Gas cap depressurization

Safe start-up of wells after shut-in

Following well shut-in, gas and liquid remaining in the flowlines segregate, with the lower-density gas rising to the top to form a “gas cap”.

The well-head pressure rises much higher than the normal flowing pressure, and the whole system (metal pipework and fluids) cools to the ambient temperature.

The combination of conditions presents significant challenges for well start-up.

Challenge

On start-up, the pressurised shut-in gas, already cold, is blown down through the choke into the production system. The Joule-Thomson effect significantly cools the gas as it flows through the choke.

This presents many safety and operational constraints, in particular:

• embrittlement risk: avoid minimum metal temperatures in pipes, manifolds and separators
• equipment capacity: excessive condensate forming in the cold gas.

These threats to the system are transient; as the blowdown progresses the pressure at the well-head decreases and the fluid temperature increases to those seen during normal operation.

Operators must determine a gas-cap policy that starts up the well within these safe operating constraints. Consideration must be given to many factors that include: blowdown rate; choke opening strategy; whether pre-warming of the flowlines is required and separator pre-pressurisation.

The optimal answer usually involves a trade-off between several factors. The gFLARE advanced process modelling solution enables all these factors and risks to be assessed quickly and accurately.

gFLARE gas-cap blowdown

PSE’s gFLARE^® gas-cap blowdown solution is based on:

• an integrated well and process model: incorporating a reservoir production rate (IPR); segregated two-phase pipe models for the subsea flowlines and riser that are linked to detailed top-side processing equipment and pipe models.
• accurate, high fidelity, models that predict 2D metal temperatures (across the pipe and vessel wall thickness) and along the length of all the pipe segments
• rigorous and fully coupled state of the art advanced thermodynamics and transport properties are used throughout the model.
• optionally, use of PSE’s detailed Oil / Gas separator model to assess low temperature risks in the separator
• advanced solvers that are able to handle dynamic multiphase flow scenarios

North Sea well start-up

A model of a typical North Sea oil well is shown below. This has a typical starting well head pressure of 115barg with choke opened such that the gas-cap is blowdown at an initial rate of 5000kg/hr.

Typical results

Results are shown for the blowdown of a gas-cap formed in a typical North Sea oil well.

• Figure 1 – the temperatures for the flowline as it joins the manifold are shown on the right (click to enlarge) for the base blowdown rate. The solid and dotted black lines give the pipe inner and outer temperatures respectively. The blue line shows the fluid temperature. The minimum metal temperature is reached between 150 and 200s.
• Figure 2 – The second figure shows the sensitivity of the system to different depressurization rates. In this case, it is seen that a faster depressurization rate increases the minimum metal temperature seen at this location.

Apply high-fidelity dynamic modelling to your well start-up

The use of a rigorous model-based safety approach based on our gFLARE technology and coupled process and flare network models can result not only in improved process safety but also an improvement in operations and a considerable reduction in CAPEX. Read our case studies to find out how Oil & Gas companies are savin