Abstract

Sour gas concentrations in DST samples from the NSB 'A' field show wide variations. This can be observed in tests from different wells, different tests from the same well and different samples from the same test. H₂S measurements show the most obvious variations, from less than 0.5% to greater than 5%. CO₂ measurements also vary beyond normal sampling or analytical errors. Geological considerations suggest that these variations are not controlled by subsurface reservoir properties.

Drill stem test (DST) rate and volume data suggest that variations in sour gas concentrations are controlled by the production of gas from formation water. Minimum sour gas concentrations occur in DST samples with water production rates which match those predicted for water vapor condensation. Elevated sour gas concentrations occur in DST's in which produced water either exceeds or falls short of water vapor condensation predictions.

In the case of excess water production, the source of the elevated sour gas concentrations is clear. Formation water in equilibrium with a gas phase containing the minimum sour gas concentration, 28% CO₂ and 0.5% H₂S, would contain a dissolved gas with 80% CO₂ and 10% H₂S. Exsolution of this dissolved gas during DST operations would result in substantial increases in sour gas concentrations. In tests with water deficits, it is probable that liquids are being trapped in the subsurface, but this cannot be proved.

These observations have been extended to a database on sour gas concentrations in north Sumatra and have had a substantial impact on assessments of the exploration and resource potential of this area.