Hydraulic diffusivity measurements on laboratory rock samples using an oscillating pore pressure method

Abstract

Borrowing from the literature on thermal diffusivity measurements with oscillating boundary conditions, we developed and tested a methodology to measure hydraulic diffusivity of rock samples in the laboratory. The method uses a pore pressure oscillation superimposed upon the ambient pore pressure at one end of the rock sample. Unlike the transient pulse technique, which requires re-equilibration between discrete measurements, and decay curve matching for true permeability, this method allows continuous measurements during slow changes in the state of the rock-environment system. This is particularly useful for time-dependent diffusivity measurements during creep or consolidation of rock. When both the amplitude ratio and the phase difference between the upstream and downstream reservoir pressures can be measured, the method allows calculation of both diffusivity and permeability at the same time. With both of these quantities known, along with the other system parameters, interconnected porosity changes may be calculated. There are some drawbacks to the methodology. The optimum frequency of oscillation and the ratio of downstream to upstream pore pressures depend upon sample size and the magnitude of permeability, so there is a practical limit to the range of measurable diffusivities. For 11 cm long samples of Berea sandstone with a diffusivity on the order of 300 cm2/sec for example, an oscillation period of about 1 sec is appropriate. In the same size sample of Tennessee sandstone, with a diffusivity on the order of 0.25 cm2/sec, periods between 500 and 1000 sec are appropriate. Granites require pore pressure oscillation periods on the order of 1 hr or more.