Foam Depression Tests

A foam depression test is recommended to calculate the producing pressure and/or the inflow performance relationship. The report includes pressure at reference depth, graph of gas-liquid interface pressure, liquid gradients, calculations and results of gas and liquid column pressures.

• Pump intake producing pressure calculation
• Annular fluid column gradient calculations
• Fluid level data supplied in graphical format

The purpose of foam depression test is:

• To determine an accurate producing bottom-hole pressure (PBHP)
• To depress any foam prior to a pressure build-up survey to ensure accurate fluid levels and pressure data.

In order to accomplish this, the fluid level is changed by closing the casing valve at surface to increase the pressure exerted by the gas column. Because the pump unit operation is not interrupted, no production downtime is involved. The fluid gradient is established by accurately recording the Casing Pressure and Corresponding fluid level at various time intervals after closing the casing. The wellbore storage capacity is sufficient in most cases to conduct depression testing without altering the production rate at the formation.

The rate of free gas flowing up the casing has a significant impact on the fluid gradient. The amount of free gas produced up the casing is related to:

• Permeability of the zone to gas (mobility)
• Properties of the produced fluid (bubble point pressure, solution gas – oil ratio, and water cut)
• The producing bottom-hole pressure (PBHP)
• Downhole separation of gas
• Wellhead tie-in (back-pressure valves, flowline pressure)

There are several factors which can result in an unsuccessful Foam Depression test:

• Casing valve opened during test or faulty
• Bottom-hole pump gas locks
• Operating conditions changed (pumping unit stopped or speed-up/slowed down during test)
• Liquid flow up the annulus
• Inaccurate or unknown completion details
• Insufficient test duration
• Unstable well

Fluid Level Pressure Gradients

• 1-5 kPa/m foam gradient
• 5-10 kPa/m fluid gradient

1 kPa/m being light foam and 10 kPa/m being solid water

Click here to view Definitive Opt Sample Producing Sub-Surface Pressure Report