Sizing an early production facility (EPF) from well test data

An early production facility (EPF) lets an operator start producing and selling oil before permanent facilities are built. Done well, it brings revenue forward and provides the reservoir data needed to design the permanent plant. Done badly, it limits production, flares gas that could have been used or sold, and fails on the first sand slug or the first cold night.

The difference usually comes down to the design basis: the short document that tells suppliers what the facility must handle. This article explains how to build that design basis from well test data.

Start with the well test, not the catalogue

Suppliers will often offer a standard package described by a single number: barrels of oil per day. That number is only one of several that matter. Before asking anyone for a price, collect the following from your well tests and fluid analysis:

• Oil rate per well and in total, at the start of production and as you expect it to change.
• Gas-oil ratio (GOR). This sets how much associated gas the facility must separate and handle.
• Water cut, now and as it is expected to rise.
• Flowing wellhead pressure and temperature.
• Shut-in wellhead pressure. This is the highest pressure the inlet may see.
• Fluid properties: API gravity, wax appearance temperature, pour point, H2S and CO2 content, and any sand production observed during the test.

If some of this data is missing, say so in the design basis and state the assumption you have made. Suppliers can then quote on the same basis, and you can see which assumptions carry the most risk.

Rate the inlet for the worst case

The inlet section of an EPF must be rated for the highest pressure it can see. That is usually the shut-in wellhead pressure, not the flowing pressure. A facility with an inlet rated below shut-in pressure relies entirely on upstream protection, and any failure of that protection puts the separator at risk.

Check the inlet rating, the choke arrangement and the safety systems together, using API RP 14C as the reference for surface safety systems.

Size the gas side as carefully as the oil side

A package described as “5,000 BOPD” may handle that oil rate only at a low GOR. If your wells produce significant associated gas, the separator, gas outlet, flare and any gas use or compression must be sized for it. Under-sizing the gas side either limits oil production or increases flaring.

Decide early what will happen to the gas: fuel for power generation, compression for sale, reinjection, or flaring within regulatory limits. Each option changes the facility scope and its approvals.

Plan for sand and wax from day one

Sand and wax are common in many Niger Delta reservoirs. Sand erodes chokes and valves and fills vessels. Wax deposits in flowlines and vessels when the fluid cools below its wax appearance temperature.

The design basis should state whether sand removal is required and where the sand will go, and how the facility will keep fluids above the wax appearance temperature: heating, insulation, chemical injection or a combination. These are far cheaper to include at specification than to add after start-up.

Define the export specification

Crude for export or trucking usually has a maximum vapor pressure. Stabilization, typically through staged separation and sometimes heating, brings the crude down to that specification. The number of separation stages and their pressures follow from the inlet conditions and the export specification together. Include the specification and its source in the design basis.

Check the materials and the codes

If the fluids contain H2S, materials must be suitable for sour service, with NACE MR0175 / ISO 15156 as the usual reference. Separators are commonly designed to API 12J, pressure vessels to ASME Section VIII and piping to ASME B31.3. Ask suppliers to state which codes the package was built to and whether it can be certified or classed for your intended use.

If the EPF will sit on a barge

Barge-mounted EPFs are common in swamp and shallow water locations. Before choosing a package, collect the skid weights, footprints and lift points and check them against the barge’s deck strength, stability and class requirements. A facility that is right for the wells but too heavy for the available barge is not right.

Turn it into a request for quotation

With the design basis written, a request for quotation can ask every supplier the same questions and require answers in the same format. That makes new, refurbished and ex-rental options directly comparable on capacity, ratings, materials, lead time and cost.

If you have well test data and want a design basis written from it, we can help. Start an enquiry.