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:
     Φ_T*(1-S_WT)=Φ_E*(1-S_WE)
     Porosity * Hydrocarbons = Porosity * Hydrocarbons
     Where:
     Φ_T = Total Porosity
     S_WT = Total Water Saturation
     Φ_E = Effective Porosity
     S_WE = 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.
     Φ_T =Φ_E+V_SH*Φ_SH
     Where:
     V_SH = Shale Volume
     Φ_SH = 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