Numerical study of variation in Biot's coefficient with respect to microstructure of rocks

• Numerical study on Biot' coefficient at the microscopic level.
• Biot’s coefficient is strongly dependent on the internal microstructure of rocks.
• Two equations were established to relate Biot's coefficient with rock microstructure.

Biot's coefficient, which is the key parameter of poroelasticity, strongly depends on the microstructure of the porous medium. Due to the complexity to conduct effective experiments at the microscale, there are very limited experimental data available, which document the influence of microstructure on Biot's coefficient. Therefore, numerical method was used in this paper to specify the influences of microstructural parameters on Biot's coefficient. Two-dimensional discrete element models with simplified microstructural geometries have been constructed with the purpose of obtaining a quantitative description of the influences of internal cavities (pores, fractures etc.) on the macroscopic deformability. Based on the numerical results some observations were revealed: 1) the Biot's coefficient is strongly dependent on the internal microstructure of the rock, and different porous media with the same composition of constituents may exhibit quite different poroelastic behavior due to their different microstructures; 2) the cracks, rather than the pores seem to have the dominant effect on Biot's coefficient; 3) the distribution of cracks and pores influences the Biot's coefficient and creates anisotropy. Two phenomenological equations were established by using the generalized mixture rule to describe the relationship between Biot's coefficient and rock microstructures, which give more accuracy results than the existing empirical equations.