Method of monitoring unconventional hydrocarbon exploration and development sites

A method of monitoring an underground formation to recover hydrocarbons using simulation and drilling and at least one rare gas present in the underground formation, comprising:

• a) selecting a diffusion model of the at least one rare gas and a diffusion model the hydrocarbons to be recovered with each model including an evolution of concentration as a function of time and depth and a diffusion coefficient;
• b) taking at least a first sample of fluid present in a subsoil zone which is being monitored that is located above the underground formation prior to recovery of the hydrocarbons by using a downhole sampler and measuring a concentration of the at least one rare gas within the first sample of the fluid;
• c) fracturing the underground formation;
• d) at least one of during and after recovery of the hydrocarbons, taking at least a second sample of the fluid present in the subsoil zone and measuring a concentration of the at least one rare gas within at least the second sample;
• e) repeating d) at different times;
• f) determining when the concentration of the at least one rare gas increases in the at least a second sample from the concentration of the at least one rare gas in the first sample and modifying the diffusion coefficient of the diffusion model of the at least one rare gas to be coherent with the increased concentration of the at least one rare gas in the at least a second sample and determining a ratio between the diffusion coefficient before modification and the modified diffusion coefficient;
• and
• g) incorporating the ratio into the diffusion model of the hydrocarbons to be recovered and thereafter determining from the diffusion model of the hydrocarbons incorporating the ratio an amount of hydrocarbons in the subsoil zone that can be recovered at a time t.