Application of advanced high power laser technology into oil and gas well drilling has been attracting significant research interests recently among research institutes, petroleum industries, and universities. Potential laser or laser-aided oil and gas well drilling has many advantages over the conventional rotary drilling, such as high penetration rate, reduction or elimination of tripping, casing, and bit costs, and enhanced well control, perforating and side-tracking capabilities.
The energy required to remove a unit volume of rock, namely the specific energy (SE), is a critical rock property data that can be used to determine both the technical and economic feasibility of laser oil and gas well drilling. When a high power laser beam is applied on a rock, it can remove the rock by thermal-spallation, melting or vaporization depending on the applied laser energy and the way the energy is applied. The most efficient rock removal mechanism would be the one that requires the minimum energy to remove a unit volume of rock. First, studies were carried out to investigate the correlation between the rock removal mechanisms and laser processing parameters. Then the test matrixes were carefully designed and performed based on the established correlation to quantitatively determine the nearly true specific energy. A 1.6 kW pulsed Nd:YAG laser, with a unique control capacity over laser parameters, is utilized to perform beam application tests on the rocks. The effect of laser processing parameters, such as beam irradiance, energy per pulse, exposure time, and pulse repetition rate, on the removal of rocks are investigated. The study found that different laser-rock interaction zones from intense melting to scorching could be produced in the rock depending on the applied beam irradiance and exposure time. However, the most efficient rock removal mechanism was found to be thermal-spallation where the rock was thermally fractured and removed from the hole before any further melting or vaporizing (which requires much higher energy). The study also found that increasing beam repetition rate within the same material removal mechanism would increase the material removal rate due to an increase of maximum temperature, thermal cycling frequency, and intensity of laser-driven shock wave within the rock.