Uncertainty over pipe and pump stroke volumes
can cause cementing errors that will affect production.
This can happen for example with unfamiliar pipe, new pump configurations
or comparatively short liners.
Over-efficient pumping or narrower than expected
drill-pipe will lead to early arrival of cement in the liner. In the event
that plugs do not bump crew will be unaware and may over-displace, reducing
production interface by creating a void zone around the sump.
Conversely a cautious approach when no bump is seen
may leave the sump cemented and or an incompletely cemented liner lap.
Best case scenario would be a cost over-run milling out the surplus cement
prior to the clean out. Worst case scenario internal blow out above the
cement due to the void zone below the liner lap.
Volumetric Drifting
The volumetric drift will land at the number of pump strokes required
to displace the drill pipe then burst at 1500-2500 psi depending on mud
and flow rate. This provides a useful reference to reduce errors in theoretical
volume and pump efficiency calculations. For example with unfamiliar pipe,
new pump configurations or for short liners.
Case History - Sample Numbers
|
GOM
|
November 2008 |
| 5" Drift Catcher | with NC50 connection 2.6" OD volumetric drift |
| LINER SYSTEM | TIW |
| Theoretical Displacement | 189.60 BBLS (error 2.9%) |
| Volumetric Drift Displacement | 184.50 BBLS (error 0.1%) |
| Actual cementing displacement | 184.30 BBLS |
| Drill Pipe Length | 10,687.00 ft |
| Mud Weight | 15.3 ppg (Type H2O) |
| Cement weight | 16.2 ppg |
| Conclusion | Drift Catcher correctly warned that actual cement volume was significantly less than theoretical expected - Fortunately plugs bumped to confirm numbers in any case |
|
North Africa
|
Summer 2009 |
| Application | Previous well displacement calculations had been extremely inaccurate. Running the volumetric drift in both 5" and 3.5" drill pipe greatly increased the confidence levels in the calibration prior to cementing and prevented any repeat over-displacement. |
| 5" Method | 2.6" Volumetric Drift into 5" Catcher |
| Pump Rate 1 | 250 gpm - 90 bbls |
| Pump Rate 2 | 100 gpm - 15 bbls (2500 psi to burst size 21 disc) |
| 5" Result | 0.01638 bbl/ft |
| 3.5" Method | 2.25" Volumetric Drift into 3.5" Catcher |
| Pump Rate 1 | 150 gpm - 24 bbls |
| Pump Rate 2 | 75 gpm - 4 bbls (1400 psi to burst size 18 disc) |
| 3.5" Result | 0.0068bbl/ft |
The number of stokes are then recorded to confirm the volume.
|
Volumetric and
choked drift are inter-changeable and
can be landed in exactly the same catcher sub as the choked drift
|
|
Method selection: The standard drift is
the preferred method for open-hole drifting where selecting a
larger choke will enable pumping at full rate for long periods
without undue pressure loss or component wear. In most cases,
drifting back at the shoe as normal means either method can be
selected.
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