Uncertainty in Fluid Substitution using Gassmann's Equation and Implication for DHI analysis and Time-lapse Monitoring

Abstract

Abstract Gassmann's equation may not be directly applicable for amplitude variation with offset (AVO) modeling due to the violation of its assumptions (low permeability, highly compressible mineral components, invasion and dispersion), the high sensitivity to input parameters which may be in error, the variations in rock frame porosity and compressibility between gas and brine saturated rocks due to the lateral or vertical geological variability or differences in digenesis or cementation due to different pore fluids, the chemical effects at very low effective stress, frame hardening or softening and the heterogeneous fluid distribution spatially and between pores of various sizes and shapes, possible in applicability of wood's equation. This study investigates the uncertainty in gassmann's equation and the implication in direct hydrocarbon indicator (DHI) analysis and time lapse monitoring. Gassmann's equations are only applicable to clean, highly permeable reservoir rocks. When gas is substituted for brine using gassmann's equations, predicted compressional wave velocity exhibits a wide range of possible values. This range is larger than the expected change in velocity. A single deterministic gassmann result is only one of many possible stochastic realizations. The maximum, mean, median, modal and minimum values of the many stochastic realization of fluid modulus, fluid density, bulk modulus, compressional wave velocity, acoustic impedance, shear impedance and fluid factor can be used in the interpretation. The resulting range of possible stochastic realizations of modelled compressional wave velocity ratio between the two bounding media, possible stochastic realizations of modelled density ratio between the two bounding media, possible stochastic realizations of poisson's ratio in the upper medium and possible stochastic realizations of poisson's ratio in the lower media gives rise to various possible stochastic realizations for direct hydrocarbon indicator (DHI) prestack data and time lapse post stack data.