Exploring Deep Carbonate Reservoir Samples: Anisotropy and Correlation of Static and Dynamic Young's Moduli

In total, 14 limestone samples, drilled in the vertical and horizontal directions from a depth >3 km, are analyzed to understand the correlation between static (operating at in-situ strain levels) and dynamic (at a sound wave strain level) Young's moduli, E-s and E-d, and also to investigate their directional dependence. Young's moduli are measured simultaneously under a confining pressure of 30 MPa to a differential axial stress of 50 MPa using a triaxial frame with linear variable differential transformers: two axial and one radial calibrated with a standard aluminum slug. Static and dynamic moduli decrease with increasing porosity whereas the static moduli are typically less than the dynamic moduli. The static measurements exhibit significant hysteresis indicating a nonlinear (irreversible) regime, whereas the dynamic moduli, measured with low-strain amplitude, indicate a linear (reversible) regime. The static Poisson's ratio increases with porosity. but the dynamic ratio remains unchanged over the whole range of porosity, which seems to not be sensitive enough to the heterogeneity in the pore structure. The induced microfractures, naturally developed under tensile stress after these deep samples were brought to the surface, are imaged in computer microtomography (micro-CT) scan experiments. The mineralogical composition is determined as higher than 95% calcite CaCO3 and mapped out using quantitative evaluation of minerals by scanning electron microscopy. Elastic anisotropy is observed with increasing porosity, where the ratio E-s/E-d increases for the vertical but decreases for the horizontal samples. Likewise, a permeability anisotropy also is observed as porosity increases, where the ratio of the vertical to horizontal permeability k(v)/k(h) is higher than one, indicating a preferred vertical structure for fluid flow. Two linear correlations between E-s and E-d are developed for the studied calcite samples, and for samples composed of up to 30% dolomite CaMg(CO3)(2) reported in the literature.