Biot’s Effective Stress Coefficient of Mudstone Source Rocks

Biot’s effective stress law has been proved to be very useful in describing the effect of pore fluid pressure on the mechanical response of rocks and understanding the poroelastic effects in unconventional shale or mudstone formations. In this work, six mudstone source rock samples from unconventional reservoir wells have been tested for measuring their grain bulk moduli, bulk moduli, and the stress dependent Biot’s effective stress coefficients. Grain bulk moduli of these samples range from 60 GPa to 90 GPa. Biot’s effective stress coefficients for all the samples decrease from 0.90 or higher to 0.70 or lower with increasing effective stress illustrating the stress dependency of Biot’s coefficient. Furthermore, the different values of Biot’s coefficients along the vertical and horizontal directions indicate that Biot’s coefficient is indeed a tensor quantity for anisotropic mudstone source rocks, so that pore pressure changes not only modify the normal stress but also induce shear stress changes. Experiments also show that mudstone source rock samples expand when nitrogen gas is used as the testing fluid (gas absorption), and the degree of such expansion is related to the absorbed gas quantity and organic matter content.