Experimental study of true triaxial stress-induced deformation and permeability anisotropy in sandstones, An

Abstract

Determination of stress-induced anisotropy of reservoir mechanical properties is essential for a number of areas that can be collectively termed as drilling risk reduction and optimization of well and reservoir productivity. Borehole stability, well completions, hydraulic fracturing, and production operations require correct analysis of deformational behavior under a general stress state (σ_1≥σ_2≥σ_3). Current attempts to capture the effect of stress state on rock deformational characteristics typically consist of conventional triaxial testing of core samples. However, there still remains an absence of experimental results on stress-induced anisotropy of deformational properties performed under true triaxial stress state. Such stress conditions allow for independent manipulation of three principal stresses and consequently, studying of the stress-induced anisotropy of static deformation, acoustic wave velocities, permeability, resistivity, and other anisotropic properties under a variety of stress states and magnitudes. A novel true triaxial testing apparatus was designed and built by Dr. Ali I Mese of Geomechanics Engineering and Research, PLLC, and has been loaned to UNGI to conduct measurements under realistic in-situ reservoir conditions using cylindrical cores samples. This study was performed to capture the true triaxial stress effects on the deformational and flow behavior of reservoir rocks. The apparatus has been calibrated and used to study the influence of realistic stress anisotropy on static deformation, acoustic wave velocity, and permeability in sandstone core samples. Through shear stress cycling at various b parameter values and octahedral normal stresses, it was determined that stress-induced anisotropy is a function of closing and opening of microfractures oriented normally to increasing stresses. Changes in the nondimensional stress parameter b, signifying the relative magnitude of intermediate principal stress to maximum and minimum stresses, influence the mechanical behavior of rock in both dry and water-saturated conditions. Permeability measurements in the axial direction also display a dependence on magnitude and state of stress.