Swami, Vivek (CGG) | Tavares, Julio (CGG) | Pandey, Vishnu (CGG) | Nekrasova, Tatyana (CGG) | Cook, Dan (Bravo Natural Resources) | Moncayo, Jose (Bravo Natural Resources) | Yale, David (Yale Geomechanics Consulting)
In this study, a state-of-the-art seismic driven 3D geological model was built and calibrated to a petrophysical and geomechanical analysis, 1D-MEM (Mechanical Earth Model), on chosen wells within the Arkoma Basin of Oklahoma. The well information utilized in this study included basic wireline logs and core analysis, including XRD (X-Ray diffraction) data. The traditional petrophysical analysis was augmented with advanced rock physics and statistical techniques to generate the necessary logs. Hydrostatic, overburden and pore pressures were calculated with a petrophysical evaluation model. The 1D-MEMs were based on the Eaton/Olson/Blanton approach with the HTI (Horizontal Transverse Anisotropy) assumption. The 1D-MEMs were calibrated to laboratory data (triaxial tests) and field observations (mud logs, wellbore failure, frac pressures). Therefore, a very good confidence was achieved on Biot's coefficient, tectonic components, anisotropy and dynamic to static conversion factors for Young's Modulus and Poisson's Ratio. Seismic inversions were performed in different time windows and merged to generate high resolution P- and S-Impedance attributes from surface down to the target interval after careful AVO compliant gather preconditioning. A density volume estimate was calibrated to well data, accounting for different geological formations, to decouple P- and S-Wave components as a 3D volume, as well as dynamic Young's modulus (E) and Poisson's ratio (PR). Dynamic E and PR were converted to static parameters using results from 1D-MEMs; and 3D models of Biot's coefficient (α) and tectonic components were built to compute 3D fracture pressure volumes calibrated to well data. The final products were seismic-driven 3D pore pressure and fracture pressure calibrated to 1D-MEMs. The correlation between measured/estimated well logs and corresponding seismic-derived pseudo logs was more than 80%, which indicates good quality of seismic inversion results and hence 3D-MEM. Also, stress barriers, anisotropy, and brittleness indices were calculated on well scale which would help to identify best zones to place hydraulic fractures. The 3D geological model will aid in identifying sweet-spots and optimizing hydraulic fractures.
Copyright 2018, Unconventional Resources Technology Conference (URTeC) This paper was prepared for presentation at the Unconventional Resources Technology Conference held in Houston, Texas, USA, 23-25 July 2018. The URTeC Technical Program Committee accepted this presentation on the basis of information contained in an abstract submitted by the author(s). The contents of this paper have not been reviewed by URTeC and URTeC does not warrant the accuracy, reliability, or timeliness of any information herein. All information is the responsibility of, and, is subject to corrections by the author(s). Any person or entity that relies on any information obtained from this paper does so at their own risk. The information herein does not necessarily reflect any position of URTeC. Any reproduction, distribution, or storage of any part of this paper by anyone other than the author without the written consent of URTeC is prohibited. Abstract Standard seismic/ acoustic log Pp prediction techniques developed for young sediments in offshore basins are not very effective in unconventional reservoirs. The age and lithification of shale reservoirs, the variability in lithology, and different overpressure generation mechanisms and basin histories all lead to poor quality predictions using standard Eaton or Bowers methods. But Pp prediction remains important in unconventional reservoirs due to the correlation between overpressured areas and productivity, and the correlations between thermal maturity and pore pressure. We have developed a method that extends the theoretical basis of the Eaton and Bowers methods to the geologic and basin history conditions of unconventional reservoirs. The method has been developed using standard log suite along with dipole acoustic logs.
For perfectly elastic materials, the mechanical properties calculated from measured compressional and shear acoustic velocities through the materials (dynamic moduli) are the same as those calculated from measurements of strain response of the material due to an applied stress during standard triaxial compression testing (static moduli) (Ledbetter, 1993). However, nearly all geomaterials are not perfectly elastic and thus the dynamic properties calculated from log or seismic velocities can be quite different from static properties calculated from triaxial testing of core material. The static properties are much more indicative of the deformation that occurs in the subsurface due to the production or injection of fluids into the subsurface, the deformation of subsurface formations due to tectonically applied loads, or the removal of material from the subsurface due to mining. Therefore, correlations between the dynamic and static properties are critical to using log or seismic based measurements to get a much wider understanding of mechanical properties in the subsurface than can be obtained from limited sampling and testing of core materials brought up from the subsurface. Geomechanical modeling to predict deformation and stress development in the subsurface needs both the broad understanding of the variability of moduli throughout the subsurface formations of interest as well as the most accurate values of properteis representing the type of deformation being modeled. Many studies over the past few decades have examined both the relationship between static and dynamic moduli and the causes of the differences between the moduli. The studies appear to point to a wide range of dynamic to static correlations and strong influence of different lithologies, depositional environments, and rock properties on the correlations. However, our analysis of the results from over 50 published papers leads to both a reasonably consistent rationale for the differences between dynamic and static moduli and a set of correlations that can be applied to a wide range of geomaterials. Young's moduli and Poisson's ratio are the most common pair of mechanical properties used in geomechanics and their relationship to the compressional (Vp) and shear (Vs) seismic or acoustic velocities of a material are given below.