Selection of Rw from Various Sources
Water resistivity data can be sparse or overwhelming, depending on where you are working at the moment. The usual sources in order of preference are:

1. Produced water from the zone being analyzed in the same well or nearby offset wells, analyzed for Rw in the lab.

2. Drill stem test or perf test water from the zone being analyzed in the same well or nearby offset wells, analyzed for Rw in the lab. The test should produce at least 1000 ft (300 m) of water before using the data, to prevent mud filtrate contamination from causing errors. The sample should be from the bottom of the test.

3. Produced or DST water from a nearby zone in the same geologic horizon (do not cross erosional boundaries), analyzed as above.

4. Water catalogues produced by local well log societies or government agencies.

5. Back calculated from log data in clean water bearing zone in the same well or nearby offset well (Rwa or Ro method).

6. Back calculated from nearby water bearing zone in same geologic horizon.

7. Calculated from SP in clean water bearing zone in same or nearby zone in same well or nearby offset well.

8. If no water has ever been produced in the area, back calculated from a laboratory measured or assumed PHIxSW product.

9. Local rule of thumb for water resistivity versus depth or versus geologic horizon.

Do not use:

1. Water from a DST or perf test that recovered mostly filtrate water (check water chemistry) or recovered only a small amount of water.

2. SP or Rwa in a shaly zone.

3. SP or Rwa in a hydrocarbon bearing zone.

4. SP in a carbonate or evaporite sequence.

5. SP in a low porosity zone.