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“All models are wrong, but some are useful” George E.P. Box

“All models are wrong, but some are useful” George E.P. Box
Merlin ERD

“All models are wrong, but some are useful” George E.P. Box

My colleague Santiago Lopez finished his Master’s Thesis at Robert Gordon University, Aberdeen in November last year. His study focused on swab and surge – how it’s handled in models, how it’s calibrated against field measurements and how it affects operational practises.

Merlin had pressure data from a client running a string of wired drill pipe and Santi used that to benchmark the output of various software models against “as measured” reality. He also accessed a few industry standard software packages which happen to run different swab/surge models. It was an interesting exercise to compare the results of the hydraulics models with reality and then look at what happens when the drill string lays on the low side of the hole, the dominant condition for most high angle or horizontal wells.

Existing models consider pipe to be positioned concentrically in the annulus for the purposes of swab/surge calculations. Pipe on the low side will invalidate that assumption, but what does it mean in reality? It has been shown* that non-concentric pipe reduces the pressures generated when the drill string moves in the fluid column. Santi also found that all models were conservative in nature compared to reality, which is desirable. After all, you don’t want to induce a problem by operating within the boundaries of the simulation.

Using a hybrid swab/surge model (see the reference below) assuming pipe on the low side, Santi was able to more closely match the measured pressures compared to the other models, effectively getting closest to reality. Operationally, improved understanding of swab and surge loads could be used to speed up tripping or to enable mud weights to be more closely optimised. In his analysis case, a 30% increase in tripping speed could have been realised,