Gas shale; Comparison between permeability anisotropy and elasticity anisotropy

The studied Gas Shale especially the silica-rich gas-shale samples displayed permeability and elasticity anisotropy behavior. These anisotropy behaviors are closely correlated in terms of the symmetry directions which means the elastic anisotropy and permeability anisotropy share the same cause. All samples, regardless of the measurement direction, show a nonlinear reduction in permeability with increase of effective pressure (up to 3 orders of magnitude), with large variations from sample to sample and measurement direction. While the elasticity showed the less sensitivity to the effective pressure change, the velocities increases with increasing the effective pressure. The permeability and the elasticity anisotropy behaviors showed also a good correlation with quartz and clay content. Introduction Recently, unconventional reservoirs such as gas shales, coal bed methane and tight-gas sand have become significant producers of domestic natural gas and offer tremendous potential for future gas reserve estimates and production. Success in tight-gas shales has renewed interest in understanding gas flow in these reservoirs as well as in predicting permeability (Nano Darcy) at different scales. Lab measurements reflect only the matrix permeability within a small volume; however, it is well accepted that the effective permeability depends, not surprisingly, on the observation scale. Meanwhile, the commonly used empirical permeability-porosity relationships such as the one that introduced by Odong (2007), Gueguen and Palciauskas (1994) have not worked correctly for shale and are unable to explain shale permeability anisotropy because they neglect the surface roughness and connectivity between pores and cracks (Metwally & Chesnokov, 2011). Therefore, establishing a relationship between the elastic wave and the permeability at different frequencies (scales) would be enormously important for the oil and gas industry. It is well known that fluid flow causes significant attenuation and dispersion of elastic waves. Therefore, measurements of attenuation and its frequency-dependence have the potential to provide estimates of permeability using elastic wave properties (Berryman and Wang, 2000; Muller and Gurevich, 2005; Goloshubin et al. 2008; Muller et al, 2010; and many other). However, the current study is devoted to highlight the correlation between permeability and elasticity behavior using some experimental results in the laboratory. Permeability Tensor Meserments Permeability measurements were done using a quasi–steady flow technique (Metwally and Sondergeld, 2011). This technique gives the ability to measure the axial permeability of three differently oriented plugs simultaneously under the same in-situ conditions. In addition and in the case of using gas as the test fluid, this technique gives the ability to measure both gas and Kinkenberg permeabilities (liquid permeability). The measurements were done using a custom built apparatus as the one that described by Metwally and Chesnokov (2010 & 2011). This apparatus can be loaded by three oriented plugs (1 inch by 2 inch). The plugs are prepared from three different directions (0, 45, and 90 relative to the shale’s symmetry axis) through the oriented cores of the shale. Preparation of the plugs was described by more details on Metwally and Chesnokov (2011). The three plugs technique allows us to reconstruct the permeability tensors for these samples. Since the bedding plane is the symmetry plane, the permeability in this plane is independent of the azimuth and the components of the permeability tensor and is equal to one another. The permeability measured normal to bedding gives the value of the tensor. If the diagonal components of the permeability tensor are known, the permeability for any direction specified by the direction cosines n1, n2, n3 will be calculate based on the following; The VTI media has symmetric second rank permeability tensor: