Estimation of dry-rock elastic moduli based on the simulation of mud-filtrate invasion effects on borehole acoustic logs
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Reliable estimates of dry-rock elastic properties are critical to accurately interpreting the seismic response of hydrocarbon reservoirs. These estimates are needed as input for Biot-Gassmann fluid substitution calculations used in a wide range of applications for present-day reservoir characterization. We describe a new method for estimating elastic moduli of rocks in-situ by simulating the effect of mud-filtrate invasion on resistivity and acoustic logs. Simulations of mud-filtrate invasion account for the dynamic process of fluid displacement and mixing between mud-filtrate and hydrocarbons. The calculated spatial distributions of electrical resistivity are matched against resistivity logs by adjusting the underlying petrophysical properties. We then perform Biot-Gassmann fluid substitution on the twodimensional spatial distributions of fluid saturation with initial estimates of dry-bulk (kdry) modulus and shear rigidity ([Greek small letter mu]dry) and a constraint of Poisson's ratio (ν) typical of the formation. This process generates two-dimensional spatial distributions of compressional and shear-wave velocities, and density. Subsequently, sonic waveforms are simulated to calculate shear-wave slowness. Initial estimates of the dry-bulk modulus are progressively adjusted using a modified Gregory-Pickett (1984) solution to Biot's (1956) equation to estimate a shear rigidity that converges to the log value of shear-wave slowness. The constraint on Poisson's ratio is then removed and a refined estimate of the dry-bulk modulus is obtained by both simulating the acoustic log (monopole) and matching the log-derived compressional-wave slowness. This technique leads to reliable estimates of dry-bulk moduli and shear rigidity that compare well to laboratory core measurements. The resulting dry-rock elastic properties can be used to calculate seismic compressional-wave and shear-wave velocities devoid of mud-filtrate invasion effects for further seismic-driven reservoir characterization studies.