This article outlines the minimum barriers requirement for any Well Operations to ensure that at any moment during any well operation, the minimum numbers of barriers are in place to prevent any uncontrolled flow from the well. This applies to all potential flow paths including annuli and control lines.
1 GENERALITIES
1. Wells and associated completions shall be designed so that, during the life cycle of any eruptive well, the reservoir is isolated from the atmosphere by a minimum of two pressure tested competent and independent barriers.
2. The following regulations apply to wells that are capable or have the potential to naturally flow to surface. Where this is not the cas, hhe Well Operations Manager must:
a. approve a Barrier Standard based on a risk assessment for the particular well.
b. risk assess the potential presence of residual or trapped gas in the well, particularly below packers.
3. Prior to removing any tree cap, Xmas Tree or BOP a minimum of two independent and tested barriers shall be established in the well (three are preferred where possible). If possible, the barriers are to be tested in the direction of potential flow. The measurable leak rate shall be zero.
If the barrier is to be inflow tested then the inflow test should be of at least 30 minutes duration. Extrapolation methodology may be used to measure the leak rate. The secondary barrier shall be capable of closing / withstanding the ensuing differential pressure should the inflow test fail.
4. An SCSSSV may be considered a barrier if it has been inflow tested for 30 minutes with a zero leak rate - Note that lower value pressure tests are more stringent than high differential pressure.
It is recommended that the SCSSSV barrier is used in conjunction with a plug set higher in order to avoid an accidentaly dropped toolstring from damaging the flapper or ball.
If this plug is considered a barrier, then it shall also be inflow tested for 30 minutes before pressure testing from above.
The use of an SCSSSV as a barrier shall be risk assessed. And be considered as the exception rather than the norm. SPE 96337 gives more background on the use of an SCSSSV as an acceptable barrier on live wells.
5. Two barriers shall be maintained at all times during any well operations. Should one of the two barriers fail or become degraded then operations must cease until the barrier is restored.
6. During well operations the combination of barriers shall be either:
a) Two independent and tested mechanical devices.
or
b) A combination of a tested mechanical device and a stable fluid column where the fluid level is visible and has sufficient density to prevent flow from the well.
The fluid column is only considered as a barrier if the level can be continuously monitored / verified (via the trip tank for example) and fluid can be added to the well to reinstate the fluid levels in both annulus and pipe / tubing. Consideration (risk assessment) should also be made on the actions to be taken if the well begins to flow from either pipe or annulus.
If the level of the fluid column is not visible then the actual fluid level must be monitored with a device which will give a read out of any sub surface fluid level e.g. a Sonolog. The monitoring period should continue for the duration of the work and measurements should be of sufficient frequency to ensure the liquid level is static.
7. Drilling fluid or Completion fluids containing solids for weighting purposes can be considered as a barrier for a limited period of time during which time the fluid should not segregate or sag. If necessary lab aging tests must be done to estimate this time.
8. In case of total losses the column of fluid can be considered as a barrier if the flow rate of the feeding fluid column has a velocity downward greater than gas bubble coming up the well.
9. To be considered as independent, the two barriers shall not have a common mode of failure (E.G. a column of fluid on top of a plug cannot be considered as independent because a plug leak will induce the loss of the hydrostatic column).
10. A barrier has to be preferably pressure tested in the direction of the flow to prove its integrity.
11. Cement plugs shall be a minimum of 100m (300ft) in length, weight and pressure tested before they are considered as barriers (Ref to WOR 28 Abandonment).
12. Cement in annuli shall be a minimum of 100ft (30m) in length and verified by pressure test or CBL or temperature log, or a minimum of 1000ft (300m) verified by pressure differential when finally displacing the cement. In case of doubt, a log will be run to ensure presence and quality of cement.
13. During any production or well testing operations a minimum of two independent and pressure tested isolation mechanical devices are compulsory.
14. Simultaneous operations may be carried out if each individual activity is protected by two independent and pressure tested barriers, neither of which may be compromised by the concurrent activity carried out.
2. WIRELINE, COILED TUBING AND SNUBBING OPERATIONS
Once they have been successfully tested are considered as barriers for:
1. Wireline:
a. Christmas tree valves for rigging up,
b. Wireline BOP,
c. Gate valve capable of shearing the wire line.
2. Coiled Tubing:
a. Stripping head,
b. A stable fluid column of adequate density,
c. CT BOP equipped with blind shear rams
d. Double check valve
3. Snubbing Operations:
a. The stripping head,
b. Snubbing BOP equipped with shear blind rams,
c. A stable fluid column of adequate density.
d. Double check valve or double plugs
3. PARALLEL STRING
1. When running parallel string, control line, wireline on side entry sub a pipe ram is not considered as a barrier unless a field test has proven otherwise.
2. The requirement of two independent tested barriers must be in force in the running configuration.
3. A dispensation has to be submitted for approval in case the two barriers requirement cannot be met.
4 DUAL STRING
1. When running dual strings (double completion) a BOP is considered as a barrier only if proper dual pipe rams are installed and satisfactory pressure tested.
5 ANNULUS INJECTION WELLS
1. Gas lift valves shall not be considered as a barrier.
2. An annulus safety valve may be considered as a barrier providing it has been successfully inflow tested.
3. Annulus safety valves shall be installed below the sea bed or ground level.
6 CASING ANNULUS ISOLATION OF HYDROCARBON BEARING FORMATIONS
In order to ensure that a permeable hydrocarbon bearing formation is adequately isolated via a casing annulus, a minimum of two good quality barriers are required:
a. Good quality cement across and above the hydrocarbon zone.
b. Good quality casing hanger/seal assembly lockdown to prevent the casing from being ejected from the well