The Meta/Kwik spreadsheet for this
model is available at
Downloads and Spreadsheets. Read comments below BEFORE using this model.
In shaly sands, convert logs into sandstone units and enter the
uncorrected density and neutron log readings into the following
equations: This method calculates the shale correction based on the density neutron crossplot shale volume (Vshx). If this is not the shale volume method you want to use, DO NOT USE THIS POROSITY METHOD. This method is pessimistic if heavy minerals exist in the sandstone. Since nearly every sandstone contains minerals other than quartz to some degree (such as mica, volcanic rock fragments, dolomite or calcite cement), the shaly sand model is almost ALWAYS PESSIMISTIC. Methods listed below are also better. However, this shaly sand model has been very widely used and it can still be found in every computer-aided log analysis package. I strongly recommend NOT using this model.
7: PHIdc = PHID - Vsh * PHIDSH 8: PHInc = PHIN - Vsh * PHINSH 9: IF PHInc < 0 10: THEN PHInc = 0 11: PHIxdn = PHIdc + (PHInc - PHIdc) / 3 Where: PHID = density porosity log reading (fractional) PHIdc = density log porosity corrected for shale (fractional) PHIN = neutron porosity log reading (fractional) PHInc = neutron log porosity corrected for shale (fractional) PHIDSH = density log shale porosity (fractional) PHINSH = neutron log shale porosity (fractional) PHIxdn = porosity from density neutron crossplot (fractional) Vsh = shale volume (fractional)
COMMENTS:This formula is effective even in gas zones and is based on the
premise that the neutron log sees deeper into the zone than the
density log, and thus has more gas effect.Shale corrections could create apparent gas crossover and this may be real or an artifact of excessive correction. Check against known data from the well if shale correction creates crossover.
PHID = 0.12 PHIN = 0.30 PHIDSH = 0.03 PHINSH = 0.30 PHIxdn2 = (0.12 * 0.30 - 0.28 * 0.03) / (0.30 - 0.03) = 0.100 The shale volume was 0.59 using this data. Since this value is too high compared to the shale content from the GR, the porosity from this method will be too low. 2. If this porosity is unacceptably low, use the complex lithology formula with Vsh of your choice, as shown below. Given Vsh = 0.33 from GR, with other data as before, then: PHInc = 0.30 - 0.33 * 0.30 = 0.20 PHIdc = 0.12 - 0.33 * 0.03 = 0.11 PHIxdn = (0.20 + 0.11) / 2 = 0.155 3. The alternate quick method gives, for Sand "D" PHInc = 0.30 - 0.33 * 0.30 = 0.20 PHIdc = 0.12 - 0.33 * 0.03 = 0.11 PHIxdn3 = 0.11 + (0.20 - 0.11) / 3 = 0.140
Shaly Sand
Crossplot (Density Neutron) with Matrix OffsetThis method is more long-winded than the quick method and accounts
for matrix offset to an arbitrary user defined matrix value.
Reconstitute density data from density porosity log. Calculate density porosity for desired matrix and fluid values: Where: Calculate neutron log reading for same matrix:
PHID = 0.33 PHIN = 0.24 (gas crossover occurs) DENSMA = 2680 kg/m3 DENSW = 1000 kg/m3 D = 0.02 C = -0.02 PHIDm = 0.33 + 0.02 = 0.35 PHINm = 0.24 - 0.02 = 0.22 PHIxdn4 = ((0.22 ^ 2 + 0.35 ^ 2) / 2) ^ 0.5 = 0.30 2. Assume data from Sand "D" and impose gas correction manually, since there is no gas crossover, due to shaliness. PHID = 0.12 PHIN = 0.28 DENSMA = 2680 kg/m3 DENSW = 1000 kg/m3 D = 0.02 C = -0.02 PHIDm = 0.12 + 0.02 = 0.14 PHINm = 0.28 - 0.02 = 0.26 PHIDSHm = 0.03 + 0.02 = 0.05 PHINSHm = 0.30 = 0.02 = 0.28 PHIxdn4 = (0.12 * 0.28 - 0.26 * 0.05) / (0.28 - 0.05) = 0.11 |
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