Impact of anisotropic poroelastic parameters estimated using well logs and core measurements on stress prediction in organic-rich mudrocks

Abstract

The evaluation of rock mechanical properties in organic-rich mudrocks is challenging because of their heterogeneity, anisotropy, and complex lithology. Hence, geomechanical analysis should include a complete definition of the anisotropic elastic stiffness coefficients for stress prediction. Furthermore, typical assumptions made for poroelastic parameters, such as assuming that Biot’s parameter is equal to one, can result in unreliable geomechanical evaluation for stress prediction and failure in completion design. Our objectives included estimating (1) anisotropic elastic properties and (2) anisotropic poroelastic parameters and minimum horizontal stresses, using laboratory measurements and well logs, in organic-rich mudrocks with different levels of mechanical anisotropy and vertical heterogeneity. We have performed mechanical tests on core plugs and obtained correlations between the dynamic and static elastic stiffness coefficients. We have analyzed advanced acoustic well logs, as well as laboratory geomechanical measurements, to estimate dynamic elastic stiffness coefficients of the formations under investigation. The estimates of mineral composition, in conjunction with the estimates of elastic stiffness coefficients, yielded the anisotropic poroelastic parameters. Finally, we have estimated the stress profile under the assumption of transverse isotropy. We have quantified the impact of anisotropic elastic rock properties and poroelastic parameters on stress prediction in the lower Eagle Ford, Haynesville, and upper Wolfcamp Formations. The estimated minimum horizontal stress gradient, assuming anisotropic elastic properties and anisotropic poroelastic parameters, varied by approximately 30% (relative difference) compared with the case in which these parameters were assumed to be equal to one. This variation decreased to approximately 15% and 6% when the poroelastic parameters were assumed to be equal to 0.7 in formations with high and relatively low levels of heterogeneity and mechanical anisotropy, respectively. The results confirmed that transversely isotropic media require a thorough depth-by-depth estimation of anisotropic poroelastic parameters to estimate stress profile accurately. We have also determined the importance of integrated interpretation of petrophysical, compositional, and mechanical properties in geomechanical evaluation in organic-rich mudrocks.