Analysis of the hydromechanical behavior of a flat-lying fracture during a slug test

Abstract

Slug tests dilate and contract fractures in aquifers. We developed a coupled hydromechanical analysis of an idealized fracture to evaluate effects of these deformations in fractured crystalline rock aquifers. The analysis shows the abrupt increase in pressure at the beginning of a slug test causes a slight opening of a fracture, but the fracture continues to open as the pressure falls in the wellbore. This happens because the pressure increases over an ever-widening area even though it decreases in the wellbore. Eventually decreasing pressure in the vicinity of the wellbore offsets the spreading pressure and the fracture starts to close, ultimately returning to its initial aperture when the pressure equilibrates. The displacement in the wellbore when the pressure head is rising is always smaller than when the head is falling, and this produces a loop-like, hysteresis when displacement is plotted as a function of pressure. A sensitivity analysis shows the displacement at the wellbore is sensitive to the magnitude and distribution of aquifer properties, suggesting that the displacement signal could be combined with pressure measurements to improve the interpretation of slug tests during aquifer characterization.