Pore volume and transport properties changes during pressure cycling of several crystalline rocks

Abstract

Samples of five crystalline rocks were submitted to confining pressure cycles during which pore volume changes were measured. A recent version of the ‘equivalent channel model’ was used to interpret these data jointly with permeability and electrical resistivity data collected on the same rocks. The variations of these three quantities with pressure were predicted very well by the model. Thus, the following parameters characterizing the pore structure of the rocks studied could be determined: the standard deviation of the asperity heights distribution h, the wetted area of the pores per unit volume A c/ V, and the product of the initial mean hydraulic radius 〈 m 0〉 by the squared initial mean tortuosity 〈 τ 0〉 2. The uncertainty on these parameters was usually rather large but decreased drastically in the cases where all the different data had been collected on a single sample. A c/ V did not appear correlated with porosity or permeability in the rocks studied here. In contrast, h and 〈 m 0〉〈 τ 0〉 2 showed some tendency to increase with increasing permeability. This suggests that permeability is mainly controlled by hydraulic radius. Finally, within the precision limits of this study, we did not observe a dependence of these parameters on either the loading and unloading cycles or on the number of cycles, although the pore volume change data showed strong hysteresis and stress history effects.