Evaluating Uncertainty in Archie's Water Saturation Equation Parameters Determination Methods

Abstract

Abstract Early in the life of reservoirs, it is required to estimate accurately hydrocarbon volumes in place. Modified Archie formula (Sw = (a Rw / φm Rt)1/n) is the basic equation to calculate water saturation. The exactness of water saturation value for given reservoir conditions depends on the accuracy of Archie parameters a, m and n. The terms of Archie relationship have been subjected to many laboratory investigations and even more speculation. There are many factors affect porosity exponent, m, saturation exponent, n and tortousity factor, a. Therefore, it is very difficult to fix Archie parameters regardless of reservoir characteristics; rock wettability, formation water salinity, permeability, porosity and fluids distribution. This paper presents a new technique to determine Archie parameters a, m and n. The developed technique is based on the concept of three dimensional-regression (3D) plot of water saturation, formation resistivity and porosity. 3D method provides simultaneous values of Archie parameters. Also, 3D method overcomes the uncertainty problems due to the separate use of formation resistivity factor-porosity and water saturation equations to get a, m and n parameters. Two field examples are given to show the applicability of 3D method and three other methods: 1) common values of Archie parameters, 2) conventional method and 3) core Archie parameter estimation (CAPE) method. The comparison between the four methods has shown that 3D method provides an accurate and physically meaningful way to get Archie parameters a, m and n for given core samples. Water saturation profiles, using Archie parameters deduced from the four methods, have been produced for the studied section in the wells. These profiles have shown a significant difference in water saturation values. This difference could be mainly attributed to the uncertainty level for Archie parameters from each method.