Electrokinetic Coupling in Hydraulic Fracture Propagation

Abstract

Electrokinetic coupling is the most popular mechanism proposed to explain observed electromagnetic (EM) signals associated with the hydraulic fracturing of rocks. Measurements in both, laboratory and in situ conditions show evidence of the phenomenon, however, as far as the authors know, there have been no reports on the description of the source mechanism, its relationship to a propagating crack, nor the electromagnetic field distribution due to such a source advancing through a conductive medium. In this paper it is shown that an electric streaming current density arising on the walls of a fluid driven crack gives rise to the EM fields observed in measurements of streaming potential in hydraulic fracturing experiments. A source function for the current density is established from the fluid pressure profile inside the propagating. Expressions for the EM fields due to such a source are derived for a crack propagating with a constant velocity, in a homogeneous isotropic conducting medium. The spatial and temporal behavior of the fields reasonably agree with measurements performed in laboratory experiments. In situ measurements are qualitatively described only, however it is shown that the magnitude of the fields and their temporal behavior can be well reproduced in a realistic hydraulic fracturing setting.