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Finite element analysis of the laser perforation process: thermal and mechanical perspectives
Alrashed, Ahmed Ali
Alrashed, Ahmed Ali
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2022
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Abstract
A great challenge to using lasers for the purpose of drilling or perforating lies in understanding the complex processes occurring simultaneously and involving thermal, mechanical, and photonic changes. As a means to better such understanding, 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. Additionally, a steel-rock model was created to simulate laser perforating in cased-hole completions.
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 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 harder to be heated, 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.
Compared to only 30 seconds to perforate the simulated sandstone rock sample, laser perforating both steel (top) and rock (bottom) layers lasted 50 seconds. The first steel element was removed after 10 seconds of laser heating, whereas removing the first rock element took approximately 18 seconds. Simulation results showed that steel was harder to laser perforate than rock, mainly attributed to steel having a larger melting/vaporization temperature and a significantly greater thermal conductivity. The stress profiles obtained in the stress-rock model were similar to the rock-only model, where compressive stresses are dominating the hotter lased area and tensile stresses are distributed in the colder surrounding area. However, the concentration of these thermally induced stresses is more pronounced in the steel layer than in the rock.
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