Elastic Properties of Shales with Respect to Silt Fraction

Summary

Elastic properties of shales are important for quantitative interpretation of seismic response, detecting high pore pressure before drilling and understanding overburden response during production. Modelling shale properties is difficult however as elastic properties of individual clay minerals are hard to measure directly. We use a transversely isotropic differential effective medium approach to calculate elastic constants of shales with different silt fractions. Increasing silt fraction from 0 to 25% results in an increase of elastic constants C₁₁, C₃₃, C₁₃, C₄₄ and C₆₆ by 15%, 25%, -0.5%, 33% and 75%, respectively, whereas a 50% increase of silt fraction increases the elastic constants by 36%, 70%, -7%, 83% and 220%, respectively, in comparison with the constants of pure clay. These non-uniform alterations of elastic constants might lead to changes in elastic anisotropy. Thomsen’s anisotropy parameters ε, γ, and δ which describe the variation of P- and SH-wave velocities as a function of polar angle with respect to symmetry axis are also calculated. ε decreases by 18% and 43% with increasing silt fraction from 0 to 25% and 0 to 50%, respectively. γ decreases by 15% and 36% by adding 25% and 50% of silt, respectively. δ decreases by 60% with increasing silt fraction from 0 to 25% and changes sign when silt fraction reaches 42%. The results show that the presence of silt in shales cannot be neglected as it substantially increases compressional and shear velocities and reduces anisotropy. Elastic moduli of shales are predicted using known silt fractions and elastic moduli of clays and compared with the results of laboratory measurements.

Key words::

• shale
• anisotropy
• clay elastic moduli
• silt fraction
• ultrasonic measurements