Assessment of Permeability from Well Logs Based on Core Calibration and Simulation of Mud-Filtrate Invasion

Abstract

This paper describes the application of a new methodology to estimate the permeability of clastic rock formations based on numerical simulation of the physics of mud-filtrate invasion. The methodology assumes a key well with a complete suite of well logs and laboratory core measurements of porosity, permeability, capillary pressure, and relative permeability. For additional wells in the same field, absolute permeability is estimated by matching shallow resistivity logs with the electrical resistivity in the flushed zone yielded by the simulation of mud-filtrate invasion. We describe the application of this methodology to the estimation of permeability in three wells penetrating the same over-pressured tight gas sands of the East Texas Bossier Formation. Conventional core porosity and permeability measurements, along with capillary pressure, are used to determine rock types and flow units in the cored key well. Estimates of permeability based on a modified version of Winland's equation agree with the measured permeability of the available core samples. Simulations of mud-filtrate invasion account for the dynamic process of immiscible flow between mud filtrate and in-situ gas as well as for salt mixing between mud filtrate and connate water. Moreover, the simulations properly reproduce the effect of mudcake buildup along the borehole wall. Two-dimensional spatial distributions of water saturation and salt concentration obtained from the simulations are used to calculate spatial distributions of electrical resistivity. The latter are checked against the measured shallow, medium, and deep resistivity logs to calibrate time of invasion and Archie's saturation and cementation exponents. Based on the analysis in the key well (Well 1), petrophysical assessment of flow units is performed in two nearby wells (Wells 2 and 3) within the same gas field. Initial Winland permeability values are progressively adjusted until the calculated spatial distributions of electrical resistivity agree with the shallow array-induction resistivity readings. We find that for Well 2, the estimated permeability is equal to the initial guess, whereas for Well 3 the estimated permeability is approximately 50-80% higher than the initial guess.