Wellbore Stability Analysis in Laminated Shaly Formations Through Anisotropic Geomechanics and Advanced Sonic Measurements
Authors: Abdi Hadi Ashtiani , Alward Wassem 
Published: May 2024

Sedimentary rocks naturally exhibit some degree of anisotropy due to deposition. Conventional geomechanical analyses frequently overlook the influence of anisotropy by employing a one-dimensional isotropic model, wherein mechanical properties are assumed to be uniform in all directions. In formations that exhibit vertical transverse isotropic (VTI) anisotropy such as layered bedding with shales, elastic properties such as Young’s modulus, Poisson’s ratio, and shear modulus within a given bed plane differ from those perpendicular to the bed plane. To develop a more accurate and reliable mud-weight window that accounts for the presence of shale anisotropy, Anisotropic stress models need to be applied.

In this study, we employ a comprehensive approach, integrating advanced sonic measurements with prior knowledge, to perform inversion for the elastic properties of transversely isotropic formations in an example well. The sonic tool multiple monopole and dipole transmitters accurately measure at multiple depths of investigation to provide a fully 3D acoustic characterization that addresses both intrinsic and drilling-induced anisotropy. The sonic measurements encompass compressional, dipole fast shear, slow shear, and stoneley shear, collectively providing essential data for determining five independent transversely isotropic formation properties: horizontal and vertical Young's modulus, horizontal and vertical Poisson ratio, and vertical shear modulus. Utilizing these elastic properties in conjunction with pore pressure and overburden gradient, we apply an anisotropic poroelastic model to compute mechanical properties and horizontal stresses. The incorporation of an integrated workflow, featuring anisotropic stress modeling, proves instrumental in effectively addressing and resolving challenges associated with predicting wellbore failure in these vertical transversely isotropic (VTI) formations.

Two Mechanical Earth Models (MEMs), isotropic and anisotropic, were constructed. Anisotropic stress modeling, incorporating 3D advanced sonic processing inputs, was specifically applied in zones characterized as Vertical Transversely Isotropic (VTI) formations. Within these intervals, horizontal stresses exhibited increased magnitudes and stress profiles diverged from the isotropic scenario. The anisotropic VTI model demonstrated enhanced predictive accuracy for breakout pressure gradients, aligning closely with the observed breakouts in the image well log. In contrast, the isotropic mechanical model failed to predict any breakouts. The utilization of an anisotropic model to define the mud-weight window resulted in significant improvement of wellbore stability while reducing risks associated with drilling wells in these VTI formations.

The paper addresses the commonly overlooked influence of anisotropy in sedimentary rocks, particularly in formations with vertical transverse isotropy (VTI). It also demonstrates the application of an integrated workflow that utilizes anisotropic stress modeling and advanced sonic measurements to address challenges related to predicting wellbore failure in VTI formations. The incorporation of anisotropic poroelastic modeling, featuring five independent transversely isotropic formation properties, contributes to a more realistic representation of the mechanical behavior of the rock.