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A Method to Quantify Gas Saturation in Gas/Water Systems, Using Density and Neutron Logs – Interpretation of Reservoir Properties When Compared With Gas Saturations from Resistivity Analysis

Holmes, Michael, Antony Holmes, and Dominic Holmes
Digital Formation, Inc, Denver, CO

     A standard approach to evaluate gas effects on porosity logs is the “density/neutron cross over” response. In the presences of gas, bulk density is reduced, and the neutron log is suppressed.
     The degree of cross over can be related quantitatively to gas saturation, so long as accurate knowledge of matrix lithology is available. In the calculations presented in this paper, porosity calculations (lithology corrected) for the density and neutron logs are compared with the cross plot density/neutron porosity. This latter calculation requires no input of matrix properties and, in gas/water systems, is relatively insensitive to fluid content.
     Differences between the individual porosity log calculations and cross plot porosity yield quantified estimates of gas saturation for each log individually. These estimates, when compared with standard resistivity modeling of gas saturation can be used to gain insight into gas reservoir characteristics:
     If gas saturations agree, the conclusion can be drawn that all sources of petrophysical data are consistent, and the model is robust. Agreement also suggests that all sources of data are equally affected by the wellbore environment, i.e. the porosity logs have not been influenced by invasion.
     If, as is common, gas saturations from porosity logs are significantly less than that derived from resistivity analysis, a number of possible explanations exist:
     Matrix properties are inaccurate.
     There has been pervasive invasion by mud filtrate, with extensive flushing of gas away from the wellbore.
     The calculations of shale volume are inaccurate – for example presence of kaolin that a gamma ray measurement might not detect.
     Presence of fresh water sands, with high values of water resistivity that have been mistaken for gas-bearing sands when analyzed by resistivity modeling.
     Examples from tight gas sands of the Rocky Mountains are presented, to show variable reservoir responses as outlined above.

 

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah