Direct and indirect laboratory measurements of poroelastic properties of two consolidated sandstones

Abstract

Experimental measurements to derive the drained bulk C and the solid bulk Csu compressibility, the poroelastic Biot coefficient α, Skempton coefficient B as well as porosity ϕ were performed under drained, undrained and unjacketed pore pressure conditions. The experiments were conducted for two different kinds of sandstones, Bentheimer and Flechtinger. During all experiments, the confining hydrostatic pressure was continuously increased up to 85MPa. To measure the undrained pore pressure response within the interior of the sample, a novel fibre optic sensors technique was applied. Subsequently, we compared the direct measurement of αdir, Bdir and ϕdir to indirect calculated values using C and Csu. For low effective pressure, it showed that all parameters display a non-linear decrease with increasing effective pressure. At high effective pressure the decrease of all poroelastic coefficients is almost linear with increasing effective pressure. In accordance with Terzaghi's effective stress concept, the comparison between direct and indirect methods showed a good to excellent agreement for all calculated parameters. Differences between the direct and indirect methods were used to quantify drained and unjacketed pore compressibilities, as well as jacket effects during the experiment.