From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

Bureau Seminar, February 27, 2004

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

Carlos Torres Verdin, Department of Petroleum and Geosystems Engineering
The University of Texas at Austin

Abstract:

The focus of this study is an active over-pressured tight-gas reservoir. Conventional core porosity and permeability, together with capillary pressure were used to determine rock types and flow units in a key cored well. The dominant non-wetting phase saturation from capillary pressure curves is used to estimate pore throat radii and permeability at different confinement pressures. Subsequently, a study is performed to assess the influence of mud-filtrate invasion on borehole array induction logging tools. Flow units defined from core analysis are taken as horizontal layers to simulate the process of mud-filtrate invasion with a two-dimensional chemical flood simulator that includes the effect of salt mixing between mud filtrate and connate water. Radial resistivity profiles are obtained from the simulated profiles of water saturation and salt concentration using Archie's law. It is found that matching the simulated flushed-zone resistivities with the shallow resistivity readings requires that Archie's parameters, m and n, be specific functions of rock type.

Based on the analysis at the key well, detailed petrophysical assessments are performed in additional wells in the same field. Simulations of mud-filtrate invasion are carried out in each well with the only free parameter being absolute permeability. All of the remaining petrophysical parameters required for the simulation of invasion are either estimated from well logs or else extrapolated from the key well. Matching the shallow and deep resistivity logs with the corresponding resistivities yielded by the simulation of mud filtrate invasion provides the desired estimate of absolute permeability. This technique embodies a novel, efficient, and accurate methodology to estimate in-situ permeability from a basic suite of logs.