Anisotropy in deformations and hydraulic properties of Colorado shale
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AbstractThe structural integrity of Colorado shale, a natural cap rock for the Clearwater Formation reservoir is of prime importance from both operational management and geoenvironmental perspectives. This research was focused on the anisotropic investigation of deformation and hydraulic properties of the Colorado shale. Development of local strain measurement devices along with the confined torsion testing technique and triaxial testing technique on horizontally drilled shale core samples enabled the determination of static stiffness parameters over a wide range of strains. The results from the triaxial tests conducted on vertically and horizontally drilled core samples (E_v, v_vh, Eₕ, vₕₕ) were combined with the results from the confined torsion tests (G_vh) to evaluate all five independent transversely isotropic elastic parameters. At small strain, the Poisson's ratios are found to be zero. The anisotropy ratios Eₕ / E_v for the Second White Specks (SWS) and Westgate (WG) Formation shales are about 1.98. Likewise, the anisotropy ratios Gₕₕ/G_vh for those formations are about 1.86 and 1.50, respectively. Anisotropic hydraulic properties of the Colorado shale have been investigated by conducting steady and transient state water displacement tests on the vertically and horizontally drilled shale core samples. The results show that the permeability of the Westgate Formation shale is higher than that of the SWS shale (k_v(WG) =7k_v(sws) ). Similarly, the permeability along the horizontal direction is greater than that along the vertical direction (k_(h(sws)) = 5k_(v(sws))). The values of pore pressure parameters B and A_v for the Westgate Formation and SWS shale samples range from 0.30-0.72 and 0.20-0.35, respectively. The theoretical analysis and the test results further show that the Colorado shale samples are not at their full pore fluid saturations. The results from the confined creep tests and the unconfined creep tests conducted on the vertically and horizontally aligned shale samples show that the creep strain along the vertical direction (perpendicular to bedding) is much higher than that along the horizontal direction (parallel to bedding). Similarly, higher creep was observed in the Westgate Formation shale than that in the SWS shale. The numerical and experimental investigation of borehole heating of low-permeability shale indicated high propensity for formation fracturing in such shales.
Bibliography: p. 353-396