See attached anex on gaslift valve testing of the ISO standard for well integrity for the operate phase that was released in August 2014. ISO/DIS 16530-2:2014 Annex
(informative)
Example of leak testing of gas-lift valves
Rigorous inflow testing or leak testing of in situ gas-lift valves to compare with the API (see API RP 14B) benchmark leak rate of 0,417 m 3/min (15 scf/min) is difficult and time-consuming due to
- the very low API leak rate [0,417 m 3/min (15 scf/min)] compared to the large gas-filled production annulus volume;
- temperature effects that potentially mask the observed pressure changes;
- complex manifolds and valve arrangements that make it difficult to determine where leaks are originating.
If it is necessary to carry out this rigorous testing, an example method is outlined below. However the
methodology can be applied only when gas is leaking into the gas-lift valve.The method does not apply to
liquid leaks. Therefore, it is necessary to ensure that any liquids are bullheaded away before testing starts.
The following steps should be taken.
a) To perform an inflow test on the gas-lift valves, the pressure in the tubing shall exceed the pressure
in the annulus. To achieve this, the tubing is displaced to gas and the annulus pressure is bled off;
this also ensures gas across the gas-lift valve(s), at least initially. Shut in the well at the choke and/or
the flow wing valve.
b) Allow the tubing pressure to build up to xx kPa (bar) (see below). Consider also bullheading gas into the tubing.
c) Shut down and isolate the gas lift and allow the pressures to stabilize.
d) Bleed off the annulus to a pressure less than 50 % of the shut-in wellhead pressure.
e) Observe the annulus pressure, and from the pressure build-up calculate the combined leak rate of the gas-lift valves.
As shown
Figure S.1, the higher the tubing pressure, the more the fluid is pushed back into the formation.
Ideally, the shut-in tubing pressure should be such that the fluid level is between the gas injection valve and the top perforation (note that over-displacing gas into the reservoir can result in reservoir impairment in some specific cases). This would give the maximum tubing pressure for performing the test and, if there is a leak at one of the gas-lift valves, it is certain that the leak is gas and not liquid. This allows making the correct leak-rate calculation. If liquids leak through the gas-lift valve or the packer into the gas-filled annulus, they will go unnoticed except when the leak is very large. Attempt to keep a constant gas pressure on the tubing to ensure that the liquid level is maintained below the gas-lift valves.
This approach does require an understanding of the reservoir pressure.
The difficulty in the interpretation of the data from this type of gas-lift valve leak test is the large volume of the annulus. For example, for a 4½ in × 9⅝in annulus with a capacity of 30 l/m, the volume in the annulus can easily be in the order of 50 m 3 to 75 m3. At a gas leak rate of 0,417 m3/min (15 scf/min) into a 60 m 3 annulus, it will take 3 h to result in a 100 kPa (1 bar) pressure increase. An increase of the average gas temperature in the annulus of 6 °C results in a similar pressure increase. So, to be able to accurately determine the leak rate, it is important that the gas temperature be stable during the test or that the temperature can be accurately monitored with surface and down-hole gauges, so that corrections for temperature changes can be made.
© ISO 2015 - All rights reserved
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Paul Hopmans
Software Portfolio Manager
Shell Technology E&P
Rijswijk
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