Friday Haggler
A reminder of last Friday’s Haggler
You have just finished drilling an 8-½” high-angle reservoir section, which is known to have intervals that are susceptible to weak bedding-plane failure. A full clean-up cycle has been performed, but residual cuttings/cavings beds are known to exist, with several over-gauge zones. The next operation will be to run a 7” liner to TD. Based on the data shown on the fixed-depth (TD) swab/surge model, which of the following tripping options would be safest overall? Assume a pulling speed of 3 minutes per stand (range 2 drill pipe), except where otherwise stated.
A. Pull on elevators at 1 minute per stand
B. Pump out (no rotation) with 450gpm
C. Pull on elevators at 3 minutes per stand in open hole, increasing speed in cased hole
D. Pump out (no rotation) with 50gpm
E. Back-ream out of open hole with parameters used to drill (450gpm, 100rpm)
F. Pump out (no rotation) at 100gpm & 10 minutes per stand
The Answer
Short Answer: D
Long Answer: While POOH on elevators is preferable the model swab pressures rule out A and C. Pumping out at drilling flow rates will move at least some of the cuttings to above the BHA, creating a high pack-off risk, ruling out B. Backreaming (E) is best avoided, if possible, especially in this case where instability has been identified as a problem. This leaves pumping out with low flow (lubricating) to neutralise the swab effect. It is also important to define a maximum flow rate to avoid the same problem as discussed above for B. The maximum lubricating flow rate needs to be calculated for a specific pulling rate. While there are various methods to do this, ranging from open-ended pipe displacement to open-hole volume, it is best to use the latter as a means of determining the maximum to avoid upward flow past the BHA. If pumping 100gpm at 10 min/std, you are over-displacing the open hole volume by 73 gpm, which is enough to mobilise cuttings and increase the pack-off risk, ruling out F. 50 gpm is more than enough to eliminate the overall swab effect, while avoiding over-displacing the open hole and mobilising cuttings. D is the best option. No space to share calcs unfortunately
