Stress Induced Release of Rn222 and CH4, to Percolating Water in Granitic Rock

Abstract

Abstract The radon, methane, and carbon d ioxide concentrations have been measured in water which has percolated through granitic rock under triaxial stresses ranging from 0.1 to 0.95 of the fracture stress. We simultaneously measured the permeability and porosity (hence hydraulic radius) of each sample. The first series of experiments were on seventeen initially dry rock samples. The radon concentration in the first 3 gin of water collected varied by a factor of 10, but was not correlated with stress. A good correlation was found between radon and permeability; rocks having high permeabilities tend to release less radon. In a second series of experiments, rock samples were saturated under stress, equilibrated for one month, then restressed and measured. These samples produced between 2 and 10 times more radon than initially dry rocks having the same permeability. In a third series of experiments, multiple successive water fractions were sampled. We found that most of the radon is removed with the first pore volume collected, while methane extraction requires several pore volumes. An experiment in which the stress was changed during a run produced an increase in CHI, but no increase in Rn222. These results are interpreted in terms of a numerical model for flow and gas extraction from a microcrack network.