Mechanical properties of the Niobrara

Abstract

The Niobrara Formation consists of alternating chalk and marl intervals. The chalk intervals comprise reservoir units while the marl intervals act as sources and seals. Mineralogy and kerogen content are vertically and laterally heterogeneous, and knowledge of mechanical properties improves calculation of minimum horizontal stress, a primary control of fracture growth. Permeability is enhanced by fracture networks, and the detection of these networks informs completion decisions. In this study laboratory ultrasonic and static measurements are used to derive relationships enabling the creation of an anisotropic stress model for a well in Wattenberg Field, CO, USA. The well log is also used to apply a fracture detection method that considers the fractured medium transversely isotropic with a horizontal axis of symmetry (HTI). Applying Hudson’s crack model to the laboratory anisotropy data serves as a quality check for the method. Dry rock velocity and strain measurements were taken for the D chalk and marl facies of the Niobrara. Mineralogy, TOC, and vitrinite reflectance data were obtained for the laboratory samples and the studied reservoir facies. The samples measured in the laboratory exhibit lower velocity pressure sensitivity than is observed in the studied well, which is related to peak maturity of the lab samples. The dry rock velocity measurements are used to obtain the Thomsen parameters for both facies. The chalk samples tested are isotropic, and the marl samples are weakly anisotropic. Anisotropy decreases with increasing confining pressure, and the observed trend is commensurate with laboratory tests of peak-mature shales (Vanorio, 2008). Saturated velocities from Maldonado (2010) were used to observe the anisotropic response to pore fluid pressure. ε and γ both decrease with increasing pore and confining pressure. γ exhibits a greater response to increasing pore fluid pressure than to confining pressure for the measured facies. Empirical relationships between the laboratory-derived Thomsen parameters are applied to the dipole shear log, which is treated as a dry rock measurement. Static and dynamic elastic parameters are measured to further describe the dry rock mechanical properties. The parameters are compared with Voigt-Reuss-Hill effective medium theory and employed in uniaxial strain approximations of the minimum horizontal stress. The anisotropy present in the study well is considerably lower than that measured in the laboratory samples. The estimation of δ allows the application of the ANNIE approximation. The approximated constants are corrected to the static case using the laboratory-derived ratios, and the errors resultant from utilizing dynamic constants and assuming isotropy are discussed. Fracture density is estimated using a method developed by Conoco (Sil, 2013). The method considers the formation a homogenous, horizontal transverse isotropic (HTI) medium consisting of vertical, parallel sets of fractures. In Conoco’s method δN, the normal fracture weakness, is used to derive the fracture density, e. δT, the tangential fracture weakness, replaces δN in this method since δT is less prone to second-order effects. Fracture density is compared to the laboratory-derived anisotropy parameters using Hudson’s crack model.