Comparison of Total and Effective Water Saturations as a Way to Verify the Validity of Effective Porosity Calculations
Holmes, Michael, Antony Holmes, and Dominic Holmes
Digital
Formation, Inc, Denver, CO
Ransom proposed that the following equality holds for standard
oil and gas reservoirs.
In place hydrocarbons in total porosity = In place hydrocarbons
in effective porosity
Or:
ΦT*(1-SWT)=ΦE*(1-SWE)
Porosity * Hydrocarbons = Porosity * Hydrocarbons
Where:
ΦT = Total Porosity
SWT = Total Water Saturation
ΦE = Effective Porosity
SWE = Effective Water Saturation
This relationship implies that there are no hydrocarbons in the
shales.
Knowing total porosity, total water saturation and effective
porosity, effective water saturation can be calculated.
Effective porosity is calculated from total porosity, shale
volume, and porosity reading in shale.
ΦT =ΦE+VSH*ΦSH
Where:
VSH = Shale Volume
ΦSH = Shale Porosity
If either of the last two terms in inaccurate, the value of effective
porosity will be inaccurate.
It can be shown that if either shale volume is overestimated and/
or shale porosity is overestimated, then calculations of effective
porosity are too low. As a consequence, values of SWE are too low or
even negative.
Examination of a large number of reservoirs shows that
traditional choices of shale porosity often give negative values of
effective water saturation. The incorrect choice of shale porosity will
often not be obvious if only total water saturation is considered.
A plot of total vs. effective water saturation can help in the
proper choice of shale porosity. A correct choice will result in data
with similar values of shale volume aligned linearly, with different
slopes converging to a single point where both total and effective
water saturation is one. An incorrect choice will lead to significant
dispersion of the alignments.
Examples from a variety of reservoirs are included.
AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah