Tier & completeness
“Is this a Tier 3 inventory?” has no one-word answer. It is a mix, weighted by methane mass rather than by source count — because one source carries about 70% of the total and another is a confirmed zero, and counting sources would let eight small ones outvote the term that dominates.
The ladder
Tier 1
Default factors on a facility proxy
A published emission factor scaled by gas throughput or time in service. Legitimate for a source whose contribution does not warrant a measurement campaign — and a screening estimate is not a failure.
To move up: A component inventory and a leak-detection survey.
Tier 2
Metered activity, published or assumed parameter
Metered volumes from the operator's monthly return, combined with a parameter taken from literature or carried as a bounded distribution. The bulk of this inventory by mass sits here, because the activity data is metered and the physics parameter is not.
To move up: Direct measurement of the governing parameter — for flaring, combustion efficiency.
Tier 3
Facility-specific estimation on measured inputs
A correlation evaluated on the facility's own assay and operating conditions — the flashing and loading terms, where the crude gravity, separator conditions and lifting record are the facility's own.
To move up: Vapour measurement, which would move these from calculated to measured.
| Source | Tier | Basis | What would raise it |
|---|---|---|---|
| Flare — uncombusted methane | Tier 2 | calculated | Direct measurement of flare combustion efficiency |
| Crude-tank flashing | Tier 3 | calculated | No route recorded — a confirmed value, not a gap. |
| Produced-water degassing | Tier 3 | calculated | No route recorded — a confirmed value, not a gap. |
| Shuttle-tanker loading | Tier 2 | calculated | Vapour measurement during a loading operation |
| Compressor rod-packing vents | Tier 1 | estimated | Vent-rate measurement or component count |
| Compressor wet-seal vents | Tier 1 | estimated | Seal-type confirmation and vent measurement |
| TEG reboiler vent | Tier 1 | estimated | Still-column vent measurement |
| Equipment fugitives | Tier 1 | estimated | Component count and leak-detection survey |
| Pneumatic devices | Tier 1 | estimated | Device inventory and drive-medium survey |
| Cold vents / blowdowns | Tier 1 | estimated | Blowdown event log |
| Sand handling | Tier 1 | estimated | No route recorded — a confirmed value, not a gap. |
| MEG-regeneration flash vent | Tier 3 | calculated | No route recorded — a confirmed value, not a gap. |
Included
- Methane from twelve production sources
- Combustion CO₂ from flared and fuel gas
- Routine operation across every reported facility-month
Not assessed
- Non-routine and abnormal events, which can dominate a single month
- Drilling and well-intervention campaigns
- Marine logistics, shuttle-tanker propulsion, aviation
- N₂O from combustion, and CO₂ already present in the produced stream
This is a production-emissions inventory, not a full Scope 1 boundary. Stating that plainly is the difference between a figure a verifier can work with and one they have to reverse-engineer the boundary of.