Cumulative and Average Reservoir Properties
The reservoir volume and flow capacity per unit area are steps toward finding total reservoir volume. Average values for comparing the quality of reservoirs are also useful results from log analysis. Pore volume (per unit area), hydrocarbon pore volume, flow capacity, and the averages of core porosity, water saturation, permeability, net pay, net reservoir, net sand, and gross sand are called mapable properties, petrophysical properties, or reservoir properties.

HPV - Cumulative Reservoir Properties
Pore volume (PV).
    5: PV = SUM (PHIe * THICK * PAYFLAG)

Hydrocarbon pore volume (HPV).
      6: HPV = SUM (PHIe * (1 - Sw) * THICK * PAYFLAG)

Flow capacity (KH).
      7: KH = SUM (Perm * THICK * PAYFLAG)

Average porosity.
      8: PHIavg = PV / Hnet

Average water saturation.
      9: SWavg = 1 - (HPV / PV)

Average permeability.
   a. Arithmetic average:
      10: Kavg = KH / Hnet

   b. Geometric average:
      11: Kgeo = (PROD (Perm * THICK)) ^ (1 / Ns)

   c. Harmonic average:
      12: Khar = Hnet / (SUM (1 / (Perm * THICK)))

WHERE:
  Hnet = net pay thickness (ft or m)
  HPV = hydrocarbon volume (ft or m per unit area)
  Kavg = arithmetic average permeability (md)
  Kgeo = geometric average permeability (md)
  KH = flow capacity (md-ft or md-m per unit area)
  Khar = harmonic average permeability (md)
  Ns = number of samples in product
  Perm = permeability (md)
  PHIavg = average porosity (fractional)
  PHIe = effective porosity (fractional)
  PV = pore volume (ft or m per unit area)
  SWavg = average water saturation (fractional)
  THICK = individual layer thickness (ft or m)

COMMENTS:
Do not use the following algorithm in thinly laminated shaly sands - see alternate method shown below.

The harmonic average most closely reflects radial flow into a borehole. If equal sample intervals are used, this geometric formula becomes: Kgeo = (PROD (Perm * INCR)) ^ (1 / INCR). where INCR = data digitizing increment. It does not give the same result as the previous version if layer thicknesses are unequal.


Sample net pay table with average and cumulative mapable properties

NUMERICAL EXAMPLE:
1. Assume three layers as follows:
       Layer       PHIe      Sw      Perm      THICK (ft)
           1           0.10      0.60        10            2
           2           0.20      0.50      100            4
           3           0.30      0.40    1000            6

Assume all layers pass all cutoffs:
  PV = 0.10 * 2 + 0.20 * 4 + 0.30 * 6 = 2.8 ft
  HPV = 0.10 * (1 - 0.60) * 2 + 0.20 * (1 - 0.50) * 4 + 0.30 * (1 - 0.40) * 6 = 1.56 ft
  KH = 10 * 2 + 100 * 4 + 1000 * 6 = 6420 md-ft
  Hnet = 2 + 4 + 6 = 12 ft
  PHIavg = 2.8 / 12 = 0.233
  SWavg = 1 - 1.56 / 2.8 = 0.443
  Kavg = 6420 / 12 = 535 md
  Kgeo = (10 * 2 * 100 * 4 * 1000 * 6) ^ (1 / 3) = 363 md
  Khar = 12 / (1 / (10 * 2) + 1 / (100 * 4) + 1 / (1000 * 6)) = 228 md

If equal sample intervals are used, (with INCR = 1.0),
  Kgeo = 215 md


 

"META/LOG" SPREADSHEET -- Log Analysis Sums and Averages
This spreadsheet provides a tool for analyzing and summarizing log data in a consistent format. It calculates porosity and permeability averages, suns pore volume, hydrocarbon pore volume, flow capacity, and net pay with user defined cutoffs in a table identical to that created by the "META/COR" spreadsheet for log analysis, making it easy to compare log analysis results to core data.

  Log Analysis Spreadsheet for Conventional Oil and Gas, includes lithology, net pay, productivity, and reserves. English and Metric Units.
 




Exanple of "META/LOG" input data and crossplots. Intermediate calculations are performed offscreen to the right. The Summary Table (shown below0 is also off to the right.


Individual Answers and Summary Table from "META/LOG" log analysis spreadsheet. Compare to core analysis results shown below.


Summary Table from "META/COR". Compare values to "META/ESP" log analysis Summary Table shown above.
 

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