out of the created hole. A high-speed video camera was employed to record the laser beam in the water. We observed the generation of initial cone shaped water bubbles through which the laser beams reached bottom rock surface with a minimum absorption energy loss. The laser beam indeed drilled the granite specimen submerged in the solution. The research will continue to collect data for designing a prototype.