iStart with the unit
Most capacity numbers in a gathering system are liquid numbers: a pipeline, a manifold, a separator inlet, a pump. Most targets are oil numbers. The conversion between them is the water cut of the wells being moved, not the field average, and it changes with time.
A forecast that adds a liquid capacity to an oil total looks right for one quarter and wrong after that. The wells that get moved are the ones with the highest water cut and the fastest decline.
iiCount each well once
When a well is moved from one facility to another, the receiving facility gains and the source facility loses. A spreadsheet that records the gain and keeps the well in the source total has created oil. The check is simple: field oil before the move equals field oil after the move, before any losses or interventions are applied.
The same applies to net additions. A shut-in well that restarts at the receiving facility is a net addition. It goes in a separate column from a well that was already flowing somewhere else.
iiiFollow the water to the ground
The receiving facility has an oil limit and a water limit. Downstream of it there are disposal pumps, injection wells with an injectivity that depends on wellhead pressure, re-injection headers and perhaps a pond. A transfer that respects the line capacity and the oil limit can still fail at the disposal pump.
In the field forecast in our selected work, applying the receiving facility's water limit moved the binding limit from the sending facility's oil capacity to the receiving facility's water treatment. The right next investment was in water, not in another transfer line.
ivWrite the rules down
The rules that stop these errors fit at the top of the workbook. Liquid is not oil. A moved well is counted once. A well test is not an allocated rate. A disposal increase is not an oil gain unless it releases a named deferral. A planned enable date is not an available date. A forecast that states them can be audited. One that does not will be believed until it is wrong.
A transfer line adds liquid capacity. How much becomes oil depends on where the water can go.