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Experimental evaluation of dynamic elastic properties and anisotropy in shales
Panfiloff, Andre
Panfiloff, Andre
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2016
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2016-08-15
Abstract
Dynamic elastic mechanical properties and transverse anisotropy in shales are very important to consider in estimation of the in-situ stress, petrophysical and geophysical analyses. Typically, they are calculated based on the acquired ultrasonic velocities under simulated close to in-situ conditions in a laboratory environment. Specifically, compressional and shear velocities are measured in 0°, 45°, and 90° orientation to bedding plane of the particular shale sample. In previous studies, these measurements were accomplished using three-plug-method, which would require to core three independent core plugs oriented horizontally, in 45°, and vertically. In this study, we designed and implemented a special core holder to perform non-destructive, efficient, reliable, multidirectional, and simultaneous ultrasonic compressional and shear velocity measurements on the single 1.5 in. cylindrical core plug. An additional important aspect of this research is to study the effects of clay, organic matter abundance and maturity, porosity and pore throat sizes on elastic properties and anisotropy in shales. In order to implement this task, eleven shale core plugs and a series of laboratory measurements are employed. The X-ray computed tomography is utilized to observe overall condition of the shale rock samples. Specifically, it is important to identify orientation of the bedding plane, possible microfractures, and potential presence of chemically altered minerals. Water immersion porosity (WIP) method is used to estimate bulk density, grain density, and porosity of the shale specimens. The nitrogen adsorption technique is applied to estimate an average pore size distribution (PSD), pore volume, and specific surface area (SSA) parameters, and, yet again, ultimately porosity. Bulk mineralogy composition is estimated using X-ray diffraction analysis. Finally, LECO TOC and source rock analysis (SRA) are utilized to provide insight into organic matter richness and maturity. My results provide an insight into the elastic mechanical behavior and the degree of anisotropy that organic shales may experience under in-situ conditions. Specifically, I find that the Young’s moduli in the direction parallel to the bedding plane is greater than perpendicular to it. Poisson’s ratios have mixed results with higher uncertainty. The degree of anisotropy in terms of Thomsen anisotropy parameters and horizontal to vertical ratio of Young’s moduli have been estimated under elevated pressures on the up and down cycles. It was observed that anisotropy decreases dramatically with increase in pressure, but does not approach zero. It was concluded that this observed phenomena at high confining pressures may potentially be explained by the existence of some degree of intrinsic anisotropy in organic matter and clay particles. The estimation of Young’s moduli in vertical and horizontal directions has been investigated based on the application of the two different sets of equations. One is the appropriate isotropic equations, and the second is VTI equations. It was discovered that the degree of discrepancy between estimation of Young’s moduli by these two methods is on the order of 15%. This result is a very important finding. Thus, it is crucial to obtain an accurate direct measurement of the C13 stiffness coefficient in order to have true estimation of the vertical and horizontal Young’s moduli. Furthermore, the effects of clay, abundance and maturity of organic matter on the degree of anisotropy has been investigated. Directly proportional correlation between clay content and the degree of anisotropy has been established. Impact of maturity of organic matter on velocity anisotropy has been observed and combined with findings by Vanorio et al. (2008). It this study it was confirmed that the degree of anisotropy is affected by maturity level of kerogen, %Ro, in a very complex nature due to laminated distribution of kerogen itself and due to creation of the microfractures and void space in organic matter during maturation. The mineralogy data and LECO TOC data for shales involved in this study have been combined with the appropriate data from Sone (2013) and Passey et al. (2010) to establish origins for the L-set, K-set, and C-set. It was concluded that these shale core samples potentially may be related to the Haynesville/Bossier and the Eagle Ford Formations.
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