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