Estimates of in situ stresses and their changes upon production can be greatly influenced by the choice in poroelastic parameter values such as Biot coefficient and pore compressibility. In the determination of these parameters, grain compressibility and its anisotropy have emerged as the most difficult quantities to ascertain. The objective of this study is to investigate the microstructural controls on the effective grain compressibility as measured in the laboratory at close to in situ conditions, with the dual goal of incorporating microstructural information into grain compressibility estimates and producing better interpretations of laboratory measurements.
A suite of ten samples from Bone Spring and Wolfcamp Formations covering a range of compositions was measured in the laboratory for anisotropic static elastic properties, including grain compressibility. A microstructural study involving X-ray radiography and SEM imaging was then carried out to investigate the contributions of texture (phase distribution and load bearing fraction) and fabric (anisotropy) to effective anisotropic grain compressibility.
The measurements reveal anisotropy in directional grain compressibility of up to 70%, which, when combined with the anisotropic rock elastic moduli, produced values of Biot coefficient between 0.4 and 0.8 with up to 30% anisotropy between the horizontal and vertical directions. Mineralogical information points to a possible control of the clay fraction on the grain compressibility anisotropy. The microstructural study was thus conducted with a focus on testing the clay contribution hypothesis. While the fabric at the sample scale does not appear to be a strong contributor to the measured rock and grain elastic anisotropy, high resolution SEM imaging suggests a close relationship between pore space morphology as a proxy for the abundance of 'wet' clay aggregates, and effective grain elastic anisotropy.
Our findings suggest that anisotropic grain stiffness values might be meaningfully inferred using microstructural information as well as mineralogy and density, thereby providing a rapid tool for the determination of poroelastic parameter sets in shale where direct measurements are not available.