Reservoir Characterization Services
Pre-stack, Simultaneous Inversion
Using an initial low frequency model based on correlated well logs, as well as optimally processed angle gathers and angle dependent wavelets, pre-stack deterministic inversion simultaneously solves the Zoeppritz equations to produce p-impedance, s-impedance and density volumes. If sufficient angles are not available, density will be calculated as a scaled version of the p-impedance volume using log-based cross plot analysis.
Stochastic (Geostatistical) Inversion
Geostatistical inversion provides many realizations of reservoir characteristics that are all consistent with the input seismic gathers and the well control. It uses vertical and lateral variograms to control the high-resolution variation away from wells. The multiple realizations can be cascaded into Lithology identification, and used in uncertainty analysis and risk assessment, generating P10, P50 and P90 cases as well as the mean case which is equivalent to the Pre-stack deterministic inversion. Connectivity of lithofacies can also be explored and uncertainties quantified.
Inversion Attribute Analysis
Other geophysical attributes such as Lambda-rho, Mu-rho, Poisson’s Ratio and Young’s Modulus can be derived from the inversion volumes. Cross plotting attributes can separate reservoir from non-reservoir or identify fluid types. Prevalence maps can be made highlighting “sweet spots” based on reservoir analysis.
Lithology identification and uncertainty analysis
Facies logs based on geophysical cutoffs are used along with the products from the pre-stack inversion to perform seismic lithology prediction. Cross plots are made of elastic parameters from the full frequency well logs, and probability distribution functions (pdfs) are created for each defined facies. These pdfs are refined using well logs upscaled to seismic frequencies, and from the results of the pre-stack inversion. Next, using Bayesian probability theory, the pre-stack inversion results are classified to create probability volumes for each facies and a “most probable facies” volume. These facies volumes can be further analyzed for connectivity of reservoir or other facies.
AVO Feasibility Studies
Using wells with p-wave, s-wave and density logs, derived attributes such as Lambda-rho, Mu-rho and Vp/Vs are cross plotted at both log and seismic frequencies to determine if sufficient resolution and geophysical differentiation exists to separate reservoir from non-reservoir. Forward models are also created to identify expected AVO response and variations with rock properties.
3C-3D – Converted Wave Analysis
PP-PS registration services provide the link between your conventional p-wave seismic and separately recorded converted mode (PS) data. Joint PP-PS inversion improves inversion results over using simple pre-stack PP inversion.
For open fault and fracture characterization, Divestco employs Amplitude vrs Azimuth (AVAz) and Velocity vrs Azimuth (VVAz) analyses to determine fracture strike and crack density estimation using the full Azimuth nature of the 3D. AVAz methods analyze the amplitude variation by azimuth to estimate the orientation of open vertical fractures as well as their density. The Fourier Coefficient AVAz analysis employed by Divestco resolves the ambiguity that is inherent in the near offset Ruger equation in the calculation of the azimuth of anisotropy symmetry axis. VVAz methods analyze the time delay between reflection data travelling parallel to fractures and that travelling perpendicular to again derive fracture azimuth and density.
Rock Physics Modeling
Rock physics Modeling provides the link between reservoir engineering and seismically definable properties such as Vp, Vs and density. Elastic properties are predicted using theoretical relationships and bounds, and are modified to investigate the effects of changes in porosity, rock composition, fluid content and pore shape.
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