Reionisation was the period during which the first galaxies emitted enough hydrogen-ionising photons to ionise the intergalactic medium (IGM). To deduce the contribution of a galaxy to the emission of ionising photons, one must know its escape fraction of ionising photons: which fraction reach the IGM before being absorbed by the gas inside the galaxy? My goal in this thesis was to explore ways of deducing the escape fraction of high-redshift galaxies, relying on indirect methods since the ionising photons are not observable. I focused my efforts on one method using metallic absorption lines, believed to be linked with the coverage of neutral gas in front of stars, thus related to the escape fraction. I post-processed RAMSES-RT simulations to produce mock absorption lines and compute escape fractions from simulated galaxies. Comparing the lines and the escape fractions, I showed that the former are not faithful tracers of the latter.