Quantitative Formation Permeability Evaluation from Stoneley Waves

Abstract

Abstract Reservoir engineers need a continuous measurement of permeability to optimize well completion and produce the formation effectively. The borehole Stoneley wave has been shown to be sensitive to fluid mobility, the ratio of permeability and viscosity. The phenomenon is well described using the Biot theory, and models to account for the effect of the mudcake have been proposed. Existing techniques use either the Stoneley slowness or its attenuation at low frequency to estimate the fluid mobility with simplified Biot models. However, the results of these techniques usually show unexplained differences from core measured permeabilities, or with well test results. The objective of the methodology presented in this paper is to improve the reliability of the estimation and to take into account important factors that have been neglected. The evaluation technique described in this paper uses the Stoneley waveforms over a wide frequency range and inverts for fluid mobility applying the full Biot theory. The mudcake is modeled as an elastic membrane. The most important parameters in this evaluation are the mud slowness, mud attenuation, membrane stiffness and pore fluid compressibility. Practical techniques are proposed to obtain these parameters with sufficient accuracy from the recorded data. Stoneley waveform inversion provides a direct and continuous fluid mobility estimate near the wellbore (i.e., in the invaded zone). The intrinsic permeability of the rock can then be derived, provided that the various fluid components, their relative permeabilities and their viscosities are known. Finally the presence of irreducible water in the rock is also discussed as it can affect the estimation of mobility through the reduction of the effective porosity. The application of this methodology to actual data sets is the subject of an accompanying paper. The Stoneley wave provides a continuous estimation of the formation permeability along the well. Core measurements are not required although they can be used for verification. This technique will find applications in reservoir engineering optimization of well production through better placement of the perforated intervals. P. 389