Abstract:
Advancement in laser technology and material engineering have progressively increased the output power and beam quality of solid-state lasers and reduced the price per kilowatt to single digit figures. These developments have the potential to make downhole photonics a reality. To reach its full potential, there are several challenges to overcome for this technology, ranging from material characterization to the accurate modeling of the underlying electromagnetic-thermal-mechanical dynamics among lasers, rocks and the downhole environment. In this paper we present the first results of our investigation, including analysis of the significant variables and their interdependence. Later we explore the advantages and disadvantages of each method, and show some preliminary results of continuous mechanics modeling of laser-rock interaction. Finally, we posit how all these physical phenomena can be coupled in a comprehensive numerical model.
Introduction
In the first femtoseconds after a laser beam impinges on the surface of a material a series of nanoscopic effects commence and give rise to the absorption, transmission and scattering of the incident rays. If the material is opaque – at the wavelength of the incident radiation – a portion of the absorbed electromagnetic energy transforms into thermal energy. In the next microseconds the material heats up, and a new chain of microscopic events takes place. The material begins to melt, then it disassociates and spalls; finally, it evaporates. Sometimes the thermal energy is large enough to sublimate the material. Through these phase changes the morphology of the material reshapes and its fundamental physical properties vary (see Fig. 1). This ensues in a complex, intense and short-lived dynamic. How to control it remains a hot topic in applied research and engineering with variegated applications; e.g., drilling, perforation, heat treatment, and nano texturing (Gahan et al., 2001; Graves et al., 2002; Batarseh et al., 2004; Lepski and Bruckner, 2009; Batarseh et al., 2012; Assuncao and Williams, 2013; Yang et al., 2015; Sharma et al., 2015; Senesi et al., 2016).
In the case of laser-rock thermodynamic interaction, Gahan et al. (2001) first proposed a direct relation between the volume of rock removed and the energy input (termed specific energy), they used it to predict the depth of a laser perforation in different rock samples. Later, Graves et al.(2002) studied the effect of laser-drilling in the porosity and permeability of rocks and observed that rocks permeability and porosity increased proportionally with thermal conductivity. Later, Batarseh et al. (2004) measured the relation between specific energy, time of laser incidence, thermodynamic phase-change and hole geometry; and showed that rocks melted under laser irradiation had larger specific energies than those that were dissociated. Thereafter, numerous studies have been conducted to observe the effects of laser irradiation on saturated and unsaturated rocks (Ahmadi et al., 2011), formation of stress waves inside the rock (Sakakura et al., 2011), and ejection of rock vapor/plasma plumes and rock dissociation (Yan et al., 2013).
