Specific energy for pulsed laser rock drilling

Abstract

Application of advanced high power laser technology to 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. Samples of sandstone, shale, and limestone were prepared for laser beam interaction with a 1.6 kW pulsed Nd:yttrium–aluminum–garnet laser beam to determine how the beam size, power, repetition rate, pulse width, exposure time and energy can affect the amount of energy transferred to the rock for the purposes of spallation, melting, and vaporization. The purpose of the laser rock interaction experiment was to determine the optimal parameters required to remove a maximum rock volume from the samples while minimizing energy input. Absorption of radiant energy from the laser beam gives rise to the thermal energy transfer required for the destruction and removal of the rock matrix. Results from the tests indicate that each rock type has a set of optimal laser parameters to minimize specific energy (SE) values as observed in a set of linear track and spot tests. As absorbed energy outpaces heat diffusion by the rock matrix, local temperatures can rise to the melting points of the minerals and quickly increase observed SE values. Tests also clearly identified the spallation and melting zones for shale samples while changing the laser power. The lowest SE values are obtained in the spalling zone just prior to the onset of mineral melt. The laser thermally spalled and saw mechanically cut rocks show similarity of surface microstructure. The study also found that increasing beam repetition rate within the same material removal mechanism would increase the material removal rate, which is believed due to an increase of maximum temperature, thermal cycling frequency, and intensity of laser-driven shock wave within the rock.

Influence of different laser operation regimes on the specific energy required for rock removal in oil and gas well drilling applications

“…In the case of laser rock drilling using cw laser sources, our results are similar to what was observed in previous works [4,5,7] , although we did not use any purging system. However, the range covered by the different laser parameters, in particular in the case of total irradiation time, is much larger in this preliminary study.…”

“…[7] Quite obviously, the most efficient laser rock removal mechanism leads to the minimum value of SE. For the sake of comparison, SE for the widely used rotary drilling approach is typically in the range of 1 kJ/cm 3 .…”

“…[6] When laser-induced thermal stresses reach the critical strength of the rock, the result is the fracture of the rock into small fragments that can be easily removed. [7] The so-called laser rock spallation or fragmentation is usually the most efficient rock removal mechanism in inhomogeneous rock materials as it occurs below the melting and vaporization temperatures, thus requiring less energy input.…”

“…The particular destruction and removal mechanism will be determined by the laser energy applied on the rock, but also by the way it is applied. [7] To measure the efficiency of a given rock removal mechanism, it is convenient to define a simple energy parameter called specific energy (SE) as SE (kJ/cm 3 ) = energy input / volume of rock removed.…”

Analysis of Thermally Altered Rock Properties Using High-Power Laser Technology

“…Laser parameters that control the laser/rock interaction are: laser type; laser power; and lasing mode. 2 However, the physical and chemical properties of the rocks also play a major role in determining the nature of the laser/rock interaction. In petroleum engineering applications, certain rock properties are critical for determining flow rate, storage capacity, and overall effectiveness of enhanced oil recovery methods.…”