Page 55 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

sulfide and carbon dioxide. These gases are called acids and
sweetening/acid removal is the process of taking them out.
Natural gas sweetening methods include absorption processes, cryogenic
processes; adsorption processes (PSA, TSA and iron sponge) and
membranes. Often hybrid combinations are used, such as cryogenic and
membranes.

Gas treatment could also include calibration. If the delivery specification is
for a specific calorific value (BTU per scf or MJ per scm) gas with higher
values can be adjusted by adding an inert gas, such as nitrogen. This is
often done at a common point such as a pipeline gathering system or a
pipeline onshore terminal.

4.4 Oil and gas storage, metering and export

The final stage before the oil and gas leaves the platform consists of
storage, pumps and pipeline terminal equipment.

4.4.1 Fiscal metering

Partners, authorities and customers all calculate invoices, taxes and
payments based on the actual product shipped out. Often custody transfer
also takes place at this point, which means transfer of responsibility or title
from the producer to a customer, shuttle tanker operator or pipeline operator.

Although some small installations are still operated with a dipstick and
manual records, larger installations have analysis and metering equipment.
To make sure readings are accurate, a fixed or movable prover loop for
calibration is also installed.

The illustration shows a full liquid hydrocarbon (oil and condensate) metering
system. The analyzer instruments on the left provide product data such as
density, viscosity and water content. Pressure and temperature
compensation is also included.

For liquids, turbine meters with dual pulse outputs are most common.
Alternatives are positive displacement meters (pass a fixed volume per
rotation or stroke) and coriolis mass flow meters. These instruments cannot
cover the full range with sufficient accuracy. Therefore the metering is split
into several runs, and the number of runs depends on the flow. Each run
employs one meter and several instruments to provide temperature and
pressure correction. Open/close valves allow runs to be selected and control
valves can balance the flow between runs. The instruments and actuators

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