Advances in Modeling of High-Power Laser Perforation

Abstract

Abstract High-power lasers could enable contactless, environmentally-friendly, and non-damaging methods for perforating and drilling of hydrocarbon and other subsurface plays. To enable understanding of the potential perforating process, a comprehensive numerical model is desired to characterize the perforation hole geometry, its evolution, and how it is impacted by various process parameters (e.g. state of stress, temperature gradients, etc.). This work develops a thermo-mechanical model of a high-power laser perforation to ascertain how different thermal and mechanical parameters alter the process. A fully coupled thermal-mechanical finite element model was developed to investigate the ensuing thermal and mechanical effects due to laser perforating a sandstone rock sample. The model was validated against experimental data based on the perforating rate and the created perforation profile. Eight sensitivity studies were conducted to understand the separate effects of the laser perforating process parameters. Laser-rock interaction involves the simultaneous coupling of thermal, mechanical, and electromagnetic physics. In all simulated cases, two distinct stress regions were formed as a result of laser perforating. The first one is the hotter lased region which was under compressive stresses opposing the induced thermal expansion, and the second one is the colder surrounding region, which was under tensile stresses to contain the thermally expanding hotter region. The eight sensitized process parameters encompass laser beam power, laser beam radius, confining stresses, Young's modulus, specific heat capacity, thermal conductivity, thermal expansion coefficient, and vaporization temperature. It was found that the perforating rate was directly proportional to the laser beam power and inversely proportional to the laser beam radius. Altering the magnitude of confining stresses, confining stresses ratio, Young's modulus, or thermal expansion coefficient, did not change the perforation rate or profile, albeit influencing the magnitude of the thermally induced stresses. A rock with a larger specific heat capacity was more difficult to heat, thus was under a slower perforating rate. Also, greatly increasing the rock's thermal conductivity resulted in somewhat a slower perforating rate. Furthermore, increasing the vaporization temperature to more than double the base value also slowed the perforating rate. Overall, changing the rock's thermal properties or laser design parameters affected the ensuing perforating rate and profile, whereas altering the rock's mechanical properties only caused changes to the generated thermal stresses. The complexity underlying the laser perforation process represented by the mechanical, thermal, chemical, and electromagnetic changes, creates a challenge toward applying this technology for field operations. As lab experiments help answer many questions, the limitations inherent within lab experiments such as the application of in-situ conditions leave other questions unanswered, necessitating numerical modeling. Besides overcoming these lab challenges, modeling saves money and time investment of lab experiments by running different sensitivity cases. Fundamentally, this type of model takes laser perforation steps closer to field applications.