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Comparison of Total and Previous HitEffectiveNext Hit Water Saturations as a Way to Verify the Validity of Previous HitEffectiveNext Hit 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 Previous HiteffectiveNext Hit porosity
     Or:
     ΦT*(1-SWT)=ΦE*(1-SWE)
     Porosity * Hydrocarbons = Porosity * Hydrocarbons
     Where:
     ΦT = Total Porosity
     SWT = Total Water Saturation
     ΦE = Previous HitEffectiveNext Hit Porosity
     SWE = Previous HitEffectiveNext Hit Water Saturation
     This relationship implies that there are no hydrocarbons in the shales.
     Knowing total porosity, total water saturation and Previous HiteffectiveNext Hit porosity, Previous HiteffectiveNext Hit water saturation can be calculated.
     Previous HitEffectiveNext Hit porosity is calculated from total porosity, shale volume, and porosity reading in shale.
     ΦTE+VSHSH
     Where:
     VSH = Shale Volume
     ΦSH = Shale Porosity
     If either of the last two terms in inaccurate, the value of Previous HiteffectiveNext Hit porosity will be inaccurate.
     It can be shown that if either shale volume is overestimated and/ or shale porosity is overestimated, then calculations of Previous HiteffectiveNext Hit 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 Previous HiteffectiveNext Hit 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. Previous HiteffectiveNext Hit 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 Previous HiteffectiveTop 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