Low-Resistivity Thin-Bed Pay: Perforate or Abandon?
An exploration well encounters a 14 m gross sand at 2,180 m TVD with thin-bedded sand-silt laminations below wireline resolution. The laterolog reads 1.9-2.4 ohm-m across the interval; a clean sand at the same depth reads 18 ohm-m. Density-neutron shows porosity of 0.26-0.30 with neutron-density crossover over 4 m. Water resistivity from a produced-water sample is 0.12 ohm-m at 78 degrees C. Archie with a=1, m=2 and n=2 gives Sw above 0.85. A sidewall core capillary-pressure composite indicates Sw of 0.35 at 60 m above free water level. NMR shows clay-bound water of 0.06 v/v, capillary-bound water of 0.04 and free fluid of 0.14. The petrophysicist recommends not perforating; the asset team sees 4 m of crossover and wants to test. A modular dynamic tester has already recovered 15% water from a nearby sand.
1. Explain the Archie discrepancy quantitatively using a laminated sand-silt model. What is the sand-layer water saturation, and what does that imply?
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The laterolog measures the bulk interval, not the sand laminae. Take a laminar model with 35% sand at Rt = 20 ohm-m and 65% silt at Rt = 1.2 ohm-m. Bulk resistivity is 1 / (0.35/20 + 0.65/1.2) = 1 / (0.0175 + 0.5417) = 1.79 ohm-m, which matches the logged 1.9-2.4 ohm-m range. Applying Archie to that bulk value is invalid because it treats the silt conductive water as formation water in the sand.
In the sand laminae, with phi = 0.24 and Rw = 0.12, Sw = square root of (0.12 / (20 x 0.24²)) = square root of (0.12 / 1.152) = 0.32. That is consistent with the capillary-pressure result of 0.35 at 60 m above free water level. The interval is therefore low-resistivity pay caused by thin-bed anisotropy, not a wet sand. The implication is that the bulk Archie result overstates Sw by more than 50 saturation units and should not be used as an abandonment criterion. The remaining risk is whether the sand laminae are laterally connected enough to flow.
2. Recommend a completion and data-acquisition decision. What would make you walk away?
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Test the interval, but do it as a controlled appraisal rather than a full commit. The NMR free fluid of 0.14 v/v and the capillary-pressure Sw of 0.35 both support movable hydrocarbon; the 15% water recovery is from a different sand and cannot be transferred across a fault or a contact without a gradient check.
Recommended programme: confirm the free water level with an MDT gradient in this sand; perforate only the upper 4 m of crossover with limited-entry charges; run a short flow-and-build-up test with surface and downhole sampling; monitor water cut and pressure transient response. If the test produces at least 30% hydrocarbon with a stabilising water cut, evaluate a horizontal or multi-frac completion. If it produces water only, or the MDT gradient shows a contact above the perforations, walk away. Also acquire dielectric or high-resolution LWD resistivity in the next well so the laminations can be resolved rather than modelled. The decision gate should be written before the test, not after seeing the result.